Ultrahigh Vacuum

Ultrahigh Vacuum P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

Radio and Electrical Engineering Division, National Research Council, Ottawa, Canada

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Physical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. T h e Significance of Surface Effects .................................. B. Physical Adsorption ............................................... C. Chemisorption .................................................... D. Sources of Gas . . . . . ....................... E. Positive Ion Impact on ..................... F. Electron Interactions ..................... G. Photo and Chemical R ..................... 111. Technology of Ultrahigh Vacuum . . . ........................... A. Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Measurement of Total Pressure ..................................... D. Measurement of Partial Pressure ................................... E. Measurement of Pumping Speed, Leak Rate, and Gauge Sensitivity . . . . . . F. Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Applications .................................. A. Surface Physics and Chemistry . . . . . . . . . . . . . . B. Thin Films . . ............... C. Thermonuclear a ............... D. Space Simulation ........................ References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page 323 325 325 328 337 343 358 368 369 371 371 386 391 405 412 414

418 419 420 422

I. INTRODUCTION Ultrahigh vacuum may be considered as the region of pressure below

lop8 Torr.* Prior to 1950, no adequate method of measuring pressures

below Torr existed because the ionization gauges then available were limited by a residual current to the ion collector. In 1950, Bayard and Alpert (1) developed an ionization gauge which reduced the residual current by two to three orders of magnitude. T h e development of this gauge permitted a systematic study of the processes limiting the ultimate pressure in vacuum systems, and led to the rapid improvements in ultrahigh vacuum technology of the last ten years. I t is presently possible to obtain pressures less than 10-l2Torr in small systems, and pressures of the order of 10-lo Torr have been achieved in large metal systems.

*

1 Torr = I m m H g .

323

324

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

T h e initial requirement for ultrahigh vacuum (u-h-v hereafter) arose in the fields of surface physics and chemistry. I n experiments where adsorbed gases have an appreciable effect on surface properties, it is necessary to maintain the surface in a clean condition for a reasonably long time. A monolayer of adsorbable gas will form on a surface in about one second at 10-6 Torr. Since cleaning a surface usually involves heating, the pressure of adsorbable gases must be less than about 10-O Torr to permit measurement of the properties of clean surfaces with observation times reasonably long compared to the cooling time of the sample. U-h-v is also necessary in any system where gases of very high purity have to be introduced (e.g., thermo-nuclear machines) or where very pure evaporated films are to be prepared. More recently u-h-v techniques have been applied to the construction of vacuum chambers for the simulation of extra-terrestrial conditions. Several reviews of the techniques and general problems of u-h-v have appeared (2-8);some discussion of u-h-v is included in more general review articles (9, 10, 11).The use of u-h-v techniques in thermonuclear machines is discussed by Munday (12) and Grove (13). T h e ultimate pressure achieved in a vacuum system is established by an equilibrium between (a) the rate of arrival of gas from various sources, and (b) the rate of removal of gas from the evacuated volume (either by removal of gas molecules from the system, or by transfer of gas to the adsorbed phase within the system). T h e partial pressure ( p ) in Torr of any one gas in the steady state ( p and temperature constant) is given by

where

L is the leak rate through holes in the envelope (molecules/sec),

FK is the permeation rate through the walls of the system (molecules/sec),

F,, is the rate of evolution of gas absorbed in the walls and component

parts of the system (molecules/sec), FA is the rate of evolution of previously adsorbed gas (molecules/sec), FR is the re-emission rate of gas molecules previously pumped by being ionized and driven into a solid surface (molecules/sec),

no = 3.27 x lO1O molecules/liter at p = 1 Torr, and T = 295"K, S is the speed of momentum-transfer pumps in the system (i.e., diffusion or molecular-drag pumps) (liters/second), S, is the speed of the ionization pumps (liters/second),

ULTRAHIGH VACUUM

325

is the specific arrival rate of gas molecules at a surface ( u = 1.98 x 1021M-1/2cm--2sec-1at 295°K and 1 Torr for a gas of molecular weight M ) , A, is the area of the j t h adsorbing surface, v

ai the adsorption probability on the j t h surface. (The adsorption probability is the fraction of the incident molecules which are adsorbed on a surface.) T h e various gases present in the system cannot, in general, be treated independently. Interaction between different gases can affect the two emission rates FA and FR,the electronic pumping speed Si and the adsorption probabilities a j . Further interaction between gases can occur through chemical reactions occurring principally at heated surfaces. T h e existing data concerning the various competing processes listed in Eq. (1) are not sufficiently extensive to permit detailed calculations of partial pressures, nor are such calculations usually necessary. T h e considerations of these various processes, which is the subject of Section 11, is of value in determining the best processing methods, materials and techniques to be used in obtaining u-h-v in a specific system. Section I1 is concerned with the various physical processes which (a) control the behavior of sources and sinks of gas and (b) affect the measurement of pressure. Section 111 describes the methods of production and measurement of u-h-v. T h e applications of u-h-v techniques are described very briefly in Section IV.

11. PHYSICAL PROCESSES A . The Significance of Surface Effects T h e pressure in u-h-v systems is very significantly affected by phenomena occurring on the solid surfaces of the system (chemical and physical adsorption, surface migration, and evolution of previously sorbed gases). T h e importance of surface effects may be demonstrated by some simple considerations. Table I shows some of the important parameters governing surface effects, as a function of pressure at T = 295°K. T h e third column gives the rate of impingement of gas molecules of molecular weight 28 on one square centimeter of surface. T h e fourth column shows the time required to form a layer of adsorbed gas containing 5 x IOl4 molecules/cm2 assuming that every molecule that stirkes the surface is adsorbed (i.e., a sticking probability of unity).

326

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

This time is an approximate measure of the time it takes for the pressure to come to equilibrium (i,e., for adsorption to cease) after a surface has been cleaned of adsorbed gas. T h e fifth column shows the ratio of the TABLE I

Pressure (Torr)

1 10-8 10-11

Molecular density in gas phase n, (molecules/cms)

Impingement rate v (molecules/cma/sec)

3.3 x 1016 3.3 x 1010 3.3 x 106

3.8 x lopo 3.8 x 1014 3.8 x 109

Monolayer time tm

NdNe

1.3 X [email protected] X 7.5 x 108 1.3 sec 7.5 x 108 36 hr

number of molecules in the adsorbed phase (Na)to the number in the gas phase ( N g ) in a one-liter sphere, assuming an adsorbed density of 5 x 1014molecules/cm2. If the monolayer of gas adsorbed on the surface of the one-liter sphere was completely desorbed, the pressure would Torr. These simple considerations indicate the increase to 7.5 x enormous ratios of the numbers of molecules in the adsorbed and gaseous phases at u-h-v and the profound effect on the system pressure produced by the desorption of only a minute fraction of the adsorbed gas. T h e adsorbed gas which desorbs spontaneously at room temperature can be easily removed by baking. T h e average time that an adsorbed molecule remains on a surface is given approximately by (14), ta, = to exp (Ed/RT)

(2)

where Ed is the activation energy of desorption and to is the period of the thermal oscillation of the adsorbed molecule normal to the surface. Table I1 shows how ta varies as a function of Ed and T,taking to = sec. Measured values of Ed lie between 20 cal/mole (the heat of vaporization of liquid helium) and 236,000 cal/mole (the activation energy of desorption for 0, on Ti). This range is often divided into two parts, called “physical adsorption” (below 8 kcal/mole) and “chemical adsorption” (above 8 kcal/mole). T h e dividing line between the two ranges is somewhat arbitrarily chosen. The nature of the bonding forces for these types of adsorption is different. T h e bonding forces of physical adsorption are always present, They occur, even in the absence of permanent electric dipole or quadrupole moments in adsorbent or adsorbate, because the charge distributions in molecules are time-dependent, and

327

ULTRAHIGH VACUUM

cause a net attraction. I n chemisorption, electron exchange occurs between the adsorbent and adsorbate and the bonding forces are similar to normal chemical bonds. Chemisorption is specific; that is, chemisorption does not occur to a measurable extent for certain combinations of adsorbent and adsorbate.

Ed (kcal/mole) T = -196°C 0.1 1.0 10

50

1.9 x 10-13 6.9 x 8 x lo6 centuries m

100

m

150

m

ta

T

=

25°C

1.2 x 10-1s 5.4 x 2 x 10-6 9

T

=

500°C

1.9 x 6.7 x lo-”

x 1013 centuries 03

(sec)

14 6

x lo6 centuries m

03

T = 1000°C T

=

2000°C

1.5 x 5.2 x lo-’% 9.2 x

4

x

4.2 hr 2

x 103 centuries

6.4 x 4.2 x lo-‘ 21

If a molecule approaching a surface from the gas phase has to surmount a potential barrier before falling into the potential well of depth E d , then this barrier height is called “the activation energy of adsorption” (Ea). T h e relation between E d and Ea is written, Ed

E.8

3-q

(3)

where q is called “the heat of adsorption” and is a quantity that arises in thermodynamic studies; E d is a quantity that arises in studies of rates of desorption; in physical adsorption Ea = 0 and Ed = q. Atomically clean surfaces within an u-h-v system have a maximum pumping speed of 3.638 (T/M)”*

(4)

liters/sec/cm* if all molecules striking the surface are adsorbed. Here M is the molecular weight of the gas. Thus, for a system with a few hundred square centimeters of clean surface, the pumping speed caused by adsorption on the surface may greatly exceed the speed of the pumps attached to the system. The pumping action by adsorption is selective, at room temperature only the chemically active gases are pumped. As

328

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

the surfaces are covered by gas, and multiple adsorbed layers are formed, Ed decreases and the rate of desorption increases. An equilibrium is finally reached, and the pressure in the system becomes constant when Ed decreases to such an extent that adsorption effectively ceases or reversible adsorption occurs. These surface phenomena are discussed in more detail in the following sections. B . Physical Adsorption General discussions of this subject have been given by Brunauer (/.5), de Boer ( I d ) , and many other authors. We now separate the various processes which take place when a molecule from the gas phase is incident upon a surface [see also Tompkins (15a)l. This separation is useful because in practical systems one process or another may be made to dominate.

1. Condensation Coeflcient, Sojourn Time, and Accommodation Coeficient. A molecule colliding with a bare surface may do one of two things on the initial impact: (1) it may rebound; (2) it may become adsorbed on the surface. The probability that a molecule will execute (2) is known as the "condensation coefficient" (c). For physical adsorption a = c [see Eq. (I)]. If the number of molecules striking a surface per cm2 per sec is v [see Eq. ( l ) ] then the number adsorbed per cm2 per sec is du = cv

dt

where cr is the number adsorbed per cm2. Foner et al. (16) have measured c for argon atoms impinging on an argon surface at 4.2"K and found a value of c = 0.6. Schafer and Gerstacker ( 1 7 ) quote measured values of c for gases on glass surfaces. These results are given in Table 111, where we have used our notation. TABLE 111. CONDENSATION COEFFICIENT OF GASESON A GLASSSURFACE IN TEMPERATURE RANGE0- 100"C.a

THE

Gas

C(O0C)

c(50"C)

c(1Oo"C)

Xenon Argon Oxygen Nitrogen Neon Hydrogen Helium

0.987 0.888 0.835 0.823 0.483 0.623 0.030

0.980 0.853 0.789 0.768 0.402 0.555 0.022

0.975 0.8 13 0.729 0.71 7 0.340 0.468 0.015

After Schafer (1 7).

329

ULTRAHIGH VACUUM

Other evidence concerning the magnitude of c comes from a rather unexpected source. It is found in chemisorption that the sticking probability of many gases on metals-in particular, tungsten-lies between 0.1 and 0.9, and moreover is independent of coverage up to nearly a monolayer. This latter property has been interpreted (18) to indicate that adsorbed molecules exist in a physically adsorbed state prior to becoming chemisorbed. If this is so, then c is at least as large as the sticking coefficient and hence lies in the same range as the results of Table I11 for glass. It may be noted that the experiments on tungsten have all been done in u-h-v systems. Mickelsen and Childs (19) have given a theoretical discussion applicable to calculation of pumping speeds in practical systems since they consider multiple collisions with wall surfaces which is the usual practical situation. Hurlbut (20) finds that for most cases of scattering of nitrogen at surfaces of steel, aluminum, and glass, the cosine distribution is obtained as has been generally assumed. Cabrera (22) has examined analytically a simple one-dimensional model of the impact 1 for between a gas molecule and a surface, and has concluded that c practical values of the parameters. Zwanzig (22) draws a similar conclusion from a similar model. Littlewood and Rideal (23) discuss the general problem of measuring c and conclude that heat transfer effects can cause serious discrepancies in measured values. They quote their own vaIues of c for long-chain fatty acids and alcohols between 0.36 and 0. I. Our conclusions from this evidence is that c lies between 0.1 and 1 for the great majority of collisions of gas molecules with surfaces, with a preference for values nearer 1. It may be noted in the above discussion that no dependence of c on the coverage of the surface with previously adsorbed gas was explicitly mentioned. I n the derivation of his famous isotherm equation, Langmuir (24)made the assumption that an incident molecule striking a site already occupied was elastically reflected. Thus if a fraction B of the available sites of a surface are already occupied, then the number of molecules adsorbed per cm2 per second is just

+

da

--vc(i -

dt

e)

In a generalization of Langmuir’s theory to multilayer adsorption, Brunauer et al. (25) permitted the condensation of molecules striking other molecules already adsorbed, and with this assumption, which is in qualitative agreement with the results above, they built up a theory of multilayer adsorption which has been widely used.

330

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

Following adsorption there is a statistical chance that the molecule will desorb from the surface. T h e mean time of sojourn on the surface is given by Eq. (2). There is an analogous relation for the mean time of sojourn on a single site before sideways or surface diffusion ts

=

to exp E,/RT

(7)

where Es is the energy barrier between adjacent sites. De Boer (14) emphasizes that since Es is normally less than E d ; an adsorbed molecule makes many sideways excursions before returning to the gas phase. Kruyer (26) has given a detailed discussion of ts. T h e magnitude of t s is determined in an analogous way to that of ta in Table 11. When the value of ta becomes comparable with the flight time between wall collisions then quantitative effects of adsorption will become noticeable in vacuum systems. I n different systems this onset will appear in different ways. For example Hayashi (27) has applied the basic work of Clausing (28) to the problems of production and measurement of high vacuum. I t may be noted that Clausing (28a) has very recently corrected an error in his earlier work. A step function of pressure is applied at the end of a pipe and Hayashi gives the solutions for the resulting pressure as a function of time and distance down the pipe, taking into account adsorption at the walls. Formally the equations are similar to diffusion equations (Section 11, D , 4). I n his calculations Hayashi uses the latent heat of vaporization to compute ta. As will be seen in Section 11, B, 2, heats of adsorption are usually several times the heats of vaporization. This will greatly emphasize adsorption effects. Hayashi recognizes this point in an application of these ideas to leak detection. De Boer (14, p. 138) discusses modifications of the considerations as a result of surface diffusion, Using methods similar to Hayashi’s, Eschbach et al. (28b) have measured the time of sojourn of a helium 8 x atom on glass at 20°K to be I sec. Provided ta is long enough at the temperature of adsorption, it is possible to trap molecules on a surface and later to cause them to desorb by raising the temperature of the surface. I t follows directly from Eq. (2) that the rate of desorption is given by

A measure of desorption rate, even an approximate one, is sufficient to

establish E d from this equation. Measurements of this type have had wider application in chemisorption (Section 11, C) where in many cases Eq. (8) is modified by second-order effects, but some notable measure-

33 I

ULTRAHIGH VACUUM

ments of low adsorption energies have been made in this way. In all cases mentioned below these measurements have employed u-h-v techniques. Ehrlich (29) reports a low temperature ( T -100°K) state of binding of nitrogen on tungsten with E d = 9 kcalimole, which has a complex interaction which Ehrlich does not interpret as true van der Waal adsorption. In a companion paper, Ehrlich (30) reports no analogous binding states for carbon monoxide even at T -1 15°K. From field emission studies, Ehrlich and Hudda (31) deduce the heat of adsorption of xenon on tungsten to be 8 kcal/mole, and also report a number of activation energies of surface diffusion for inert gases on tungsten. These were all below 5 kcal/mole. Ehrlich et al. (32) g’ive a desorption energy for argon on tungsten, measured by flash filament methods, of 5 kcal/mole. Gomer (33) has studied the adsorption of neon, argon and xenonon tungsten in a field-emission microscope. Gomer gives the activation energy of desorption for argon from tungsten, obtained from an Arrhenius plot, as 1900 & 200 cal/mole. Gomer comments that physisorption on clean metals results in energies not very different from other substrates. Table IV below is taken from Gomer’s paper. We have omitted from the table the results on work function changes which do not concern us directly here, but which are normally obtained from field emission experiments. TABLE Iv. DIFFUSION AND D E S O R P T I O N TEMPERATURES AFTER GOMER (Temperatures refer to completion of diffusion or desorption in 10-100 sec; “mono” and “mult” refer to monolayer and multilayer, respectively) ~~

Diffusion temperature Gas Neon Argon Xenon

Desorption temperature

(“K)

(OK)

40 mono

5 18-21

10

28 mult 60 mult

50 mult

35 mono 100 mono

T h e temperatures given in Table IV may be converted to energies with Eq. (1 3). In a subsequent paper, Gomer (34) discusses his work on the adsorption and diffusion of argon on tungsten in detail. T h e accommodation coefficient is a measure of the energy exchange between an impinging gas molecule and a surface; it thus combines the effects of the condensation coefficient and the sojourn time. If the temperature of the surface is T s O K , while that of the desorbing gas is T,, then the accommodation coefficient is defined as:

x = ( T , - T,)/(Ts

-

To)

(9)

332

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

where To is the temperature of the impinging gas molecules. Reviews of accomodation coefficients have been given by Devienne (3.9, and Hartnett (3%) who quote results for a variety of gases and solids. Thomas and Golike (36) have compared two methods of measuring accommodation coefficients (the temperature jump and low pressure methods) and quote results for helium, neon, and carbon dioxide on platinum. A summary of these results has been taken from their paper and is shown in Table V, where we have disregarded the small temperature dependence observed by Thomas and Golike. TABLE V. ACCOMMODATION COEFFICIENTS OF HELIUM, NEON,AND CARBON DIOXIDE ON PLATINUM BY Two METHODS

Accommodation coefficient Gas Helium Neon Carbon dioxide

Method LP

method TJ

0.1747 & 0.009 0.433 0.779 j~0.006

0.1764 f 0.012 0.326 0.787 & 0.017

l'O1

z

-

0 0

0

5

I

0.4-

-

0 0.20 0

a

-

-

-

250

350

300

Th

400

l

FIG. I . Accommodation coefficients of various gases on glass as a function of temperature [after K. Schiifer and H. Gerstacker, 2. Elektrochem. 60, 874 (1956)l.

ULTRAHIGH VACUUM

333

Schafer and Gerstacker (37) have measured the accommodation coefficient of various gases on glass, and these are shown in Fig. 1. T h e order of the accommodation coefficient follows approximately that of the boiling points of the gases, and a similar pattern emerges in the adsorption equilibrium results of Section 11, B, 2. Schafer and Cerstacker relate these accommodation coefficients to the results on condensation coefficients given in Table 111. T o our knowledge, no accommodation coefficient measurements have been done in u-h-v systems, although the results of Schafer and Gerstacker explicitly show that the accommodation coefficient depends upon the degree of outgassing of the surface. This point is also stressed by Hartnett (352).

2. Systems in Thermodynamic Equilibrium. When a quantity of gas is adsorbed on a surface, and an equilibrium has been reached in which there is no net transfer of matter from one portion of the system to another, there will be in the system a pressure analogous to a vapor pressure, but differing in one essential respect. T h e pressure above an adsorbed layer depends in general upon the density of molecules in the layer. Thus there is a relation between p , T, and the relative coverage 8, and for a given system we may write:

f(& P , T ) = 0

(10)

There are two important cases where the dependence of pressure on 6' at fixed T becomes simple. The first occurs when many layers have been adsorbed and the adsorbent is indistinguishable from the liquid or solid phase of the molecular species concerned. In this case, p is identified with the vapor pressure or sublimation pressure respectively, and there is no dependence on the number of molecules involved. T h e second occurs when the adsorption of one molecule does not alter the adsorption of another, as has been assumed in most of our preceding discussion. I n this case p is proportional to 8, a condition known as Henry's Law. With T fixed Eq. (10) is termed the adsorption isotherm and many different relations have been proposed for it. Dushman (38) reviews a number of these. As may be seen from this simple discussion, Eq. (10) combines the kinetic processes which we have discussed separately in Section 11, B, 1 and it is often difficult to say which of the kinetic parameters is the essential variable. From a practical viewpoint, a knowledge of the relation f (0,p , T ) permits an estimate of the limiting pressure achievable in a given system. Halsey and his co-workers (39-42), in a series of investigations have

334

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

performed experiments in which a small reduction in pressure was measured when the adsorbent was cooled. Halsey carried out experiments at temperatures well above the boiling point of the adsorbate and at pressures of the order of hundreds of Torr. While these conditions appear far removed from those of an u-h-v system, the experiments yield a parameter E* which we may identify with the heat of adsorption q and which we believe applicable to the same gas-solid combinations in u-h-v systems. Heats of adsorption obtained by Halsey and co-workers are listed in Table VI. TABLE VI. VALUES OF

lGas

c * (KCAL/MOLE) FOR

VARIOUS GAS-SOLID COMBINATIONS~ ~

Solid

He

H,

Ne

Porous glass Saran charcoal Carbon black Alumina Graphitized Carbon black P. 33 (2700")

