Uhv and xhv pressure measurements

Vacuum/volume44/numbers 5-7/pages 559 to 564/1993 Printed in Great Bntam

O042-207X/93S6 00+ O0 © 1993 Pergamon Press Ltd

Uhv and xhv pressure m e a s u r e m e n t s P A Redhead,

National Research Councd of Canada, Ottawa, Ontario, Canada KIA OR6

The advances m the measurement of both total and parttal pressures below 10 -9 torr (uhv and xhv) m the last six years O.e smce the uhv Gaugmg Workshop held m 1986) are rewewed and methods for the reduction of errors caused by soft X-ray photoemmston and electron sttmulated desorption are examined

1. Introduction

Table 2. Immersed-collector gauges

U l t r a h i g h v a c u u m (uhv, < 10 - 9 torr) a n d extreme high v a c u u m (xhv, < 1 0 12 torr) have been receiving increased a t t e n t i o n recently, )n particular for application in semiconductor device processing The m e a s u r e m e n t s o f b o t h total a n d partial pressure in the u h v / x h v range is limited by several effects including the Xray limit, electron stimulated desorption, c a t h o d e evaporation, alKt chemical a n d thermal effects at hot cathodes New designs of u h v / x h v gauges a n d R G A s are reviewed as well as measurements o f the limiting processes The m e a s u r e m e n t of pressure in the uhv range was the subject of the U l t r a h i g h V a c u u m G a u g i n g W o r k s h o p held in N o v e m b e r 1986 at G a i t h e r s b u r g , U S A ~ This p a p e r attempts to review briefly the significant advances in u h v a n d xhv pressure measurem e n t s m a d e since the 1986 W o r k s h o p

2, Recent developments in total pressure gauges M o s t new developments in h o t - c a t h o d e gauges for the uhv a n d xhv ranges are concerned with the reduction of hmitations o n 1Qw pressure m e a s u r e m e n t caused by p h o t o e m l s s l o n from soft X-rays (the X - r a y effect) a n d electron stimulated desorptlon of a d s o r b e d gas from electrode surfaces (the ESD effect) 2~1. Immersed-collector gauges. The ion-collector in this category o f h o t - c a t h o d e gauge is immersed in the ionizing region (e g B~yard Alpert a n d similar gauges) T a b l e 1 summarizes results o b t a i n e d o n B a y a r d - A l p e r t gauges indicating the significant reduction in X-ray hmit t h a t can be o b t a i n e d by either reducing the collector d m m e t e r or by m o d u lalaon methods, the significance of self-modulation a n d selfcancellation will be discussed in Section 3

Spherical grid BAG

Point collector gauge

20 20 1 1 x 10-" 10 [0 150]

22 53 2 x 10 ~3 95 0 050 [0 030]

Geometric factor

~ 2 x 10 -3

4 5 x 10 -6

Reference

5

67

Grid dmmeter (mm) Sensitivity (torr- ~) X-ray hmlt (torr) Modulation factor (%) Collector length [dmmeter] (mm)

W a t a n a b e has developed two new immersed-collector designs using spherical grids Table 2 shows the characteristics of these two gauges a n d Figure 1 shows the gauges schematically The first is a spherical grid B A G h a v i n g an X-ray limit a b o u t one h a l f that o f a typical B A G 5 The second is a p o i n t collector gauge where the collector is shielded so t h a t ions can only reach the tip of the collector wire 6 7 T h e X-ray limit o f the p o i n t collector gauge is a b o u t 2 × 10 13 torr which is two orders o f m a g n i t u d e lower t h a n a typical B A G , with m o d u l a t i o n the pressure limit o f this gauge is below 7 × 10- ~4 torr The m o d u l a t e d p o i n t collector gauge has been c o m p a r e d with a n extractor gauge d o w n to a b o u t 10 J 2 t o r r T 8 2.2. External-collector gauges. These h o t - c a t h o d e gauges have the ion-collector external to the ionizing region, the principal examples of this type o f gauge are the extractor a n d the bentbeam gauge

