Ion trapping and gas release phenomena

Ion trapping and gas release phenomena

lon trapping and gas release phenomena received6 August 1965;accepted25August 1965 W A Grant and G Carter, Departmentof ElectricalEngineering,Universi...

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lon trapping and gas release phenomena received6 August 1965;accepted25August 1965 W A Grant and G Carter, Departmentof ElectricalEngineering,Universityof Liverpool,Liverpool3

When energetic gas ions are injected into a solid surface there is generally a high probability of their becoming trapped within the lattice and this process is of great technological importance in ion pumps and in physical applications where a gas discharge is in contact with a solid surface. In addition to the trapping process, there exist unfortunately, two mechanisms via which the trapped gas can re-evolve. The first is a purely thermally activated phenomenon which occurs without operation of further ion bombardment, whilst the second occurs during bombardment itself and is some form of sputtering phenomenon. In the present review we examine the basic physical interactions involved in both the trapping and re-emission processes and survey the relevant fiterature which gives the magnitude of the effects. In particular we summarize the measured probability of trapping of inert gas ions at metal and insulator surfaces and show how this is related to interatomic forces. We then investigate the process responsible for thermal gas release, which is shown to be diffusion controlled, and correlate the available data for bombardment induced gas release, for which the physical phenomenon is less definable. Finally, we study the manner in which trapped atoms dissolve in the solid target and show how individual atoms can migrate and cluster to form bubbles. Our attention is directed wholly towards examination of inert gas ion--solid systems since chemical reactions can be discounted but suggest that the basic physical interaction with more active gases is similar. Introduction In recent years considerable data has been accumulated concerning the interactions between gas atoms and solids from experiments involving the injection of energetic ions into various prepared target surfaces. Information obtained in this way can be related to the basic physics of the interaction forces between energetic gas atoms and a solid lattice, to the subsequent history of the gas projectile and to possible damage in the target. On approaching the experimental surface an ion is neutralised and electron ejection may occur (Hagstrum 1956) 1 and during the subsequent collisions with the target atoms sufficient momentum can be transferred to result in damage to the lattice through displacement of the lattice atoms and removal (ie sputtering) of the surface atoms. The gas atoms finally come to rest in the target or, due to momentum reversal, are re-emitted. The interaction between activated gases and glass or metal surfaces was first reported over a century ago2, but it is within the last two decades that the phenomena has become of technological importance as a pumping mechanism (Bayard and Alpert, 1950)3 useful in achieving ultra-high vacua. This has prompted more extensive studies of the processes involved. Much of the initial investigations were concerned with the sorption o f activated gases in ionization gauges with glass envelopes and reviews of earlier work using these instruments have been made by Alpert (1958) 4, Carter (1959) 5 and Strotzer (1958, 1959)6. It is the intention of the present review to summarize more recent results obtained in similar experiments and in investigations of the interaction between ion beams and various solid surfaces. Most experiments have been conducted using inert gas ions as projectiles since use of these gases reduces the possible

influence of chemical reactions. Although ionic combinations of inert gases have been reported they are extremely unlikely under the conditions prevailing in the studies under consideration. On the other hand, it is well known that oxygen, for example, reacts with hot tungsten filaments and filaments containing carbon to produce various tungsten and carbon oxides which evaporate to the gauge wall and are absorbed 7. Similarly hydrogen dissociates at hot filaments and the resulting atoms are strongly absorbed at the gauge surface or may initiate reactions producing CO, CO2, H20, etcS, which may in turn react with the filament. In the case of nitrogen it appears from the investigations of Donaldson9 and others 10, 11 that the production of metastable molecules (as well as ions) and their dissociation at the experimental surface before adsorption is important in determining its reaction characteristic. Because of these complications, experiments involving active gases are more difficult to interpret and we will therefore exclude these from further consideration. Nevertheless, what is said about the initial penetration of inert gas ions in the following review, is equally applicable to any energetic gas ion; the added complications of chemical reactivity generally occur only when the ion has come to rest, but this can in fact lead to complications which we will not consider further. We should mention, however, that molecular ions frequently dissociate upon striking a solid surface and the fragments proceed with a total energy equal to the incident molecular ion energy, this again can lead to complexity in analysing trapping processes.

Experimental methods As mentioned above, much of the information on ionic absorption has been obtained during observation of pumping pheno-

Vacuum / volume 15/number 10. PergamonPress Lid/Printed in GreatBritain

477

W A Grant and G Carter: Ion trapping and gas release phenomena mena in ionization gauges, mainly because of their simplicity of design, but also because of their technological importance as pressure transducers. Data from such studies, although readily obtained, is complicated by poor definition of experimental conditions. The tube may be of the triode Bayard-Alpert type, the Comsa-Musa diode typOZ, the tetrode design as used by Varnerin and CarmichaeU3, or a Penning type used by Brown and Leck 14. In all tubes, gaseous ions may be impelled into a variety of surfaces including the gauge envelope and the various electrodes and their supports. Specific ion currents to each surface, and ion energies, are difficult to specify whilst the various surface temperatures are generally unknown and may result in preferential impurity evaporation, sputtering and condensation. In addition, it has been shown by Carter et al~5, 16, 17 and others 18 that, due to bombardment by both ions and electrons, insulating surfaces, such as the tube envelope can stabilise at one of two potentials; either close to the cathode or to the electron collector potential. This bistability can modify the ion energy and bombardment rates considerably and hence the tube pumping characteristics. Despite these difficulties, however, it is possible to deduce useful results if care is exercised in the operation of the gauges and in interpretation of the data. Many of the above difficulties can be circumvented in ion beam experiments of the type described by Kornelsen 19, Colligon20, 21 et al and others 22 in which a well defined ion beam, formed by electron impact, is directed onto a carefully prepared and maintained target surface. Data from such experiments is easier to interpret and much current investigation is performed in this manner particularly with metal targets. For glass surfaces, however, the convenience of the ionization gauges has led to their continued investigation. Other experimental methods of ion production have been employed in addition to acceleration of thermionic electrons. Maddix23 and his co-workers, for example, have used high-power, pulsed micro-wave plasmas in studying the absorption of inert gases in quartz, whilst Kelly24 has employed fast neutron irradiation techniques to examine rare gas attachment to various powders. In all these experiments, however, the basic processes are the production of energetic gaseous ions or atoms and their impact on specific target areas. In the case of ionization gauges, one of two experimental procedures is usually followed. Either the experimental volume is closed so that the pressure falls as ions are impelled or pumped into the walls, or the pressure is kept constant by continuous introduction of fresh gas from a reservoir. In the former, static method, observation of the fall in pressure provides information on the pumping action of the gauge and the efficiency of trapping of the ions. Similar data can be obtained from the latter dynamic experiments with the added advantages that the bombardment rate is constant and saturation may be quickly achieved while the relatively high operating pressures minimize the effects of impurity background gases. Following bombardment, a release of gas from the trapping surfaces is generally observed both at the bombardment temperature and at elevated temperatures as the gauge is heated. It has been established also that subsequent operation of a gauge with a second ion species can promote release of the primary trapped species and one should anticipate that such a bombardment induced gas release process will operate during primary trapping. It is, therefore, apparent that in addition to ion trapping a 478