0.68 0.63 0.60

1.97 1.87

1.54 1.28 1.36 0.87

NO 4.26 3.70

~~~~

A

3.78 3.66 4.34 2.80 2.46

0% CH, 4.09

Kr

Xe

4.64 3.46 3.30

4.23

After Halsey and Co-Workers (39-42).

T h e gases have been arranged in the order of increasing boiling points and we might expect the heats of adsorption to vary in the same order. Generally this is true, but hydrogen and nitrogen appear to be exceptional, possibly because of an electric quadrupole contribution to their heats of adsorption (43).While the heats of adsorption of a given gas on porous glass, saran charcoal and carbon black are approximately the same there appears to be a marked reduction for alumina and the particular graphitized carbon black, P. 33, which is specially prepared to yield a uniform surface. T h e great bulk of physical adsorption measurements have been carried out in the pressure range from several hundred Torr to about Torr with temperatures generally less than those of Halsey and coworkers. Relative surface coverages in these experiments were usually greater than B = 0.01. Dispersed adsorbents were generally used (an exception is the work of Rhodin, 44) and the data were usually analyzed with the B. E. T. theory (25) to yield a value of the true adsorbing area. There appears little doubt that the value of the area obtained in this way is essentially correct, and is probably the most widely used method for the measurement of the surface area of disperse solids (25). However,

335

ULTRAHIGH VACUUM

heats of adsorption obtained from such conventional data have to be examined carefully if they are to be used to predict the behavior of u-h-v systems. An example of this type has been examined by Hobson (45) for the physical adsorption of helium on pyrex at 4.2"K in an u-h-v system. T h e highest value of the heat of adsorption of helium on any adsorbent quoted from conventional adsorption measurements at or below the boiling point (2' 4.2"K) was 148 cal/mole (46)for helium on NiSO, . 7H,O. T h e immersion in liquid helium of 100 cm2 of adsorbent in a one-liter system containing He gas at a pressure in the u-h-v range should have given a barely perceptible drop in pressure if the heat were of the order of 150 cal/mole ; whereas, if the heat were of the order of that given in Table VI, the pressure should have dropped to an immeasurably low value. There was indeed a spectacular drop in pressure (see also Hobson and Redhead, 47), but the final pressure (approximately 10-l2 Torr) still appeared measurable. Hobson's conclusion was that this final pressure was the result of a dynamic flow of helium in the system and he placed a lower bound on the heat of adsorption of helium on Pyrex at 250 cal/mole, in qualitative agreement with Table VI and other measurements at temperatures well above 4.2"K (48). This result suggested that u-h-v techniques might make significant contributions to the existing data on physical adsorption and this idea has been pursued (49,50). T h e latter reference was a measurement of the adsorption

<

--

I

I

HOBSON (PYREX)

I

I

I

I

I

I

I

I

I

I

w DRAIN 8 MORRISON IRUTILE)

0

0

P

-

-I

-

-2

-

LOPEZ-GONZALEZ E T AL.(CHARCOAL) STEELE 8 HALSEY (POROUS GLASS)

0

CONVENTIONAL DATA

-3 -

I

'

-5 -10

ULTRA-HIGH VACUUM 1

I

-8

I

I

-6

I

I

-4

I

,

-2 log,, p [Torrl

DATA I

I

0

I

I

2

'

-5

FIG.2. Adsorption isotherms for N, on various adsorbents [after J. P. Hobson and R. A. Armstrong, Rept. 2lst Ann. M.Z.T. Conf. on Phys. Electronics p. 236 (1961)l.See J. P. Hobson, J . Chem. Phys. 34, 1850 (1961); L. E. Drain and J. A. Morrison, Trans. Faruduy Soc. 29, 654 (1953); J. de Dios Lopez-Gonzales, Carpenter, F. G., and Deitz, V. R., J . Research Nutl. Bur. Standards 55, 11 (1955); W.A. Steele and G. D . Halsey. Jr., J . Phys. Chem. 59, 57 (1955).

336

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

isotherms of nitrogen on Pyrex at seven different temperatures between

63.3"K and 902°K. A selection from these results is compared with other adsorption isotherms obtained by other techniques in Fig. 2. T h e

results of the u-h-v experiments have caused Hobson and Armstrong (51) to propose a general hypothesis in which to describe physical adsorption isotherms. This hypothesis rests at present on rather sketchy data, although it has been reinforced recently by measurements by Hansen (5lu) of the adsorption of A, Kr, and Xe on zirconium. In principle the hypothesis may be used for calculating the result of a wide variety of adsorption isotherms including those in the u-h-v range. An interesting result arises when the hypothesis is applied (51b) to an experiment which has been performed by Gomer et al. (52). Their experiment is idealized to the immersion in liquid helium of a sphere of Pyrex, volume 500 cm3, which had been exhausted to 5 x 10-l0 Torr of helium gas ; the predicted pressure is Torr. Gomer, et al. had no means of checking this prediction. At this pressure, in any interval of time a single atom of helium is present in the gas phase for 10-13 of the interval, and the average gaseous density is about grams/cm3. Allen (53)quotes the mean density of interstellar gas as 1 x grams/cm3. Gomer (54) has developed a universal gas source based on physical adsorption of gases at 4.2"K. While this method of achieving very low pressures appears very powerful, it has serious limitations. T h e central difficulty is that the pressures are achieved only at very low surface coverages, and in practical systems the amounts of gas involved may be sufficient to drive LIP the pressure, or to cause limitations due to dynamic flows of gas in the system. Of course reduction of T below 42°K can always produce extremely low pressures, the vapor pressure of helium at T = 0.1"K Torr. being 3 x Hengevoss and Huber (542) describe experiments in which gases are absorbed on a cold trap in an u-h-v system and note that residual gases desorb from the trap at different temperatures. They suggest that this result may have practical application as a rough method of residual gas analysis. Garbe et al. (55) have published isotherms for water on glass in an u-h-v system, but they mention serious hydrogen evolution and it appears uncertain whether these data can be used as a check on the hypothesis mentioned above. They find the heat of adsorption of water on glass to be about 12 kcal/mole at low coverage. This value decreases with coverage with an average value 6-7 kcal/mole. Tuzi and Okamoto (552) have measured the adsorption of water vapor on glass in highvacuum apparatus and find it t o consist of two processes, the first being

ULTRAHIGH VACUUM

337

physical adsorption with a heat of adsorption of 11 kcal/mole, and the second being activated adsorption with an activation energy of 9 kcal/ mole and a heat of adsorption of 1 3 4 0 kcal/mole. In another paper Tuzi (5%) discusses the diffusion of water molecules into a hygroscopic surface layer. At first sight it might appear possible to lay down a new adsorbing surface on top of an adsorbed layer in a manner analogous to that used in getter-ion pumps (see Section 111, A, 3). T h e results of Becker (56)on additional pumping by mercury vapor in a cold trap, do not appear encouraging. However the results of Balwanz (57) indicate that some additional pumping can be achieved in this way. Penchko et al. (58)have immersed a tetrode ionization gauge in liquid helium at 13°K and obtained pressures of about 10-lo Torr. T h e gauge was sealed off at relatively high pressures (4 x Torr) and the authors give evidence that several gases were probably present at the time of immersion. The results then appear to be a case of physical adsorption of a complex mixture of gases. Adsorption of gases on preadsorbed layers of other gases has been studied [e.g., Keesom and Schweers (59); Singleton and Halsey (60)] and one systematic work in the u-h-v range has appeared ( 5 1 ~ ) .

C . Chemisorption Chemisorption is the dominant process controlling the partial pressures of chemically active gases (i.e., gases other than the inert gases) in most u-h-v systems. Chemisorption of neutral molecules of the chemically active gases occurs predominatly on the metal surfaces in the system. T h e adsorption of ions or other active species will be considered in a later section, Chemisorption of gases on metals has been reviewed recently by Gundry and Tompkins (61). The rate of adsorption into a chemisorbed layer at constant temperature may be written as

where u is the number of adsorbed molecules per cm2, and s(u) is the sticking probability which is a function of surface coverage (s = a in this case). For the common active gases a clean metal surface has a sticking probability of 0.1 to 0.5, thus a few square centimeters of a clean metal surface has a very appreciable pumping speed [see Eq. (3)]. The sticking probability of a gas on a clean metal surface remains constant until a certain fraction of the available sites are filled; this

338

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

fraction depends on the gas-metal combination, on the crystal face and on temperature. The initial constancy of the sticking probability implies that chemisorption takes place from a reservoir of physically adsorbed molecules, As the coverage is increased the sticking probability drops very rapidly. The best measurements of sticking probability have been made on tungsten; Fig. 3 shows the variation of sticking probability for 1 .o

~

t

lo-’

-I -

m 4

m

0

CL

a 0 10-2

z

Y

I!

I-

w

II

In

lo5

0

1

2

TOTAL NUMBER

3

4

5

6

7 X 10j4

OF ADSORBED MOLECULES PER CM2

FIG. 3. Sticking probability as a function of surface coverage of various gases on the 411 plane of tungsten at room temperature [after J. A. Becker, Solid State Phys. 7, 379 (l958), courtesy of the Bell Telephone Laboratories].

various gases as a function of surface coverage on a tungsten ribbon exposing the 41 1 plane predominantly. Becker (62) interprets these data as indicating that the sticking probability changes abruptly when the adsorbed species has made a first set of valence bonds with every surface metal atom and again when they have made second valence bonds with all the surface atoms. At higher coverages, adsorption takes place in the outer layers and bonds are being made to the adsorbed atoms in the first and second layers. The sticking probability and bond strength is much smaller in these outer layers. Sticking probability curves measured

ULTRAHIGH VACUUM

339

on polycrystalline adsorbents do not show the second region of constant s. Adsorption into the outer chemisorbed layers is quite slow because the sticking probability has dropped to very low values. Considerable quantities of gas can, however, be adsorbed into these outer layers and although the effective pumping speed may be very low, a very long time is required to reach an equilibrium condition in the u-h-v pressure range, Of the four gases for which most data are available, 0,, N,, and H, are dissociated upon adsorption on tungsten and are chemisorbed in the atomic form. T h e atoms recombine and desorb as molecules, thus the desorption reaction is second order. T h e fourth gas, carbon monoxide does not dissociate on tungsten. T h e chemisorption of the above four gases on tungsten, and most other metals, is unactivated, i.e., the activation energy of adsorption approaches zero. Thus for most cases of interest in vacuum problems the heat of adsorption approximates the activation energy of desorption. T h e sticking probability is observed to decrease with increasing temperature for most cases. For nitrogen on polycrystalline tungsten the temperature dependence of the sticking probability is shown in Fig. 4(a). T h e sticking probability of carbon monoxide on tungsten is not strongly affected by temperature [see Fig. 4(b)]. T h e time taken to reach a monolayer coverage (tm) is sometimes a useful method of estimating the pressure of a chemically active gas. T h e monolayer time is here defined as the time until the sticking probability starts to drop from its initial constant value. This time can be measured by initially flashing a tungsten filament at a temperature high enough to desorb all gases (-2400°K) and then cooling the filament. T h e pressure drop on cooling the filament, remains constant and then starts to rise slowly. T h e time at which the pressure starts to rise, from its initial constant value, is the monolayer time. This “flash-filament” method has been widely used for the measurement of sticking probabilities (see for example Becker, 62). T h e rate at which the number of molecules per unit area (u)is depleted by desorption is given by - da - u7vr ~ exp

dt

~

[-

1-RT -Ed

at constant temperature. Here x is the order of the desorption reaction and Ed is the activation energy of desorption; v, is the rate constant, independent of temperature. For a first-order reaction (x = I ) , v1 is approximately 10’“ sec-I.

340

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

A useful rule of thumb is that desorption becomes significant at a temperature T where T("W ,20 Ed (kcal/mole) Recent measurements of the heat of adsorption of the simple gases on metal single crystals show that it is constant until one "layer" of gas is adsorbed. The heat then decreases for the second layer and again for the outer layers. For example, Becker (62) has found that the heat of adsorption of hydrogen on the 320 plane of tungsten is 53, 37,

0.12

: 9 8 OK

I I

I

0.08

SAMPLE # 16

I

\ 0.04

0

0

0.5

I .o

c [MOLECULES/

I.5

2.0

C M ' ~10-1~1

FIG.4(a). Sticking probability of nitrogen on polycristalline tungsten as a function of surface coverage for various temperatures of absorption ( T o ) .

ULTRAHIGH VACUUM

34 1

37, and 25 kcal/mole for the first, second, third, and fourth stages, respectively. T h e first stage corresponds to one hydrogen atom per surface tungsten atom and the following stages are multiples thereof. The measured values of the important parameters of chemisorption on metals (sticking probability and heat of adsorption as a function of coverage) are not complete and there is considerable divergence between the results of different experimenters. Part of this experimental uncertainty is caused by differences in the crystal structure of the surfaces of the various experimental samples and by contamination of the surfaces, the latter is particularly true of the older measurements made with inadequate vacuum techniques [see Becker (63) for comments on this problem]. Measurements made on filaments or ribbons of refractory metals which can be rigorously outgassed are likely to be least affected by 0.5

0.4

>

t

-1 0.3

m

2

0 [L

a

a

z $ 0.2 $

0.1

0

FIG. 4(b). Sticking probability of carbon monoxide on polycrystalline tungsten as a function of surface coverage for various temperatures of adsorption (To).

342

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

contaminations. Measurements on evaporated metal films may be in doubt unless very great care was taken in the preparation of the films. Measurements made with oxygen and hydrogen using hot-filament ionization gauges may be in error because of decomposition of the gas by the action of the hot filament (see Section 11, G). T h e predominant impurity is usually carbon monoxide with the above gases. Table VII lists the measured values of the initial heats of adsorption of various gases on several metals. Most of these data were obtained on evaporated films. I n the case of oxygen, the initial heats of chemisorption are very similar to the heats of formation of the corresponding stable oxides (64). Measurements on evaporated films show that the total heat of adsorption decreases as the coverage increases. Recent measurements on polycrystalline wires of metal show that adsorption occurs into several TABLE VII. EXPERIMENTAL VALUESOF INITIALHEATSOF ADSORPTION qo kcal/mole) ON POLYCRYSTALLINE SAMPLESO 0 2

No

< 3 < 3 < 3

Ag

A1 Au Cd

21 1

co

100

H2

co

C*H,

8.7b

< 3

Cr cu Fe

174 136

40"

Hg In

45 9 32 < 3

102

9.3b

32"

68

31 < 3 27 30 27 45

32"

58

45

80"

< 3

150

Mn

172 208

Mo Nb Ni Pb

41

107

Pd

67 67 115 212 236 194

Pt

Rh Ta

Ti W

140'

140' 95c

138

< 3

Zn

data from Brennan (64); Hz data from Ehrlich (65); CpH4data from Beeck ( 6 6 ) . Trapnell (67). Beeck (68).

OI

Bagg and Tompkins (69). Baker and Rideal (70). f Beeck et al. (71). 0 Redhead (72).

102

343

ULTRAHIGH VACUUM

different phases with different heats of adsorption. The phase with the highest heat is filled first, followed in order by the phases of lower heats of adsorption. These various phases correspond to adsorption on the different crystal planes exposed on the surface. Thus the total heat of adsorption, for all phases, appears to decrease with coverage. Trapnell (67) has examined the adsorption of several gases on evaporated metal films and shows that the activity of. the various gas-metal combinations can be tabulated as shown in Table VIII. TABLE VIII."

W, Ta, M o , Ti, Zr, Fe, Ca, Ba Ni, Pd, Rh, Pt Cu, A1 Zn, Cd, In, Sn, Pb, Ag Au

+

-

-

+ + -

-

+ gas chemisorbed; - gas not chemisorbed.

+ + ++

+ + + +

+ + ++

+ + + +-

~

a

It can be seen from this table that the order of activity of these gases for all metals is 0 2 > CzH2 > C2H4 > CO > H, > N, , if a metal chemisorbs a particular gas it will also chemisorb all gases higher on the scale, whereas if it does not chemisorb the gas it will not chemisorb gases lower in the scale. The only exception to this rule is gold. The adsorption of CH4 and C,H, on evaporated metal films has been examined by Trapnell(73). He finds that W, Mo, Ta, Cr, Rh, Ti, and Pd adsorb CH, and C,H, strongly; Fe, Co and Ni adsorb no CH, and little C2H,. Roberts (73a) finds that C,H, decomposes on Rh to yield CH, and an adsorbed hydrocarbon residue. Two processes of great importance in u-h-v systems have received little attention, they are: (a) the simultaneous adsorption of two or more gases, and (b) the replacement of one adsorbed gas by the introduction of a second gas. The second process is frequently observed in u-h-v systems, but little quantitative data is available.

D. Sources of Gas This section is concerned in the main with the sources of background pressure in an u-h-v system.- A number of authors (74-78) provide

344

P. A. REDHEAD, J. P. HOBSON, E. V. ICORNELSEN

extensive data and methods of analysis for the degassing of materials under vacuum. These discussions, while not normally directed toward u-h-v, nevertheless are applicable to u-h-v problems. Rather than examine these works in detail, however, we seek below to isolate the various physical processes which give rise to the degassing of materials and to illustrate their orders of magnitude with simple examples. As a specific example we consider a system of volume 1 liter, with an applied pumping speed S = 1 liter/sec, and we shall discuss the mechanisms involved and the procedures .required in reducing the partial pressure Torr. The total input leak-rate for each of any gas to 10-lo, IO-l5, of these pressures is given in Table IX [derived from Eq. (l)]. TABLE IX. PERMISSIBLE LEAKRATESFOR V = 1

FI(

P (Torr) ema (STP) s e c -

molecules/sec

I

!1

lo-'"

LITER,

10-16

1.32 x 10-l"

1.32 x

3.3 x lo*

3.3

x 10'

S =1

LITER~SEC

10-*0

1.32 x 0.33

Next, we examine individually the various sources of background pressure in an u-h-v system which has been made leak-tight. These sources are usually critically dependent on particular choices of components. Often the main parameters are very sensitive functions of temperature, and in these circumstances detailed numerical agreement between various authors is not found, and it is difficult to make precise design calculations for a particular case. We have omitted from the list of processes treated below the re-emission of gas already pumped by the various pumps described in Section 111, A, and also gases which arise directly from pump action, such as hydrocarbons in systems using oil-diffusion pumps (79). An interesting practical discussion of some of the processes considered is provided by Farkass and Barry (80).

1. Desorption of Adsorbed Gases. Most materials, when exposed to the atmosphere, acquire one or more surface layers of gas held to the surface by chemical or physical forces (see preceding sections). When these materials are placed under vacuum, these layers tend to desorb, but the process may not go to completion (81) and if precautions are not taken, this desorption may continue for long periods, making the achievement of u-h-v impossible. As an example we consider below an idealized model in which a solid surface of area A = 100 cm2 is initially covered with a monolayer of gas bound to the surface with an activation energy of desorption of E d cal/mole. We imagine the desorbing gas from this

ULTRAHIGH VACUUM

345

surface to be pumped away by a pump speed I liter/sec. We disregard repumping by the surface; the latter assumtion greatly simplifies the calculation but is unrealistic in practice. We also neglect surface diffusion effects which may be of importance in the nonequilibrium situation we are examining. However, the conclusions we draw from the model appear so decisive from a practical viewpoint that even these poor assumptions will not alter them. By using Eqs. (1) and (8) we compute the time necessary to achieve the design pressures of Table IX as a function of E d . T h e results of this calculation are given in Fig. 5(a) for a temperature ( T I )of 295°K. Figure 5(a) demonstrates that for E 5 20 kcal/mole, desorption takes place quickly and the surface becomes bare, while for E 2 40 kcal/mole the surface binding is sufficiently strong to permit u-h-v to be reached without the surface becoming bare. However there is a range of heats between these limits which can make the achievement of u-h-v a very lengthy process under the conditions described. Also, it seems quite probable that the troublesome range of heats will be found in practice in the second chemisorbed layer. T o investigate the effect of raising the temperature of the desorbing surface we have calculated from the model the heating time necessary at T, = 300°C so that design pressures can be reached after cooling the surface instantaneously to room temperature at the end of the heating time, T h e result is given in Fig. 5(b). It is readily seen that the time scale has been drastically reduced. Thus even modest outgassing appears adequate to prepare surfaces approximated by our model for u-h-v purposes. Variations on these considerations have been given by Venema (81a) and Hobson (81b). However, the work of Todd (82) shows that not all surface gas can be treated as simply as we have done above. Todd finds that to remove surface water on Corning 0800 glass requires outgassing for several hours at 480°C. He postulates a mechanism involving formation of hydrates. Garbe et al. (55) give qualitatively similar results for the desorption of water from lime glass, although their times for the desorption of surface water are shorter. Their results on the adsorption isotherms (see Section 11, B , 2) of H,O on glass also show that glass once cleaned will act as quite a rapid adsorption pump for small amounts of further water vapor even at room temperature. This result is in essential agreement with Todd (83) who demonstrates that the adsorption of water by glass is a reversible process. Garbe and Christians (83a) have made a detailed study of the gases evolved by glass upon heating, and find CO,CO,, N,, H,, A, and CH, given off, in addition to H,O. Dayton (836)has discussed the desorption of gas from porous surface layers. Hunt et nl. (83c) argue that surface desorption can be significantly reduced

/

TI =295'K

P 5 10-20 Torr

10-15 irr

10

30

20

Ed

7

kcal /mole1

FIG. 5. Time required to reach given pressures as a function of E d ( A = 100 cma, S = 1 literlsec, V = 1 liter): (a) desorption occurs at room temperature; (b) desorption occurs at 573°K.

347

ULTRAHIGH VACUUM

by evaporating a molybdenum film on the interior surface of the apparatus. I t should be emphasized that the problem we are discussing in this section is distinct from that of obtaining atomically clean surfaces. T h e latter is a field in which u-h-v has found wide application. T h e specific problem of producing and demonstrating clean metal surfaces has been discussed by Hagstrum and D’Amico (84,and Dillon (842) has given a general discussion of the clean surface approach to chemisorption studies.

2. Gus Permeation. I n general, gases permeate or pass through solid who materials. Reviews of this subject are given by Norton (85) (8.5~) draws two general conclusions: (1) no rare gas permeates any metal; (2) diatomic molecules dissociate into atoms when permeating metals. It is consistent with these conclusions to write for the permeation rate: FK =

KAP 7 gases through nonmetals,

FK =

KAP 7 gases through metals,

(1 4 4

where K is the permeation constant and has the dimensions of cm2/sec and where n is usually about 0.5 (see Barrer, 86). I n these units K is the quantity of gas in the cm3 (STP) passing per second through a wall of area 1 cm2, thickness 1 cm, when a pressure difference of 1 atm exists across the wall. Equations 14 (a) and (b) break down at high pressures but appear adequate for this discussion. For a wall of A = 100 cm2 and d = 0.1 cm, the values of K to give the design pressures of Table IX must not exceed the values of Table X. TABLE X. MAXIMUM VALUESOF K FOR A WALLOF AREA 100 C M ~ ,THICKNESS 0.1 CM GIVEDESIGN PRESSURES IN A SYSTEM OF VOLUME1 LITER, WITH PUMPING SPEED 1 LITER;SEC AT 1 ATM EXTERNAL PRESSURE

TO

K

10-10

1.3 x

1043

10-15

1.3 x 10-18

10-20

1.3 x

10-25

Torr crn*/sec

If the external gas pressure is less than 1 atm, the permissible values of K are increased accordingly. Figure 6(a) gives values for the permeation constant for various gasinon-metal combinations, and Fig. 6(b) for various gaslmetal combinations. Altemose (87a) has recently published permeation constants for He in 20 different glasses as a function of temperature. Less extensive results are given for hydrogen, neon,

348

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

10-10n

,c 0

E

&I10-12-

Y

10-l~

-

FIG.6(a). Permeation constants for various gas-nonmetal combinations as a function of temperature. Units of K are cms/sec [quantity of gas in cm3 (STP) passing per second through a wall of area 1 cmz, thickness 1 cm, when a pressure difference of 1 atm exists across the wall]. Curve number

Gas-nonmetal combination

-

1. Oa or N, Pyrex 2. Air - Pyroceram 3. Air - 97% alumina ceramic 4. Air-Pyrex 5. He - Lead borate glass G 6. He - 97% alumina ceramic 7. Ne - Vycor 8. N8 -50, 9. He 1720 glass 10. He - Pyroceram 9606 11. H, - S O p 12. He - Pyrex 7740 13. He - Vycor 14. H, -Pyrex 15. Air - 1720 glass

-

Reference

85 876 876 87b 85 876 107

86 85 87b 86 92 107

85 87b

349

ULTRAHIGH VACUUM

methane, argon, and nitrogen in glasses. The results of Altemose have not been included in Fig. 6(a). Similarly the results of Altemose on the solubility and diffusion of He in various glasses has not been included in Figs. 7(a) and 9(a). Permeation data are available for the combinations given below: H, - Pd (87c);0, - Ag (87d); H, - Ni, D, - Ni (87e); H, - Ni, H, - OFHC Copper, H, - Kovar, H, - stainless steel, H, - cold drawn steel, H, - gas-free iron, H, - Inconel (87f).From Fig. 6 it may be seen that at room temperature choice of wall material will generally meet the requirements of Table X for P = 10-lo Torr; for P = 10-l6Torr only particular combinations will meet the requireTorr scarcely any will meet the requirement. ment; for P = However, cooling the walls or evacuation outside the walls (88,89, 89a) may solve the problem. A further consideration in permeation problems is the time required to reach the steady-state condition of Eq. (14).

0

1

2

3

FIG. 6(b). Permeation constants for various gas-metal combinations as a function of temperature. (Units as in Fig. 6(a).) 1 . Ha-Pd 5. N, - M o 2. HZ- Ni 6. CO - Fe 3 . Ha-Mo 7. H, - F e 4. NP- Fe 8. H, -CU Ref.: 96.

350

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

Y

0

0

FIG. 7(a). Solubilities for various gas-nonmetal combinations as a function of temperature. Units of C, are dimensionless [quantity of gas in cm3 (STP) in 1 cm3 of material at 1 atm pressure]. Curve number

Gas-nonmetal combination

1. H,O - 0800 glass, Co 2. Ha -SIOa 3. He -Pyrex 7740

Reference

82 not shown on graph 86

0.6 at 300°K

92 107 107 107 not shown on graph

4. He -Vycor

5. H, -Vycor 6. Na - Vycor, lo-&< C,

<

at 673°K

Glass composition in

yo (108)

~~

SiOg BpOs Ala0, Na,O

7.

,%I

11.

12.

--

\ He-glass

76.1 75.9 64.1 75.3 56.2 69.1

16.0 16.0 23.2 7.6 -

-

1.75 0.4 4.0 6.2 1.2 3.3

5.4 4.9 4.0 5.7 7.6 13.2

KaO PbO

4.1 0.8 0.6

0.8

4.5 1.7

30.0

-

i

\

I

Io3

3

2

T (‘K)

FIG.7(b). Solubilities for various gas-metal combinations as a function of temperature [units as in Fig. 7(a); values From E. Waldschmidt, Metal1 8, 749 (1954)l. 1. Ha-Ti 2. Ha- Ta 3. Ha-Pd

4. o a - c u 5 . Ne - Fe 6. HZ-CU

10

-4

10

\ I

Io3 T ( K)

2

3

a

FIG.7(c): Solubilities for various gas-metal combinations as a function of temperature [units as in Fig. 7(a); values 1 - 5 from E. Waldschmidt, Metoll 8, 749 (1954), and 6 from S. Dushman, “The Scientific Foundations of Vacuum Technique.” Wiley, New York, 19491. 1. H , - N i 4. H,- Fe 2. Np-Mo 5. HZ-MO 6. N z - W 3. Op-Ag

352

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