Table 1. Bayard- Alpert gauges BAG uhv 24 Collector diameter (#m) S=asmvlty (torr- ~) X-rayllmlt (torr) Modulation factor (%) ESD separation l~ference

125 20 28x10

tl -no 2

50 18 4 x 1 0 12

MBAG 1MR 103

Self-modulated*

127 ~ 12 7 x 1 0 ~2 38 partial 3

125 50 3xl0 75 9 4

~3t

*Filament potential switched between +20 and +70 V tWlth self-cancellation 559

P A Redhead

Pressure measurements Outer deflector (q~4Omm)

/-Rim

Znner detlector

'\\

/

] l

//

I

F

I

F~ L___J '-V,~ ~ L__ : G

CEM (c)

(a)

ShieLd

Defl_~ 6.~ C Sphencet gr~d P t - R h Ring fiLament I CoLLector ThO2-Re~ ] Rim P t - R h / E v a P o r at ed -- - ~ - - ~ ' - I ~

(a)

Figure l Schemanc diagrams of two new designs of Immersed-collector gauges (a) Spherical grid BAG s and (b) point collector gauge 6' C collector F filament, G grid M modulator, S shield See Table 2 for cha~ acterlstlcS

'/' F

(b)

The characteristics of three recently developed b e n t - b e a m gauges are s h o w n In Table 3 together with the c h a r a c t e n s n c ~ o f an improved form o f the original Helmer design (improved BBG) as a reference The original Helmet design of BBG has a senS]tl~ity factor o f a b o u t 7 t o r t ~, Benvenuti a n d H a u e r 9 modified this design by (a) replacing the tungsten filament with a thor],]coated filament, reducing the low pressure hmit due to tungsten e v a p o r a t i o n by a factor of ten a n d (b) by optimizing various electrode dimensions resulting in a n increase of senslnvlty factor to 40 t o r t ~ The four gauges are s h o w n schematically an Figure 2 Three o f these l b u r gauges have X-ray hmits of 10 t4 torr ol less, the m I n I - B B G has a n X-ray limit o f less t h a n 10 ~ torr The b e n t - b e a m gauge Is capable of separating gas-phase ions from runs p r o d u c e d by electron stimulated desorptlon (ESD lens) at the grid surface on the basis o f their difference m energy, since the deflection plate system acts as a crude energy analyzer In the case of the three gauges with two-dimensional deflection systems, the resolving power (defined as the ratio o f the deflection voltage at the peak to the width ( F W H M ) o f the ion-current vs deflection voltage peak) is only a b o u t 2 a n d separation of E S D and gas p h a s e ions IS incomplete W a t a n a b e ' s hemispherical B B G ~0 has a deflection system o f concentric hemispheres resulting in a resolving power o f a b o u t 10, yielding virtually complete separation of ESD a n d gas-phase lens Fagure 3 shows the collector current as a f u n c n o n of deflector voltage for the hem]-

560

Improved BBG

MIm-BBG + multlpher

90 40 ~10 ~4 partial 2 9

180 2 2 x 10' <10 ~' parnal <2 14

9O 33

10 14 partial 2

'\'

spherical B B G (ion spectroscopy gauge) for various electron currents Increasing the electron current increases the spacecharge depression o f potential at the center o f the grid a n d thus increases the energy difference between the E S D ions (formed at the grid surface) a n d the gas-phase lens (formed within the grid volume) Figure 4 indicates the resolving power of the BBG ~ (Helmer gauge) The space-charge depression o f potential at 10 m A electron current is a b o u t 55 V ~2 and thus an ESD ion peak would a p p e a r at the p o s m o n indicated by the vertical dashed hne m Figure 4, the resolving power is a b o u t 1 2 (no experimental data for the Helmer gauge in the presence o f a n E S D peak has been pubhshed) It appears that the separation of E S D and gas-phase lens would be incomplete for all B B G designs with a resolving power less than a b o u t 10 The axml emissmn B B G ~ [see Figure 4(b)] Is a variant of the original B B G design with slmdar characteristics A miniature version of the B B G design has been developed by L] and Z h a n g ~4