simultaneous gas release due to both thermal influences and bombardment induced processes will occur, and in order to try to understand any one process it must be separated from or correlated with the influence of the others. In the following we review the investigation of each process in turn, and note that both dynamic or static and ion beam experiments may be used in such investigations. Pumping

equations

In addition to the ion trapping process and the various release processes of thermal and bombardment induced origin, other effects such as surface sputtering of the target, saturation of the available gas trapping sites and variation in the target stopping power due to damage will all play an active role in determining pumping phenomena. A typical experimental curve for pumpdown of argon in a static system is shown in Figure 1. An equation for the pressure variation may be derived on the assumption that the bombardment rate of the surfaces is directly proportional to the gas density (pressure), that there are no induced or spontaneous release processes and that the trapping efficiency remains constant, thus p = P0 exp{ --(s/v) t} where S is the characteristic pumping speed of the system in l./sec and is directly proportional to the ion trapping efficiency. The inadequacy of this equation is revealed in Figure 1 where there is rapid departure from initial linearity of the log pressure/ time curve. The initial linearity implies, however, that when only small quantities of gas are trapped, release processes are negligible and that true trapping efficiencies can be obtained from the initial slope of such curves. I

I

-

l

. 10- 4

10-5

10-6

I

I

5O

I00

Time,

150

min

Figure I, Typical curve for the pumpdown of argon in a static system.

Von MeyernaS(a),(b), Young26 and Alpert4 proposed a pumping equation which accounted for the possibility of a limited number of available sites for gas trapping and of bombardment induced release processes, but ignored spontaneous release. The equation is

--dn=I( 1-n)dt

no

W A Grant and G Carter: Ion trapping and gas release phenomena where n = n u m b e r of entities t r a p p e d at time t no = original n u m b e r o f t r a p p i n g sites t = time(see) I -- b o m b a r d m e n t c u r r e n t a n d solution of this shows t h a t p a n d n vary in complex exponential fashions. G r a p h s o f p u m p i n g speed against q u a n t i t y p u m p e d at a c o n s t a n t pressure s h o u l d be linear while those of n versus time or b o m b a r d m e n t dose s h o u l d increase e x p o n e n tially. C a r t e r a n d others27 in their w o r k o n p u m p i n g into glass have s h o w n t h a t these variations are n o t in fact followed. Several theories h a v e been suggested for t h e t h e r m a l release process involving various f o r m s o f the time d e p e n d e n c e o f the release rate, r a n g i n g f r o m a simple inverse p o w e r of time relationship to a c o m b i n a t i o n of this a n d e x p o n e n t i a l functions. F o r the inverse time f o r m s the m e c h a n i s m e v o k e d is t h a t of diffusion o f t r a p p e d gas f r o m various c o n c e n t r a t i o n profiles w i t h a single activation energy. F o r the e x p o n e n t i a l f o r m s a step d e s o r p t i o n m e c h a n i s m is envisaged in one or m o r e j u m p s b u t involving a s p e c t r u m of a c t i v a t i o n energies for desorption. Similarly there is a complexity in the theories o f b o m b a r d m e n t i n d u c e d a n d sputtering release processes, so t h a t the final p u m p i n g e q u a t i o n for the pressure v a r i a t i o n is extremely c o m p l e x a n d until the exact f o r m of each of these processes is f o u n d , their c o m b i n a t i o n into a p u m p i n g e q u a t i o n is impossible. W e will n o w examine e a c h of these p h e n o m e n a in greater detail, a n d n o t e t h a t whilst s i m u l t a n e o u s t r a p p i n g a n d release occur one m a y well expect d y n a m i c e q u i l i b r i u m effects where t r a p p i n g a n d release processes balance. W h e r e a s initial p u m p ing e x p e r i m e n t s allow d e d u c t i o n o f ion t r a p p i n g efficiencies, a n y t e n d e n c y t o w a r d s s a t u r a t i o n effects m i g h t be r e g a r d e d as evidence of release p h e n o m e n a . T h e t r a p p i n g efficiency and saturation q u a n t i t y T h e t r a p p i n g efficiency n defines the p r o b a b i l i t y o r efficiency of c a p t u r e o f a n ion incident u p o n a target surface, b u t since there

is s i m u l t a n e o u s d e s o r p t i o n o f gas the definition refers only to gas which r e m a i n s t r a p p e d for the d u r a t i o n of the e x p e r i m e n t a l measurements. T a b l e 1 s u m m a r i s e s the available d a t a for the t r a p p i n g efficiencies o f low energy inert gas ions incident u p o n various surfaces. T h e value of ~/ varies with t h e incident i o n energy a n d Figure 2 shows ~ as a f u n c t i o n o f energy for inert gases o n the Pyrex envelope of a B a y a r d - A l p e r t gauge as r e p o r t e d by C o b i c et a127. E q u i v a l e n t curves for ions incident o n a m o l y b d e n u m target h a v e been o b t a i n e d by V a r n e r i n a n d Carmichae128. F r o m these curves a n d f r o m T a b l e 1 it c a n be seen t h a t the t r a p p i n g efficiency increases, u p to a limiting value of o r d e r unity. F o r glass surfaces rl is o f the o r d e r t0-1 at I0

g

A

a

Ne

"~

e

o

I 50o Electron collector voltage Figure 2. The trapping efficiency as a function of incident ion energy for inert gases pumped into a Pyrex surface.

tO00

T a b l e 1. Trapping efficiencies for various inert gas ions on different targets Ion type Helium

Helium Helium Helium

Ion energy 150 ev 500 ev 1 Kev 1.5 Kev 2.5 Kev 150 ev 2 Key and 4 Kev 150 ev

Target material Molybdenum Molybdenum Molybdenum Molybdenum Molybdenum Nickel Tungsten

Target temperature ~300°C ~300 °C ~300°C ~300°C ~300°C ~300 °C ~ 20 °C

Molybdenum ---

60 °C 80 °C 196 °C

r/

Investigators

Reference Remarks

0.35 0.57 0.67 0.71 0.79 0.65 0.19

Pumping device Tetrode Tetrode Tetrode Tetrode Tetrode Tetrode Ion beam

Varnerin and Carmichael Varnerin and Carmichael Varnerin and Carmichael Varnerin and Carmichael Varnerin and Carmichael Carmichael and Knoll Corkhill and Carter