This time may be very long. All these considerations have been quite thoroughly investigated for the helium-Pyrex system by the Westinghouse group of Alpert and associates (90, 91, 92). This system received wide attention because the permeation of helium, present in the atmosphere with a partial pressure of 5 x Torr, through the walls of a Pyrex system determines the final pressure of this system when other processes have been eliminated. McAfee (93, 94) has investigated the dependence of helium diffusion in Pyrex under various conditions of stress in the glass.

3. Diffusion of Gas from Inside Solids. General texts on this subject are Jost (95) and Barrer (86). In general, gases are soluble in solids, the amount of gas in solution depending upon the pressure of gas outside the solid. This phenomenon is closely related to that of permeation, and indeed the solubility equations are similar in form to the permeation equations (14a, b), with the same qualifications: C

= COP gases

in nonmetals,

C = C,Pn gases in metals,

(1 5 4 ( 15b)

where C,,is the solubility and is dimensionless. I n these units it is the quantity of gas (cm3 at STP) in 1 cm3 of solid at 1 atm external pressure; C in Eqs. (15a, b) is in similar units. For example, metal parts exposed to an atmosphere of nitrogen over a long period, will contain a concentration of nitrogen in solution given by Eq. (15b). Values for C, for typical gassolid combinations are given in Figs, 7(a) and 7(b) as a function of temperature. When parts containing dissolved gases are placed under vacuum this dissolved gas diffuses to the surface and desorbs until a new equilibrium governed by Eq. (15a or b) is established. During the transition between the two equilibria the diffusion is generally governed by Fick’s law of diffusion [for an exception see McAfee, (93, 94)]. T h e transition times, as will be shown, may be very long, particularly when the second equilibrium pressure is in the u-h-v range. Often the second equilibrium state is never achieved in practice, and it is necessary to consider the desorption from the solid as a time dependent intermediate between the two equilibrium states. T h e analysis of Todd (82)in his work on the desorption of water vapor from glass provides some simple and useful relations for the desorption problem. At t = 0, the pressure outside a body is reduced to a low value. Figure 8 is reproduced from Todd’s paper and shows the ratio of the amount of material VI, which has desorbed from the body after time t to

3 53

ULTRAHIGH VACUUM

the original amount V,,. T h e curves have an initial linear portion and it is of interest to examine the time required before deviations from the

0.5

a/ a

I .o

FIG. 8. Ratio of the amount lost by diffusion to the initial amount of diffusing material plotted against (Dt)'l2/iafor a slab of thickness 2a, an infinite circular cylinder of diameter 2a, and a sphere of diameter 2a [after B. J. Todd, J . Appl. Phys. 26, 1238 (1955)l.

linear portion take place. At this time approximately half of the dissolved gas has desorbed. For a slab of thickness d this time is given by: '

0.28 d2

tc = __

D

where D is the diffusion constant and has units of cm2/sec. Values for the diffusion constant are shown in Figs. 9(a) and 9(b). Diffusion data are available for the following combinations: He - fused quartz (9.52); Ne - fused quartz (9%); H, - Pd ( 9 5 , 9.54; H, - Ge (95e); H, hardened steel (9Sf);H, - mild steel (95g); He - T i (9%); 0, - Si, 0, - Ge (95i);A - Ag, A - Au, A - Al, A - Pb (95k). I n Table XI we have calculated values of tc from this formula for the three combinations: H,O - 0080 glass, N, - Fe, and H, - Ni for a slab 0.1 cm thick, surface area 100 cm2 at room temperature. For times less than those given by tc in Table XI desorption will be

3 54

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

-

,610

I

2

I o3

4

3

T (OK

FIG. 9(a). Diffusion constants for various gas-nonmetal combinations. Units are cm*/sec. Curve number

Gas-nonmetal combination

Reference

1.

He - Pyrex 7740 He-Vycor Hp-Vycor NI-Vycor HI-SiOl 6. He - Duran glass

92 107

2. 3. 4. 5.

107 107 86 108 Glass composition in yo (108) .-

SiO,

B,O,

Al,O,

Na,O

K,O PbO

76.1 75.9 64.7 75.3 56.2 69.1

16.0 16.0 23.2 7.6

-

1.75 0.4 4.0 6.2 1.2 3.3

5.4 4.9 4.0 5.7 7.6 13.2

0.6 0.8 4.1 0.8 4.5 1.7

76.1 75.9

16.0 16.0

1.75 0.4

5.4 4.9

0.6 0.8

-

30.0 -

355

ULTRAHIGH VACUUM Id4

-Y

10-6

N '

E

Y

a

10-8

-10

10

7'

I

2

3

FIG. 9(b). Diffusion constants for various gas-metal combinations. Units are cm*/sec. Reference

1. 2. 3. 4.

H, -Pd NP - F e CO - N i H2 - N i

86 86 38 86

Reference

5. Ha- Fe 6. 0 2 - N i 7. O p - F e

86 38 86

governed by the linear region of Fig. 8. In this region the rate of desorption is given by, F - dVL A (D/t)"2 dt - 2d TABLE XI. OUTGASSING A SLAB 1

vo

AREA100 C C = 0.1 INITIALLY

MM THICK, OF

FOR

(17) M ~AT ROOM

TEMPERATURE;

F,,[cma(STP)/sec] Gas-solid

H,O - 0080 glass N, - Fe H2 - Ni

tc

(hr) 1089

1 o=e

400

After 1 hr

2 6

x 10-10

x

4.5 x 10-8

After 10 hr x x 10-18 1.5 x 10-6

6

2

356

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

where X is a geometric factor dependent on the shape of the specimen and is 1.13 for a slab. Values of dV,/dt have been calculated from Eq. (17) for the slab, and are given in Table XI for t = 1 hr and t = 10 hr. T h e value of C used in calculations of this type is a subject of some interest and is discussed below. T h e initial value of C used for the table was 0.1. A comparison of Table XI with Table IX shows that the desorption rates of the H,O - 0080 and N, - F e systems are sufficiently low to permit the achievement of u-h-v, whereas the H, - Ni system provides a very serious source of gas from the point of view of u-h-v. T h e dependence of the desorption rate on the time is so weak that it does not represent a practical solution to the outgassing problem. Thorough outgassing of metal parts thus appears mandatory for most u-h-v components. Waldschmidt (96) has given the time/temperature combinations for typical metal samples for outgassing to 5 % of their original gas content. I n Table XI we used a value of C = 0.1. As may be seen from Figs. 7(a) and 7(b), this corresponds more closely to the solubility of gases in metals at temperatures nearer their melting points than room temperature. T h e figure is a rough upper limit on the solubility found in practice, based on data of the type given by von Ardenne (97). Hashimoto et al. (98)and also Bliss (99)give data similar to those of von Ardenne and discuss the methods of measuring outgassing rates. A method for automatic determination of vacuum outgassing rates which might be applicable to the u-h-v ranges has recently been described by Fish (100) Varadi (101) has constructed an apparatus for following partial pressures as a function of time during thermal outgassing. T h e simple discussion of diffusion above has assumed that a certain gas had a clearly defined diffusion constant, While there is no difficulty with this assumption when the gas consists of one element, the diffusion of a gas containing two elements, such as CO, is more complex, and it has been shown (102) that the carbon and oxygen atoms which are released as carbon monoxide diffuse at different rates. Also, reactions may take place after a gas is released into the gas phase. These are discussed in Section 11, G and serve to emphasize the complex nature of gas-metal reactions found in practice. Della Porta (103, 104) has shown that adsorption and diffusion are closely related in the gettering process.

4. Vapor and Dissociation Pressure. For many materials in a vacuum system the residual pressure will be the vapor pressure of the material. Von Ardenne (97) and Honig and Hook (104a) give useful data on the vapor pressure of solids, liquids, and gases at various temperatures, and for pumping fluids and sealing materials. Balwanz et al. (57) summarize the vapor pressure of gases commonly found in u-h-v systems. For most

ULTRAHIGH VACUUM

357

solids likely to be used the vapor pressure is exceedingly low at room temperature. The vapor from an incandescent filament, however, may be a factor in u-h-v systems. For example, Alpert and Buritz (90) have found that the equivalent nitrogen pressure of tungsten vapor in a Torr. Bayard-Alpert gauge with a tungsten filament at 2300°K was (A particular vapor pressure of interest is that of Hg in a liquid N, trap. This is about Torr.) Recently Borovik et al. (105) have applied u-h-v techniques to the measurement of the vapor pressure of nitrogen at the normal boiling point of liquid hydrogen (2.2 x 10-l1 Torr) and the vapor pressure of hydrogen at the normal boiling point of liquid helium (3.5 x lo-’ Torr). Clearly, u-h-v techniques provide wide opportunities for the extension of vapor pressure data. One of the causes of limiting pressure in high-vacuum systems has been described as “dissociation pressure” (106, 38). At a given temperature every compound MO can be in equilibrium with its components

If one or both of the components M and 0 are volatile they will create a pressure known as a “dissociation pressure”. An example of a compound (in which both components are volatile) is HgO, whereas the more common compounds encountered in vacuum work are metal oxides, nitrides, and hydrides. The basic parameter governing the magnitude of the dissociation pressure is the heat of dissociation which, for metallic oxides, has been shown to be approximately equal to the heat of chemisorption of oxygen (64). Thus the dissociation pressure of oxygen will be closely related to the pressure above a chemisorbed layer of oxygen on the metal and we believe the analogy is sufficiently close for practical purposes to eliminate the need for further discussion of dissociation pressure.

5. Measurement of Diffusion Coeficient, Solubility, and Permeability of Gases in Solids. T h e central conclusion drawn from the foregoing discussion of the sources of background pressure in u-h-v systems is that at room temperature the diffusion of gases from the interior of solids represents the limiting process in the achievement of u-h-v, provided the permeation problem has been solved. Thus the main parameters required for. design are the diffusion coefficient, the solubility, and the permeability. Rogers et al. (92) have described a method for measuring these three quantities with a single experimental arrangement. T h e method has been used by Leiby and Chen (107) and by Eschbach (108).

358

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

The arrrangement used u-h-v techniques and we review it briefly here because the method appears to have great value in u-h-v studies. We estimate that leak rates of 10-l2 Torr liters/sec are measurable with mass spectrometers operating continuously, and that leak rates of Torr liters/sec are measurable by integrating the entering gas on a filament by chemisorption, or on a cold surface by physical adsorption, and then releasing it in a burst whose height can be measured. If sufficient time is available to establish steady-state permeation across a slab 100 cm2 in area and 1 mm thick, with a pressure differential of I atm, these leak rates suggest that values of K = cm2/sec and K = lO-l* cm2/sec are measurable with continuous and integrating techniques, respectively. T h e measurement of D and C however require a measurement of the time dependent approach to the steady state. For the “late approximation’’ method of Rogers et al. (92), if we restrict the time duration of the measurement to several weeks and the value of tc to 1 week = 6 x lo5 sec (where t c is the time of onset of the late approximation), then for a 1 mm slab the measurable value of D is restricted to D 2 d2/6t, = 3 x cm2/sec. (19) by the Since C = K/Dthe lower limit on C is given by C = 3 x integrating method of measurement. T h e method thus appears potentially able to give values of K , D , and C several orders below those given in Figs. 6, 7, and 9. Such data would be useful for design calculations for u-h-v systems.

E. Positive Ion Impact on Surfaces T h e range of ions in solids for energies usually encountered in u-h-v systems ( < 30 kev) is less than -lo3 lattice distances (I09),and approximately proportional to ion energy. Thus surface effects can be expected to play a significant role in almost any ion impact phenomenon, their influence becoming greater as the ion energy decreases. Aside from the application of u-h-v to their detailed study, ionic impact phenomena are of interest in u-h-v systems for two reasons: (1) they affect the vacuum conditions in the system ; (2) they affect ionic current measurements (notably that of pressure) which depend on the neutralization of the ion’s charge at a solid surface. I n the former category entrapment and sputtering are the most significant processes; in the latter one needs to consider electron ejection, ion reflection, and the production of secondary ions.

1. Entrapment. Ionic entrapment results in the transfer of gas

ULTRAHIGH VACUUM

359

molecules to the solid phase where they can remain for relatively long periods. A pumping mechanism is thus provided which requires no direct connection to the external atmosphere. There are, however, two ways in which trapped particles can be returned to the gas phase: ( 1) by spontaneous re-emission, a process probably involving both diffusion and desorption; (2) by the sputtering action of incoming ions, For the energy range of interest here (0-30 kev), an ion striking a solid transfers most of its momentum to the lattice atoms by elastic scatter in their composite electrostatic (i.e., time average) potential field (110, 111). T h e atoms which have absorbed the momentum recoil, causing lattice vibrations, defects, or sputtering, depending on the momentum absorbed and the direction of recoil. For initial ion energies greater than -10 ev, there exists a significant probability that the original particles will come to rest in the solid. The trapped particles are then bound to the solid by forces that can be considered electrostatic. I n most cases the trapped particle possesses considerable potential energy (up to -10 ev) indicating that severe distortion of the lattice must occur (112). T h e time spent by the particle in the trapped state depends strongly on the height of the potential barrier which separates it from the gas phase, and on the temperature of the solid. Penetration depths of ions in solids have been measured by Young(113) for H+ and He+ in aluminum, and by Beliakova and Mittsev (114) for Li+ ions aluminum and gold. In both cases the transmission of ions of energy 1 kev and higher through thin evaporated films to cm) was observed. T h e results indicated that ions of 1 kev energy were a b k t o penetrate about ten lattice distances into aluminum. Depths obtained indirectly in this laboratory from ionic pumping onto thin evaporated titanium layers (115) are in general agreement with this result. For somewhat higher energies (45 kev), Nielsen (116)has derived range expressions from the general relations of Bohr (110) which he concludes are in faire agreement with the available data (109). Bredov and co-workers (117) have done a Monte Carlo type of calculation, based on Bohr’s potential, to find the depth distribution of ionically pumped potassium ions (4 kev) in germanium. They obtained a curve of approximately the form predicted by Nielsen, but with a characteristic cm (180 atomic layers). This is larger depth of approximately 5 x by about a factor of five than would have been predicted by Nielsen, but is still smaller than their experimental value obtained from etching the germanium target and measuring the (radio-active) potassium remaining in the target. Davies and Sims (117~2)have used similar techniques to obtain the depth distribution of ionically pumped alkali

3 60

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

metals in aluminum for ion energies in the range 0.7 kev to 60 kev. They have slightly modified Nielsen’s theory to account for the large energy decrement per collision, and obtain good agreement with their experiments. It is generally conceded ( 2 2 1 ) that the equation for the atomic potential field suggested by Bohr gives values which are too small at separation distances larger than about 0.5 A (i.e., energies less than about 1 kev). Further calculations of the type done by Nielsen, but using more accurate potentials would yield useful expressions for the penetration of low energy ions. Ion entrapment probabilities at low concentration have been reported for only a few gas-metal combinations. Varnerin and Carmichael (118) measured the ionic pumping speed and the target ion current to determine the “trapping efficiency’’ of positive helium ions on molybdenum at energies of 150 ev to 2600 ev. Similar measurements were made by Buritz and Varnerin (129) for the combination A+ -+ Mo. Carter and Leck (120) studied the trapping of inert gas ions in glass by measuring the quantity of gas re-evolved during subsequent heating of the target to 350°C.They used ion energies of 80 ev to approximately 1000 ev, and measured the desorbing gas with a mass spectrometer. I n another paper (121) the same authors describe similar pumping and desorption experiments on metal ribbons. They found that the total amount of gas recovered, while dependent on the number and energy of the bombarding ions was “essentially the same” for the metals nickel, tungsten, molybdenum, platinum, and aluminum. I n both of these papers, however, the number of ions used in bombardment (> 1014/cm2)was too large to allow the trapping probabilities for low coverage to be determined. A similar technique has been used in this laboratory to measure the entrapment probability of positive argon ions on tungsten between 80 ev and 4.0 kev (122). Except for the 80 ev case, the number of bombarding ions was < 1013/cm2which was found to ensure proportionality between number desorbed and number incident. T h e results of these measurements and some of those mentioned above are summarized in Fig. 10. T h e most notable features are: the fairly distinct energy threshold at -100 ev for A+ + W, the almost energy-independent region above -1.5 kev, and the close similarity of the A+ + Mo data of Buritz (119) with that for A+ -+ W. Of considerable interest in the application of entrapment to pumping is the maximum number of particles which can be trapped per unit target area, As the concentration of trapped atoms in the target increases, the pumping speed is observed to decrease, falling to very nearly zero at some maximum trapped concentration. It is probable that this saturation effect, rather than involving the reflection of incoming ions, is

361

ULTRAHIGH VACUUM

the result of an equilibrium in which the ion releases on the average one previously pumped atom by sputtering or direct ejection. Convincing measurements of such replacement phenomena have been made by

10

*

10

103

10

ION ENERGY ( e v )

FIG. 10. The probability of entrapment of inert gas ions at metal surfaces as a function of ion energy. Reference

-0-0-0

-0-0-0

-x-x-x

A+ -+W

He+ + Mo

A+ + M o

122 I 18 I19

Carmichael and Trendelenburg (123) using mass spectrometric techniques. They induced the re-emission of one ionically pumped inert gas from a nickel target by bombardment with ions of another, Figure 1 I , taken from their paper, shows the number of atoms of a trapped gas which are re-emitted (An,) when no trapped atoms of this gas are bombarded with 5 x I O I s ions of a second gas. In all cases the ion energies were spread over the range 40 to 180 ev. This same replacement or exchange process was observed qualitatively by Schwarz (124) as early as 1944. Bills and Carleton (125) measured pumping saturation in a normally operating ionization gauge for the gases nitrogen and oxygen, and

362

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

found that about 2 x Torr liter of gas was required for saturation (about 3 x IOI5 molecules/cm2 of bulb surface). They developed a theory based on a fixed number of adsorption sites and assigned sticking

FIG. 11. The ion induced re-emission of ionically pumped inert gases [after J. H. Carrnichael and E. A. Trendelenburg, J . Appl. Phys. 29, 1570 (1958)l.

probabilities to particles striking occupied and unoccupied sites. Reasonable agreement with their experimental data resulted. T h e data of Carter (120) and of Leck (121) give saturation numbers of about 1014/cm2to lOI5/cm2on metals for argon ions of energies 400 ev to 5000 ev. Saturation values for a number of materials and ions in the energy range 5 kv to 65 kv have been reported by AlmCn and Bruce (126). A theory of ionic pump saturation based on simultaneous penetration, diffusion and sputtering has been given by Kuchai and Rodin (127). These authors make the following simple assumptions: all ions penetrate to the same depth; the diffusion coefficient of the pumped particles is independent of coordinates and concentration; and because of diffusion, the maximum allowable concentrations are never approached. (This assumption is made implicitly.)

363

ULTRAHIGH VACUUM

They then solve a “Fick’s law’’ diffusion equation to obtain the steadystate depth distribution and the total amount pumped, in terms of the diffusion coefficient, the penetration depth, the bombarding flux, and the sputtering rate. While their particular assumptions may require modification, the general form of their resulting expressions should provide valuable information on the saturation characteristics of ionic pumps. Predictions of the spontaneous re-emission rate could be obtained from the same formulation by deriving an expression for the concentration gradient at the metal surface, although this is not done by these authors. Carter et al. (227a) have developed a theory which allows them to derive a penetration probability function from the saturation characteristic of an ionic pump. This model takes no account of diffusion of the pumped gas, but rather considers the saturation to be due to sputtering alone. The spontaneous re-emission of previously pumped gas begins to affect the performance of an ionic pump before saturation of the pumping action. Varnerin and Carmichael (118) first reported quantitative data on re-emission probabilities of ionically pumped atoms. They observed the time variation of pressure in a closed volume during the ionic pumping of helium into a molybdenum target. Such pump-down curves have been observed by many workers and are of the form shown in

W

i g P

W

t la

-1 W

a

I0

o;

30 40 TIME WIN)

50

FIG.12. The variation of pressure with time in a sealed I-liter system when a coldcathode discharge is turned on ( t = 0) in the presence of a pressure P,, (approximately 10-4 Torr) of argon.

364

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

Fig. 12. By interrupting the pumping, Varnerin and Carmichael were able to show that for large t, where dp/dt had become small, the pressure was determined by an equilibrium between (a) a slowly decreasing gas source proportional to the total amount of gas pumped, and (b) the original pumping speed. The source was attributed to spontaneous re-emission of the gas trapped earlier in the pump down. A calculation ) the relation, of re-emission probabilities ~ ( tyielded 1 < t < lo3 min (20) where t was measured from the moment of pumping. This re-emission work was extended by Carmichael and Knoll (128) to the four lightest inert gases on targets of nickel and molybdenum. In every case they found Eq. (20) to hold for t > 40 sec. They give values of “k” ranging from 2 x lop3 to 0.15. More recently, Fox and Knoll (129) found, by a “pulse pumping” technique, that for the combination A+ -+ Mo, Eq. (20) was valid for t > 1.5 sec. It is clear, however, from integration, that Eq. (20) cannot hold rigorously to zero time nor to infinite time: in each case an infinite amount of gas would be re-emitted. The limits of applicability of the equation in time are as yet unknown. The physical basis of the observed decreasing n(t) is also uncertain. It is not consistent with desorption from states of a single energy. Diffusion of ionically pumped inert gases does take place within metals (130), even though these gases do not spontaneously permeate metals (85). However, if diffusion alone is to account for the change of re-emission with time, a specific form of depth distribution, valid for all gases, targets and energies, must be postulated. Experiments in this laboratory using a “desorption spectrometer’’ technique (1.31) have shown ionically pumped argon to be bound in rigorously clean tungsten with several preferred energies. Desorption spectra for various bombarding energies are shown in Fig. 13 (122). Assuming that Eq. (8) is applicable, the calculated desorption energies of the peaks in the figure range from 40 kcal/mole to 110 kcal/mole. T h e mean lifetimes of atoms bound with these energies should be extremely long (> IO*’sec) and the re-emission rates completely negligible. A small fraction of the gas must exist in sites of much lower desorption energy to account for the usually observed re-emission. It is not known to what extent the presence of adsorbed gases on the target affects the phenomena of trapping and re-emission. For low energies such as were used in the reemission and particularly the induced desorption work described (118, 123) the ion penetration depths are expected to be only a very few lattice spacings, and variation with the amount and type of gas adsorbed would not be surprising. ~ ( t= ) kt-1

365

ULTRAHIGH VACUUM

T h e application of ionic entrapment to u-h-v pumping is discussed in Section 111, A, 3. Ionic pumps have the cardinal advantage of containing no fluids which might be sources of gas. Their main shortcoming is the limited amount of gas which they can remove. When pumping action is

i S ii'

-

Y)

c

c rJ

) .

=e

-e 0

i" i il'

x

0 X

W

0

9 -

X

G a

ev

k a

ev

250 e v

z 0

I-

150 e v

z

a 0 cn

100 e v

+ a W

a

z

P n

W 0

ev

W

n

0 I TEMPERATURE

2 (OK

3

x 10-3)

is i i d

2 Ea X

z

0

! i a 0

v)

W

n

1

0

I

2

1

3

FIG. 13. Desorption rate spectra of ionically pumped argon from clean tungsten for various ion energies. The rate of temperature rise was SO"K/sec [after E. V. Kornelsen, Trans. Natl. Symposium on Vncrrzrm Tech. 8 , 281 (1961).]

366

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

undesirable, as for example in ionization gauges, Fig. 10 shows that with proper choice of ion energy and target material, trapping probabilities can be drastically reduced.

2. Sputtering. Sputtering is the ejection of atoms of a target from its surface upon the impact of positive ions. This transfer of solid material from one place to another within the system affects the vacuum in the following ways: (1) It removes chemisorbed layers from the target. (2) It releases gas atoms previously dissolved or trapped in the target. ( 3 ) It forms, in other parts of the system, metal films capable of chemisorbing active gases and burying trapped gases. The extent to which the vacuum conditions are altered will depend on the sputtering rate, the gas composition and the previous history of the target. For recent developments in this field, the reader is referred to a comprehensive series of papers by Wehner and co-workers dealing with low energy sputtering (132-135). Some work has also been published recently on sputtering by ions of higher energy (136-138, 126). At low ion energies (< 1000 ev) it seems well established that the sputtering mechanism involves momentum transfer between individual atoms (132). The most important parameters governing threshold and yield (atomslion) are the mass ratio of ion to target atom, the elastic constants and sublimation energy of the target, and the type of crystal lattice (133). During the sputtering of metal single crystals, atoms are ejected with strong preference along directions of closest atomic spacing in the lattice (239, 136) in which direction the most efficient momentum transfer would be expected to occur. For higher ion energies, it is thought that evaporation from small regions of intense local heating may also contribute to the sputtering (140). Using the most sensitive detection method, Stuart and Wehner (135) have found that sputtering threshold energies lie in the range 15 ev to 45 ev. They find that, with few exceptions, the product of threshold energy and momentum transfer coefficient is a constant:

where Et is the threshold energy in ev, mi is the mass of the ion, and mt is the mass of the target atom. As the energy is increased above about 100 ev sputtering yields (atomslion) increase with gradually decreasing slope to a maximum at energies in the range 10-100 kev. For

ULTRAHIGH VACUUM

367

300 ev normally incident A+ ions the yields for 28 pure metals fall

between the limits 0.3 (for Si and Re) and 2.2 (for Ag) (234). Yields for glasses do not appear to be available, but Hines and Wallor (241) report, for vitreous silica under Xef ion bombardment, a yield of 0.5 at 20 kev and a peak yield of 2.0 at -45 kev. These yields are considerably lower (about a factor ten) than would be expected from metal targets. T h e peaks in the yield curves for a given metal target seem to occur at approximately constant ion velocity but are higher (in atoms/ion) for the heavier ions. Thus, for example, on a copper target (237) the sputtering yields for positive argon ions reaches a broad maximum of about 9 at 30 to 50 kev, while for positive helium ions a maximum yield of 0.20 occurs at approximately 16 kev. It is generally agreed that the presence of adsorbed gases on the target can strongly influence sputtering yields even at energies of several kev (132, 137). For accurate sputtering measurements the sputtering rate must therefore greatly exceed the rate of adsorption of atoms on the target surface. Either high current densities (as used by Wehner) or very low active gas pressure (142) must be employed. T h e removal of chemisorbed gases or “cleaning” of a surface by sputtering has been used with some success. Farnsworth and co-workers (143, 144) have produced clean silicon and germanium surfaces by sputtering with argon ions. T h e equivalence of cleaning by sputtering and high-temperature outgassing was recently demonstrated by Hagstrum and d’Amico (84) using the secondary electron ejection yield of tungsten under helium ion bombardment. By sputtering it is possible to clean surfaces which are covered by a chemisorbed layer that cannot be thermally desorbed below the melting point of the metal. T h e release of previously trapped or dissolved atoms by sputtering can cause serious gas sources and the saturation of ionic pumps. These effects were briefly described earlier (see Section 11, D, 1).