~xldl emission BBG

/ \

/ Ton b'eom/ Hermspher,caL / ~ inner ¢JlefLector ShieLd m e s h ~, Hemlspr~erlcoL outer d e f l e c t o r (d)

Figure 2. Schemanc diagrams o[ four bent-beam gauges (a) lmpm,,ed BBG ~, (b) axial emission BBG w (c) mlmature BBG with multiplier TM and (d) hemlsphencal BBG "~ C collector F filament G grid, S ~hleld Su suppressol See Table 3 lor charactenbtlCS

Table 3 Bent-beam gauges

Deflection (degrees) Sensitivity (torr ~) X-rayllmlt (torr) ESD separanon Resolving power V[~,/AV~) Reference

[

~

(b)

Hemispherical BBG 180

8 < 10 ~4 complete 10 10

P A Redhead Pressure measurements I00 -

4--

Me grid After O~ exposure at about I x IO-rPa Gas

~

,o o. E o

ESD mn

®

o= g

3--

Electron current IOmA Pressure

~'~

4 x io-IO~orr

/

/

b

/lOmA

x v +, E

t ~ Pos=t=on of peak ~ / o r ESD . . . .

2--

g ~

/SrnA

_,=

25 -

L) ~

120

160

200

2mA

240

200

DefLector b,as (V)

400

I

600

DefLectmn voltage (v)

Figure 3. Ion collector current vs deflector voltage for the hemlsphencal bent-beam gauge (ion spectroscopy gauge) for various electron currents ~°, after exposure of the Mo grad to O. at about 10 9 torr

Figure 4. Collector current in bent-beam gauge (Helmer design) as function of deflection voltage at electron currents of 10 mA and pressure of 4 x 10 ~0 torr (adapted from Figure 3 ofref 11) Arrow indicates positron where peak due to ESD runs would appear if present

[see Figure 4(c)] containing a spiral C E M which results in a very high effective sensitivity O t u k a has recently reported a 255 ° deflection system for the B B G ~5 with i m p r o v e d resolution which has m a d e m e a s u r e m e n t s tO 10- ~3 torr T a b l e 4 hsts the characteristics of three recent designs of extractor gauges (EG) c o m p a r e d with the characteristics of the a n g i n a l d ~ i g n of E G J6 as exemphfied by the Leybold IE511, these gauges are s h o w n schematically in Figure 5 The modified E G ~7 is similar tO the original design of E G with m i n o r changes in geometry o f t~¢ electrodes T h e X - r a y limit o f this gauge is lower t h a n the otfiglnal E G by a factor o f 25 The m o d u l a t e d - i o n - c u r r e n t gauge ( M I C ) contains a m o d u l a t o r electrode In front o f the collector ~s, With m o d u l a t i o n at 12 Hz complete separaUon o f E S D a n d gasphase ions is possible by varying the bias o n the m o d u l a t o r electrode (resolving power a b o u t 11) Varying the m o d u l a t o r bias In effect produces a retarding-field analysis o f the ion energies a n d the m o d u l a t i o n forms the derivative of the retarding-field

curve The m i n i m u m measurable pressure for this gauge xs hmlted to a b o u t 10 ~2 torr by the noise level a n d by the m o d u l a U o n process The c o m p e n s a t i o n gauge ~ reduces the X-ray limit by balancing the photoelectron current from a large area collector with the p h o t o e l e c t r o n current to the collector from a close-spaced grid ( t h e c o m p e n s a t l o n g n d ) By careful cholce o f t h e g r l d p o t e n tlal it is possible to reduce the net p h o t o c u r r e n t at the collector to near zero T h e c o m p e n s a t i o n gauge is strongly affected by electron stimulated desorptlon from the grid The principle o f c o m p e n s a t i o n for a ' f o r w a r d ' p h o t o c u r r e n t by balancing it with a 'reverse' p h o t o c u r r e n t p r o d u c e d at a n o t h e r electrode (or the envelope) o f the gauge was first described in 19632°, it has been used to reduce the X-ray limit o f B a y a r d Alpert gauges by C h o u a n d T a n g 4, a n d H s e u h a n d L a n n i 2 There is n o p u b h s h e d d a t a on the m e d i u m a n d long term stability o f this c o m p e n s a t i o n m e t h o d which m a y be affected by exposure to chemically active gases causing a change o f contact potential