28

1 2.3 3.6

Triode Triode Triode

Fox and Knoll Fox and Knoll Fox and Knoll

69 22 43

Value at 60 °C not known exactly, but values at 80 °C and --196°C relative to arbitrary value of 1.0 at 60°C --

e x p r e s s e d

Helium

250 ev ~_200 ev

Pyrex Pyrex

glass glass

~ ~

20 °C 20°C

0.87 0.2

Triode Cobic, C a r t e r a n d L e c k BayardAlpert Alpert gauge

27 4

Neon

150 ev

Molybdenum

~ 3 0 0 °C

0.05

Tetrode

Carmichael and Knoll

69

Neon

150 ev

Nickel

~ 3 0 0 °C

0.73

Tetrode

Carmichael and Knoll

69

Neon

50 ev 100 ev 1 Kev

Tungsten Tungsten Tungsten

~ ~ ~

20 °C 20 °C 20°C

Ion beam Ion beam Ion beam

Kornelsen Kornelsen Kornelsen

32

~ 10-5 0.06 0.6

Neon

250 ev

P y r e x glass

~

20 °C

0.55

Triode

Cobic, C a r t e r a n d L e c k

27

Argon

150 ev

Molybdenum

~ 3 0 0 °C

0.05

Tetrode

Carmichael and Knoll

69

Argon

0.7-4 Kev

Molybdenum

~

0.60

Ion beam

CoUigon a n d L e e k

20

20 °C

479

WA GrantandG Carter: Ion trapping and gas release phenomena Ion type

I o n energy

Target material

r/

Pumping device

Investigators

Reference

Argon

150 ev

,~300°C

0.40

Tetrode

Carmichael and Knoll

69

Argon

150 ev Titanium 300 ev a n d a b o v e T i t a n i u m

~ ~

20 °C 20°C

0.25 0.5

Triode Triode

Jenkins a n d T r o d d e n Jenkins and Trodden

70

Argon

500 ev 1 Key 5 Kev a n d above

Aluminium Aluminium Aluminium

~-'- 2 0 ° C -~- 20 °C ~ 20 °C

0.06 0.5 1.0

Ion beam Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies B r o w n a n d Davies

30

Argon

100ev 1 Kev and above 250 ev 500 ev 1 Kev 2 Kev 1 Key 5 Kev 10 Kev 0 . 7 - 4 Kev

Tungsten Tungsten

:~- 2 0 ° C ~ 20 °C

7 x 10-4lonbeam 0.6 Ion beam

Kornelsen Kornelsen

32

Tungsten Tungsten Tungsten Tungsten Tungsten Tungsten Tungsten Tungsten

~' ~ ;-: :-~ ~ s-, ~ "~

20 °C 20 °C 20°C 20 ~C 20 °C 20~C 20°C 20 °C

0.0 l 0.04 0.10 0.16 0.16 ~,0.21 ~-,0.7 0.25

Ion Ion Ion Ion Ion lon Ion Ion

Corkhill a n d C a r t e r Corkhill and Carter Corkhill and Carter Corkhill and Carter B r o w n a n d Davies B r o w n a n d Davies B r o w n a n d Davies Colligon a n d L e c k

22

0.7-4 Kev

Platinum

~

20 °C

> 0.7

Ion beam

Colligon a n d L e c k

20

P y r e x glass

~-" 20 °C

0.31

Triode

Cobic, C a r t e r a n d L e c k

27

TiO2 a n d o t h e r metallic o x i d e powders

,~" 20 °C

~'0.15

Recoils f r o m c~ irradiation

Kelly

24

Molybdenum Molybdenum

~ 3 0 0 °C 20 °C

0.09 0.15

Tetrode Ion beam

Carmichael and Knoll Colligon a n d Leek

69 20

Argon Argon

250 ev

Argon

~ 2 0 0 ev

Nickel

Target temperature

beam beam beam beam beam beana beam beam

30

20

Krypton

150 ev 0 . 7 - 4 Kev

Krypton

150 ev

Nickel

~ 3 0 0 °C

Tetrode

Carmichael and Knoll

69

Krypton

100 ev

Tungsten

~

20°C

3 × 10-6

Ion beam

Kornelsen

32

1 Kev 5 Key 1 Kev 5 Kev 10 K e y 0.7-4 Key

Tungsten Tungsten Tungsten Tungsten Tungsten Tungsten

~_" ,-' :~ ~, ~ ~

20°C 20°C 20°C 20°C 20°C 20 °C

0.5 0.7 0.1 0.52 0.88 0.1

Ion Ion Ion lon Ion Ion

beam beam beam beam beam beam

Kornelsen Kornelsen B r o w n a n d Davies B r o w n a n d Davies B r o w n a n d Davies Colligon a n d L e e k