3. Secondary Effects. A number of other impact phenomena can affect the detection of ions in vacuum systems even though they exert no strong influence on the vacuum conditions. One of the most important such effects, the ejection of electrons from solids by positive ions, has received considerable attention. A careful examination of Auger or “potential” ejection by noble gas ions has been carried out by Hagstrum (145, 246, 247) and the reader is referred to his excellent series of papers for quantitative data. T h e ejection yields were found to vary only slightly with energy in the range 0-1 kev (for clean metals) and decreased with decreasing ionization potential. Thus, on clean tungsten, the yields dropped from approximately 0.25 for positive

368

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

helium ions to approximately 0.02 for positive xenon ions. For ions whose ionization potential is less than twice the target work function, Auger ejection cannot occur. Kinetic electron ejection is still possible, but for energies < 2 kev the yields are generally much lower than for potential ejection, and are strongly energy-dependent (248, 249). A great many papers have been published in which the state of cleanliness of the target surface is either definitely bad or uncertain [see, for example, the review paper of Little (ISO)].Ejection yields are extremely sensitive to the presence of adsorbed gases (146) and even for energies in the Mev range the sensitivity is still evident (152). Auger yields are in general lower for gas covered surfaces than for clean ones and show a different energy dependence. T h e ion reflection which occurs at metal surfaces also shows a strong dependence on the relative values of ionization potential V , and work functionrj. For V , > 24, as is the case for noble gas ions, Auger neutralization of the ion occurs with high probability unless the energy is very large, and reflection coefficients are correspondingly small (152). For alkali metal ions, on the other hand, V, < 24, and reflection coefficients rise sharply with decreasing energy below 1 kev (253). Brief mention should be made of the production of secondary ions by positive ion impact. Such effects have been reported by Bradley et al. (154) and by Stanton (255) who used mass spectrometric methods to study secondary positive ions. In general these effects do not occur with large probability and do not often cause serious measurement errors. All the secondary effects discussed above show radical dependence on the presence of adsorbed gases, making a large fraction of early experiments of questionable quantitative value. Only within the past decade have proper vacuum techniques, in particular, u-h-v techniques, been applied to the study of ion effects at truly clean surfaces. Bills (156, 257) has reported results in which alkali ions were produced during the flashing of a filament in an u-h-v system after the system glassware had been subjected to heating or charged particle bombardment. The source of these ions was assigned to decomposition of the glass and subsequent transfer of the decomposition products to the filament. Bills points out the possible errors that may arise in physical electronics measurements as a result of these effects. Further details on glass decomposition are given by Donaldson (257~).

F . Electron Interactions Ionization of gas molecules by electrons, emission of soft X-rays by electron impact on surfaces, secondary electron emission, the elastic

ULTRAHIGH VACUUM

,

3 69

reflection of primary electrons, and the modification of work function by absorbed layers of gas, are all important in u-h-v systems, but these phenomena are not special to u-h-v systems and have been treated extensively in other publications. A bibliography of these subjects has been compiled by Nottingham (158). In this article we examine these phenomena only where they play an important role in a particular u-h-v instrument. There is, however, one general field of electronic interactions in which little work appears to have been done, but which is of direct interest in u-h-v. This is the evolution of gas by electron bombardment of a surface. I n a mass spectrometer, Moore (159) has studied the fragments emitted when electrons bombard carbon monoxide adsorbed on molybdenum and tungsten. T h e dominant fragment emitted is the positive oxygen ion, and the threshold incident energy for this process is about 17.5 ev. Moore also found other fragments (particularly with new filaments) wich were independent of the carbon monoxide. T h e time-dependence of the positive oxygen peak under various bombarding conditions was studied, and it was found that this peak reached a maximum near monolayer coverage of CO and decreased thereafter. The reactions investigated must presumably take place at the grid of every BayardAlpert gauge, and data on other gases would appear to be of importance. Reynolds (160) has reported multiply charged ions of tungsten and mercury in a mass spectrometer, which he attributed to bombardment of the walls of the ion source by secondary electrons. Jacob (161) describes the bombardment with electrons of barium on an anode and the resulting physical transfer of the barium to the cathode. T h e threshold for the reaction was 300 ev primary energy. Todd et al. (162) have studied the outgassing caused by electron bombardment of glass in a television tube with 20 kev electrons. Ninety-five per cent of the evolved gas was oxygen for five types of glass examined.

G. Photo and Chemical Reactions When an adsorbed layer of gas is irradiated, three processes may occur, viz, (a) photodesorption of the gas molecules, (b) photosorption of gas under the influence of light adsorbed by the solid, and (c) photodecomposition of the adsorbed gas. A review of Russian work on these photoeffects has been published by Terenin (163). T h e effect of principal interest in u-h-v systems is the photodesorption and decomposition of gases from metals and glass, these processes are only significant in u-h-v systems where a reasonably high flux of ultraviolet radiation exists. Valnev (164) has shown that carbon monoxide is photodesorbed from

3 70

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

nickel by the action of light of wavelength less than 240 mp. Valnev also showed that water vapor is photodesorbed from cadmium and zinc at wavelengths less than 250 mp, only 50% of the desorbed gas was condensable at - 180°C, indicating that photodecomposition of the water was occurring. Data on the photodesorption of gases from glass are very scanty (1642). Kenty (165) has shown that irradiation of Pyrex glass by light of wavelength less than 300 m p caused photodecomposition of water and carbon dioxide in the body of the glass, the hydrogen and carbon monoxide so formed then diffused out of the glass. This process is particularly noticeable in mercury arc lamps. Experiments in these laboratories have shown that hydrogen and carbon monoxide are desorbed from Pyrex glass surfaces when illuminated by a xenon flash lamp, in this case the gas was desorbed from the surface only, since the flash was too short (approximately 1 msec) for diffusion to occur (see Fig. 9a). T h e threshold wavelength for this effect was found to be about 270 mp. Interactions in which one gas is converted to another will be referred to as “chemical” reactions. The predominant chemical effects occur when the gas is exposed to an incandescent filament. Hydrogen, oxygen, water, and some hydrocarbons are dissociated at a hot tungsten filament, the dissociated fragments are extremely reactive, and may react with impurities in the filament or with other surfaces of the system to produce gaseous products. Hickmott (166) has studied the interaction with a glass surface of atomic hydrogen, formed at an incandescent filament. It was found that carbon monoxide, water, and methane were formed in a system containing hydrogen and a tungsten filament operated above 1000°K. As an example, in a system initially filled to 1.5 x lo-’ Torr of hydrogen the partial pressure of carbon monoxide was about 1.4 x Torr with the filament cold, with the tungsten filament heated to 2000°K the CO partial pressure had risen to about 3 x lo-* Torr. Becker (167) has shown that if the carbon impurities in the tungsten are reduced by prolonged heating of the tungsten in oxygen, the amount of carbon monoxide produced in a hydrogen atmosphere is greatly reduced. T h u s the dominant source of carbon must be the impurities in the filament while the oxygen in the carbon monoxide and water come from the glass. A modified water cycle may be involved in which the atomic hydrogen reacts with the glass to produce water, the water then decomposes on the hot tungsten to form carbon monoxide with carbon impurities on the tungsten surface. It is also possible that the atomic hydrogen releases some carbon monoxide directly from the glass. It has been observed in these laboratories that whenever two glass surfaces in an

ULTRAHIGH VACUUM

371

u-h-v system are rubbed together, carbon monoxide is evolved. This indicates the presence of an adsorbed layer of carbon monoxide on the glass surface which can be released by “tribodesorption” and which may also be evolved by the action of atomic hydrogen. Young (168) has examined the reaction of oxygen with hot filaments of tungsten, rhenium and molybdenum, and found that carbon monoxide and carbon dioxide are formed by interaction with the carbon impurity in the filaments. All three metals produced carbon monoxide and carbon dioxide in almost identical amounts. When the system was filled to lop6 Torr with oxygen and a tungsten filament heated to 2000°K the partial pressure of CO increased to 1.2 x lo-’ Torr and the CO, pressure to 6 x Torr. Before heating the tungsten filament the pressures of CO and CO, were extremely small. T h e glass walls were shielded from the hot filaments by a tantalum cylinder. When the shield was removed no change in the amount of carbon monoxide and carbon dioxide was observed, thus the source of carbon was established as the impurity in the filaments. Rhodin and Rovner (169) have studied the interaction of oxygen with a hot tungsten filament. Becker (167) has examined the reactions of oxygen with tungsten in detail. Water vapor reacts with a hot cathode to form hydrogen, carbon dioxide, carbon monoxide, and methane, it also reacts with a barium getter to give hydrogen, methane, and higher hydrocarbons (55). These chemical reactions which occur at hot metal filaments can cause drastic changes in the active gas composition in an u-h-v system. T h e problems raised by these effects in the measurement of pressure with hot-filament gauges will be discussed in Section 111, C.

111. TECHNOLOGY OF ULTRAHIGH VACUUM The apparatus and techniques for the production and measurement of u-h-v will be described in this section.

A . Pumps

I. Diffusion and Molecular-Drag Pumps. Oil and mercury diffusion pumps are widely used for the production of u-h-v. No special design of pump is necessary to achieve u-h-v; an adequate trapping system is however essential to prevent back-streaming of the pump fluid into the system. T h e choice of a diffusion pump for any particular u-h-v application is controlled by two requirements: (a) adequate pumping speed for the application, and (b) minimum rate of back-streaming of the pump fluid. Large systems, requiring very high pumping speeds, are usually

372

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

pumped with diffusion pumps, sometimes in combination with cryopumps (see Section 111, A, 4). Systems of a few liters volume can equally well be pumped by diffusion pumps, but for many applications getter-ion pumps may be more convenient (see Section 111, A, 3) and avoid the problem of contamination from pump fluids. In general it is possible to make the compression ratio of a diffusion pump sufficiently high that the ultimate pressure is not determined by diffusion from the fore-vacuum back through the vapor jet. Typically, the compression ratio is lo4 for hydrogen and 10l2for nitrogen in an oil diffusion pump. Thus the ultimate pressure that can be obtained in practice is limited by the outgassing rate of the system and the vapor pressure of the pump fluid or any of its breakdown products. Mercury diffusion pumps have three principal advantages for u-h-v use; (a) the pump fluid is very easily trapped at liquid nitrogen temperatures and if the trap is inadvertently allowed to warm up, the mercury can be readily removed from the system by baking, (b) the mercury pump will operate against a high backing pressure, permitting the backing pump to be turned off for long periods, and (c) the fluid is stable, i.e., there are no products produced by thermal breakdown. Pump designs will not be discussed here since pumps used at high vacuum are quite suitable for u-h-v, and have been described frequently elsewhere (see for example Pollard, 9). The use of mercury diffusion pumps to achieve pressures as low as 10-l2 Torr has been described by Venema (4). Mercury diffusion pumps are to be preferred over oil diffusion pumps when it is essential to avoid contamination from hydrocarbon vapors. Oil diffusion pumps are more widely used than mercury pumps because (a) the pump fluid is less dangerous, and (b) recently developed traps for oil vapor (copper foil or molecular sieve traps) do not require refrigeration. As in the case of mercury pumps, no special design of oil pump is necessary for u-h-v, it is however advisable to use a pump with the lowest possible backstreaming of the pump oil to reduce the trapping problem. Smith (Z70)has considered the thermodynamics of the processes occurring in the pump fluid of an oil diffusion pump and has obtained considerable improvement in diffusion pump performance by superheating the vapor in the stacks, I t is suggested that backstreaming would also be reduced by this technique but no experimental data has been reported. Dreyer (171) has reported measurements of the backstreaming rate of several oil and mercury pumps of 25 cm diameter. T h e design of high speed oil pumps, where a low backstreaming rate has been obtained by adding a cooled shield around the high vacuum jet, has been described by Hablanian (172).

ULTRAHIGH VACUUM

373

Hickman (173) has recently investigated polyphenyl ethers as pump fluids. These compounds have very low vapor pressures at room temperature and it is expected that the surface migration rates of these materials should be much smaller than for conventional pump oils. Hickman claims pressures of about 5 x 10-lo Torr with an untrapped, three-stage glass diffusion pump without baking the system. Molecular-drag pumps have not been widely used in u-h-v systems; however, some new designs show considerable potential for u-h-v applications. Becker (274) has described a molecular-drag pump which is capable of achieving an ultimate pressure of 5 x 10-10 Torr without trapping. T h e residual gas was mostly hydrogen because of the low compression ratio of a molecular-drag pump for gases of low molecular weight. This pump resembles an axial-flow turbine using nineteen stages of flat plate blades and a rotor-tip speed of 145 meters/sec; the spacing between rotors and stators has the very large value of 1 mm. T h e speed of the pump is 140 liters/sec for air and 170 liters/sec for hydrogen. T h e apparent pumping speed for hydrogen decreases at low pressures (below lo-' Torr) because the compression ratio for hydrogen is only 250. ) T h e compression ratio for air is as high as 5 x 10'. Becker ( 1 7 5 ~ has developed the theory of these pumps and Pupp (275b)has evaluated the commercial pump. A design of bakable molecular pump using a magnetically suspended rotor has been described by Williams and Beams (176). A steel rotor is freely suspended by the axial magnetic field of a solenoid. A sensing coil is used to regulate the solenoid current so as to maintain the desired vertical position of the rotor, T h e rotor is spun by a rotating magnetic field produced by the drive coils, at low pressures the rotor will coast for many days without the drive. Pumping takes place between the upper surface of the rotor and a spiral groove in the stator. A pump with a rotor diameter of 23 cm was operated at 300 revolutions/sec (peripheral speed, 220 meter/sec) and gave a compression ratio of lo2 at input pressures of 6 x lo-* Torr with clearances as large as 0.5 mm. T h e limitation on the clearance of rotor and stator was caused by vibration of the building and supports. T h e pumping speed was 6-10 liter /second. T h e lowest pressure obtained was 4 x 10-lo Torr. T h e biggest advantage of diffusion and molecular-drag pumps over getter-ion pumps lies in their ability to pump inert gases with speeds commensurate with the speed of pumping active gases. The pumping speed of getter-ion pumps for inert gases is always considerably lower than their speed for chemically active gases. T h e speed of diffusion pumps increases with decreasing molecular weight of the gas, which is

374

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

particularly advantageous for u-h-v applications where the principal residual gases are frequently of low molecular weight (hydrogen and helium). T h e compression ratio of molecular-drag pumps decreases with decreasing molecular weight, thus these pumps are at a disadvantage in most u-h-v applications.

2. Getters. I n many u-h-v systems it often becomes practical to pump by transferring the gases to the solid surfaces of the system rather than to the external atmosphere. Pumps can then take extremely simple forms (a getter, an ionic pump, or a cold surface), and problems of the evolution of pump fluids into the system are completely avoided. Since the pumped gas remains within the system, however, the problems of saturation and re-evolution become of prime importance in determining the range of applicability of these pumping methods. I n this section the use of evaporated films as getters in the production of u-h-v will be considered. Only a limited number of getter materials has been used to any extent in the production of u-h-v and these are all pure metals (Ti, Zr, Ta, Mo, W). There does not appear to have been any thorough investigation to determine the getter materials most suitable for u-h-v. T h e materials listed above have been found adequate, by various experimenters, to produce u-h-v in their particular systems. A review of the properties of various getters has been given by Wagener (177). Titanium, in the form of evaporated or sputtered films, has been found to be a very convenient getter for pumping the chemically active gases in u-h-v systems. Klopfer (178) has measured the maximum capacity of evaporated titanium films for various gases at 20°C (see Table XII). TABLE XII. GETTER CAPACITY IN

ToRR-LITERs/mg X

T h e adsorption of air and oxygen by evaporated films of titanium and zirconium has been studied by Zdanuk (179). Luckert (180) has studied the physical and chemical adsorption of oxygen and nitrogen on ) studied the formation of evaporated titanium films. Holland ( 1 8 0 ~has hydrocarbons from carbon and hydrogen impurities in titanium getters. Stout has measured the characteristics of bulk titanium metal as a getter (181) and showed that, above 70O0C,0,, N,, and CO, were rapidly adsorbed by titanium metal. Hydrogen was adsorbed in the range 25 to 400°C, and released at higher temperatures. Hydrogen was

ULTRAHIGH VACUUM

375

the only gas found to be released on heating the titanium. Gibbons (182) has reported on the gettering properties of alloys of titanium and zirconium in bulk form. These alloys are capable of dissolving any oxide film and sorbing hydrogen at temperatures below 400°C. Evaporated films of molybdenum and tantalum have also been used successfully in u-h-v systems. Milleron (183) has evaporated molybdenum films by electron bombardment of a molybdenum wire and obtained pumping speeds to hydrogen greater than lo4 liters/sec with a film area of about 1.5 x lo5 cm2. Measurement of the pumping speed of evaporated molybdenum films (produced by heating a molybdenum filament) have been made by Hunt ( 8 3 ) . Maximum speeds to hydrogen of about 4 x lo4literslsec were observed with a film area of 3 x lo4cm,. Pressures of about Torr could be obtained in an unbaked stainless steel system of 85 liters volume by means of the molybdenum getter. There is insufficient experimental data to permit a decision on the most suitable getter material for any specific u-h-v application. Titanium has been proved adequate for most purposes ; the principal disadvantage of titanium is its behavior with hydrogen, i.e., at temperatures about 500°C hydrogen is re-evolved from titanium.

3. Ionic Pumps. This section will be concerned with pumps which employ electrical excitation of the gas to enhance its transfer to the solid. In addition to the ionic entrapment discussed in Section 11, E, I, another phenomenon can play an important role in practical ionic pumps; the electrons which produce the ions can at the same time produce considerable numbers of excited neutral particles which may (particularly in the case of the active gases N,, 0,, H,, etc.) be able to adsorb when neutral molecules would not (184, 185). T h e performance of ionic pumps is further modified by sputtering (Section 11, E, 2) and in some cases by deliberate evaporation of metal onto the pumping surface. T h e terms “electrical clean-up” or “getter-ion pumping” have been applied to pumps which utilize various combinations of the phenomena mentioned above. We first discuss pumps in which deliberate sputtering and evaporation are absent, and then proceed to those in which these processes play dominant roles. T h e production of u-h-v by the use of an ionization gauge as the major pump was reported by Alpert (186, see also 90) in his early definitive work in this field. By ionically pumping the atmospheric helium which permeated the glass envelope, he was able to maintain a Torr even though the gauge pumping speed for pressure of about helium was only approximately lo-, liter/sec.

376

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

For inert gases the total pumping speed is a measure of the ionic pumping speed because chemical effects are then absent. In a BayardAlpert gauge most of the ion-pumping occurs at the envelope; ion current to it being 5 to 10 times the collector current. An evaporated film of metal on the glass is necessary for appreciable pumping of helium to occur (187, 188). Such a film is usually produced during outgassing of the grid structure. Young (188) has measured a maximum liter/second for helium in a Bayard-Alpert pumping speed of 4 x gauge with a grid voltage, V , = 145 volts and electron current, 1- = 10 ma. Hobson (189) has measured the ionic-pumping speed of nitrogen in a Bayard-Alpert gauge and finds an initial speed of 0.25 liter/sec (at V g = 250, 1- = 8 ma) which remains constant until lo1' molecules have been pumped, and then decreases rapidly. Cobic et al. (189a) have reported extensive data on the ionic pumping of both inert and active gases in a Bayard-Alpert gauge. Re-emission and saturation characteristics (see Section 11, E, I ) directly determine how much gas can be pumped by an ionization gauge before the vacuum conditions begin to deteriorate. T h e maximum number of molecules N s which can be pumped by a gauge will depend on the type of gas, the material and area of the pumping surface, and the ion energy. Some measurements of pumping saturation are summarized in Table XIII. Normally pumping speeds are not seriously reduced from their maximum value until the number of molecules pumped is approximately 0.1 Ns. Re-emission effects are difficult to assess because of the strong dependence of the re-emission probability on the time since pumping took place. For the simple case of pumping away a known quantity of gas ( N molecules) in one relatively short interval, the system pressure (in the absence of other gas sources) at a later time t can be predicted from the equation, S N dP -- - vp _ + -kKt-' dt no v

in which the last term represents a rate of re-emission of the pumped gas and is obtained from Eq. (20). When gas is pumped continuously or over some extended period of time, account must be taken of the contribution to the re-emission at time t of pumping at all earlier times. In general, numerical methods would be required to carry out such calculations, although certain special cases can be solved analytically. For the maximum gas input rate likely to occur due to re-emission, let us consider the rapid pumping of a quantity of gas by an ionization gauge. If the t-l law [see Eq. (20)]holds, and K (a typical value),

TABLE XIII.

Authors

Type of gauge

Bills and Carleton (125)

BAG (WL5966)

Brown and Leck (190)

cylindrical anode Penning

Carter and Leck (120)

BAG

Kornelsen

IMG

(115)

(IMP)

Pumping area (cm')

-

200

Material of pumping surface glass W on glass

-6

-

Maximum ion energy (ev)

Ns, Saturation number (molecules x lo-'') ~

He

Ne

A

Kr

Xe

150

4Ooo

10

4.0

2.3

0.17

glass

250

> 10

2.0

2.0

2.0

titanium

6Ooo

A1

200

26

75

Nz -60

-60

2.2

1.o

2.0

4.0

w

4 4

378

P . A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

Torr-liter (Le., a number of molecules then 100 sec after pumping N = 3.3 x 1016) the re-emission rate would be r ( t ) = khijt =

3.3 x 1012 molecules/sec, or 10-7 'I'orr-liter/sec

(23)

I n contrast, the atmospheric helium permeation rate in a one-liter Pyrex system is typically 10-l2Torr-liter/sec. If the available pumping speed is a constant S = V/r,the course of the system pressure with time in the above example will be given by

P

=*--

kN

t n,V

=-

St

fort>r

Torr

where T is the pumping time-constant of the system (sec). When the total quantity of gas to be pumped ionically exceeds the saturation amount (see Table XIII) modifications of the pumping method are necessary. One of the simplest methods of increasing the capacity of an ionic pump is to deliberately evaporate, when required, a fresh layer of metal onto the pumping surface. This provides new sites in which incoming ions can be trapped and at the same time greatly reduces the rate of re-emission of previously pumped gas. The total capacity of the pump should then depend only on the amount of metal available for evaporation. Alpert (292) has briefly described titanium evaporation in a Bayard-Alpert gauge type of structure for the pumping of nitrogen. T h e high pumping speeds achieved in his experiment (up to 20 liters/sec) indicate that electrical excitation of the gas, other than ionization, contributes significantly to the pumping action. A more decisive demonstration of such "activated" pumping has been given by Holland (185) who pumped oxygen onto an evaporated titanium getter which had been saturated with unactivated 0,. Similar observations with nitrogen have been made in this laboratory using a cold-cathode discharge and a titanium getter. Increased capacity for helium and argon has been obtained in this laboratory (ZZ5) using titanium evaporation within an inverted-magnetron gauge (47). The pump is shown schematically in Fig. 14. It was found that 7.5 x 10l8helium atoms or 7.5 x 10'' argon atoms could be pumped for each milligram of titanium evaporated. This implies that the number of evaporated titanium atoms per gas atom pumped was only 2.0 for helium and 20 for argon. Pumps of this design used recently in this laboratory have had approximately 30 mg of

AMOUNT PUMPED (TORR LITER)

FIG. 14. Schematic diagram of inverted-magnetron pump.

lo-'

I 1

I

-I W

2

ZE 10-9

a

W

n

cn cn

3

w

OZ

0

-n n -I-

10-8

10-i

-

10-6

IO-~

10- 4

HELIUM PRESSURE ATTAINABLE WITH 16 HR OF PUMPING

I O - ~

10-2

IMP

D

Voc-Ion TYPE (4 CELLS1

titanium available for evaporation and have pumped approximately 0.5 Torr-liter of argon, an increase of more than lo2 over the saturation number for the same gauge operated without metal evaporation. Perhaps of greater significance, the re-emission rate of the pumped atoms

380

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