Tilde 4. Extractor gauges

SllSltlVlty (torr ~) X~-rayllmlt (torr) (k~metnc factor E~I) separation Rt~olvmg power (V/A V) P*m (torr) Reference

Extractor gauge IE511

Modified EG

MIG gauge*

Compensation gauge

6 1 5 x 1 0 12 2 5 x 10- 5 partial

25 6 x 1 0 14 2 4 x I0 6 9

3 <10-~2

27 75x10_14 none

~ 10 137 16

i0-14~ 17

complete 10 10 12§ 18

19

"12 Hz modulaUon tWlth correcUon for X-ray hmlt **With modulation of reflector voltage §Lmm set by noise level 561

P A Redhead

Pressure measurements

jS

OV

_a

_a

50V 500V

50V

IOOV - I O V

i

/reflector

(a)

i

,

i

+

//" / ' - ' "x ~ x ' ~ F • / \ L " L ELectron

./G

500V

OV -~OOV i

OV

-500V

I

'

t__l I

Electron gun

,

I

Lenses for e l e c t r o n

Lenses for ion

1on

collecto r

F)eld ermss)on cathode ,lu 25c 25o 15o ' 8o

Focus electrodes

(a)

--- ~

(c)

Anode ring ~So

Spher~ceL gr~d (q~22mm) /~f,.

~--

"Xx\

I

/"

~"

'"Y

y

(b)

/

I

x\

"

Supplemental anode

\

~,

"%x I

"~5o

F

/ t/

I

~o

/' /

Spherical shell

Compensa~lon

"A___tU" °"°

I

~on c o l l e c t o r

Figure 6 Schemanc diagram ot two long electron-path gauges (a) ( a~cade ~lanc lens gauge 2~ and (b) electron oscillator gauge wflh heldem~slon c,Lthode :6 showing equlpotentlaN (b)

(d)

Figure 5. Schematic diagram offourextractorgauges (a) Extractor gauge (IE5ll) ~ , (b) modified EG ~7, (c) modulated ~on current gauge ~ , (d) compensanongauge~ C collector, F hlament, G grld, M modulator, S shield See Table 4 for charactenst)cs

between the 1on-collector and the electrode from which the compensating p h o t o c u r r e n t is emitted The original extractor gauge design is very msenslnve ~ ~2 to ESD ions when the reflector is at grad p o t e n n a l E S D ions formed at the grid surface have a very small p r o b a b l h t y o f striking the hne collector a n d are collected at the reflector G a s - p h a s e ions formed within the grid have a lower energy, because of the spacecharge depression o f potential, a n d are not capable of reaching the reflector a n d thus are c a p t u r e d at the collecto~ 2.3. Long electron-path ganges. Figure 6(a) shows the electrode a r r a n g e m e n t m the so-called cascade static lens gauge, electrons are m a d e to oscillate longitudinally between the cathode and ~on collector regions C o m p u t e r simulations of the electron a n d Ion trajectones2~ 24 indicate that as m a n y as l04 oscillations are possible P r e h m m a r y e x p e n m e n t g 5 indicate t h a t senslnvlty factors of 2870 torr ~ at an electron current of 4 # A are possible, 1 e a sensltlwty o f a b o u t 0 01 A t o r t ~ It is not yet clear whether this design is statable for uhv use since the electrode structure is complex a n d requires m a n y different p o t e n n a l s Ll a n d Z h a n g :6 have developed a gauge m which electrons from a field emitter oscillate back a n d forth t h r o u g h a ring a n o d e [see s c h e m a n e d i a g r a m in Figure 6(b)] A h n e a r response with pressure has been measured d o w n to a pressure of a b o u t 5 × 10 ' ~ t o r r w~th a sensmvlty factor o f 2000 t o r t ' at an electron current of a b o u t 2 yA. 1 e a senslnvlty of a b o u t 0 003 A torr