20

0.7-4 Kev

Platinum

~

20°C

0.1

Ion b e a m

Colligon a n d Leek

20

Krypton Krypton

250 ev

Krypton

~ 2 0 0 ev

~ 1.0

30

Pyrex glass

~

20 °C

0.62

Triode

Cobic, C a r t e r a n d Leek

27

TiO2 a n d o t h e r metallic oxide powders

~

20 °C

0.03

Recoils from irradiation

Kelly

24

Ion Ion Ion Ion

Brown Brown Brown Brown

Xenon

100 ev 500 ev 1 Key 5 Kev and above

Aluminium Aluminium Aluminium Aluminium

-,~ ~ ~-~ ~

20°C 20 °C 20°C 20 °C

0.05 0.30 0.60 1.0

Xenon

500 ev 1 Key 5 Kev 1 Kev 5 Kev and above 2 Kev 4 Kev

Tungsten Tungsten Tungsten Tungsten Tungsten

~ ,~ ~ ~ ~

20 °C 20°C 20 °C 20°C 20 °C

~ 10-2 0.2 0.6 0.1 1.0

Tungsten Tungsten

~ 20 °C ~:- 20 °C

beam beam beam beam

and and and and

Davies Davies Davies Davies

30

I o n beana Ion beam Ion beam Ionbeam Ion beam

Kornelsen Kornelsen Kornelsen B r o w n a n d Davies B r o w n a n d Davies

32

0.10 0.23

I o n beana Ion beam

Corkhill a n d C a r t e r Corkhill and Carter

22

30

Xenon

1 Key 5 Key and above

Beryllium Beryllium

~ ~

20 °C 20 °C

0.23 ~ ' 1.0

ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies

30

Xenon

1 Kev 5 Key and above

Nickel Nickel

~ 20°C _~ 20 °C

0.20 ~ 1.0

Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies

30

Xenon

1 Key 5 Kev and above

Zirconium Zirconium

~ ~

20°C 20 °C

0.43 ~ 1.0

Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies

30

Xenon

1 Kev 5 Key and above

Tantalum Tantalum

,~- 2 0 ° C ~ 20°C

0.37 21.0

Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies

30

Xenon

5 Key 10 K e v

Silver Silver

:~ 20 °C ~ 20°C

_~_0.4 ~0.5

Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies

30

Xenon

1 Kev 5 Kev 10 Kev

Gold Gold Gold

~ ~ ~

20°C 20 °C 20°C

0.05 -~0.65 ~0.7

Ion beam Ion beam Ion beam

B r o w n a n d Davies B r o w n a n d Davies B r o w n a n d Davies

30

Pyrex glass

~

20 °C

0.41

Triode

Cobic, C a r t e r a n d L e e k

27

TiO2 a n d o t h e r metallic oxide powders

~

20°C

~0.15

Recoils f r o m c~ irradiation

Kelly

24

Xenon

250 ev

Xenon

~ , 2 0 0 ev

480

Remarks

Extrapolated f r o m slightly higher energy

W A Grant and G Carter: Ion trapping and gas release phenomena energies less than 100 eV and then increases to the order of unity in the region of 1 keV. For metal targets ~/ follows a similar pattern with similar orders of magnitude but with a wide variation from metal to metal. The variation of trapping efficiency at higher energies has been evaluated largely from ion beam experiments. Burtt et a121 and Colligon and Leck20 bombarded Mo, Pt and W targets with inert gases of energy from 0.68 keV to 3.45 keV and subsequently desorbed the trapped gas by heating the target to 2000°C. They report a value of 10-1 for ~ for all ion/target combinations at low energies but it is believed that this constancy is a result of poor experimental resolution in determining small quantities of trapped gas at low ion doses. Kornelsen29 irradiated polycrystalline tungsten targets with mono-energetic inert gas ions from 40 eV to 3 keV and noted release upon flashing the targets to 2400°C at a constant temperature rise rate. The variation of r/ with ion energy obtained by this author is shown in Figure 3 which indicated that at energies less than 100 eV ~ is less than 10-1 but then rapidly increases to about 0.6 at 3 keV. For energies less than 1 keV the trapping efficiency decreases with increasing ion mass while the rate of increase of ~7 with energy is approximately constant for all ions. At energies greater than about 2 keV ~ tends to a saturation value for Ne and A but continues to increase for the heavier Kr and Xe. Further work at still higher energies has been conducted by Brown and Davies30 and Almen and Bruce 31, and the results indicate that at energies < 5 keV, ~ is always fractional for many metal surfaces but tends rapidly towards unity for energies greater than about 5 keV. I0

Kornelsen32 has calculated the energy required by inert gas atoms to just penetrate, at half the mean lattice spacing, a randomised tungsten target by equating the ion kinetic energy to the ion-metal repulsive potential energy and has obtained values which are in good agreement with his experimental data. The results also indicate that the trapping efficiency for inert gas ions on tungsten should increase at a constant relative rate thus explaining the parallel curves in Figure 3. The larger values of r/ for glass can be explained on the basis of larger average interatomic spacing and weaker interatomic potentials resulting in easier penetration of the impinging ion. Figure 4 shows the decrease of trapping efficiency with increasing target temperature from the work of Cobic et a127. This variation of r/with temperature may be anticipated from our definition which indicates that r/includes only those ions which remain trapped for the duration of the experiment. Spontaneous desorption of trapped atoms occurs at the target temperature and can be expected to increase as the temperature is raised, resulting in a reduction in the effective value of the trapping efficiency. The increase of ~ as the temperature is decreased below room temperature also indicates that some ions have very short trapping times at normal temperatures and can only be trapped for longer times at reduced temperatures. IC

E 3

o

.," ,# r2,,"

I!//

i0-1

'

-

.Q

i

E

o_-

[

10-2

0.5 --

Q.

Xe

g 1--

o 10-3

!

10-4

/:

375 Gauge t e m p e r a f u r e ,

Figure 4. Gauge temperature (°K).

10-5 ~le

IKr

10-6 I01

750 OK

102

103

"Ion energy,

eV

10 4

Figure 3. The sticking probability of inert gas ions in tungsten as a function of ion energy.

In experiments conducted in a closed volume the pumping speed appears to decrease with the quantity of gas removed until it reduces to zero when an apparent saturation quantity ns has been pumped. In experiments in which the surface bombardment rate is kept constant, however, a saturation level in the quantity of gas pumped is also observed. This apparent saturation occurs when the net rate of removal of gas is just balanced by the net rate of any desorption. In static experi481

W A Grant and G Carter: Ion t r a p p i n g and gas release p h e n o m e n a I0

I

/He

Ne

IHe

/

o

o

xe/

c

E

g S

g s 1Jr g o

u~

y, 0

5O0

I000

Electron collector voltoge

\\ k\

Figure 6. The variation of n, with electron collector voltage for inert gas ions incident on Pyrex.

Xe

0

375 750 Gouge -~emperoIure, °K Figure 5. The variation ofns with target temperature for 250 eV inert gas ions incident on Pyrex.

ments the apparent saturation is largely due to the decreasing number of ions available for pumping which are readily balanced by thermal re-emission processes and for this reason, constant bombardment experiments are preferable in deter-

mining true values of ns. It has been shown by James and Carter17 that spontaneous desorption of gas is only relatively important when small quantities of gas have been pumped while bombardment induced processes become important at large pumped quantities. Thus the true saturation quantity ns will be determined primarily by these induced processes, and is thus determined in dynamic experiments. The variation of ns with target temperature for 250 eV inert gas ions incident on Pyrex as reported by Cobic et a127 is shown in Figure 5, while Figure 6 shows the variation of ns with energy for these ions. Typical values of ns are from 0.1 to 1.0 equivalent monolayers

/ /Ao=l I015

/

4keV 2 ke", s~,....4-

~'-"'Tkev

/,

500 eV

%

1014

/ "//,~

X 5 250 eV I0 __o~O- - 4 - 15 20 30 • q~ • - 150eV

o / //'A.: ~

1013

,o,2

~ o ~

/

,./-////

,t"/''

IOOeV

z

leIr ~ r ~ .

/" 1010 109 i011

.#

/q "

/"

1012

.,,, /

./

// i013

i014

Number incident,

482

i015 ions/cm z

iOI6

10I?

Figure 7. The number of argon atoms desorbed from the tungsten target as a function of the number of incident argon ions of various energies.