was sufficiently low that a base pressure below 2 x 10-lo Torr could be maintained regardless of the amount pumped (up to 0.5 Torr-liter), even though the pumping speeds for helium and argon were only 0.03 and 0.25 liter/sec, respectively. A pump for the noble gases with very much higher speed and capacity has been described by Alexeff and Peterson (292). It is based on the “Evapor-Ion” principle first exploited by Herb (293). Using a magnetically confined electron flow to produce the ions, and titanium evaporation, speeds of 90 literslsec for helium and 260 liters/sec for argon at Torr have been achieved. While these speeds are much lower than those for the active gases in similar pumps (approximately lo4 literslsec), they are nevertheless among the highest entrapment pumping speeds yet reported for the noble gases. With appropriate modification and processing of the pump components, and thorough bakeout, there seems no reason to expect the performance of this type of pump to be inferior in the u-h-v range. During the past five years, extensive development work has been done on cold-cathode discharge pumps which rely on sputtering to avoid saturation of the pumping action (194, 195,196). These pumps are presently sold in a variety of sizes from fractions of a liter/sec to thousands of liters/sec. They all consist of parallel arrays of Penning discharge cells with titanium cathode plates. Ions created in the discharge strike the cathodes with several kev kinetic energy, causing sputtering of the titanium onto the electrodes. Generally speaking, inert gases are pumped by ionic entrapment at locations on the cathodes where there is a net accumulation of sputtered material, while chemically active gases are, in addition, pumped by chemisorption predominantly at the anode. A discussion of the main pumping mechanisms for various gases in such pumps has been given by Rutherford et al. (297). Rutherford concludes that penetration depths, sputtering efficiencies and chemical properties of the gas are involved in rather complex combination in the pumping action. Two modified “sputter-ion” pumps have been described recently (195, 196). These pumps do not show the pressure instabilities during argon pumping which were sometimes troublesome in the original type (294). T h e ultimate capacity of these “sputter-ion” pumps is limited only by the amount of titanium which is available for sputtering from the cathodes. In the nominally 10-literslsec pumps described by Hall (29#), the cathode plates weighed approximately 160 grams. Assuming that 30 grams of this is available for sputtering, the expected capacity might be of the order of lo3 times that of the “IMP” (115) in which 30 mg of titanium could be evaporated. Thus Hall’s pump should be able to

38 1

ULTRAHIGH VACUUM

remove as much as 500 Torr-liters of argon or 5000 Torr-litres of helium, assuming comparable ratios of gas to titanium atoms. Since the pumping speed per discharge cell does not vary greatly, this capacity should be proportional to the number of cells for other pumps of the same type. In the u-h-v range the performance of the pumps described above is fundamentally limited by the re-emission of a portion of the previously CATHODE SAPPHIRE ROD TITANIUM EVA PORATOR

GLASS BASE

I

\

2

3

CM SCALE

FIG. 15. T h e helium pressure obtained with two types of ionic pumps after 16 hr of pumping following the introduction of various amounts of gas.

pumped gas (see Section 11, E, I). It is useful to imagine two types of area on the cathode surface: one in which a net build-up of sputtered metal occurs (area A), and one in which a net excavation takes place (area B). I n area A, gas can be removed permanently through burial by subsequently sputtered metal. T h e ions trapped in area B, however, give rise to spontaneous re-emission, as discussed earlier (Section 111, A , 3)

382

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

and are also liable to be re-evolved by the sputtering action of later arriving ions. T h e dynamic equilibrium between the pumping action in area A, and the re-evolution in area B, results in an apparent “limiting pressure” which decreases only very slowly with time after a large amount of gas has been pumped. Similar arguments apply to more recent modified pumps (195, 196) although quantitatively they may show improvements. In addition, measurements in this laboratory have indicated that, for argon and helium, the ratio of discharge current to pressure in a typical diode “sputter-ion” pump decreases by about a factor of 10 as the pressure decreases from Torr. This Torr to has the effect of accentuating the apparent “limiting pressure” mentioned ) recently reported that the pumping speed above. Klopfer ( 1 9 6 ~ has does not decrease at low pressures if the cathode surfaces are kept free of adsorbed gas. It should be noted that this internal equilibrium limitation is independent of the pumping speed (i.e., of the number of cells in the pump), being determined only by the pumping and reemission characteristics of the individual pump cells. Figure 15 demonstrates the limiting effect in a four-cell diode pump for helium. The pressure obtained after 16 hr of pumping in a sealed system is plotted against the amount of helium removed by the pump near the start of the 16-hr interval. Similar results for an IMP (ZZ5) are included for comparison. The results for argon were qualitatively similar except that the upper plateau occurred at a pressure of approximately 3 x Torr, as compared with 2 x Torr for helium in Fig. 14. The significance of the pumping re-emission equilibrium in a “sputterion” pump becomes clearly evident if the pump voltage is turned off after pumping inert gases. The pump then immediately becomes the Torr-liter/second per pump cell of the source of approximately gas previously pumped. As mentioned earlier, helium premeation in a Pyrex glass u-h-v system (the dominant gas source) amounts to approximately 10-l2 Torr-literlsecond in a one-liter system. I n summary, pumps of the “Hall” type are extremely convenient for pumping systems of low or moderate gas through-put in the pressure range Torr to 10-s-10-9 Torr. For the most demanding u-h-v conditions (approximately 10-lo Torr), however, their performance can be considered adequate only when the amount of gas they are required to pump is extremely small (less than Torr liter/cell). Ionic pumps have also been described (198, 199), which operate by the more conventional method of transferring the gas from the system to the external atmosphere. Compared with diffusion pumps, the power consumed per unit pumping speed is extremely high. No attempt has yet been reported to use ionic pumps of this type in the u-h-v range,

ULTRAHIGH VACUUM

383

but such extension may prove feasible with a number of pumping stages in series, as is the common practice in diffusion pumps.

4. Traps and Cryogenic Pumps. Physical adsorption and/or condensation has two main applications in u-h-v systems: traps and pumps. Both rely upon the physical processes described in Section 11, B and the classification is made on the basis of application and design rather than upon the physical principles involved. A clear distinction between the two is not always possible. T h e principles of trap design are not special to u-h-v systems but care may be required for special problems such as surface migration of the adsorbate (200). An example of refrigeration trapping in the u-h-v range is provided by the work of Venema and Bandringa (4). These authors use three liquid nitrogen traps in series with a mercury diffusion pump to reach pressures in the range lo-” to 10-l2 Torr in a small glass system. They give reasons why more than one trap is necessary. Ullman (200) has described a liquid nitrogen trap used with an oil diffusion pump in a large u-h-v system (570 liters). This trap is designed to prevent surface migration along the warmer parts of the trap. Smith and Kennedy (201) describe mechanical refrigeration systems for high vacuum traps and baffles, Caswell (202,203) has used a liquid helium trap in an apparatus for thin film evaporation and finds that it is about 100 times faster than an evapor-ion pump. It is difficult to classify his helium trap as a trap or a pump. Caswell estimates the condensation coefficient of nitrogen and carbon monoxide on this trap as 0.2 to 0.3, which is lower than the results of Section 11, B, I suggest. However, Caswell’s incident molecules come from a source which is hotter (1400°C) than is normal. Henderson et al. (204) have discussed refrigerated trap design in the u-h-v system of the Model C Stellarator. T h e volume of this system was 1500 liters and pressures in the 1O-lo Torr range were achieved with oil diffusion pumps and liquid nitrogen traps. Milleron (205) has achieved a base pressure below 1 x 10-lo Torr in a metal system of volume 70 liters, using liquid nitrogen traps and oil diffusion pump. Simons (206) reports a base pressure of approximately Torr in a volume of 1100 liters, with oil diffusion pumps and liquid nitrogen traps. Metal foil traps (207) are effective in trapping oil at room temperature and maintain their effectiveness for considerable periods (208, 209). Small copper-foil traps have a “stay-down” time of about 15 days. T h e stay-down time is the useful life of the trap before the surface of the trap becomes saturated and the pressure in the system rises. Traps employing molecular sieves (zeolite or activated alumina) have a con-

384

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

siderably longer stay-down time than metal foil traps. Biondi (210, 211) has shown that small glass traps using molecular sieves have a stay-down time of more than 100 days and that large metal traps are effective for periods ranging from 20 to 100 days. Goerz (212) has described a molecular sieve trap with an internal heater. The ion trap described by Haefer (212a) has not been used in u-h-v but might find application in this range because of its similarity to an ion pump (Section 111, A , 3). Thus there is no doubt that a variety of trap systems exist with which Torr may be achieved with diffusion pumps, u-h-v pressures of The room temperature traps exhibit saturation with time which is a qualitative result to be expected from the discussion of adsorption isotherms given in Section 11, B, 2. Refrigerated traps, in principle, also exhibit saturation properties, but the vapor pressure (po) of the component being trapped may be so low at the temperature of the trap that the effects are not observed. A feature of traps that becomes increasingly important in the u-h-v range is that they may hold relatively large quantities of the gas being pumped, as well as undesirable vapors. The general considerations governing the quantity adsorbed as a function of the pressure are similar to those of the adsorption isotherm (Section 11, B, 2) and account must be taken of these effects when any sudden changes of pressure occur in u-h-v systems containing traps, particularly when measurements of gas quantity are important. Cryogenic pumps or “cryopumps,” have found application in several large scale systems such as thermonuclear machines, space simulation apparatus, and wind tunnels. As will be shown below, these pumps can reach and maintain u-h-v pressures. Surveys which contrast the general properties of cryopumps with the other types of u-h-v pumps are given by various authors (10, 57, 213, 214). This last reference gives data on a simple liquid helium pump in a 1000-liter system. The results show that good agreement with experimental pressures can be obtained by assuming an accommodation coefficient of unity on the pumping surface. Lazarev et al. (215), in work completed in 1951, have described two designs of cryogenic pumps using liquid hydrogen. In the first the pumping surface is the exterior of a 20-cm diameter sphere which contains liquid hydrogen. A schematic diagram of this pump is shown in Fig. 16. With this pump Torr at a pumping speed of they obtained a base pressure of 6 x 13,200 liters/sec. T h e maximum possible pumping speed for nitrogen was 14,500 liters/sec, indicating a value near unity for the condensation coefficient, in agreement with the results of Table 111. The loss of hydrogen was 0.25 liters/hr. The second pump was smaller (4000 liters/sec) and differed in geometry but not in design principle. I n a later publication Borovik et al. (216) point out that the limiting pressure of a

ULTRAHIGH VACUUM

385

hydrogen condensation pump should be lower than 6 x Torr, since the vapor pressure of the main components of air is about 10-l1 Torr at 20°K. They describe a careful experiment in which elaborate precautions were taken to cut off impurities arising in the diffusion LIQUID N,

EVACUATED SPACE

FIG.16. Schematic diagram of hydrogen cryogenic pump [after B. G . Lazarev et al., Ukrain. Fiz. Zhur. 2, 175 (1957)l.

pumps and to reduce the base pressure to its limiting value, and they then achieved a base pressure of 10-lo Torr with a hydrogen pump, and Torr with a helium pump. They present an interesting but inconclusive discussion of why the pressures were not lower still. While molecular sieve pumps such as those described by Jepsen et al. (227), and Varadi and Ettre (218) would appear to have application in the u-h-v range, the central difficulty at present is probably one of thermal conduction. It is difficult to cool a disperse adsorbent when gaseous thermal conduction tends to zero. Degras (219), following the work of Prugne and Garin (220), has combined evapor-ion and liquid helium pumping in a small system and has achieved maximum speeds of 2000 liters/sec for hydrogen and 1000 liters/sec for air. An interesting contrast illustrating the range of cryogenic pumps is afforded by the work of Ames et al. (221) versus that of Hobson and Redhead (47). Both use typical small u-h-v systems,

386

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

but the former employ liquid helium pumping to reduce the pressure from atmosphere to Torr, and then proceed to 10-lo Torr by ionic pumping. The latter reach 10-lo Torr with a mercury diffusion pump and ionic pumping, and employ liquid helium pumping to reduce the pressure to Torr. Behrndt (222) describes an u-h-v evaporation apparatus in which pressures of 7 x 10-lo Torr were achieved with an oil diffusion pump and liquid nitrogen traps, and a final pressure of approximately 1 0-lo Torr with liquid helium pumping.

B. Processing The objective of processing is to reduce the emission of gas into the gas phase from the system parts below a tolerable maximum. The value of the tolerable maximum will vary with the particular system being used, and will depend on such variables as the available pumping speed, the type of gas being emitted, the type of experiment being done, etc. We give here a general description of system processing, which is common to all u-h-v systems, and in part common to other high-vacuum devices such as electronic tubes. Specific procedures for particular materials may be found in recent books by Kohl (223) and by Knoll (224). Espe (225) is publishing a comprehensive series on materials used in high vacuum technology. For large-scale operations it is advisable to adopt the general rules of cleanliness used in the electron tube industry: minimum handling of parts, reduction of dust level to a minimum, use of lint-free clothing by personnel, etc. A description of these precautions is given in the transactions of a recent symposium on cleaning electron device components and materials (226).Papers of this symposium also discuss other related problems in the preparation of device components. After parts have been machined or otherwise prepared, they are generally chemically cleaned. This involves careful rinsing in a grease solvent such as trichlorethylene, and possibly other chemical procedures. Following chemical cleaning, parts should no longer be handled. For laboratory operations we have found the foregoing procedures less critical than the thermal procedures described below. The next stage in processing is usually heating in hydrogen or vacuum. The main purpose of hydrogen firing is cleaning, annealing, or brazing, but this firing may also contribute to outgassing. Firing in vacuum is essentially an outgassing procedure. Conditions for vacuum and hydrogen firing are given by Knoll (224, who also shows several designs of vacuum and hydrogen furnaces. Vacuum outgassing can be done by suspending the work in metal container in a nonmetallic vacuum

387

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chamber and heating the work by ardio-frequency induction from outside the vacuum. No radiation shielding of the work is usually possible in this case, and the energy efficiency of the operation is low. Resistance-heated vacuum furnaces offer a more efficient use of the available power. Resistance furnaces have been described by Libin and Rocklin (227), Kramers and Dennard (228), and Kornelsen and Weeks (229). These furnaces all used voltages 5 20 volts and currents 2 100 amp. Figure 17 taken from Varadi (101) shows a typical partial pressure versus time plot during the outgassing of a small sample of 220 nickel.

DEGASSING OF 220 NI TEMPERATURE 850'C

TOTAL PRESSURE

I 1

2

3

4

5

6

7

TIME minutes]

FIG. 17. Degassing of 220 Nickel [after P. F. Varadi, Trans. Nutl. Symposium on Vucuum Tech. 7, 149 (196O)l.

After vacuum outgassing, it is advisable to keep the parts in a vacuum dessicator until final assembly. Leak-testing after assembly and prior to the final u-h-v processing often saves time in the long run and is essential in industrial applications. T h e physical appearance of an u-h-v system after assembly will of course depend upon the application. Two forms which at present appear to be limiting cases are shown in Figs. 18(a) and 18(b). Figure 18(a) shows a small glass system of the type used in our laboratory. This system is similar in general to other small glass systems, but differs in

388

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

that it is pumped during bake by a Vac-Ion pump, which may subsequently be removed, making the system thereafter movable from one bench to another. Redhead and Kornelsen ( 5 ) have given the operating procedures used with these systems. Figure 18(b) shows a large thermonuclear machine.

FIG. 18(a). Photograph of a small, glass u-h-v system including a mass spectrometer, an inverted-magnetron pump and a Bayard-Alpert gauge.

T h e next step in the achievement of u-h-v is the baking under vacuum of the entire high-vacuum portion of the apparatus to as high a temperature as the components will tolerate. (An exception to this procedure is provided by the work of Hunt et al. (83c) who achieved u-h-v conditions in an 85 liter metal system by evaporation of Mo onto the water cooled walls of the apparatus.) Usually the baking temperature does not exceed 500°C and a bake lasts from 2 to 24 hr. For small glass systems baking is done underneath an oven. Several methods are used. Alpert (2) describes a method in which w e n s consist of panels which are assembled around the apparatus. In our laboratory we use an oven weighing about

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389

3 90

P. A. REDHEAD, J. P. HOBDON, E. V. KORNELSEN

30 l b which can be lifted over the apparatus by two men. T h e latter ovens consume an average power of 1 kw with the system at 500°C. For large systems a pre-formed muffle may be assembled around the apparatus (183) or individual heater elements may be used for different parts of the apparatus. The complexity of outgassing arrangements is well illustrated by Mark and Dreyer (230) who describe the bakeout arrangements for the Model C Stellarator as follows: “Because each flange, observation window and section of tubing is to be maintained within 25°C of the desired bake temperature, 126 separate heater units are required to bake the vessel.and pumping system. Each heater circuit has its own temperature controller and each controller is monitored by the temperature recorder. Each bakeout through 450” Crequired approximately four days. T h e vessel components and traps are raised to 450°C in 25°C increments, and are held at 450°Cfor 18 hr. T h e vessel’s pressure at 450°C with one trap cold is Torr.” T h e baking procedure may be carried out in steps as in the work of Venema ( 4 ) who used a glass system with two diffusion pumps and three liquid nitrogen traps in series arranged vertically, with an oven which could be lowered continuously over the column. During the first part of the overnight bake at 450°C,the upper diffusion pump was not operating and was partly included in the baking region. During the second part of the bake the oven was raised and this pump was operating while the rest of the systemstayedin the

I

,‘u~ A S E PRKSSURE

10-4

J

BEFORE BAKE OUT MAX SYSTEM TEMP: 400°C ON FIRST BAKE

I

I

CUT HEAT 1st. B A Y l S E T E M

3 ANSI

-

WEEKS /

BAKE OUT STARTED

TIME [Hours]

FIG. 19. Pressure versus time during bake of an u-h-v system [after N. Milleron, Trans. Natl. Symposium on Vacuum Tech. 5, 140 (1958), courtesy of Lawrence Radiation Laboratory, Livermore, California, under auspices of U.S.A.E.C.].

ULTRAHIGH VACUUM

39 I

oven at 450°C. After a few hours the oven was again lifted and the first liquid nitrogen trap put into action. Then, with intervals of some hours, the other traps were cooled. T h e final pressure was less than Torr. T h e results of a bake upon the system’s pressure are illustrated by a curve due to Milleron (205) which is shown in Fig. 19 and which appears to be typical of most u-h-v systems, large or small, on the initial bake. Holland (231) has provided an interesting comparison between bakeable and unbakeable metal systems. Normally the final step to u-h-v is the individual outgassing of parts which are likely to be gas sources, such as the grid structures of hot cathode gauges. This is usually accomplished by electron bombardment, ohmic heating, or radiofrequency induction. This localized outgassing is sometimes executed during baking, and following the outgassing further baking may be required.

C.Measurement of Total Pressure Only the ionization gauge, in its various forms, has adequate sensitivity for the measurement of total pressure in the u-h-v region. Various types of viscosity gauges have been proposed for u-h-v, but none has so far been proven useful. Measurements of total pressure at these low pressures must be treated with great caution since the residual gas composition is strongly dependent on the processing methods and the type of system being used. In most cases the total pressure, as indicated by an ionization gauge, can only be used as a rough indication of the general performance of a system and of the effectiveness of any changes in processing methods or system design. Great care must be taken if the ionization gauge readings are to be used as an accurate measure of the gas pressure. I n the latter case it is preferable to use a mass spectrometer as a partial pressure measuring device [see Section 111, D). It should be noted that the output from an ionization gauge is proportional to the gas density within the ionizing region. Conversion of the ionization gauge readings to pressure requires a knowledge of the gas temperature. Leck (232) has reviewed the methods of total pressure measurements, and Brombacher ( 2 3 2 ~ )has published a bibliography of pressure measurements. Review of u-h-v pressure measurements have been published by PQtjr(233) and Grigor’ev (234). 1. Hot-Cathode Ionization Gauges. Nottingham (235) first suggested in 1947 that a lower limit existed to the pressure measureable with a hot-cathode ionization gauge ; he suggested that there was a photocurrent, independent of pressure, produced at the ion-collecting electrode by the

392

P. A. REDHEAD, J . P. HOBSON, E. V. KORNELSEN

action of soft X-rays caused by electron bombardment of the grid. Thus , when the pressure was sufficiently low that the positive ion current and the photocurrent were of the same order, pressure measurement was no longer possible. T h e hot-cathode ionization gauges available at that time had a lower limit of about Torr. Within a few years, Nottingham's suggestion was confirmed and gauges were designed to minimize the X-ray effect by Bayard and Alpert (I), Lander (236) and Metson (237). T h e designs of Bayard and Lander reduced the X-ray effect by decreasing the solid angle subtended by the ion collector at the X-ray source. Metson's design used a suppressor grid in front of the collector to prevent the photoelectrons from leaving the collector. T h e design of Bayard and Alpert was the simplest and most effective, and is now most widely used. T h e existence of the X-ray effect, and its reduction by the new gauge design, were clearly demonstrated by Bayard and Alpert ( I ) by a comparison of the ion-collector current ( I c ) vs. grid voltage ( V g ) characteristics at different pressures for a conventional ionization gauge and a Bayard-Alpert gauge. For the conventional ionization gauge with a large collector, the I c vs. V , curves at high pressures have a shape similar to that of ionization cross section curves. At lower pressure (about Torr) the I , vs. V , curve is a straight line on a log-log plot with a slope between 1.5 and 2, as would be expected for an X-ray induced photocurrent. T h e curves for the Bayard-Alpert gauge show that the X-ray effect has been reduced to such an extent that even at pressures of less than 5 x 10-l' Torr the log I , vs. log V , curve is not a straight line; i.e., the true ion current has not been completely obscured by the residual current even at these low pressures. T h e design of a typical Bayard-Alpert gauge is shown in Fig. 20. T h e ion-collector consists of a fine tungsten wire (typically 150 micron diameter) on the axis of the cylindrical grid structure. T h e filament is outside the grid cage. This inverted geometry has two predominant advantages: (a) the surface area of the ion-collector is minimized, thus reducing the X-ray effect, and (b) the potential distribution between the grid and collector is such that almost all the volume within the grid is available for ionization and the collector acts as an efficient ion-trap. T h e efficiency of ion-trapping is increased, and hence the sensitivity improved, by closing the ends of the grid as shown in Fig. 20. Gauges of the design shown in Fig. 20 have a sensitivity (k)to nitrogen of

k

=

i+ x -1 N 20 Torr-I

2-

P

(25)

for a grid voltage, V , = 105 volts and an electron current, i- = 8 ma.

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393

T h e residual current is about 6 x 1 0 ~ amp, ' ~ corresponding to a pressure of 3.6 x lo-" T o r r (equivalent, nitrogen). Electrode potentials throughout are given with respect to the filament. Very rigorous outgassing of the electrodes of the gauge is essential in the u-h-v region. Outgassing is usually accomplished by electron

FIG. 20. Schematic cut-away diagram of modulated Bayard-Alpert gauge.

bombardment of the grid and ion-collector (typically 300 ma at 1 kv for a molybdenum grid). Bayard-Alpert gauges have been designed where the grid is a spiral, without supporting side-rods, which can be directly heated by passing a current through the grid wire. Adequate degassing of the grid is difficult with this design without causing the grid to sag. Various modifications to the original three-electrode design of Bayard and Alpert have been described. Addition of another grid, outside the filament, prevents surface charging of the glass bulb from affecting the gauge calibration and increases the gauge sensitivity slightly (238). These improvements must be balanced against the increased difficulty of thoroughly outgassing the additional grid. T h e electron current can be controlled by an additional control grid between the filament and cylindrical grid; this arrangement simplifies the design of the electronic regulator for the electron current (239). T h e X-ray effect can be reduced below the level of the normal BayardAlpert gauge design by decreasing the diameter of the ion-collector wire to the smallest practical size ( 4 ) ( 2 3 9 ~ )Van . Oostrom ( 2 3 9 ~has ) achieved an estimated X-ray limit of below 10-l2 Torr with a collector diameter of 4 microns. For measurements of pressure below about 5 x Torr it is

394

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

essential to know the residual current accurately. The residual current is the current to the ion-collector which is independent of pressure; it consists of two components: (a) the X-ray induced photocurrent, and (b) the photocurrent caused by radiation from the hot filament and external light sources. T h e residual current can be estimated from a plot of the collector current (Ic) versus V , taken at low pressures where the residual current is a large fraction of the total ion-collector current. T h e I c vs. V gcharacteristic is plotted on a log-log scale, and the straight line portion at high voltage is extrapolated downwards, the extrapolated value of I , at the normal operating grid voltage is a measure of the residual current (ir).T h e accuracy of this method is poor because the large grid voltages, which must be applied to establish the upper portion of the curve, cause changes in the pumping speed of the gauge and thus may change the pressure in the system. A second method for the measurement of the residual current (244, which does not cause any change in the pumping action of the gauge during the measurement, requires the addition of one electrode to the standard Bayard-Alpert gauge. A modulator wire (see Fig. 20) is inserted parallel to the ion-collector inside the grid of a standard Bayard-Alpert gauge. When the modulator is at grid potential, the sensitivity of the gauge is the same as that of the unmodified gauge. When the modulator potential (V,) is equal to the ion-collector potential (V,), the modulator collects a fraction of the positive ion current. By switching the modulator from V g to V,, the positive ion current can be modulated by 30 to 40% without any change in the residual current. T h e ion-collector current in the two cases is given by

Zl = i,

+ ir ( V , = V,)

and I ,

= mi,

+ ir (Vm = Vc)

(26)

where i, is the positive ion current and a is the modulation factor, which is independent of pressure, Thus the true ion current is given by, i, = (4 - Z,)/U

-4

(27)

and can be measured by a difference method once (Y has been found from measurements at higher pressures where i, ir. T h e residual current is given by, ir = (I, - aZ,)/(l - a). (28)