562

The low senslnvlty of these gauges m a k e them only marginally useful for uhv a n d xhv without electron m u l n p h e r s 2.4. Magnetic gauges. There has been httle d e v e l o p m e n t of magnencally confined gauges in the period u n d e r review Chen et a1:7 has described a variant of the h o t - c a t h o d e m a g n e t r o n gauge which has the filament at the end o f the cyhndrical a n o d e (called axial-emission) rather than on the axis The X-ray limit of the axial-emission gauge is 4 × 10 ~4 torr w~thout suppression, It is expected t h a t with suppression the low pressure limit will be reduced to 10 ~6 torr At 10 6 A emission current the sens~tlvlt~ is 9 × 10 : A t o w ' Peacock has described an mverted-magnetron cold c a t h o d e gauge :s :~ with a senslnvlty a b o u t 1 A torr ' , pressure m e a s u r e m e n t s to 10 ,0 t o r t are possible t h o u g h the c u r r e n t - p r e s s u r e curve is non-lineal M a g n e n c a l l y t~onfined gauges are notoriously unstable Some of the causes of these mstabfllnes have been reviewed TM and m e t h o d s to minimize lnstabfllnes a n d n o n - h n e a m t e s suggested, these include (a) ensuring that the magnetic a n d electmc fields are accurately o r t h o g o n a l a n d eyhndrlcally symmetric, (b) all electrodes s u r r o u n d i n g the plasma should be of high conductivity, and (c) electrically separating the c a t h o d e end-plates ~o that only the ion current to the cylindrical portion of the cathode is measured 2.5. Laser ionization. K o k u b u n a n d his colleagues ~~ have studied the lomzaUon of rare gases, O:, C O a n d CO2 by rare gas/halogen exclmer lasers with a s o p h l s n c a t e d m n c o u n t i n g system, it ~s expected that pressure m e a s u r e m e n t s could be made over the range 10 : 10 ~2 torr T h o u g h highly complex, this pressure m e a s u r e m e n t system may have advantages for partlculal uhv apphcanons

P A Redhead Pressure m e a s u r e m e n t s

3. Measurement of residual currents The residual current in an ionization gauge is defined as the current at the ion-collector when the gas pressure is instantly reduced to zero It consists of three components (a) the X-ray lladuced photoelectron current, (b) the current of ESD ions from raatenal on surfaces struck by electrons, and (c) the ion current resulting from cathode evaporation Measurements of residual currents in Bayard-Alpert gauges have been reported by Flhppelh 3 and Peacock 32 Flllppelll has measured the residual currents of four identical commercial Bayard-Alpert gauges and two different modulated BAGs by Comparison with an extractor gauge The results are complicated by the fact that all the measurements were done in hydrogen resulting in complex chemical effects Residual currents were found to vary by a factor of l0 among the nominally identical BAGs, residual currents varied from 1/3 to 4 times the manuthcturers specified values Residual currents In the modulated B A G s measured by modulation methods and by comparison with the extractor gauge were in good agreement Peacock has measured the residual currents m six B A G s by comparison with an extractor gauge and by the Alpert method (measuring collector current vs electron energy at very low pressure, the resultant curve on a log-log scale can be extrapolated I~tck to the operating electron energy to yield a value of the rVSldual current 33) The agreement between the two methods was g~od for gauges with tungsten filaments Agreement varies, and residual current depended on operating history for the four gauges with oxide coated cathodes (thorla and yttrla) The X-ray limit of the extractor gauge (IE511 and IE514) has I~en measured 16 by modulating the potential of the ion reflector which surrounds the collector [see Figure 5(a)] With the reflector sufficiently positive ( > 400 V) no ions can reach the collector and the collector current is the X-ray photoemlsslon only The X-ray lilllltS are about l 5 x l0 12 torr with a scatter of about 30% Repa 34 has measured the modulation of the ion current resultllag from electron-impact lomzatlon of evaporated cathode material from pure metal cathodes (W or Mo) m a modulated B A G Modulation of this current due to cathode evaporation ean be the cause of residual current modulation in the modulated Bayard-Alpert gauge, resulting in errors in modulation measurerlaents if, as is usual, the residual current is assumed to be tlamodulated Use of low temperature cathodes can eliminate tins error The measurement of residual currents in ionization gauges and r ~ l d u a l gas analyzers has recently been rewewed 3s