W A Grant and G Carter: Ion trapping and gas release phenomena while for ions of a given energy the saturation quantity approaches zero at temperatures similar to those at which the trapping efficiency becomes zero as would be expected. Carmichael and Trendelenburg33 have measured saturation values for 100 eV inert gas ions incident on nickel targets and obtained fractional equivalent monolayer values of ns for all ions except helium where many equivalent monolayers could be trapped. Maddix and Allen23 have also observed that many equivalent monolayers of the Ne, A and Kr ions could be trapped in a quartz surface without any sign of saturation even though the bombarding energy was low (<300 eV). In addition it was observed that the clean-up rate followed a (time) -~ relation, indicating that the gas was not only being trapped but was diffusing into the quartz at the bombardment temperature ( ~ 2 0 0 °C). Such a possibility exists for the cases of the trapping in glass and Ni. At higher energies the variation of ns with ion energy has been measured by Colligon and Leck20 and Kornelsen29, 32 using ion beam methods. Colligon and Leck found that ns increased almost linearly with ion energy from 0.7 keV to 3.75 keV for inert gases on Ni, Mo and Pt targets. At the highest energies the saturation levels were of the order of three to thirty equivalent monolayers while linear extrapolation to low energies indicated fractional monolayer saturation values. The variation of ns with ion energy from 40 eV to 5 keV for argon ions incident on tungsten as reported by Kornelsen is shown in Figure 7. The saturation level increases rapidly from 100 eV up to 0.5 keV and then less rapidly up to a value of approximately two equivalent monolayers at 5 keV. Extensive work has been conducted by Brown and Davies30 and Almen and Bruce31 in the energy range 5 to 65 keV for inert gases on various metal targets, and have found essentially similar results to those of Kornelsen and Burtt, except that the values of ns are larger. The ns/energy relationship is almost linear in most cases and a saturation value of ns itself is finally reached. Interesting observations by Almen and Bruce were the dependence of ns upon the surface orientation of single crystal targets and a dependence of ns upon the bombardment rate for Ag and Sn targets. In most other experiments ns has been observed to depend only upon the total ion dose and not upon the individual parameters of bombardment rate and time. These two effects are explicable on the basis of ion channelling along preferred open crystal directions, and upon trapped gas diffusion respectively. The question of the depth of ion penetration will not be elaborated here except to note that, quite recently, a wealth of data on ion ranges in amorphous, polycrystalline and single crystal targets has been accumulated by Davies and his eollaborators34a,b,c,d,e,f,g,h and other workers. Generally speaking the measured ion ranges in amorphous and polycrystaUine materials are in good agreement with theoretical deductions based upon the assumption that a solid is equivalent to a very dense randomised gas and which employ conventional atomic stopping power concepts (Lindhard and Scharff35, Leibfried36). Ranges turn out to be distributed about a mean value for a particular ion energy, because of the statistical nature of the ion collision processes and a rough rule of thumb in that the mean ion range is of the order of 10A °/keV. In single crystal targets, however, it has been noted experimentally37a,b,c,d that an appreciable fraction of incident ions can travel very large distances into the target, and it has been established that this is a result of penetration along axes in the crystal where the energy loss to target atoms is small because

of crystal symmetry. These are denoted as channels38 and are probably the cause of the observations by Almen and Bruce of the orientation dependence of ns.

Spontaneous release After irradiation of a target subsequent spontaneous desorption of gas at the same target temperature occurs. Many investigations have been carried out to evaluate this phenomena, generally using the glass envelopes of Bayard-Alpert gauges as the target surfaces. In static pump-down and recovery experiments Robinson and Berz39 found a recovery rate of the form - - d n / d t = A e at 4- Be -bt whilst Baker and Giorgi40 reported a form - - d n / d t = Ce -ct (where A, B and C are constants) the value of the constants depending on the quantity of gas previously trapped. Blodgett and Vanderslice found that log (dn/dt) = --½ log t + D provided that a saturation quantity of gas had been trapped, but for times before saturation log (dn/dt) = m log t 4- D where - - ½ < m < - - 1 depending upon the quantity of gas pumped. Smeaton, Carter and Leek 42 also found a release rate equation of the form log (dn/dt) -- m log t 4- D for release times t in the range 10see < t <103 seconds. The particular values of m and D in a particular experiment depended on the quantity pumped up to one tenth the saturation value and the manner in which the gas had been pumped, ie the emission depended on the individual parameters of electron current, pressure and time which determined the ion dose. Since in all these experiments a static system was used there was a pressure decrease during the experiment so that the bombardment rate of the surface decreased. Thus, although similar amounts of gas may have been trapped, the bombardment conditions may have differed considerably from experiment to experiment and this could affect the subsequent release rate data. Carmichael and Trendelenburg33 studied the bombardment and subsequent spontaneous emission of inert gases in Ni and Mo targets in a static system, and report an equation of the form d n / d t = K n o / t for t <1000 min and for pumped quantities up to about one tenth of the saturation value. Fox and Knoll43, in a study of the re-emission of helium for a molybdenum target, also report a 1/t time dependence of the reemission rate for times as short as 0.03 min. Smeaton, Carter and Leek 44, and Cavaleru, Comsa and Iosifescu45 used the dynamic method in which pressure and hence bombardment rate was kept constant. Curves for the recovery rate of 250 eV A, He and Xe ions from a Pyrex surface at 20 °C from the results of Smeaton et al can be described by equations of the form log (dn/dt) = m log t 4- C where m varies from --0.3 to --1.0 according to the pressure during bombardment, ie the bombardment rate. The index m also depends on the time of bombardment (at a given bombardment rate) and for re-emission times greater than 100 min m fell rapidly. The results of Cavaleru et al are qualitatively similar but an emission equation of the form - - d n / d t - A e -at 4Be -bt is deduced to explain the data. Other workers have obtained equations similar to those quoted above and the general conclusion to be drawn from these results is that the re-emission rate can in general be represented by - - d n / d t = - - n o / t m for values of r/up to about one tenth of the saturation value and for times between one and ten thousand seconds. The values of m depend on the bombardment conditions prior to the spontaneous release. Maddix and Allen, however, observed on inert gas release from quartz,

483

W A Grant and G Carter: Ion trapping and gas release phenomena a quite unequivocal (time) ~ dependence of the release rate, a fact suggesting normal diffusion to be the operating mechanism in this system.