>

T h e validity of this method depends on the escape probability of a photoelectron from the ion-collector being independent of the modulator potential in the range from Y , to Vc. This escape probability is determined by the shape of the potential well around the ion-collector.

ULTRAHIGH VACUUM

395

Because of the large ratio of grid to ion-collector diameters, this potential well is very steep and is relatively unaffected by changes in the modulator potential. Using this method it has been found that the residual current in a Bayard-Alpert gauge may change by large factors in a few hours, particularly in the period following outgassing of the gauge. T h e cause of these changes has not been definitely established. As an example, in one case the residual current was 1.5 x 10-l2 amp after outgassing, and increased to 6 x 10-l2amp in 14 h. Riemersma (241) has designed an ionization gauge in which the source of electrons is the output of a photomultiplier whose first dynode is illuminated with ultraviolet light. A mercury vapor lamp illuminates the first dynode through a quartz window. T h e multiplier is operated at 200 to 300 volts per stage to produce an electron current of the order of amp in the ionizing region. By careful adjustment of the voltages, the output of the gauge can be made linear with pressure from lop5 to Torr. T h e sensitivity of this type of gauge is about 2 x amp/Torr for air. T h e pumping speed of the gauge is about lop3 liters/sec. This type of gauge would be most useful in experiments where the presence of a hot filament is undesirable because of its interaction with the gas in the system. T h e sensitivity of the hot-cathode ionization gauge has been greatly increased by applying a magnetic field to increase the electron path length. Figure 21 shows Lafferty's design (88) which consists of a cylindrical magnetron operated in a magnetic field of 250 gauss (2.5 times the cutoff field). T h e ion current is measured at one of the two negative end plates. In normal operation the anode potential is +300 volts, the shield potential is - 10 volts, and the ion-collector potential is to lop9amp, -45 volts. Very small electron emission, in the range is used to ensure stable operation and to prevent the production of high-energy electrons which would reach the ion-collector. Below lo-* Torr the ion-collector current is a linear function of pressure. For an electron emission of lo-' amp the ion current is 9 x loe2 amp/Torr. T h e X-ray induced photocurrent is given by

ix

= 1.4 x lo-* i,

(29)

where io is the electron emission. Thus the gauge should be linear to pressures of about 4 x 10-14 Torr and could detect a pressure of 10-15 Torr. T h e sensitivity of this type of gauge has been increased by . of about 10-15 the addition of an electron multiplier ( 2 4 1 ~ ) Pressures Torr can be detected with an output current of 10-l' amp.

396

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

The electron emission from the filament in Lafferty’s gauge can only be directly measured by removing the magnet, thus the electron emission cannot be directly regulated in normal gauge operation. At pressures below 10-8 Torr the anode current becomes independent of pressure and

FIG.21. Hot-cathode magnetron gauge [after J. M. Lafferty, 424 (1961)l:

I.

Appl. Phys. 32,

dependent only on the electron emission, thus at pressures below Torr the electron emission can be maintained constant by regulating the anode current. At pressures above Torr continuous regulation is not possible.

2. Cold-Cathode Ionization Gauges. The Penning type of ionization gauge, a cold-cathode discharge in a magnetic field, has been modified to permit operation in the u-h-v region (47,242,234). T h e advantages of a Penning type of gauge are: (a) there is no X-ray limit because the

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397

electron current which produces the X-rays is proportional to pressure ; (b) the gauge contains no hot filament, thus photoeffects and chemical changes produced by a hot filament are absent; (c) the sensitivity is higher than that of any hot filament gauge. T h e attendant disadvantages are: (a) stable operation can only be achieved over a limited range of voltage and magnetic field; hence the pumping speed of the gauge can only be changed over a small range; (b) the ion current versus pressure characteristics are, in general, nonlinear ; thus the Penning gauge must be calibrated over a fairly wide range of pressure; (c) at very low pressures (below 5 x lo-" Torr) the Penning gauge takes some minutes to strike; (d) oscillations occur in the discharge at all pressures, and care must be taken to prevent these oscillations from causing errors in the measuring circuit. For a Penning discharge to be useful for u-h-v pressure measurements, two requirements must be met: (a) the electrodes must be designed to trap electrons in the discharge so that the discharge will be maintained at very low pressures, and (b) the ion-collector must be shielded from the high electric fields so that no field emission can occur from the ion collector. Two types of modified Penning gauge have been developed which are capable of measuring to at least 10-l2 Torr. T h e first type of gauge, the cold-cathode magnetron gauge (169, 242), is shown in Fig. 22. T h e anode consists of a cylinder (20 mm long and 30 mm diameter) which is B ?

ION CURRENT AMPLIFIER

FIG. 22. Schematic diagram of cold-cathode magnetron gauge.

398

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

perforated to improve gas flow through the gauge. T h e ion collector is shaped like a spool, consisting of an axial cylinder (3 mm diameter by 20 mm long) joined to two circular end-discs. The end-discs are shielded from the high electric field by the auxiliary cathodes, which consist of two annular electrodes shaped and polished so as to reduce field emission to a minimum. T h e auxiliary cathodes are operated at ground potential. This gauge is normally operated with an axial magnetic field of 1000 gauss and an anode voltage of 5 to 6 kv. T h e ion current in the magnetron gauge is linearly proportional to pressure in the to 10-lo Torr range, and the sensitivity for nitrogen is about 10 amp/Torr. In some cases a change of slope of the ion-current versus pressure curve occurs at about 10-lo Torr, and a relationship, I = up**', is observed for p < 10-lo Torr. T h e cause of the change of slope has not been established and in some gauges of different dimensions it does not occur. T h e second type of modified Penning gauge, the inverted-magnetron gauge (47,243) is shown in Fig. 23. T h e ion collector is a cylinder (30 mm diameter by 20 mm long), partially closed at both ends, with its axis parallel to the magnetic field. T h e anode is a tungsten rod (1 mm diameter) passing axially through the holes in the end-plates of the ion-collector. T h e auxiliary cathodes are circular discs with short spouts placed between the anode and the end-plates. T h e auxiliary cathodes prevent field-emission from the edges of the holes in the endplates. This gauge is normally operated with the auxiliary cathodes at ground, the anode at 5 to 6 kv, and a magnetic field of 2000 gauss. T h e inverted-magnetron gauge has been calibrated from to 10-l2 Torr and the ion current obeys the relationship,

I = bp" (30) where b is a constant and the exponent n is about 1.10. T h e value of b varies slightly from gauge to gauge and has a value of about 10 amp/(Torr)n for nitrogen. For the measurement of pressures below 10-lo Torr, these gauges must be completely shielded from ambient light and operated from a well stabilized high-voltage supply. A cold-cathode gauge without magnetic field has been described by Barness (244) which uses a set of fine tungsten points in a very high electric field as field-ion emitters. T h e positive ions are detected by scintillation counting when they strike a fluorescent screen. Barnes shows that in his unbaked system the counting rate from his gauges reaches a constant value while the pressure, as indicated by a BayardAlpert gauge, decreases by several orders of magnitude (245). Barnes

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ULTRAHIGH VACUUM

interprets these results as indicating that the pressure readings of the Bayard-Alpert gauge are in error by several orders of magnitude, thus casting doubt on the use of Bayard-Alpert gauges at u-h-v. Measurements by many other investigators are in disagreement with this interpretation (see for example Crawford, 246). A more probable explanation -ANODE

COLLECTOR

FIG. 23. Schematic diagram of cold-cathode inverted-magnetron gauge.

of Barnes’ results is that water molecules, adsorbed on the shank of the field-emitter tips, migrate to the point and are there ionized and counted by the scintillation counter. This background of ions formed from adsorbed water could cause a constant counting rate from the coldcathode gauge independent of pressure. This effect renders this gauge useless for the measurement of low pressures.

3. Ionization Gauge Sensitivity to Various Gases. Table XIV lists the measured sensitivity ratio of various ionization gauges to several gases, the sensitivity to nitrogen being taken as unity. T h e values given for the chemically active gases (hydrogen, oxygen, and water) should be used with caution since the hot filament of a hot-cathode ionization gauge causes decomposition of these gases (see Section 111, C , 3).

400

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

TABLE XIV. RELATIVE SENSITIVITY OF IONIZATION GAUGES TO VARIOUS GASES Gas

Nz

HZ 0%

co

COZ

H2O He Ne A Kr Xe Hg

SFB

1

3

4

1 1 1 1 0.46 0.47 0.38 0.53 1.14 0.85 1.06 1.36 0.9 0.16 0.16 0.25 0.23 0.25 1.16 1.23 1.06 1.86 1.80 2.69 2.78 3.18 3.93

CH4 CsHa Column Reference

10 11

2

(254) (47)

5 1 0.42

6 1

7 1

1.25 1.25

0.21 0.20 0.17 0.33 1.5 1.25 1.33

2.5

Type of gauge

8 1

9

10

0.2808

1 0.52 0.99

1.09 0.823 1.163 0.1283 0.2407 1.000 1.333 2.72 2.190

11

I

1.29 0.24

0.15

1.76

1.015 1.992

-

Operating Conditions

Vg = 125 v, 1- = 0.5-5 ma Hot-filament triode (FP-62) Vg = 150 v, 1- = 0.5-5ma Hot-filament triode (VG-1) Hot-filament triode (BAR type 507)Vg = 145 v, 1- = 5 ma Hot-filament triode Bayard-Alpert (WL-5966) Vs = 140v,I- = 0.1 ma V, = 125 v, 1- = 100 pa Bayard-Alpert (RG-75) Va = 125 v, 1- = 100 pa Hot-filament triode (826 A) Bayard-Alpert (WL-5966) V, = 145 v, I- = 0.5 ma Vg = 18Ov,I- = 1 ma Hot-filament triode (Leybold IM-1) Cold-cathode magnetron gauge Va = 6 kv, B = los gauss Cold-cathode inverted-magnetron Va = 6 kv, B = 2 x lo3 gauss magnetron gauge

4. Pumping Effects in Ionization Gauges. All ionization gauges behave as pumps to some extent, the removal of gas from the volume is caused by four processes: (1) Ionic pumping: the entrapment of ions that impinge on any solid surface. This is the only pumping mechanism for inert gases. (2) Chemicalpumping:the removal of neutrals by chemisorption on the electrodes or the bulb. ( 3 ) Activated chemical pumping: the removal of excited or dissociated molecules by chemisorption. Excitation or dissociation is caused by the electrons in the discharge.

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40 1

(4) Pumping at an incandescent jilament: in the case of hot-filament

ionization gauges, an additional pumping process is produced by dissociation of gas molecules (in particular, hydrogen, oxygen, and water) at the hot filament, and reaction with impurities in the filament. The dissociated fragments may then be readily chemisorbed at any solid surface.

Ionic and activated chemical pumping in ionization gauges was discussed in Section 111, A, 3. The remaining two types of pumping, insofar as they affect the measurement of pressure, will be discussed briefly in this section. Chemisorption of active gases (Nz, H,, 0,, etc.) on the metal surfaces of an ionization gauge may produce a chemical pumping speed which greatly exceeds the ionic pumping speed. Hobson (189)has measured an initial chemical pumping speed for nitrogen in a Bayard-Alpert gauge ( V , = 250 volts, I - = 8 ma) of 2 liters/sec when the gauge was first operated after the initial outgassing. After molecules of nitrogen had been pumped the chemical pumping speed dropped to zero. It was concluded that most of the chemical pumping took place at the grid by adsorption into the second layer, where the sticking probability is about Bills and Carleton (125) have reported a maximum pumping speed for nitrogen in a Bayard-Alpert gauge of 0.5 liter/sec, and Young (188) has reported a speed of 0.1 liter/sec. These measured values are total speeds but are predominantly caused by activated and ionic pumping. A tungsten filament operated at a temperature exceeding 1100°K will dissociate hydrogen, the atomic hydrogen so formed is readily adsorbed at metal or glass surfaces. Hickmott (255) has shown that for pressure below Torr and filament temperature exceeding 1475"K, the fraction of hydrogen molecules striking the hot surface which are dissociated is constant at 5 yo.Thus, if the sticking probability of the atomic hydrogen is unity and the area of the hot filament is 0.2 cm2, a maximum pumping speed of 0.1 liter/sec for hydrogen will be produced at pressures less than Torr and temperatures greater than 1475°K. This process is complicated by reactions between the atomic hydrogen and carbon impurities in the tungsten filament and the glass which produce CO, H,O, and CH, (166). Eisinger (256) has measured the pumping efficiency of a hot tungsten surface in an oxygen atmosphere. T h e pumping efficiency is defined as the probability that an oxygen molecule striking the hot surface is Torr this efficiency removed from the gas phase. At pressures below has a maximum value of 2 % at a filament temperature of 1700°K. Thus

402

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

for a filament area of 0.2 cm, the maximum pumping speed for oxygen will be 0.04 liter/sec. The pumping efficiency decreases quite rapidly at higher temperatures; at 2100°K the efficiency drops to 0.2%. Oxygen also reacts with the carbon impurities in the hot tungsten filament to produce carbon monoxide and a small amount of carbon dioxide (268). Bistable behavior of the pumping speed of Bayard-Alpert gauges has been observed (121,257) and attributed to changes in potential of the inner surface of the glass bulb. T h e glass surface can stabilize in potential at one of two values, controlled by the secondary emission characteristics of the glass surface. This effect can be prevented either by adding an additional grid between the gauge structure and the glass (238) or by placing a conductive coating on the glass and controlling its potential externally. Only a limited number of measurements has been made of the pumping speed of cold-cathode gauges (258, 259, 269) and in most cases only total pumping speeds have been measured, i.e., chemical and ionic pumping speeds have not been separated. Barnes (254) has measured the pumping speed of the cold-cathode magnetron gauge (242) for various gases and obtains the following results: He-0.17, N,-2.5, H,-2.0, 0,-3.4, A- 1.7, C0,-2.04 liters/sec. For the inverted-magnetron gauge the speed for helium is 0.02 to 0.04 liter/sec, for argon 0.25 liter/sec, and for nitrogen (when the chemical pump is saturated) 0.5 to 1.O liter/sec. Rhodin (269) has measured the pumping speed of the cold-cathode magnetron gauge for various gases and finds that the speed depends on the previous treatment of the gauge. For a gauge of the dimensions given in ref. (242), immediately after bakeout the speeds were 0.14 liter/sec for N,, 0.15 liter/sec for 0,, and 0.2 liter/sec for argon. After operation of the gauge for 16 h in 10-6 Torr of O,, the speeds had dropped to 0.1 liter/sec for N,, 0.12 liter/sec for 0,, and 0.05 liter/sec for argon. I t can be seen from the above data that the pumping speed of an ionization gauge may be comparable with the speed of other pumps in a system, For a hot-filament ionization gauge the ionic pumping speed can be reduced by decreasing the ionizing electron current and/or the electrode potentials. Electron currents of 100 p or less are desirable in a Bayard- Alpert gauge when accuracy of pressure measurement is required. Chemical pumping speeds cannot be controlled and, to obtain accurate measurements of pressure, these speeds must be measured and taken into account. 5. Factors Causing Errors in Pressure measurement. Some of the sources of' error in pressure measurement with ionization gauges have been discus-

ULTRAHIGH VACUUM

403

sed by Redhead (260). T h e commonest source of error is the pressure drop across the gauge tabulation caused by the pumping action of the gauge. T h e high pumping speed of cold-cathode gauges is particularly troublesome in this respect. Significant changes in gauge calibration can result from changes in the secondary emission coefficient (for ions and/or electrons) of the gauge electrodes. I n particular, changes in secondary emission coefficient (electrons per ion) of the ion-collector of a triode ionization gauge may cause significant changes in gauge sensitivity (see Section 11, E, 3). Another source of error is the possible production at hot filaments of alkali metal ions arising from glass decomposition products (156, 157) (Section 11, E, 3). T h e glass bulb of an ionization gauge assumes a potential dictated by secondary emission effects at the glass surface. Uncontrollable changes in potential of the bulb cause changes in the gauge sensitivity (257).This effect can be prevented by depositing a conducting film on the bulb which is held at a known potential. High-frequency BarkhausenKurtz oscillations (40 to 80 Mclsec) occur in Bayard-Alpert gauges under almost all conditions (260). In gauges with an uncoated glass bulb these oscillations can attain sufficient amplitude so that some electrons gain enough energy from the rf field to reach electrodes considerably negative with respect to the filament. If these electrons strike the glass bulb they cause it to go negative, producing a change in gauge sensitivity. A conductive coating on the bulb reduces the amplitude of the Barkhausen-Kurtz oscillations and prevents the above effects. Oscillations also occur in Penning-type gauges (47). T h e efficient trapping of electrons in a cross-field discharge results in strong plasma oscillations, typically in the frequency range 1 to lo3 kclsec. These plasma oscillations are the cause of the sudden breaks and instabilities that are frequently observed in the ion-current versus pressure characteristics of Penning gauges. No way has been found to prevent these oscillations and they represent the biggest difficulty in using a Penning type of gauge for accurate pressure measurements. Pressure measurements with a hot-filament ionization gauge in a system containing appreciable amounts of chemically active gases (H,, O,, H,O or hydrocarbons) are complicated by the chemical changes in gas composition caused by the hot filament; these effects were discussed in Section 11, G. These unwanted reactions at the hot-filament can be reduced by three methods: (1) T h e hot filament can be surrounded with a metal surface at which atomic species recombine readily. T h e production of carbon

404

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

monoxide by a hot filament in an atmosphere of hydrogen or oxygen can be greatly reduced this way (166). (2) The carbon impurity content of a tungsten filament can be reduced by heat treatment in an oxygen atmosphere. The filament Torr of oxygen should be heated at 2200°K for 10-60 h in

(167).

(3) The operating temperature of the filament can be lowered by the use of a low work-function coating. Rhenium filaments coated with lanthanum boride (261) are suitable for u-h-v applications. A typical coated filament for a Bayard-Alpert gauge gives 10 ma electron emission at 1300°K. Another advantage of the lanthanum boride coating is that its vapor pressure is only that of tungsten for the electron emission density necessary to give 10 ma in a Bayard-Alpert gauge. Thoria coatings on tungsten produced by cataphoresis are also suitable (262). Care must be taken when using low work-function filaments to ensure that the ion-collector is kept clean of evaporated material, A reduction in work-function of the ion-collector by evaporated material causes large increases in the residual current because of photoeffects.

Mizushima and Oda (263) have reported observing a non-linear relation between the positive ion current and the electron current of a Bayard-Alpert gauge. It has been shown by Baker (264) and others that this effect is caused by a change in gauge temperature when the filament temperature is used to control the electron current. Baker shows that when the electron current is controlled by an additional grid and the filament temperature maintained constant, then the positive-ion current is always linearly related to the electron current. In a system pumped by oil diffusion pumps and containing some oil contamination, there may be large differences in the pressure indicated by: (a) an ionization gauge enclosed in a bulb and connected to the system by a small diameter tube, and (b) an ionization gauge whose electrodes are directly inserted into the chamber in which the pressure is to be measured (“nude gauge”). This effect has recently been studied in some detail by Haefer and Hengevoss (265). In Haefer’s experiments it was found that immediately after bakeout the reading of the two gauges was identical, but after about 24 h the reading of the nude gauge (NG) was an order of magnitude higher than that of the gauge connected externally to the system through a small tube (EG). Haefer interprets his results an indicating that the NG measures the pressure of permanent gases and the oil vapor pressure, whereas the EG measures essentially the pressure of permanent gases

ULTRAHIGH VACUUM

405

alone. This interpretation leads to the conclusion that the tube joining the EG to the system has a conductance to oil molecules which is lower by a factor of lo4 than its conductance for permanent gases. Although the exact mechanism of this effect is not certain, it is clear that in any system where oil pumps are used and trapping of the oil vapor is not complete the pressure should be measured with a nude ionization gauge.

D. Measurement of Partial Pressure I n u-h-v systems, residual gas sources and pumping speeds vary widely for different pumping methods, structural materials, temperatures, previous history, and different gases (see Section 11, D). In addition, serious interdependent effects can occur ; the introduction of a known gas causing large variations in the partial pressures of other gases through chemical conversion, ionic replacement, or pump saturation. It is evident that in such systems measurement of the partial pressures of the individual gases has decisive advantages over total pressure measurement which indicates only the net equivalent pressure resulting from these numerous and complex processes. The most important and most widely used instrument for the measurement of partial pressures in high vacuum is the mass spectrometer in one of its many forms. Two additional advantages of mass spectrometers over ionization gauges are: (1) they usually have lower ionic pumping speeds and thus cause less disturbance of the vacuum conditions, and (2) they have very small residual currents to the ion collector-usually < amp. In the u-h-v region it becomes extremely difficult to provide a mass spectrometer of adequate sensitivity and sufficiently low outgassing rate to fully exploit the advantages of partial pressure measurement. Consider first the problem of sensitivity. An electron current i- (amp) passing a distance I through a gas at pressure p will produce ions at a rate corresponding to a current i+ = LpQl (amp)

(31)

where values of the differential ionization cross section Q lie between 1.0 and 10 cm-' Tor+ for the most common gases (97). Typical operating parameters for a high sensitivity mass spectrometer might be i- = amp, I = 1 to 2 cm. One would then expect the total to ion current produced to lie in the range p . I n practice, due p to to imperfect focusing and ion transmission through the analyzer, or to low i-, collected ion currents seldom exceed approximately lod4 p . A

406

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

partial pressure sensitivity of 10-13 Torr is adequate for most u-h-v applications, thus requiring that ion currents of approximately 1O-l' amp must be registered with reasonable accuracy and speed. Such performance is beyond the capabilities of simple current collection techniques, and it becomes necessary to make use of the kinetic energy of the individual ions in counting or current amplification systems. Even if the ions are detected individually, 1O-I' amp corresponds to an arrival rate of only 62 ions/sec. Errors of approximately 10% due to statistical fluctuations are therefore to be expected for an observation time of 1 sec; or approximately 3 % for 10 sec. This fundamental limit to partial pressure measurement can be avoided onIy by increasing the ion source yield (i.e., increasing i+/p). A general discussion of the outgassing of solids is given in Section 11, Z I of this paper. I n broad outline, one can draw the conclusion that if pressures due to outgassing are to be reduced to below approximately Torr, the components must first be raised to the highest allowable temperature to remove dissolved gases. Such treatment is particularly important for the mass spectrometer ion source which is normally heated to a temperature of a few hundred degrees by the filament input power. Metal components which receive no heat treatment other than the vacuum processing (450-500°C) are frequently major sources of residual gas. Prior heating in a vacuum furnace to about 1000°C has been found in this laboratory to be extremely effective in reducing the outgassing of spectrometer components (266). An additional gas evolution can occur in mass spectrometers due to the sputtering effect of the ions striking solid surfaces. Adsorbed and dissolved gas atoms are released into the gas phase at rates which depend on the sputtering ion flux and energy. Sputtering has been considered in more detail in Section 11, El 2. Because of the relatively low rate of ion production in spectrometers, the effect of sputtering on gas composition is not usually large unless a relatively high pressure of some gas is introduced. Reynolds (160) describes yet another source of outgassing: the impact of high energy electrons on gas-covered surfaces. He found multiple-charged mercury ions to be present due to electrons produced in the analyser and accelerated by the ion beam potential into the source. U-h-v systems do not usually contain residual gases with mass numbers higher than 44 (carbon dioxide). Thus, unless heavier gases are deliberately introduced, the resolution of adjacent masses up to mass 50 will usually suffice. Indications stable to about 1-2 yo are normally acceptable, so that very precise electrical stabilization is not required. It is, of course, important that the collected current vary linearly with

ULTRAHIGH VACUUM

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partial pressure. This does not appear to present any serious difficulty for permanent gases below about to Torr. Gunther (267) has reported linear operation of an electric quadrupole spectrometer up to 3 x Torr. T h e calibration of a mass spectrometer in terms of gas pressure presents a number of problems. Because operation is limited usually to pressures below Torr, absolute calibration is especially difficult. Most frequently comparisons are made with a previously calibrated ionization gauge, thus limiting the accuracy to that of the gauge calibration (see Sections 111, E and 111, C, 5 ) . T h e main characteristics of a number of reported u-h-v mass spectrometers is summarized in Table XV. A short discussion of the advantages and shortcomings of the various types seems in order. T h e omegatron has been popular for partial pressure measurements because of its small size, relatively simple construction and ease of degassing. Its optimum operating conditions, however, are rather critical, and reproducibility is difficult to achieve without frequent adjustment. Variations of electrode surface potential are thought to be responsible for these effects. Also, though its collection efficiency is high, the necessity for using low electron currents (approximately loF5amp) limits the sensitivity to about lo-" Torr. T h e geometry of the omegatron makes the addition of current multipliers, which would increase sensitivity, almost impossible. T h e magnetic-sector deflection spectrometer (276) which has come into such wide use as an analytical instrument has been adapted by several workers to u-h-v operation. This type of spectrometer has two important advantages: (1) a great deal of experience in design, construction and operation is readily available; (2) both the source and collector regions are easily accessible, making the addition of sensitive current detectors or special ion source assemblies relatively easy. T h e 60" sector spectrometer, which is the most commonly used, tends to be a rather large assembly of a shape which makes attachment to small u-h-v apparatus difficult. When a glass envelope is used, careful dimensioning and jigging are required to obtain proper alignment of the source and collector assemblies. Subsequent performance is dependent to some extent on the stfuctural stability of the glass during processing. All the 60" sector spectrometers mentioned in the table have resolving powers of 150 or higher, analyser radii of 10 to 15 cm, and overall lengths of approximately 60 to 100 cm. Recently a much smaller bakable 90" sector spectrometer has been reported by Davis and Vanderslice (269).T h e tube, shown in Fig. 24, has a maximum dimension of 35 cm, an analyzer radius of 5 cm, a resolving power of 100, and a minimum