practical Mltsul and Shlgehara have used metal-insulator-metal films as a cathode in a B A G 36, the films were A1-A1203-Au and emission currents of about ten mlcroamps were obtained at pressures down to 10-10 torr These cathodes still need much improvement in lifetime, insensitivity to baking, and increased emission They are also strongly affected by chemically active gases, for example, O2 reduces the emission very markedly Vacuum microelectronlc devices using arrays of etched silicon field emitters 37 show promise of becoming useful electron sources for ionization gauges and R G A s Oglwara and Shiho 38 have recently reported the use of a Spindt-type field emitter array in a quadrupole at uhv Although these developments look promising there is much development work needed to achieve a cold cathode electron source suitable for ionization gauges In particular, the problem of minimizing the effects of chemically active gases (e g 02) is difficult

5. Residual gas analyzers The ubiquitous quadrupole is now almost the only type of mass spectrometer used for residual gas analysis The problems with the quadrupole special to the uhv and xhv ranges are (a) outgassing of the electrodes and envelope caused by the hot filament in the Ion source, (b) the separation of gas phase ions from ESD ions, and (c) variation of multiplier gain (in the uhv and xhv range a multiplier is essential) Problem (a) above has been addressed by Watanabe 39 in a redesigned ion source for a quadrupole using an aluminum alloy envelope and a thorla-coated rhenium filament to reduce its operating temperature Since AI has an emissivity only 1/10 of stainless steel, less power is needed to heat a filament near an A1 surface, this results in the envelope operating at a lower temperature All electrodes In the Ion source can be heated above 1000°C by electron bombardment The Ion source is separated from the quadrupole rods so that heat is not conducted from the ion source to the rods When the ion source was turned on and off at 2 x 10-12 torr at 20 mln intervals no change in pressure was observed Figure 7 shows a mass spectrum taken with this quadrupote at a pressure of about 10-12 torr, as measured by a

H2

4. Cold electron sources The hot cathode in an ionization gauge or R G A is a prolific source of problems in the uhv region including (a) heating of the gauge structure and envelope causing outgasslng, (b) dissociation of H2, H20 and other molecules causing the creation of new gaseous molecules (CxHy, etc ), and (c) the evaporation of material from the cathode causing false pressure indications m some cases Magnetic cold-cathode gauges do not suffer from these difficulties but they are frequently non-linear and unstable Cathodes capable of adequate electron emission at r o o m temperature have been long sought Field emission sources (e g see ref 26) can be used but they are fragile and strongly affected by chemically active gases There has been some recent experiments which suggest that Cold cathode sources suitable for ionization gauges may soon be