Gas release at elevated temPeratures If the target is subjected to a heating schedule after irradiation then the release rate of gas is greatly enhanced. Experiments 101

240°C

J:l 0

o

8

"o

g

3150C

CB
lO0°C

i

12

0 Desorpfion

24 lime,

min

Figure 8. The variation of the desorption rate with temperature for the release of 250 eV argon ions from glass.

using this technique are usually conducted in dynamic systems and a typical result for the variation of des0rption rate with temperature is shown in Figure 8 for the release of 250 eV Ar ions from glass3S. The release rate rises rapidly to a peak and then falls to zero at about 450°C. The temperature width of this gas release transient precludes one step desorption from sites with a single activation energy as the operative mechanism, since, as the Frenkel equation for the sticking time of sorbed gas at a temperature T and for an activation energy Q, r = r 0 exp (Q/RT) shows, for T to be the order of the experimental time, the temperature release width must be small (:~20°K). Possible explanations of the peak width involve a depth distribution of trapped atoms, so that multiple jumps occur before release is effected, or a spectrum of activation energies for gas desorption. For the low ion energies used in most of the work summarised here the depth of ion penetration is small ( < 10°A), and an analysis by Kelly46 shows that such depths are insufficiently large to lead to the temperature peak widths observed. The alternative explanation of an activation energy distribution can account for such a broad desorption peak, however, even with a comparatively small energy spread, and Carter and Leck 15 and many subsequent workers19,29,32,20 have analysed their data on this basis. From the form of the desorption transient, Carter and Leck 47 have shown how it is possible to derive this activation energy distribution, and these authors and Cobic, Carter and Leckl6, 27 derive values of 25 to 50 kcal/mole for the binding energies of inert gases to glass. Cobic, Carter and Leck 16, 27 and Fox and Knoll43 also present evidence for the existence of sites of lower energies, which do not participate in desorption at temperatures above room temperature, since an increase in pumping speed was observed when the gauge walls were kept at temperatures lower than ambient. James and Carterl7, 4s found that the form of the desorption transient was slightly dependent on the ion species, indicating a variation in relative energy site distribution, while the quantity of gas pumped also changed the transient shape. A shift in the

1 4xlO I ni

ni

(ions/cm2l

(ions/cmz) 5 T,

IO0°K

Ne+, 400eV 484

IO 15 20 24 T, IO0°K (

Kr+, 500 eV

Figure g. The influence of the number of incident ions on desorption spectra.

14/A Grant and G Carter: Ion trapping and gas release phenomena transients towards higher temperatures with the quantity pumped indicated a preferential build up of gas in higher binding energy sites while increasing the ion energy from 100 eV to 1 keV resulted in a general broadening of the transients, again towards higher release energies.

2000

,50o-x~fIlll)l/W A~ Ion energy

(eV)

Zol 2000"~ ^ I00

~

5 I0 15 20 24 T, IO0°K Neon

ener, (e~

5

I0 15 2 0 2 4

T,

I00 °K

Argon

Figure 10. Desorption spectra of neon and argon from tungsten for various incident ion energies.

Kornelsenl9, 29, 32 investigated the desorption of inert gases previously trapped in a tungsten target when heated at a constant rate and typical curves are given in Figures 9 and 10. The peaks in the transient are associated with preferred trapping energies from approximately 30-105 kcal and, since they show little variation with ion type, must be largely attributed to properties of the target itself. The relative populations of the sites associated with these peaks was a function of the ion energy and quantity of trapped gas. An increase in incident ion energy resulted in a shift in the population density of the spectra towards higher energy peaks for all the gases considered while the amount of spectral distortion resulting from an increase in the trapped quantity also depended on the gas species. For neon the shift was towards higher energies and this was associated with knock-on collisions driving trapped gas more deeply into the target, while for krypton the shift was in the opposite sense and was explained by the increased stopping power of the krypton saturated surface layer. An investigation of the depth distribution of the ions trapped in the target both before and after heating, showed that a slight diffusion of gas away from the target surface occurred so that the high energy peaks in the transients may be correlated with atoms which have diffused into the target. Colligon and Leck20 also reported desorption transients (exhibiting multiple peaks), for inert gases from tungsten, while similar results for the release of these gases from various metal targets, at temperatures up to their melting points, have been given by Tucker and Norton 64. Kelly and Brown49a,b,c, have studied post-bombardment thermal release of trapped inert gases from several metallic

oxide powders, aluminium, silver and gold and alkali halide crystals, and again interpreted their results in terms of trapping activation energy spectra. In materials which form an oxide skin it was found that gas release was inhibited until this skin dissolved, suggesting that the surface layer could prevent escape of gas. In certain cases the gas release could be associated with structure and phase charges in the target, whilst evidence was found for some release of gas not in single atomic form, but in atomic clusters or bubbles. AuskernSO has also observed a continuous gas release from room temperature to 1050°C of Xe produced by fission in UC and investigations with UO2 and graphite have yielded essentially similar results. Jech52a,b,c has reported the existence of activation energy spectra for the release of trapped inert gases from alkali halides and other materials and has noted that, if during heating, the target temperature increase is halted and the temperature maintained constant, the release rate falls rapidly to zero. Similar results have been reported by Carter53 for glass and a further observation of interest by Smeaton et a154 was that if pumping into glass is carried out at elevated temperatures and the target subsequently cooled, gas release does not commence upon reheating until the bombardment temperature is surpassed. These results mitigate against a depth distribution trapping of gas but are readily explicable in terms of the activation energy spectrum model. Gas release at room temperature has also been interpreted on the basis of diffusion from a depth distribution of trapped atoms, but, as noted above, the analysis by Kelly shows that the required penetration depths are inordinately large for low energy ions (<1 keV). On the other hand Smeaton et a144 suggest :hat the spontaneous release at room temperature is the result of desorption from that part of the activation energy spectrum which has sticking time constants of the same order as the experimentally observed release times (18-23 kcal/mole or release times of 1-104 sec). On this basis it is possible to explain the t m spontaneous release rate behaviour reported by many authors, and the model is supported by observations by Carter53 (thesis), Grant (unpublished) and Hobson 55, that following pumping at temperatures below ambient (down to --196 °C), gas release can then be effected simply by removal of the refrigerant in addition to the normal release above room temperature. The decrease in sticking efficiency with increasing temperature is further favourable evidence for a release activation energy spectrum, since as the temperature is increased the sticking times of atoms only become of the order of the experimental times for trapping energies Q greater than R T loge r / r 0. Thus as the temperature is increased gas is trapped only in higher energy trapping sites, and the lower part of the spectrum is eliminated. On the other hand trapping and release rates for inert gases in quartz 23 at target temperatures where normal diffusion is possible, strongly suggest that this is the dominant feature in this system. It is probable that the injected ions, which were of low energy, caused little damage on entry and simply diffused deeply into the quartz. B o m b a r d m e n t induced gas release

Schwarz56 and later Brown and Leck 14, showed that gas trapped in a solid could be released by further ionic bombardment. This phenomenon has been investigated using a technique where the target was first bombarded with one ion species, and the release of this species was then studied mass spectrometri485

W A Grant and G Carter : Ion trapping and gas release phenomena cally on i r r a d i a t i o n o f the target by a second species. T h u s Carmichael a n d Trendelenburg33 h a v e o b t a i n e d data for induced release of inert gases t r a p p e d in nickel, a n d J a m e s a n d Carter48a,b,c h a v e studied the release of inert gases t r a p p e d in glass. C a r m i c h a e l a n d Trendelenburg33 b o m b a r d e d a nickel target with inert gas ions of a p p r o x i m a t e l y 100 eV a n d t h e n n o t e d their release w h e n the target was exposed to s h o r t pulses of a second species. T h e release efficiency (ie the rate of release of t r a p p e d a t o m s per b o m b a r d i n g ion) was f o u n d to be a m a x i m u m at the c o m m e n c e m e n t of secondary b o m b a r d m e n t a n d Figure 11 shows this m a x i m u m efficiency plotted as a f u n c t i o n of the q u a n t i t y of p r i m a r y gas trapped. It can be seen t h a t initially the release efficiency increased linearly a n d t h e n reached a s a t u r a t i o n level where it was i n d e p e n d e n t of the q u a n t i t y of gas trapped.