TABLE XV. CHARACTERISTICS OF SOME ULTRAHIGH VACUUMMASSSPECTROMETERS

Reference

(90)

Spectrometer type Omegatron Diatron 20 (180" magnetic deflection) 90" sector deflection 60" sector

Approximate partial pressure sensitivity (Ton) 10-0

2x

< 10-15 10-1'

10-13

60"sector

3 x 10-10

Omegatron

lo-" 5x

-

-

Resolving power 20 at mass 20

30

10-10

60" sector

Trochoidal double focusing 60" sector

special current detection

Electron multiplier (Ag-Mg)

60" sector

10-14

Omegatron

lo-"

Omegatron

5 x 10-10

Time-of-flight

10-12

5 x 10-9

x

-2

envelope Glass Stainless steel (gold wire gaskets) Glass and metal

-

s: 150

3 x 10-9

Metal

Electron multiplier (Ag-Mg)

5 150

6 x lo-*

Glass

7 150

3 x 10-9

Glass

3 x

Glass

-

30 at mass 30 100

-

10-12

10-13

100

Lowest total pressure (Tom) 10-9

Electron multiplier or scintillation counter Electron multiplier (Ag-Mg) -

Electron multiplier (Ag-Mg)

150

5

150

-

-

30 at m a s s 30 30 at mass 30

-40

1.5

10-9

Glass

X

2 x 10-8

5 x

Glass

P

52

pumps Ionization gauge Vac Ion

* EU

Ion pump

P

Hg and oil

?

diffusion pump Oil diffusion P-P Oil diffusion Pump Ba getter

.?r

Ti. getter and ion gauge Getter and ion gauge

5

Glass (Pyrex)

Hg diffusion pump

5 x 10-9

Soft glass

Ba getter

2 x 10-9

Glass

Hg diffusion pump

9 x lo-'"

Stainless steel (Cu gaskets)

Ti getter and ion pump

10-l0

'd

U

3: 0

Ez

m

c

P

3E 9

ULTRAHIGH VACUUM

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Trans. Natl. Symposium on Vacuum Tech. I , 417 (1960)l.

FIG.24. 90” sector bakable high-sensitivity mass spectrometer [after W. D. Davis and T. A. Vanderslice,

detectable partial pressure below Torr when using an electron multiplier ion collector. The total pressure during operation is in the low Torr range when connected to the diffsion pump; approximately Torr when valved-off and pumped with an ionization gauge. The trochoidal mass spectrometer reported by Kornelsen (266) is shown, without its glass envelope, in Fig. 25. I n normal operation it Torr to the total pressure in a getter-ion contributes less than 2 x gauge pumped system (see Table XV).

410

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

FIG. 25. Trochoidal ultrahigh vacuum mass spectrometer [after E. V. Komelsen. Rept. 19th Ann. M.I.T. Conf. on Phys. Electronics p. 156 (1959)l.

The ion current multipliers used in the sector mass spectrometers in Table XV have been the secondary electron type with electrostatic focusing (277). These have the disadvantage of introducing into the vacuum system a rather large quantity of material which is difficult to outgas. An attractive alternative method of counting collected ions has

41 1

ULTRAHIGH VACUUM

been used by Bernhard et al. (278). In their method the ions fall at approximately 20 kev energy onto a metal plate, and the secondary electrons produced are accelerated by the same potential onto a fluorescent screen. T h e scintillations are counted by a sensitive external photomultiplier. Pikus (274) has reported the use of such a detector in an u-h-v mass spectrometer with results comparable to those with the secondary electron multiplier. T h e composition of the residual gas can vary considerably with the type of construction and processing. Residual compositions for the twelve representative systems described i n Table XV are presented in Table XVI. T h e numbers refer to relative ion currents except in the case of Wagener (75) in which case true pressures were used. T h e small number of gases which form the bulk of the residual atmosphere for all TABLE XVI. RESIDUAL GASCOMPOSITION I N THE SYSTEMS OF TABLE XV Approxiniate percentage composition

Reference

Total pressure (Torr)

HZ ~~

z

x

10-9

5 x 10-0 4 x 10-10 3 x JO-D 6 x lo-@

70 17 30 70

CH,

He

-

17

5 x 10-10 5 x 10-9 2 x 10-9 9 x 10-10 ~~~

a

~

6 I5 4

65 90

69

~~

In addition, mass numbers 19 and (35

coz

15

2 40

26 50

50 2 10

42 15 2

52

5

4

3

+ 37) contributed

-

31

30

2

-

Nz

7

5

10

co 10 30 25

90

3 x 10-9

3 n 10-9 1.5 > 2 k. 10-8

H,O

-

80

15

21 100

2

19

7

-

30% and 3% respectively.

the various types of systems is most striking. With one exception (269), only seven gases are significant. Components contributing less than 2 yo of the total pressure have been omitted. In most cases, the mass spectrometers themselves have been the major sources of residual gas in the systems in which they were used. I t has, however, been shown that more vigorous processing of components can afford drastic reduction in the

412

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

gas evolution rates, The sensitivities obtained with sector field instruments equipped with ion-current multipliers (approximately 10-13 Torr) seem quite adequate for measurements presently of interest. I n the immediate future the study of residual gases with spectrometers adequate in both sensitivity and “cleanliness” seems likely to greatly improve our understanding of u-h-v systems and processes. A less general, but highly sensitive method for partially analyzing the gases in u-h-v has been described by Redhead (231). I t consists of a modified flash filament technique in which a tungsten wire is heated at a uniform, relatively slow rate. Chemically active gases previously adsorbed onto the wire are thus made to desorb at times (i.e., temperatures) characteristic of their chemical binding energy to the tungsten. T h e pressure transients are recorded by a total pressure gauge. By introducing known gases, or by employing a mass spectrometer on initial tests, individual peaks can be identified with particular gases. Unfortunately, a single gas gives rise, in general, to more than one desorption peak (see Section IT, C ) . Despite this complication and very limited resolution, this technique provides a useful monitor of the active gas pressures in a system. Its sensitivity is such that with a 2-min adsorption time, partial pressures of approximately Torr can be detected. Because of its extreme simplicity, such a desorption spectrometer is routinely included in almost every u-h-v system built in this laboratory.

E. Measurement of Pumping Speed, Leak Rate, and Gauge Sensitivity T h e most convenient method of speed measurement in u-h-v systems is to observe the change in pressure following a step-function change in gas pressure or pumping speed. I n general, the partial pressure ( p ) of a specific gas in a system is given by, dt

where L is the rate of influx of the gas into the system from all sources (molecules/sec), S is the total speed of all pumps (literslsec), V is the volume of the system (liters), and no = 3.27 x 1019 molecules/liter at 1 Torr and 295°K.T h e solution of the above equation yields,

P -P m

= (Pi- PaJ exP (-

tl4

(33) where p m is the steady pressure reached when dpldt = 0, pi is the pressure at t = 0, and r = V / S is the characteristic time of the exponential pump-down curve. At time t = 0 a rapid change is made in either the leak-rate ( L )or the speed (S) and the exponential pressure-time curve is measured. T h e

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speed is obtained by estimating the characteristic time of the exponential pump-down curve. T h e sudden change in conditions at t = 0 can be made in several ways. If gas is being fed into the system through a valve, at t = 0 the valve can be rapidly closed. A second method, which is only applicable to gases which can be chemisorbed, is to allow the gas to adsorb on a metal wire; the wire is suddenly heated to desorb a burst of gas, and the subsequent pump-down curve measured. T h e wire must be maintained at a high temperature ( > 2000°K) during the pump-down, to prevent readsorption of gas on the wire. A third method is to physically adsorb gas onto a cold surface and then rapidly heat the surface at t = 0. A fourth method is applicable when one of the pumps in the system is an ionization pump (or an ionization gauge behaving as a pump). T h e ionization pump is switched off, and at t = 0 is switched on again. T h e pump-down curve so obtained has a characteristic time determined by the sum of the speeds of all pumps inthesystem, notthespeedof the ionization pump alone. T h e rate of influx of gas (leak rate) can be found from a measurement of speed ( S ) and ultimate pressure (pa). Referring to Eq. (32) it can be seen that when dpldt = 0, and p = pm, then L

=

nos&

=

V no7p

(34)

T h e most sensitive method of measuring small leak rates is to accumulate the gas that has leaked in and then measure the pressure of the accumulated gas. Gas may be accumulated in the gaseous or adsorbed phases. I n a system using ionization pumps the leak rate for inert gases can be easily obtained by switching off the ionization pumps (or gauges), thus the pumping speed for inert gases becomes zero. T h e inert gas is allowed to accumulate for a known time and then an ionization gauge is turned on momentarily to measure pressure. This method is particularly suitable for measuring the rate of permeation of helium through the glass walls of a system. In some cases, the gas can be accumulated by adsorption onto a cold surface (physisorption) or by chemisorption onto a metal surface at room temperature. T h e gas is later released by suddenly heating the adsorbing surface. As an indication of the sensitivity of this method, leaks of about lo3molecules/sec of hydrogen have been detected by adsorption and storage on a tungsten filament. T h e accurate calibration of ionization gauges in the u-h-v range is a problem which awaits a completely satisfactory solution. Calibration with the inert gases can be achieved with reasonable accuracy but calibration with the chemically active gases is beset with many of the difficulties

414

P. A. REDHEAD, J . P. HOBSON, E. V. KORNELSEN

described in Section 111, C, 5 . The only satisfactory standard is the McLeod gauge, thus direct calibration against a standard is only possible at pressures above about Torr. Having calibrated the gauge against Torr range, the pressure vs. ion-current a McLeod gauge in the relationship for the lower pressure range must be established by an indirect method. T h e method described by Alpert (90) is satisfactory for Torr. Three volumes are used in calibration in the range 10-lo to this method, separated by adjustable valves. T h e pressure in the third volume will increase proportionally to the square of time. T h e pumping speed of the gauges must be essentially zero; this implies that the gauges must be operated intermittently and all chemical pumps must be saturated. By plotting the ion-current of the gauge in the third volume against t2, the ion-current vs. pressure relationship can be obtained with the use of calibration points obtained with a McLeod gauge in the Torr region. Hobson (45) has described a method for calibrating one ionization gauge against a second pressure gauge, which can be a McLeod gauge if desired. This method is not restricted by the pumping speed of the gauges. Two volumes are used: the first containing the gauge to be tested and a high-speed pump (Hobson used a liquid-helium-cooled finger); the second containing the gauge to be used as a standard. T h e second volume is filled to a pressure near the lower limit of the standard gauge. A valve between the two volumes is opened slightly and the pressure allowed to equilibrate, T h e pressure in the first volume (pl), is then proportional to that in the second volume (p,), independent of the gauge pumping speed, provided that the speed of the high-speed pump greatly exceeds that of the gauge, T h e valve opening is kept fixed and the pressure in the second volume is increased. p , is then plotted against p,. Provided the current-pressure relation is known for the gauge in the second volume in the higher pressure range, then the current-pressure relation for the gauge in the first volume can be found in the lower pressure range. This method has been used to calibrate gauges from 10-11 to 10-7 Torr. Bayard-Alpert gauges have been found to have a linear ion-current vs. pressure characteristic for inert gases from the lowest measurable pressures to about Torr. T h e cause of non-linearity at higher pressures has been discussed by Schulz (250).

F. Components T h e main components of u-h-v systems (pumps, gauges, traps) have been discussed in some detail in other sections of this paper. Descrip-

ULTRAHIGH VACUUM

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tions of other components specifically designed for u-h-v applications are quite numerous and have been reviewed by others (2,12) ( 2 7 8 ~ )This . section will be devoted to a brief survey of some auxiliary components not discussed elsewhere in this paper.

1. Valves. Considerable attention has been given to the design of valves suitable for gas handling and isolation functions in u-h-v. T h e primary requirements for such valves are bakahility and very small closed leakage conductance. Because of the bakability requirement, all valves employ metal or glass rather than organic compounds for the seat material. Closure is achieved either by transmitting force to a metal valve seat through a flexible portion of the vacuum envelope or by moving a pair of precisely mating glass surfaces into contact by an external magnetic field. Small all-metal u-h-v valves (with open conductances -1 liter/sec) employing flexible metal diaphragms have been described by Alpert (279), Bills and Allen (280), Thorness and Nier (281), and several others. Somewhat larger all metal valves (-100 liters/sec open conductance) which use metal bellows rather than diaphragms have also been developed (282). More recently a much larger valve with a throat diameter 8-3/4 in. and an open conductance -2000 literslsec has been used successfully (283). I n metal seat valves, closing forces must be rather large since the closure depends on plastic flow of one of the seat metals. T h e valves must consequently be structurally very strong and solidly mounted. Glass seat valves (284) are frequently used to isolate volumes within a vacuum system. T h e closure, which depends on the precise mating of two polished glass surfaces, does not give extremely low conductances (usually liter/sec) so that such valves are unsuitable for isolation against the external atmosphere. I n contrast to metal valves, seating forces are very small, and open conductances up to -20 literlsec are easy to realize in a small assembly. Another class of magnetically actuated valves use the sealing action of a molten metal of low vapor pressure such as indium or tin (285,286). A magnetically moved body is made to displace the molten metal in such a way that isolation is achieved and the metal is then usually allowed to cool and solidify, Closed conductances are reported to be immeasurably small (287) and no large forces are involved. T h e metal to be melted must, however, be of very high purity to ensure proper “wetting” and avoid excessive gas evolution during opening or closing. T h e small all-metal valves first described in this section have conductances which can be fairly easily controlled in the range lO--’O to literlsec. They can thus serve as excellent controlled leaks for gas

416

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

handling in u-h-v. A valve of low closing torque (-lo4 cmgm) specifically for such applications is available commercially. It is .claimed that to lo-, liter/sec can be reproduced with with it conductances of good accuracy. Other controlled leaks have been described which rely on the permeation properties of specific gas-solid combinations, as for example 0,-Ag (288), H,-Ni (289), H,-Pd (290), and He-SiO, (291). Such leaks have the advantage of preferentially transmitting the desired gas, thus effectively enhancing its purity. Leak rates are limited to Torr liter/sec), controlled by the temperrelatively small values (< ature of the solid. Electrical control of the flow rate (through the heater power) lends itself to pressure regulition in some cases.

2. Gaskets, Seals, and Transitions. The problem of demountably connecting sections of metal envelope together and of providing transitions from metal to ceramic or glass requires special attention in u-h-v systems. Again, the most important requirements are very low leakage and bakability. For high temperature baking (> 4OO0C), gaskets of gold wire, copper, and aluminium of a variety of shapes and sizes have been successfully used (205,230,292-294). The large forces necessary to produce low leakage seals at metal gaskets require rather heavy flanges and a ring of evenly spaced bolts. Surface finish and machining tolerances on the flange structures vary somewhat but are in general only moderately stringent. The use of neoprene O-rings cooled to - 25°C in systems with maximum bakeout temperatures -250°C has been reported to give pressures as low as 1.5 x 10-10 Torr in a moderate-sized metal system evacuated by a trapped diffusion pump (80). This technique should be adaptable to very much larger systems of similar design. For electrical or optical reasons it is frequently necessary to provide, in a metal u-h-v envelope, a transition to a ceramic or glass section. The technology of effecting seals between metals and glasses or ceramics has been extensively developed in the electronic tube industry [see Kohl (223), Chapters 13 and 141. No special techniques are normally required for u-h-v apparatus. An interesting compression seal technique has been used (295) to produce large diameter joints between cylindrical metal sections and either high alumina ceramic cylinders or sapphire discs. These were components of the u-h-v system of the Princeton C Stellarator.

ULTRAHIGH VACUUM

417

IV. APPLICATIONS

A . Surface Physics and Chemistry T h e applications of u-h-v technology to the problems of surface physics and chemistry are much too numerous to allow a detailed enumeration here. I n general, as we have seen in earlier sections, whenever measurements are to be made on an atomically clean surface, u-h-v techniques are essential. Some of the recent work on the adsorption of gases on solids using u-h-v technology has already been mentioned in Sections 11, B and 11, C. Becker (63) has evaluated the effects of poor vacuum techniques on some of the earlier work on the chemisorption of gases on metals and clearly establishes the need for u-h-v techniques in this field. I n particular, he points out the difficulties of obtaining clean metal surfaces by evaporation, a method very widely used for the measurement of chemisorption on metals. Hickmott (296) has also demonstrated that evaporated films, in particular tungsten, do not have surfaces of the same degree of cleanliness as can be achieved with filaments or ribbons of the metal. T h e use of u-h-v methods for obtaining stable field emission has been demonstrated by the work of the group at Linfield Research Institute (297,298, 299). T h e general problems of field emission have been reviewed by Dyke (300)and the u-h-v methods employed have been discussed by Martin (301). By the use of alumino-silicate glass (Corning 1720) the rate of permeation of helium into the field emission tubes has been reduced and very low pressures can be maintained for extended periods (about Torr of adsorbable gases). Field emission work is one of the few cases where the helium present in the residual gas is troublesome; in this case the helium causes sputtering of the field emitting tip and a slow increase in its radius. T h e effect of surface contamination on the measurement of secondary emission has been pointed out by Jonker (302) and he shows that if care is taken to ensure adequate surface cleanliness the secondary emission coefficient data for many metals will fit a universal curve. T h e extreme care necessary to prepare clean films for work-function measurements was demonstrated some time ago by Anderson (303) in what was probably the first realization of u-h-v conditions. T h e exoemission of ,electrons is another surface phenomenon affected by the presence of surface gas ; very few experiments on exo-emission have becn carried out under vacuum conditions, which would ensure reproducible surface conditions. For a review of exo-emission phenomena, see

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Grunberg (304) and Huguenin (305). U-h-v techniques have been used in recent work on the interaction of slow electrons with atomically clean surfaces (306-312). U-h-v techniques have also been used in studies of the condensation of atomic beams of metals on surfaces. Rapp et al. (313, 324) find that the condensation coefficient of silver, cadmium, and'zinc from the vapor is essentially unity, and that the ambient pressure does not affect this conclusion at high incident beam flux from pressures of 10-lo to Torr.

B. Thin Films T h e important role played by vacuum conditions in determining the physical properties of evaporated metallic films has recently become widely recognized. I n brief summary, the presence of residual active gases prior to and during the deposition of a film (a) affect the nucleation site density, the mobility, and the adhesion of the depositing atoms by forming adsorbed layers on the substrate, (b) tend to create a high density of imperfections within the film by adsorption during the course of deposition. T h e electric and magnetic properties of t h e deposited film can, in consequence, show serious deviations from those expected from the nature and temperature of the substrate and the annealing procedure subsequent to deposition. Since film deposition rates are usually from one to a few hundred atomic layers per second, the residual gas conditions required to avoid these complicating effects are similar to those for measurements on metal surfaces free of adsorbed gases, namely u-h-v conditions in most cases. T h e maintenance of u-h-v during film deposition is difficult due to the outgassing of the evaporating metal, Rigorous prior outgassing of the evaporator is in general necessary to achieve the desired conditions. An excellent cross section of recent contributions to thin film research can be found in the proceedings of a recent conference (315). U-h-v techniques have been used in several of the reported investigations and it can be said that an understanding of the fundamental factors which determine thin film properties is beginning to emerge. Among the most important factors seem to be:

(1) The nature and state of roughness of the substrate. (2) T h e temperature of the substrate during deposition. (3) T h e annealing temperature of the film material relative to the substrate temperature. (4) T h e mobility of the deposited atoms on the substrate.

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419

Once ambiguities due to residual gas influence have been removed, it appears possible to obtain films of specific desired properties by proper choice of film, substrate, deposition temperature and annealing procedure. Thus Mayer (326) has studied the fundamental problem of electronic conductivity of metals using u-h-v eyaporated alkali metal films at 90°K. Evans and Mitchell (317) have studied the resistivity of thin copper films on glass during oxygen adsorption, and the saturation magnetization of very thin pure nickel films on glass has been examined (318).A detailed examination of the effects of various residual gases on the superconducting properties of thin tin films has been made by Caswell (329). T h e effects appear to be strongly dependent on the particular gas, as might be expected from their chemisorption characteristics. I n a recent review of the application of thin films to electronic component production, Greenland (320) expresses the opinion that u-h-v techniques may be of great practical importance in this field if they allow the production of thin films possessing nearly bulk properties. In no such application is the anomalous structure resulting from residual gas adsorption considered to be anything but detrimental. Such techniques are expected to undergo rapid development in the near future, and thin film electronic component production may well become the first large scale commercial application of u-h-v.

C . Thermonuclear and Plasma Devices T h e effect of impurities in the operation of controlled thermonuclear devices has been discussed by several authors (13, 321). T h e impurities in the plasma increase the rate at which energy is radiated from the plasma. This effect increases with the atomic number (2)of the impurities. T h e presence of very small amounts of impurities of large z can prevent the attainment of the required plasma temperature. T h e vacuum problem in a controlled thermonuclear device is twofold; (a) to achieve u-h-v in the system before the introduction of the plasma gas (usually deuterium or tritium), and (b) to minimize contamination of the plasma caused by its interaction with the walls of the chamber. Very great advances have been made in the past few years in achieving u-h-v in the large and complex systems required for controlled thermonuclear machines. T h e general technical problems involved have been discussed by Munday (12). Work on the Stellarator project at Princeton has produced significant improvements in u-h-v technology, and has been reported in considerable detail (23,230,204, 271). T h e Model C Stellarator has a vacuum chamber with a 400-liter volume, which is

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bakable at high temperatures, and which consists of a stainless steel tube 20 cm inside diameter, in the shape of an oval with a peripheral length of 120 meters. Pressures below 3 x 10-lo Torr have been obtained with a 25 cm diameter mercury diffusion pump. Fig. 18(b) shows an over-all view of the Model C Stellarator. Sledziewski (322) describes a French thermonuclear device which has a volume of 400 liters and is pumped by six oil diffusion pumps with a total speed of 1400 liters/sec. The system can be baked only at 100°C because of the use of indium for some of the seals. The minimum pressure Torr. obtained is about 3 x The Russian experimental thermonuclear device “Ogra” has been described by Kurchatov (323). This device can be baked at 450°C and is pumped to Torr by mercury diffusion pumps,. Titanium getterion-pumps are then turned on and the pressure reduced to lo-* Torr. Titanium is evaporated directly onto the walls of the chamber so that the walls of the confining chamber become an adsorbing getter surface. Vacuum systems for the Mirror Machine and associated experiments at Berkeley have been described by Milleron (205, 183).

D. Space Simulation I n the last few years the need has risen to simulate extraterrestrial conditions in the laboratory. We are concerned here only with the simulation in the laboratory of the pressure conditions of outer space. The chief requirement for such systems is in the testing of complete space-vehicles and their components under the pressure conditions they will experience in flight. This requirement has led to the design of very large u-h-v systems, in the largest of which it is possible to test complete artificial satellites. There is still considerable uncertainty in the value of pressures to be expected at a given altitude and in the gaseous composition of the upper atmosphere, but the generally accepted pressureversus-altitude curve is expressed by the “model atmosphere” curves Torr is [ARDC Model Atmosphere, 1959 (324)l. A pressure of reached at an altitude of about 320 km. Construction of the space chambers is a particularly difficult job, not only because of their large size but also because of the large number of observation ports, doors, and other openings that must be provided in the walls of the chamber. Moreover, in some systems the problems are further compounded by the introduction of mechanical motion, vibration, high radiant flux, etc. All these facilities must be provided in a chamber capable of being pumped to pressures of about 10-lo Torr. In discussing pressure simulation of outer space Santeler (325) points

42 I

ULTRAHIGH VACUUM

out that molecules emitted from a vehicle in space rarely return to the vehicle. If this condition is to be simulated in the laboratory, then it is necessary for the walls surrounding a test vehicle to have an effective condensation coefficient approaching unity. To achieve this with diffusion pumps for a test chamber 25 meters in diameter, would be prohibitively expensive. T h e cost for direct liquid hydrogen or liquid helium cooling of the walls would also be too high. Santeler (325) proposed as a solution walls consisting of panels cooled to liquid hydrogen temperatures but shielded from radiation from the test vehicle by panels cooled with liquid nitrogen. With this arrangement the wall condensation eficiency is reduced by a factor of two, but the cost is reduced by about a factor of thirty. Bennett (326) has proposed a system in which the test object is placed in a volume surrounded by the vapor stream from the annular jet of a large diffusion pump. Molecules from the test object would be entrapped in the vapor stream and removed from the system. w 5 0 FEETSOLAR LIGHT SOURCE

DIFFUSION P U M P S VACUUM WALL

100' K COLD WALL

20" K CRYOGENIC PUMPS

looo K

CRYOGENIC P U M P SHIELDS

FIG. 26. Schematic diagram of space simulation chamber. (Courtesy of R.C.A.).