VerticaL x I0 Hor~zon't.aL x 2

N2

CH 4 .... H20

CO, I

35

CL,_ArC02

Figure 7. Residual gas mass spectrum at about 10- ~2torr40 563

P A Redhead

Pressure measurements

m o d u l a t e d point collector gauge, the peaks at masses l, 19 a n d 35 are p r o b a b l y E S D peaks 4° H u b e r et al ~ ~have m a d e measurements with a Q M S at pressures o f a b o u t 2 x 10 ~'- torr, by c o m p a r i n g the sum o f the Q M S peaks (corrected for mass dependence o f sensitivity) with the readings of a m o d u l a t e d B A G it was s h o w n that the M B A G can rehably measure pressure to a b o u t 10 ~2 torr in a clean system C o m p l e t e separation o f gas-phase a n d E S D ions in a quadrupole can be achieved by m o d u l a t i o n of the Ion accelerating potential a n d the use o f lock-in detection ~2 The two types of ions are separable on the basis of their difference in energy, as discussed above with respect to b e n t - b e a m gauges G e m s c h has de~eloped a n o m e g a t r o n with gold-coated electrodes t h a t are p e r m a n e n t l y heated to minimize gas interaction at the electrode surfaces 43 This inert o m e g a t r o n has a resolution of m / A m = 1900/m and ~s capable of m e a s u r i n g to 10 ~2 torr Because o f its high resolution at low mass it is capable o f leak detection with helium In the presence of d e u t e r m m a n d tritium

6. Conclusion

In reviewing the advances in the m e a s u r e m e n t s of pressure in the u h v a n d xhv range m a d e in the last six years some significant themes are evident (a) C o n s i d e r a b l e advances have been made in separating gasphase ions from E S D ions The work o f W a t a n a b e in i m p r o v i n g the resolving power o f b e n t - b e a m gauges ~° a n d in developing a m e t h o d for separating the two categories o f ions in R G A s 42 is particularly n o t e w o r t h y A t very low pressures the e h m l n a t l o n of errors caused by E S D ions IS frequently more o f a problem t h a n the X - r a y limit (b) The use o f c o m p e n s a t i o n m e t h o d s to reduce the effective X-ray limit o f h o t - c a t h o d e ionization gauges has received some a t t e n t i o n 2 4 i0 after years o f neglect This m e t h o d requires the balancing of a ' f o r w a r d ' p h o t o c u r r e n t from the ion-collector with the 'reverse' p h o t o c u r r e n t from some o t h e r electrode to the ioncollector, this balance is sensitive to the potential between the two electrode a n d thus to their contact potential difference C h a n g e s in surface conditions (gas a d s o r p t i o n etc ) may shift the contact potential a n d affect the balance M e a s u r e m e n t s are needed to establish the long-term stability of this c o m p e n s a t i o n m e t h o d particularly in the presence o f chemically active gases (c) Some o f the new designs of gauges are so sophisticated that their price a n d complexity is hkely to a p p r o a c h those of an R G A In m a n y cases the preference would be to use a n R G A in such circumstances while the highly sophisticated total pressure gauges would be used only for the calibration of R G A s al low pressures (d) The original design o f extractor gauge 2~ _,2 was s h o w n to be insensitive to E S D Ions resulting from 0 2 exposure This insensitivity to ESD ions needs to be experimentally confirmed for the newer extractor gauge designs a n d for gases o t h e r t h a n O, (e) M a n y of the p r o b l e m s o f h o t - c a t h o d e gauges at very low pressures result from the presence o f heated c a t h o d e s Some very preliminary a t t e m p t s have been m a d e to use c a t h o d e structures capable of emitting near r o o m t e m p e r a t u r e M u c h remains to be done in this promising area

564

(f) C o m p a r i s o n s of the b e h a v i o r o f cold-cathode discharges with the experiments on thermlonlcally p r o d u c e d pure electron plasmas ~° have resulted in some suggestions to improve the stabihty a n d hnearity of cold-cathode, magnetically confined gauges In particular, the need for uniform, rotatlonally symmetric magnetic fields is evident Those possibilities have yet to be tested experimentally Cg) New designs of electrostatlcally focussed, long electronpath gauges show some potential for use in the uhv/xhv range, however further tests are needed to determine their stability and hnearity All previous designs of long electron-path gauges (e g the o r b l t r o n ) have s h o w n mstablhtles which p r o b a b l y occur as a result of the large, t r a p p e d electron charge References