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T h e release of a gas by b o m b a r d m e n t c a n be characterised by a gas sputtering coefficient Sg. This defines the p r o b a b i l i t y of release o f a t r a p p e d a t o m per b o m b a r d i n g ion a n d is normalised to equivalent m o n o l a y e r c o n t e n t of t r a p p e d gas. Values of Sg for the w o r k described a b o v e are s h o w n in T a b l e 2. R e p o r t e d values of the target sputtering coefficient St T a b l e 2. Values of the gas sputtering coefficient Sg and the target sputtering coefficient St for 100 eV ions bombarding nickel. Gas-Gas Combination Kr He Kr Ne Kr Ar He Kr Ne Kr Ar Kr He He

Sg 0.2 0.96 2.0 1.5 0.24 0.24 5.0

Gas-Target Combination Kr Ni Kr Ni Kr Ni He Ni Ne Ni Ar Ni He Ni

St 0.18 0.18 0.18 0.072 0.25 0.30 0.072

are included for c o m p a r i s o n . It can be seen t h a t for N e or A r releasing Kr, or for K r releasing H e or N e f r o m Ni, one N i a t o m s h o u l d b e s p u t t e r e d for every gas a t o m released. B u t K r ions release four N e a t o m s a n d H e ions release seven K r a t o m s 486

"G == o

x A/He

=-

A/Ne

L

A/Kr

s

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0

14 Quantify sorbed, x 105 I.forr

28

Figure 13. The release rate of gases released by argon as a function of the quantity trapped,

W A Grant and G Carter: Ion trapping and gas release phenomena for each Ni atom sputtered. These high ratios of Sg/St suggest that target sputtering (ie removal of target material and exposure and desorption of trapped gas) is not the dominant mechanism for bombardment induced release, but that a separate gas sputtering mechanism is responsible. The investigation of Ni was extended, by Carmichael and Waters 57, for the case of equal bombarding and trapped gas masses using He3 + and He4+ ions. The values of Sg resulting from this work are again indicative of a gas sputtering mechanism. Using a similar technique James and Carter 48 investigated the release of inert gases from glass. The experimental method also included post bombardment heating to determine the percentage of gas left in the target. Typical initial release rate/ quantity of gas trapped curves for Ar released by He, Ne, Kr and Xe are given in Figure 12 while the release of He, Ne, Kr and Xe by Ar is shown in Figure 13. The initial gas release rate increases to a saturation value, while the percentage of gas recoverable during bombardment decreases monotonically. By interpolating between the curves in Figures 12 and 13 the selfsputtering coefficients were calculated and the full characteristics are shown in Figure 14, while values of Sg calculated from these curves are shown in Table 3.

from 100 eV to 1 keV to release 250 eV trapped ions resulted in a small increase of Sg between 10O eV and 400 eV, but thereafter there was no further increase. Figure 15 shows the release rate of argon as a function of the tube temperature both without and following bombardment by a second species. It can be seen that bombardment by a second species causes the subsequent thermal release to be greatly reduced in the lower temperature range but to a smaller extent at higher temperatures. James and Carter 48 interpreted their results tor bombardment induced release on a gas sputtering, as opposed to target sputtering, basis. If the release was dependent on target sputtering only, the sputtering coefficients for a particular trapped atom would depend only on the target sputtering coefficient of the bombarding ion and would, therefore, tollow the same sequence for all bombarding ions. Furthermore, on this basis the gradual removal of the target would result in the complete liberation of all the trapped gas. Both these effects are absent in James and Carter's48a,b, c results.

== J°l'

T a b l e 3. Gas sputtering coefficients Sg for inert gases

released from glass. (Ions sorbed and released at 230 eV). Sorbed gas Helium Helium 5.5 Neon 9.0 Argon 10.0 Krypton 7.5 Xenon 13.0

Bombarding ion Neon Argon 12.0 50.0 20.5 35.0 35.5 22.0 38.0 12.0 24.0 40.0

Krypton 40.0 33.0 18.0 I h5 62.0

Xenon 5.0 10.0 12.5 6.0 63.0

Variation of the trapping energy from 100eV to 1 keV before release by 250 eV ions, produced an apparent decrease in Sg by a factor of 2.5, while variation of the releasing energy

5

(II )

g $

g

A T

0

E

I

I0 Time,

E

v

20 rain

Figure 15. The desorption of argon trapped at 950 V as a function of the target temperature both without and following bombardment by a second species. (1) Desorption without second species bombardment. (2) Desorption following second species bombardment.

(o O

g o

oo o

O

IO 20 Qu0nfify of orgon sorbed (x 105 I forr )

Figure 14. The release of argon by argon as a function of the quantity trapped. (1) Deduced from Figure 12. (2) Deduced from Figure 13.

A general theory, assuming target sputtering to be the release mechanism has been proposed by Carter, Colligon and LeckS8 and applied by Colligon to explain, with some success, saturation effects of 2 keV A + and Kr + ions in tungsten. When applied to the low energy data of Kornelsen19, 29, 32 and the high energy data of Almen and Bruce 38 and Brown and Davies30, however, the theory meets with less success. According to this theory saturation is anticipated when the target has been eroded to a depth equal to the maximum ion penetration depth where the gas exposure rate due to sputtering just equals the trapping rate, but values for these maximum ranges calculated in this way are much smaller than known recorded values34. This and other discrepancies are undoubtedly due in part to inaccuracies in the simple assumptions of the theory. For example, no account is taken of possible diffusion of trapped gas (viz Almen and Bruce 24 data), channelling or knock-on processes (for which there is evidence in the work of 487