A chamber of about 1300 liters volume, capable of reaching a pressure of 4 x 10-1O Torr, has been described by Simons (206). This system consists of a stainless steel cylinder 1 meter in diameter with a removable door at one end. T h e system is baked at 200°C. A 25 cm diameter oil diffusion pump is used to evacuate the system. Some of the technical developments leading to the design of this system have been described by Farkass (2Z4).

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Considerably larger chambers for space simulation are now being built, technical details of their design and performance have not yet been published. Figure 26 shows a schematic diagram of a large space simulation chamber.

REFERENCES I. Bayard, R. T., and Alpert, D., Rew. Sci. Znstr. 21, 571 (1950). 2. Alpert, D., in “Handbuch der Physik” (S. Flugge, ed.), Vol. 12, p. 609. Springer, Berlin, 1958. 3. Schweitzer, J., Le Vide 14, 165 (1959). 4. Venema, A., and Bandringa, M., Philips Tech. Rev. 20, 145 (1958). 5. Redhead, P. A., and Kornelsen, E. V., Vukuum-Technik. 10, 31 (1961). 6. Yanvood, J., J . Sci. Instr. 34, 297 (1957). 7. Kleint, C., Expte. Tech. Physik 8, 193 (1960). 8. Lafferty, J. M., and Vanderslice, T. A,, Proc. Z.R.E. 49, 1136 (1961). 9. Pollard, J., Repts. Progr. in Phys. 22, 33 (1959). 10. Men’shikov, M. I., Pribory i Tekh. Eksptl. No. 4, 3 , July-Aug. (1959); English transl., Instr. and Exptl. Tech. No. 4, 511, July-Aug. (1959). 11. Jancke, H., Exptl. Tech. Physik 7, 241 (1959). 12. Munday, G. L., Nuclear Instr. & Methods 4, 367 (1959). 13. Grove, D. J., Trum. Nutl. Symposium on Vucuum Tech. 5, 9 (1958). 14. de Boer, J. H., “The Dynamical Character of Adsorption.” Oxford Univ. Press, London and New York, 1953. 15. Brunauer, S . , “The Adsorption of Gases and Vapors.” Princeton Univ. Press, Princeton, New Jersey, 1945. 15u. Tompkins, F. C., Le Vide 17, 72 (1962). 16. Foner, S. N., et ul., J. Chem. Phys. 31, 546 (1959). 17. Schafer, K., and Gerstacker, H., Z. Elektrochem. 60, 874 (1956). 18. Becker, J. A., in “Structure and Properties of Solid Surfaces” (R. Gomer and C.S . Smith, eds.), p. 459. Univ. of Chicago Press, Chicago, Illinois, 1952. 19. Mickelsen, W. R.,and Childs, J. H., Rew. Sci. Instr. 29, 871 (1958). 20. Hurlbut, F. C., J. Appl. Phys. 28, 844 (1957). 21. Cabrera, N., Discussions Furaduy SOC.28, 16 (1959). 22. Zwanzig, R. W., J. Chem. Phys. 32, 1173 (1960). 23. Littlewood, R., and Rideal, E., Trans. Furaduy SOC.52, 1598 (1956). 24. Langrnuir, I.,-J. Am. Chem. SOC.54, 2798 (1932). 25. Brunauer, S., Emmett, P. H., and Teller, E., J. Am. Chem. SOC.60, 309 (1938). 26. Kruyer, S., Proc. Koninkl. Ned. Akud Wetenschap. B58, 73 (1955). 27. Hayashi, C., Trans. Nutl. Symposium on Vacuum Tech. 4, 13 (1957). 28. Clausing, P., Ann. Physik [5] 7. 489 (1930). 28u. Clausing, P., Physicu 28, 298 (1962). 28b. Eschbach, H. L., Jaeckel, R., and Mliller, D., T~uns.Nutl. Symposium 011 Vucuum Tech. 8, 1110 (1961). 29. Ehrlich, G., J . Chem. Phys. 34, 39 (1961). 30. Ehrlich, G., J . Chem. Phys. 34, 29 (1961). 31. Ehrlich, G., and Hudda, F. G., J . Chem. Phys. 30, 493 (1959). 32. Ehrlich, G., Hickmott, T. W., and Hudda, F. G., J . Chem. Phys. 28, 977 (1958).

ULTRAHIGH VACUUM

423

33. Gomer, R., J . Chem. Phys. 29, 441 (1958). 34. Gomer, R., J . Phys. Chem. 63, 468 (1959). 35. Devienne, F. M., Mimorial Sci. phys. (Acad. sci. Paris) No. 56 (1953). 35a. Hartnett, J. P., “Rarefied Gas Dynamics,” p. 1. Academicpress, New York, 1961. 36. Thomas, L. B., and Golike, R. C., J. Chem. Phys. 22, 300 (1954). 37. Schafer, K., and Gerstacker, H., 2. Elektrochem. 59, 1023 (1955). 38. Dushman, S., “The Scientific Foundations of Vacuum Technique.” Wiley, New York, 2”“ edition 1961. 39. Steele, W. A., and Halsey, G. D., Jr., J. Chem. Phys. 22, 979 (1954). 40. Steele, W. A., and Halsey, G. D., Jr., 3. Phys. Chem. 59, 57 (1955). 41. Freeman, M. P., and Halsey, G. D., Jr., J. Phys. Chem. 59, 181 (1955). 42. Sams, J. R., Jr., Constabaris, G., and Halsey, G. D., Jr., J. Phys. Chem. 64, 1689 (1960). 43. Drain, L. E., Trans. Faraday SOC.49, 650 (1953). 44. Rhodin, T. N., Jr., J. Am. Chem. SOC.72, 5691 (1950). 45. Hobson, J. P., Can. J. Phys. 37, 300 (1959). 46. Stout, J. W., and Giauque, W. F., 3. Am. Chem. SOC.60, 393 (1938). 47. Hobson, J. P., and Redhead, P. A., Can. J. Phys. 36, 271 (1958). 48. Aston, J. G . ,Mastrangelo,S.V.R., andTykodi, R. J.,J. Chem. Phys. 23, 1633 (1955). 49. Hobson, J. P., Can. J . Phys. 37, 1105 (1959). 50. Hobson, J. P., J. Chem. Phys. 34, 1850 (1961). 51. Hobson, J. P.,and Armstrong, R. A., Rept. 21st Ann.M.Z.T. Conf. on Phys. Electronics p. 236 (1961). 5 l a . Hansen, N., Vakuum Technik (in press). 516. Hobson, J. P., Trans. Natl. Symposium on Vacuum Tech. 8, 146 (1961). 52. Gomer, R., Wortman, R., and Lundy, R., J . Chem. Phys. 26, 1147 (1957). 511. Allen, C. W., “Astrophysical Quantities,” p. 227. Univ. of London Press (Athlone), London, 1955. 54. Gomer, R., J . Chem. Phys. 28, 168 (1958). 54a. Hengevoss, J. and Huber, W. K., Trans. Natl. Symposium on Vacuum Tech. 8, 1332 (1961). 55. Garbe, S., Klopfer, A., and Schmidt, W., Vacuum 10, 81 (1960). 55a. Tuzi, Y. and Okamoto, H., J. Phys. SOC.Japan 13, 960 (1958). 556. Tuzi, Y., J. Phys. SOC.Japan 17, 218 (1962). 56 Becker, G. E., Trans. Natl. Symposium on Vacuum Tech. 6, 197 (1959). 57. Balwanz, W. W., Singer, J. R., and Frandsen, N. P., Trans. Natl. Symposium on Vacuum Tech. 6, 159 (1959). 58. Penchko, E. A., Khavkin, L. P., and Borodkin, A. S., Pribory i Tekh. Eksptl. No. 4, p. 146, July-Aug. (1959); English transl., Znstr. and Exptl. Tech. No. 4, 667, July-Aug. (1959). 59. Keesom, W. H., and Schweers, J., Physica 8, 1032 (1941). 60. Singleton, J. H., and Halsey, G. D., Jr., J. Phys. Chem. 58, 330 (1954). 61. Gundry, P. M., and Tompkins, F. C., Quart. Revs. (London) 14, 257 (1960). 62. Becker, J. A., Solid State Phys. 7 , 379 (1958). 63. Becker, J. A., Adwances in Catalysis 7 , 135 (1955). 64. Brennan, D., Hayward, D. O., and Trapnell, B. M. W., Proc. Roy. SOC.(London) A256, 81 (1960). 65. Ehrlich, G., 3. Chem. Phys. 31, 1111 (1959). 66. Beeck, O., Discussions Faraday SOC.8, 118 (1950). 67. Trapnell, B. M. W., Proc. Roy. SOC.(London) A218, 566 (1953).

424

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

68. Beeck, O.,Advances in Catalysis 2, 151 (1950). 69. Bagg, J., and Tompkins, F. C., Trans. Faraduy Soc.. 51, 1071 (1955). 70. Baker, M. McD., and Rideal, E. K., Trans. Faraday SOC.51, 1597 (1955). 71. Beeck, O., Cole, W. A., and Wheeler, A., lX.~cussionsFaraday Soc. 8, 314 (1950). 72. Redhead, P. A., Trans. Faraday SOC.57, 641 (1961). 73. Trapnell, B. M. W., Trans. Faruduy Soc. 52, 1618 (1956). 73a. Roberts, R. W., Nature 191, 120 (1961). 74. Collins, R. H. and Turnbull, J. C., Vacuum 11, 114 and 119 (1961). 75. Wagener, J. S. and Marth, P. T . , J . Appl. Phys. 28, 1027 (1957). 76. Dayton, B. B., Trans. Natl. Symposium on Vacuum Tech. 6, 101 (1959). 77. Jaeckel, R., and Schittko, F. J., “Forschungsberichte des Wirtschafts und Ver-

kehrministeriums Nordrhein-Westfalen” No. 369. Westdeut. Verlag, Koln and Opladen, 1957. 78. Santeler, D. J., Trans. Natl. Symposium on Vacuum Tech. 5, 1 (1958). 79. Wolsky, S. P., and Zdanuk, E. J., Vacuum 10, 13 (1960). 80. Farkass, I., and Barry, E. J., Trans. Natl. Symposium on Vacuum Tech. 8, 66 (1961). 81. Koenig, H.,Vacuum 3, 3 (1953). 81a. Venema, A., Trans. Natl. Symposium on Vacuum Tech. 8, 1 (1961). 81b. Hobson, J. P., Trans. Natl. Symposium on Vacuum Tech. 8. 26 (1961). 82. Todd, B. J., J. Appl. Phys. 26, 1238 (1955). 83. Todd, B. J., J. Appl. Phys. 27, 1209 (1956). 83a. Garbe, S. and Christians, K., Vakuum Technik 11, 9 (1962). 83b. Dayton, B. B., Trans. Natl. Symposium on Vacuum Tech. 8, 42 (1961). 83c. Hunt, A. L., Damm, C. C., and Popp, E. C., J ’. Appl. Phys. 32, 1937 (1961). 84. Hagstrum, H. D.. and d’Amico, C., J. Appl. Phys. 31, 715 (1960). 84a. Dillon, J. A. Jr., Trans. Natl. Symposium on Vumum Tech. 8, 113 (1961). 85. Norton, F. J., J. Appl. Phys. 28, 34 (1957). 85a. Norton, F. J., Trans. Natl. Symposium on Vacuum Tech. 8. 8 (1961). 86. Barrer, R. M., “Diffusion In and Through Solids.” Cambridge Univ. Press, London and New York, 1951. 87a. Altemose, V. O., J. Appl. Phys. 32, 1309 (1961). 87b. Muller, C. F. and Shepard, R. W., Vacuum 11, 58 (1961). 87c. Young, J. R. and Whetten, N. R., Rew. Sci. Instr. 31, 1112 (1960). 87d. Whetten, N. R. and Young, J. R.. Rev. Sci. Instr. 30, 472 (1959). 87e. Belyakov, Yu. I. and Ionov, N. I., Zh. Tekh. Fi2 ( U S S R ) 31, 204 (1961. Soviet Phys.-Tech. Phys. 6, 146 (1961) [English Trans.] 87j. Gorman, J. K. and Nardella, W. R., Vacuum 12, 19 (1962). 88. Lafferty, J. M., J. Appl. Phys. 32, 424 (1961). 89. Rivera, M.,and Le Riche, R., Trans. Natl. Symposium on Vacuum Tech. 6, 55 (1959). 89a. Metcalfe, R. A. and Trabert, F. W., Trans. Natl. Symposium on Vacuum Tech. 8, 1211 (1961). 90. Alpert, D., and Buritz, R. S., J. Appl. Phys. 25, 202 (1954). 91. Varnerin, L. J., and White, D. H., J . Appl. Phys. 25, 1207 (1954). 92. Rogers, W. A., Buritz, R. S. and Alpert, D., J. Appl. Phys. 25, 868 (1954). 93. McAfee, K. B., Jr., J. Chem. Phys. 28, 218 (1958). 94. McAfee, K. B., Jr., J. Chem. Phys. 28, 226 (1958). 95. Jost, W., “Diffusion in Solids, Liquids, Gases.” Academic Press, New York, 1952. 9Sa. Swets, D. E., Lee, R. W., and Frank, R. C., J . Chem. Phys. 34, 17 (1961).

ULTRAHIGH VACUUM

425

95b. Frank, R. C., Swets, D. E., and Lee, R. W.,J. Chem. Phys. 35, 1451 (1961). 9 5 . Hurlhert, R. C. and Konecny, J. O., J. Chem. Phys. 34, 655 (1961). 95d. Tiedema, T. J., De Jong, B. C., and Burgers, W. G., Proc. I(. Ned. Akad. Wetensch. B 63, 422 (1960). 95e. Frank, R. C . and Thomas, J. E. Jr., J . Phys. Chem. Solids 16, 144 (1960). 95f. Nechai, E. P., Popov, K. V., and Panenkova, L. S., Fiz. Metallov i Metallovedenie ( U S S R ) 10,838 (1 960). 95g. Foster, P. K., Naiure 188, 399 (1960). 95h. Rodin, A. M . and Surenyants, V.V., Fiz. Metallow i Metallovedenie ( U S S R ) 10, 216 (1960). 9.52’. Haas, C., J . Phys. Chem. Solids IS, 108 (1960). 95j. O’Keefe, M. and Moore, W. J., J. Phys. Chem. 65, 1438 (1961). 9Sk. Tucker, C. W. and Norton, F. J., J. Nuclear Materials 2, 329 (1960). 96. Waldschmidt, E., Metalf. 8, 749 (1954). 97. von Ardenne, M., “Tabellen der Elektronenphysik, Ionenphysik und uberrnikroskopie.” Deut. Verlag. Wiss., Berlin, 1956. 98. Hashinioto, K., Iwayanagi, H., and Fukushima, H., Vacuum 10, 92 (1960). 99. Bliss, B. J., Trans. Natl. Symposium on Vacuum Tech. 7, 145 (1960). 100. Fish, I. P. S., Rev. Sci. Znstr. 30, 889 (1959). 101. Varadi, P. F., Trans. Natl. Symposium on Vacuum Tech. 7, 149 (1960). 102. Smithells, C. J., and Ransley, C. E., PYOC.Roy. Soc. (London) A155, 195 (1936). 103. della Porta, P., and Ricca, F., Trans. Natl. Symposium on Vacuum Tech. 5, 25 (1958). 104. della Porta, P., Trans. Natl. Symposium on Vacuum Tech. 6 , 317 (1959). 104a. Honig, R. E. and Hook, H. O., R C A Rev. 21, 360 (1960). 105. Borovik, E. S., Grishin. S. F., and Grishna, E. Ya., Zhur. Tekh. Fiz. 30, 539 (1960); English transl., Soviet Phys.-Tech. Phys. 5 , 506 (1960). 106. Wagener, S., Vucuum 3, I 1 (1953). 107. Leiby, C. C., Jr., and Chen, C. L., J . Appl. Phys. 31, 268 (1960). 108. Eschhach, H. L., in “Advances in Vacuum Science and Technology” (E. Thomas, ed.), Vol. 1, p. 373, Pergamon Press, New York, 1960. 109. Thulin, S., Arkiv. Fysik 9 , 107 (1955). 110. Bohr, N., K g l . Danske Videnskab. Selskab, Mat-fys. Medd. 18, 8 (1948). 111. Gibson, J. B., Goland, A. N., Milgram, M., and Vinegard, G. H., Phys. Rev. 120, 1229 (1960). 112. Rirnrner, D. E., and Cottrell, A. H., Phil. Mag. [8] 2, 1345 (1957). 113. Young, J. R.,J. Appl. Phys. 27, 1 (1956). 114. Beliakova, V. V., and Mittsev, M. A., Zhur. Tekh. Fiz. 27, 803 (1957); English transl., Soviet Phys.-Tech. Phys. 2, 729 (1957). 115. Kornelsen, E. V., Trans. Natl. Symposium on Vacuum Tech. 7, 29 (1960). 116. Nielsen, K. O., in “Electromagnetically Enriched Isotopes and Mass Spectrometry” (M. L. Smith, ed.), p. 68. Butterworths, London, 1956. 117. Bredov, M. M., Lang, Z. O., and Okuneva, N. W., Zhur. Tekh. Fiz. 28,252 (1958); English transl., Souiet Phys.-Tech. Phys. 3, 228 (1958). 117a. Davies, J. A. and Sims, G. A,, Can. J. Chew. 39, 601 (1961). 118. Varnerin, L. J., and Carmichael, J. H., J. Appl. Phys. 28, 913 (1957). 119. Buritz, R. S., and Varnerin, L. J., Dept. of the Air Force (U:S.) Report 71F191-R4 (1955). 120. Carter, G., and Leck, J. H., Trans. Natl. Symposium on Vacuum Tech. 7, 339 (1960).

426

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

121. Leck, J. H., and Carter, G., in “Advances in Vacuum Science and Technology” (E. Thomas, ed.), Vol. 1, p. 463, Pergamon Press, New York, 1960. 122. Kornelsen, E. V., Trans. Natl. Symposium on Vacuum Tech. 8, 281 (1961). 123. Carmichael, J. H., and Trendelenburg, E. A., J. AppZ. Phys. 29, 1570 (1958). 124. Schwarz, H., Z. Physik 122, 437 (1944). 125. Bills, D. G., and Carleton, N. P., J. AppZ. Phys. 29, 692 (1958). 126. Almkn, O., and Bruce, G., Nuclear Instr. @ Methods 11, 257 (1961). 127. Kuchai, S. A., and Rodin, A.M., Atomnaya Energ. 4, 202 (1958). English transl., Soviet J . Atomic Energy 4, 277 (1958). 127a. Carter, G., Colligon, J. S., and Leck, J. H., Proc. Phys. SOC.79, 299 (1962). 128. Carmichael, J. H., and Knoll, J. S., Trans. Natl. Symposium on Vacuum Tech. 5, 18 (1958). 129. Fox, R. E., and Knoll, J. S., Trans. Natl. Symposium on Vacuum Tech. 7, 364 (1960). 130. Le Claire, A. D., and Rowe, A. H., Rev. mPt. 52, 94 (1955). 131. Redhead, P. A,, Trans. Natl. Symposium on Vacuum Tech. 6,. 12 (1959). 131a. Reich, G., Vakuum Technik 10, 109 (1961). 132. Wehner, G. K., Advances in Electronics and Electron Phys. 7, 239 (1955). 133. Wehner, G . K., Phys. Rev. 102, 690 (1956). 134. Laegreid, N . , and Wehner, G. K., J. Appl. Phys. 32, 365 (1961). 135. Stuart, R. V., and Wehner, G. K., Phys. Rev. Letters 4, 409 (1960). 136. Rol, P. K., Fluit, J. M., Wiehbock, F. P., and de Jong, M., in “Proceedings of the Fourth International Conference on Ionization Phenomena in Gases, 1959” (N. R. Nilsson, ed.). Vol. 1. p. 257. North Holland, Amsterdam, 1960. 137. Yonts, 0. C., Normand, C. E., and Harrison, D. E., Jr., J. AppLPhys. 31, 447 (1960). 138. Wehner, G. K., and Rosenberg, D., J. Appl. Phys. 32, 887 (1961). 139. Anderson, G. S . , and Wehner, G. K., J. Appl. Phys. 31, 2305 (1960). 140. Keywell, F., Phys. Rev. 97, 1611 (1955). 141. Hines, R. L., and Wallor, R., J. Appl. Phys. 32, 202 (1961). 142. Wolsky, S. P., and Zdanuk, E. J., Trans. Natl. Symposium on Vacuum Tech. 6, 6 (1959). 143. Farnsworth, H. E., Schlier, R. E., George, T. H., and Burger, R. M., J . Appl. Phys. 29, 1150 (1958). 144. Dillon, J. A., Jr., and Farnsworth, H. E., J. Appl. Phys. 29, 1195 (1958). 145. Hagstrum, H. D., Phys. R w . 104, 672 (1956). 146. Hagstrum, H. D., Phys. Rev. 104, 1516 (1956). 147. Hagstrum, H. D., Phys. Rev. 119, 940 (1960). 148. Waters, P. M., Phys. Rev. 109, 1466 (1958). 149. Waters, P. M., Phys. Rev. 111, 1053 (1958). 150. Little, P. F., in “Handbuch der Physik” (S. Fliigge, ed.), Vol. 21, p. 574. Springer, Berlin, 1956. 15I. Aarset, B., Cloud, R. W., and Trump, J. C., J. Appl. Phys. 25, 1365 (1954). 152. Hagstrum, H. D., Phys. Rev. 123,758 (1961). 153. BrunnCe, C., Z . Physik 147, 161 (1957). 154. Bradley, R. C., Arking, A., and Beers, D. S., J. Chem. Phys. 33, 764 (1960). 155. Stanton, H. E., J. Appl. Phys. 31, 678 (1960). 156. Bills, D. G., Pkys. Rev. 107, 994 (1957). 157. Bills, D. G., and Evett, A. A., J. Appl. Phys. 30, 564 (1959). 157a. Donaldson, E. E., Vacuum 12, 11 (1962).

ULTRAHIGH VACUUM

427

158. Nottingham, W. B., “Bibliography on Physical Electronics.” Addison-Wesley,

Reading, Massachusetts, 1954. 159. Moore, G. E., J. Appl. Phys. 32, 1241 (1961). 160. Reynolds, J. H., Rev. Sci. Znstr. 27, 928 (1956). 161. Jacob, L., Proc. Phys. SOC.(London) B70,235 (1957). 162. Todd, B. J., Lineweaver, J. L., and Kerr, J. T., J. Appl. Phys. 31, 51 (1960). 163. Terenin, A., and Solonitzin, Yu., Discussions Faraday SOC.28, 28 (1959). 164. Valnev, J., Zhur. Fiz. Khim. 30, 1308 (1956). M a . Lange, W. J. and Riemersma, H., Trans. Natl. Symposium on Vacuum Tech. 8, 167 (1961). 165. Kenty, C., Rept. 13th Ann. M.I.T. Conf. on Phys. Electronics p. 138 (1953). 166. Hickmott, T. W., J. Appl. Phys. 31, 128 (1960). 167. Becker, J. A,, Becker, E. J., and Brandes, R. G., J . Appl. Phys. 32,411 (1961). 168. Young, J. R., J. Appl. Phys. 30, 1671 (1959). 169. Rhodin, T. N., and Rovner, L. H., Trans. Natl. Symposium on Vacuum Tech. I . 228 (1960). 170. Smith, H. R., Trans. Natl. Symposium on Vacuum Tech. 6, 140 (1959). 171. Dreyer, K., and Mark, J. T., R.C.A. Rev. 21, 508 (1960). 172. Hablanian, M. H., and Landfors, A. A., Trans. Natl. Symposium on Vacuum Tech. 7, 55 (1960). 173. Hickman, K. C.D., Trans. Natf. Symposium on Vacuum Tech. 8, 307 (1961). 174. Becker, W., in “Advances in Vacuum Science and Technology” (E. Thomas, ed.), Vol. I , p. 173, Pergamon Press, New York, 1960. 17.5~.Becker, W., Vakuum Technik 10, 199 (1961). 175b. Pupp, W., Vakuum Technik 10, 195 (1961). 176. Williams, C . E., and Beams, J. W., Trans. Natl. Symposium on Vacuum Tech. 8, 295 (1961). 177. Wagener, J. S., Proc. 4th Natl. Conf. on Tube Techniques p. 1 (1958). 178. Klopfer, A., and Ermrich, W., Va’akuum-Technik8, 162 (1959). 179. Zdanuk, E. J., and Wolsky, S. P., Trans. Natl. Symposium on Vacuum Tech. 8, 188 (1961). 180. Luckert, J., Vakuum Technik 10, 1 and 40 (1961). 180a. Holland, L., Laurenson, L., and Allen, P.G.W., Trans. Natl. Symposium on Vacuum Tech. 8, 208 (1961). 181. Stout, V . L., and Gibbons, M. D., J. Appl. Phys. 26, 1488 (1955). 182. Gibbons, M. D., and Stout, V. L., Rept. 19th Ann. M.I.T. Conf. on Phys. Electronics p. 117 (1959). 183. Milleron, N . , and Popp, E. C., Trans. Natl. Symposium on Vacuum Tech. 5, 153 (1958). 184. Jaeckel, R., and Teloy, E., Trans. Natl. Symposium on Vacuum Tech. 8,406 (1961). 185. Holland, L., Nature 185, 911 (1960). 186. Alpert, D., J. Appl. Phys. 24, 860 (1953). 187. Varnerin, L. J., and Carmichael, J. H., J. Appl. Phys. 26, 782 (1955). 188. Young, J. R., J . Appl. Phys. 27, 926 (1956). 189. Hobson, J. P., Vacuum 11, 16 (1961). 1890. Cobic, B., Carter, G., and Leck, J. H., Brit. J. Appl. Phys. 12, 288 and 384 (1961). 190. Brown, E., and Leck, J. H., Brit. J. Appl. Phys. 6, 161 (1955). 191. Alpert, D., in “Advances in Vacuum Science and Technology” (E. Thomas, ed.), Vol. 1, p. 31. Pergamon Press, New York, 1960.

428

P. A. REDHEAD, J. P. HOBSON, E. V. KORNELSEN

192. Alexeff, I., and Peterson, E. C., Trans. Natl. Symposium on Vacuum Tech. 2, 87 (1955). 193. Herb, R. G., Davis, R.H., Divatia, A.S., and Saxon, D., Phys. Rev. 89, 897 (1953). 194. Hall, L. D., Rev. Sci. Instr. 29, 367 (1958). 195. Brubaker, W. M., Trans. Natl. Symposium on Vacuum Tech. 6, 302 (1959). 196. Jepsen, R. L., Francis, A. B., Rutherford, S. L., and Kietzmann, B. E., Trans. Natl. Symposium on Vacuum Tech. 7, 45 (1960). 196a. Klopfer, A,, Vucuum Technik 10, 113 (1961). 197. Rutherford, S. L., Merser, S. L., and Jepsen, R. L., Trans. Natl. Symposium on Vacuum Tech. 7, 380 (1960). 198. Foster, J. S., Jr., Lawrence, E. O., and Lofgren, E. L., Rev. Sci. Instr. 24, 388 (1953). 199. KovalSkii, G . A., and Kuchai, S. A., Pribory i Tekh. Eksptl. No. 2, 110, MarchApril (1960); English transl., Instr. and Exptl. Tech. No. 2, 293, March-April (1960). 200. Ullman, J. R., Trans. Natl. Symposium on Vacuum Tech. 4, 95 (1957). 201. Smith, H. R., and Kennedy, P. B., Trans. Natl. Symposium on Vacuum Tech. 6, 271 (1959). 202. Caswell, H. L., Trans. Natl. Symposium on Vacuum Tech. 6, 66 (1959). 203. Caswell, H. L., Rew. Sci. Instr. 30, 1054 (1959). 204. Henderson, W. G., Geiger, G. S., and Mark, J. T., Trans. Natl. Symposium on Vacuum Tech. 6, 170 (1959). 205. Milleron, N., Trans. Natl. Symposium on Vacuum Tech. 5, 140 (1958). 206. Simons, J. C., Jr., Trans. Natl. Symposium on Vacuum Tech. 6, 48 (1959). 207. Alpert, D., Rev. Sci. Instr. 24, 1004 (1953). 208. Trendelenburg, E. A., Le Vide 14, 74 (1959). 209. Carmichael, J. H., and Lange, W. J., Trans. Nail. Symposium on Vacuum Tech. 5, 137 (1958). 210. Biondi, M. A., Rev. Sci. Instr. 30, 831 (1959). 211. Biondi, M. A., Trans. Natl. Symposium on Vacuum Tech. 7, 24 (1960). 212. Goerz, D. J., Jr., Trans. Natl. Symposium on Vacuum Tech. 7 , 65 (1960). 212a. Haefer, R. A., Trans. Natl. Symposium on Vacuum Tech. 8, 1346 (1961). 213. Beecher, N., and Hnilicka, M. P., Trans. Natl. Symposium on Vacuum Tech. 5, 94 (1958). 214. Farkass, I., and Vanderschmidt, G. F., Trans. Natl. Symposium on Vacuum Tech. 6, 42 (1959). 215. Lazarev, B. G., Borovik, E. S., Grishin, S. F., and Tsin, N. M., Ukrain. Fix. Zhur. 2, 175 (1957). 216. Borovik, E. S., Grishin, S. F., and Lazarev, B. G., Pribory i Tekh. Eksptl. No. 1, 115 , Jan.-Feb. (1960); English transl., Znstr. and EJcptl. Tech. No. 1, 128, Jan.-Feb. ( 1960). 217. Jepsen, R. L., Mercer, S. L., and Callaghan, M. F., Rev. Sci. Instr. 30, 377 (1959). 218. Varadi, P. F., and Ettre, K., Trans. Natl. Symposium on Vacuum Tech. I , 248 (1960). 219. Degras, D. A., Le Vide 14, 128 (1959). 220. Prugne, P., and Garin, P., in “Advances in Vacuum Science and Technology” (E. Thomas, ed.), Vol. 1, p. 439. Pergamon Press, New York, 1960. 221. Ames, I., Christiansen, R. L., and Trule, J., Rev. Sci. Instr. 29, 736 (1958). 222. Behrndt, K. H., Trans. Natl. Symposium on Vacuum Tech. 6, 255 (1959). 223. Kohl, W. H., “Materials and Techniques for Electron Tubes.” Reinhold, New York. 1960.

ULTRAHIGH VACUUM

429

224. Knoll, M., “Materials and Processes of Electron Devices.” Springer, Berlin, 1959. 225. Espe, W., “Werkstoffkunde der Hochvakuumtechnik,” Vols. I , 2, and 3. Deut. Verlag Wiss., Berlin, 1960. 226. A.S.T.M. Spec. Tech. Pu61. No. 246 (1959) (21 papers). 227. Libin, I. Sh., and Rocklin, G. N., Pribory i Tekh. Eksptl. No. 3, 150, May-June (1959); English transl., Instr. and Exptl. Tech. N o . 3, 497, May-June (1959). 228. Kramers, W. J., and Dennard, F., Vacuum 3, 151 (1953). 229. Kornelsen, E. V., and Weeks, J. O., Rev. Sci. Znstr. 30, 290 (1959). 230. Mark, J. T., and Dreyer, K., Trans. Natl. Symposium on Vacuum Tech. 6, 176 (1959). 231. Holland, L., Trans. Natl. Syyposium on Vacuum Tech. 7, 168 (1960). 232. Leck, J. H., “Pressure Measurements in Vacuum Systems.” Chapman & Hall, London, 1957. 232a. Brombacher, W. G . , Natl. Bur. Standards ( U S . ) Monograph 35 (1961). 233. Paty, L., Pribory i Tekh. Eksptl. No. 6, 3 , Nov.-Dec. (1959); English transl., Instr. and Exptl. Tech. No. 6, 863, Nov.-Dec. (1959). 234. Grigor’ev, A. M., Pribury i Tekh. Eksptl. No. 6, 10, Nov.-Dec. (1959); English transl., Instr. and Exptl. Tech. No. 6, 870, Nov.-Dec. (1959). 235. Nottingham, W. B., 7th Ann. M.I.T. Con$ on Phys. Electronics (1947). 236. Lander, J. J., Rev. Sci. Instr. 21, 672 (1950). 237. Metson, G. H., Brit. J. Appl. Phys. 2, 46 (1951). 238. Nottingharn, W. B., Trans. Nail. Symposium on Vacuum Tech. 1, 76 (1954). 239. Penchko, E. A., and Khavkin, L. P., Pribory i Tekh. Eksptl. No. 1, 128 (1959); English transl., Instr. and Exptl. Tech. No. 1, 132 (1959). 239a. Van Oostrom, A., Trans. Null. Symposium on Vacuum Tech. 8, 443 (1961). 240. Redhead, P. A,, Rev. Sci. Instr. 31, 343 (1960). 241. Riemersma, H., Fox, R. E., and Lange, W. J., Trans. Natl. Symposium on Vacuum Tech. 7, 92 (1960). 241a. Lafferty, J. M., Trans. Natl. Symposium on Vucuum Tech. 8, 460 (1961). 242. Redhead, P. A,, Can. J. Phys, 37, 1260 (1959). 243. Redhead, P. A., Can. J. P h y ~ .36, 255 (1958). 244. Barnes, G., Rev. Sci. Instr. 31, 608 (1960). 245. Barnes, G., Rev. Sci. Insty. 31, I121 (1960). 246. Crawford, C. K., Rev. Sci. Instr. 32, 463 (1961). 247. Dushman, S., and Young, A. H., Phys. Rev. 68, 278 (1945). 248. Riddiford, L., J. Sci. Znstr. 28, 375 (1951). 249. Wagener, S., and Johnson, C. B., J. Sci. Instr. 28, 278 (1951). 250. Schulz, G. J., J. Appl. Phys. 28, 1149 (1957). 251. McGowan, W., and Kerwin, L., Can. J . Phys. 38, 567 (1960). 252. Ehrlich, G., J. Appl. Phys. 32, 4 (1961). 253. Moesta, H., and Renn, R., Vakuum-Technik 6, 35 (1957). 254. Barnes, G., Gaines, J., and Kees, J., Vacuum, 12, 141 (1962). 255. Hickrnott, T. W., J. Chem. Phys. 32, 810 (1960). 256. Eisinger, J., J. Chem. Phys. 30, 412 (1959). 257. Carter, G., and Leck, J. H., Brit.J . Appl. Phys. 10, 364 (1959). 258. Leck, J. H., J. Sci. Instr. 30, 271 (1953). 259. Pitj., L., Czechodov. J. Phys. 7, 113 (1957). 260. Redhead, P. A., Trans. Natl. Symposium on Vacuum Tech. 7, 108 (1960). 261. Lafferty, J. M., J. Appl. Phys. 22, 299 (1951). 262. Weinreich, 0. A., and Bleecher, H., Rev. Sci. Instr. 23, 56 (1952).

430

P.

A. REDHEAD, J. P. HOBSON, E.

V. KORNELSEN

263. Mizushima, Y.,and Oda, Z., Rev. Sci. Instr. 30, 1037 (1959). 264. Baker, F. A., Rev. Sci. Instr. 31, 911 (1960). 265. Haefer, R. A., and Hengevoss, J., Trans. Natl. Symposium on Vacuum Tech. 7, 67 (1960). 266. Kornelsen, E. V., Rept. 19th Ann. M.Z.T. Conf. on Phys. Electronics p. 156 (1959). 267. Giinther, K. G., Vacuum 10, 293 (1960). 268. Caswell, H. L., I.B.M. Journal 4, 130 (1960). 269. Davis, W. D., and Vanderslice, T. A., Trans. Natl. Symposium on Vacuum Tech. 7, 417 (1960). 270. Drawin, D. W., and Brunnbe, C., Vakuum-Technik, 9 , 65 (1960). 271. Huber, W. K., and Trendelenburg, E. A,, Trans. Natl. Symposium on Vacuum Tech. 6, 146 (1959). 272. Kanomata, I., Oguri, T., Kaneko, Y.,Hayakama, T., @S Butsuri ( J . Appl. Phys., Japan) 28,584 (1959). 273. Klopfer, A., Garbe, S.,and Schmidt, W., Trans. Natl. Symposium on Vacuum Tech. 6, 27 (1959). 274. Pikus, G . Ya., Pribory i Tekh. Eksptl. No. 2, 104, Mar.-Apr. (1960); English transl., Instr. and Exptl. Tech. No. 2, 286, Mar.-Apr. (1960). 275. Kendall, B. R. F., PYOC. 9th Ann. Meeting on Mass Spectrometry, A S T M Committee E-14, yune 1961. 276. Nier, A. 0. C., Rev. Sci. Instr. 11, 212 (1940). 277. Allen, J. S., Phys. Rev. 55, 966 (1939). 278. Bernhard, F., Krebs, K. H., and Rotter, I., Z. Physik 161, 103 (1961). 278a. Papirov, I. I., Pribory i Tekhnika Eksper. 7, 5 (1962). 279. Alpert, D., Rev. Sci. Instr. 22, 536 (1951). 280. Bills, D. G., .and Allen, F. G., Rev. Sn'. Instr. 26, 654 (1955). 281. Thorness, R. B., and Nier, A. O., Rev. Sci. Instr. 32, 807 (1961). 282. Lange, W. J., Rev. Sci. Znstr. 30, 602 (1959). 283. Parker, W. B., and Mark, J. T., Trans. Natl. Symposium on Vacuum Tech. 7, 21 (1960). 284. Vogl, T. P., and Evans, H. D., Rev. Sci. Instr. 27, 657 (1956). 285. Pdtjr, L. and Schurer, P., Rev. Sci. Instr. 28, 654 (1957). 286. Axeirod, N. N., Rev. Sci. Instr. 30, 944 (1959). 287. Pritjr, L.,Nature 185, 674 (1960). 288. Whetten, N. R., and Young, J. R., Rev. Sci. Instr. 30, 472 (1959). 289. Hobbis, L. C. W., and Harrison, E. R., Rev. Sci. Instr. 27, 238 (1956). 290. Landecker, K., and Gray, A. J., Rev. Sci. In&. 25, 1151 (1954). 291. Young, J. R., and Whetten, N. R., Rew. Sci. Instr. 32, 453 (1961). 292. Lange, W. J., and Alpert, D., Rev. Sci. Instr. 28, 726 (1957). 293. Holden, J., Holland, L., and Laurenson, L., J . Sci. Instr. 36, 281 (1959). 294. Brymner, R., and Steckelrnacher, W., J. Sci. Znstr. 36, 278 (1959). 295. Zollrnan, J, A., Martin, I. E., and Powell, J. A , , Trans. Natl. Symposium on Vacuum Tech. 6, 278 (1959). 296. Hickmott, T . W.. and Ehrlich, G., Phys. and Chem. Solids 5, 47 (1958). 297. Dyke, W. P., I.R.E. Trans. on Military Electronics MIL-4, 38 (1960). 298. Martin, E. E., Trolan, J. K., and Dyke, W. P., J. Appl. Phys. 31,782 (1960). 299. Dyke, W. P., Charbonnier, F. M., Strayer, R. W., Floyd, R. L., Barbour, J. P., and Trolan, J. K., J. Appl. Phys. 31, 790 (1960). 300. Dyke, W. P., and Dolan, W. W., Advances in Electronics and Electron Phys. 8, 89 (1956).

ULTRAHIGH VACUUM

43 1

301. Martin, E. E., and Collins, F. M., Proc. 4th Nut. ConJ on Tube Techniques p. 21 302. 303. 304. 305. 306. 307. 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322.

323.

324. 325. 326.

(1958). Jonker, J. L. H., Philips Research Repts. 9, 391 (1954). Anderson, P. A., Phys. Rev. 47, 958 (1935). Grunberg, L., Brit. J. Appl. Phys. 9, 85 (1958). Huguenin, E. L., and Valat, J. G., J . phys. radium 17, 965 (1956). Bronshtein, I. M., and Roschin, V. V., Zhur. Tekh. Fiz. 28, 2200 (1958); English transl., Sooiet Php-Tech. Phys. 3, 2023 (1958). Bronshtein, I. M., and Roschin, V. V., Zhur. Tekh. Fia. 28,2476 (1958), English transl., Soviet Phys.-Tech. Phys. 3, 2271 (1958). Dobretsov, L. N., and Matskevich, T. L., Zhur. Tekh. Fiz. 27, 734 (1957); English transl., Soviet Phys.-Tech. Phys. 2, 663 (1957). Fowler, H. A., and Farnsworth, H. E., Phys. Rew. 111, 103 (1958). Gorodetskii, D. A., Zhur. Eksptf. Teoret. Fiz. 33, 7 (1958); English transl., Soviet Phys. J E T P 34, No. 7, 4 (1958). Germer, L. H., Scheibner, E. J., and Hartman, C. D., Phil. Mag. [8] 5, 222 (1960). Farnsworth, H. E., Schlier, R. E., and Tuul, J., Phys. and Chem. Solids 9, 57 (1959) (and many other papers). Rapp, R. A., Hirth, J. P., and Pound, G. M., Can. J. Phys. 38, 709 (1960). Rapp, R.A., Hirth, J. P., and Pound, G. M., J. Chem. Phys. 34, 184 (1961). Neugebauer, C. A., Newkirk, J. B., and Vermilyea, D. A., eds., “Structure and Properties of Thin Films.” Wiley, New York, 1959. Mayer, H., in “Structure and Properties of Thin Films” (C. A. Neugebauer et al., eds.), p. 225. Wiley, New York, 1959. Evans, C. C., and Mitchell, 1. W., in “Structure and Properties of Thin Films” (C. A. Neugebauer et a/., eds.), p. 263. Wiley, New York, 1959. Neugebauer, C. A., in “Structure and Properties of Thin Films” (C. A. Neugebauer et al., eds.), p. 358. Wiley, New York, 1959. Caswell, H. L., J. Appl. Phys. 32, 105 (1961). Greenland, K. M., J. Sci. Inttr. 38, 1 (1961). Spitzer, L., Astrophys. J. 95, 329 (1942). Sledziewski, Z., and Torossian, A., Le Vide 14, 107 (1959). Kurchatov, I. V., Atomnayu Energ. 5, 105 (1958); English transl., Sowiet J. Atomic Energy 5 , 933 (1958). Minzer, R. A., and Ripley, W. S.,U.S. Air Force Surveys in Geophys. No. 115 ( 1959). Santeler, D. J., Trans. Natl. Symposium on Vacuum Tech. 6 , 129 (1959). Bennett, W. H., Trans. Natl. Symposium on Vacuum Tech. I , 378 (1960).