Many ot the papers from the 1986 uhv Gauging Workshop can be found A5, 3214-3249 (1987) 2H C Hseuh and C Lannl, J Pat Sit Tethnoi, A5, 3244 (t987) ~A R Flllpelh J Vat Sit Technol, A5, 3234 (1987) ~T S Chou and Z Q Tang, J Vat Sit Te~hnol, A4, 2280 (1986) ' F Watanabe, Personal comrnumcatlon (ICF070 gauge, Sukegawa Electric Co) ~'F Watanabe, J Vat Sit Te~hnol AS, 242 (1987) "F Watanabe, J Vat Sot Japan 34, 17 ( 1991) ~H lshlmaru, J Vat S~I Tcchnol, A7, 2439 (1989) ~'C Benvenutl and M Hauer Pro( 8th lnt Vacuum Conqre~s Vol 2 p 199, Cannes (1980) H~F Watanabe, J Vat At; Tethnol, AI0, 3333 (1992), J ~2t~ Sot Japan 35, 422 (1992) S-W Han, W Jltschm and G Grouse, Vacuum, 38, 1079 (1988) 2K Urbanek, A V S 14th Nat Vat Svmp Abstratts, p 49 New York 1967) C D Suen. J Z Chen and Y H Kuo, J Vat St ; Tc( hnol A8, 3888 (1990) Va~uunl, 41, 1805 (1990) ~4W L1 and Z Zhang, J Vat St; lethnol, AS, 2447 (1987) ~ A Otuka and C Oshlma J ~ t Sit Technol, All, 240 (1993) i(, F Watanabe, J Vat Sit Tethnol, A9, 2744 (1991) Ivz Q Tang, H Y Chen and Z H Lu, J Vac Sit Technol, AS, 2384 (1987) ~ F Watanabe and H l shlmaru, J Vac Sol Te~hnoL A5, 2924 (1987) I')F Watanabe, J Va~ Sot Japan, 34, 25 (1991) 2(~W H Hayward, R L Jepsen and P A Redhead, Trans AI~S IOth Nat Vat Svmp, p 228 (1963) ~'~U Beeck and G Reich, J Vac St; Technol, 9, 126 (1972) ~':P A Redhead, J Vat St; Tethnol 3, 173 (1966) : ' T Kanaj1, T Urano and S Hongo, Vacuum, 41, 2144 (1990) 24K Takanashl, H Fukuoka A Yoshloka, S Hongo, T Urano and T Kanall, J Vat Sot Japan, 34, 10 (1991) 2• T Kanajl, T Urano and S Hongo, Abstract, Worl~shop on uhv Measurement, Grange (1992) 2~W LI and D Zhang J Va~ Sit Technol, A5, 2376 (1987) 27j Z Chen, C D Suen and Y H Kuo, J Va~ S~z Technol A5, 2373 ( 19871 2~N T Peacock, J Vdt St1 Technol, A6, 1141 (1988) 3~'N T Peacock and R N Peacock, J Vac Set Technol, A9, 2806 (1990) ~"P A Redhead, Va~ uum, 38, 901 (1988) ~K Kokubun, S Ichlmura and H ShlmlZU 1 Vat So¢ Japan, 33, 7 & 15 (1990) ~'-N T Peacock, J VIe Stl Technol, AI0, 2674 (1992) ~ R T Bayard and D Alpert, Ret S~I In~trum 21,571 (1950) ~4p Repa, Vacuum, 36, 559 (1986) ~ P A Redhead, J Vac Sit Tethnol, AI0, 2665 (1992) ~'T MltSUl and T Shlgehara, Vatuum, 41, 1802 (1990) ~; 1 Brodle and C A Spmdt, Adv Electron Phys', 83, I (1992) ~SN Oglwara, M Shlho and Y Ueda, Vacuum, 44(5 7), 661 ([993) ~ F Watanabe, J Vat S u Technol, A8, 3890 (1990) 4°H lshlmaru, M R S Bulletin, 15, 17 (1990) 4~W K Huber, N Muller and G Rettlnghaus, Vacuum, 41, 2103 (1990) 42 F Watanabe and H Ishlmaru, J Vac St~ Te~hnol, A3, 2192 (1985) 4'H Gentsch, Vak Teth, 36, 224 (1987) In J Vat S~l Tcthnol,