WA GrantandG Carter: Ion trapping and gas release phenomena Brown and Davies) 30, whilst the trapping probability is assumed to be independent of the quantity trapped. This latter assumption is unlikely to be true since the stopping power of the target is likely to change as the damage to it increases. Since it appears that a target sputtering mechanism is unacceptable in some cases we should enquire as to possible gas sputtering processes. It is known from Radiation Damage theory59, that an ion of energy E ejects approximately E/2JEd lattice atoms of the same mass as the ion from their normal positions via collisions in which kinetic energy is transferred. (Ed is a dynamic binding energy of the order of 25 eV.) If it is assumed that energy is partitioned to trapped gas in a similar way, one would expect gas sputtering coefficients, in the region of 5 for 250 eV ions. This is higher than some of the values summarised in Table 2 but lower than others. However, the value E/2Ed is well known, in damage theory, to considerably over-estimate the observed damage (because of various crystal symmetry effects and simultaneous damage annealing) and is certainly an over-estimate in our present case since the trapped gas, ion and target species may differ widely in mass and therefore reduce energy transfer efficiencies. A further mechanism known in radiation damage is the production of thermal spikes60, in which an ion gives up a large amount of energy in a small volume and there is a rapid and large temperature increase in this volume. Such a process is believed to account for the observation by Nelson6t of the rapid increase with temperature of the target sputtering coefficients of some metals. This process should be particularly effective in a material such as glass where the heat generated in the thermal spike cannot be rapidly conducted from the spike. In these spikes, it is probable that thermally activated processes such as gas migration and release can occur efficiently (ie 25 eV of ion energy partitioned in 1 eV packets can affect 25 trapped atoms and ensure their release) and thus effective gas sputtering coefficients can be large. One might equally well expect, on this basis, high values for the target sputtering coefficient of glass, which in fact is well known to be small. It is probable that insufficient energy is imparted to the glass atoms to ensure their evaporation (2-3 eV) but enough (1-1.5 eV) to ensure gas migration. This is reflected in the observations of James and Carter48, that it is the lower part of the release energy spectrum which is preferentially denuded by ion induced re-emission. It is notable also that Navez and Sella62 have reported a pit formation (observed microscopically) on ion bombarded glass which is very reminiscent of thermal spike production. In the case of metals the enhanced thermal diffusivity may result in lower spike temperatures and thus, as observed, lower gas sputtering efficiencies. Further work, at different temperatures, ion energies and target materials is evidently desirable on this topic. The location of trapped gas and the effect of target structure The target structure, as well as the ion mass, will decide the configurations in which the foreign atoms reside after entering the target, whilst re-orientation of the target lattice during desorption experiments will influence these configurations and hence the movement of gas prior to its release. Rimmer and Cottrel163 have considered the nature of the solution of inert gases in copper and have shown that the type of trapping site occupied depends on the gas species and the structure and state 488

of the target. For example, Ar, Kr and Xe should always dissolve substitutionally whilst He and Ne should dissolve substitutionally only if vacancies are available but interstitially in their absence. However, the desorption of inert gases from tungsten (Kornelsen) and from glass (Carter, etc) shows only slight variation with gas species indicating that the target structure dictates the gas location. In the experiments considered, this is to be expected since the structural definition of the targets were poor and contained grain boundaries, dislocations, cracks, etc, as well as surface irregularities, all of which are possible sites for gas trapping. In addition the damage introduced by the bombarding particles will provide various trapping sites for various configurations and these can readily account for the ranges of activation energies required for gas release, and such an explanation has been invoked by James and Carter48a,b, c, Kelly and Brown 49 and Auskern50. The effect of the gas species is not completely absent as can be seen from the data of Kornelsen for the desorption of inert gases from tungsten, where there is a slight variation in the location of peaks in the desorption transients according to the ion species trapped. This is to be expected from an homogenous type of target material where atomic diffusion will be somewhat characterized by the nature of the diffusing species. For the case of glass, which has a random open structure however, the ion mass would be expected to be of lesser importance and this is supported by the results of James and Carter 48 which indicated only a small dependence of the desorption transients on the ion species. The location of trapped gas is thus largely determined by the state (either naturally occurring or induced by irradiation), and type of the target material, and the corresponding activation energies for desorption may have a subsequent wide variation. In the work of Jech 52, peaks in the desorption spectra were found to be associated with activation energies which are similar to those for the annealing of radiation damage and this would indicate that gas trapping and target damage are often adjunct. In the release of Kr from Ur as reported by Tucker and Norton64 the desorption rate is dependent on the rate of transition from the ~ to the/~ phases of the target and this also indicates a release mechanism dependent on lattice re-orientation. The desorption transients reported by Kornelsen exhibit distinct, well-defined peaks which appear to be characteristic of the target rather than the trapped gas. This indicates that the target has preferred trapping configurations, either intrinsically due to normal lattice structure, or extrinsically due to radiation induced damage. As well as the normal substitutional and interstitial states there is evidence of a further trapping configuration. Barnes and Mazey65 have observed, by electron miscroscopic examination, that during irradiation by 38 keV ~ particles of various metal targets at 350°C, small vacancy clusters formed. On heating, these grew and coalesced, trapping helium atoms to form helium bubbles (-~ 40A ° radius). Nelson61 also observed the formation of these bubbles during irradiation of various targets with 60 keV inert gas ions, the bubble radius varying from 20°A at 20°C to about 140°A at 500°C. Bubble formation has been observed at lower energies (-'~4 keV) by Jouffrey 66, and by Ogilvie et a167 and others 68 who again noticed that the bubble diameter increased with the target temperature. Barnes and Mazey65 have also reported the migration of bubbles at approximately 103 A°/sec when an Au target was pulse-heated, the velocity depending on the bubble radius. Bubbles were also

W A Grant and G Carter: Ion trapping and gas release phenomena o b s e r v e d to coalesce a n d to burst at the target surface. T h e strain energy associated with a t r a p p e d a t o m / v a c a n c y aggregate is less t h a n t h a t for s u b s t i t u t i o n a l o r interstitial dissolution a n d since, in irradiated targets, m a n y vacancies will be produced, a g g l o m e r a t i o n of t r a p p e d a t o m s a n d v a c a n cies is to be a n t i c i p a t e d as a t r a p p i n g m e c h a n i s m . A s the target t e m p e r a t u r e is raised, vacancies will be g e n e r a t e d at a n increasing rate a n d the s u b s e q u e n t diffusion of these will result in f u r t h e r b u b b l e precipitation a n d enlargement. These general tendencies have been observed by B a r n e s a n d his c o l l a b o r a t o r s 65 A t low target t e m p e r a t u r e s the b u b b l e radii are small ( ~ 2 0 A °) a n d occur in such regions as grain b o u n d a r i e s a n d dislocation l o o p s where vacancies already exist. O n b e a t i n g the target, vacancies are p r o d u c e d in regions such as the free surface a n d grain b o u n d a r i e s , a n d m i g r a t e to a g g l o m e r a t e with t r a p p e d gas to precipitate bubbles. T h e n u m b e r of t h e r m a l l y p r o d u c e d vacancies increases with target t e m p e r a t u r e so t h a t the size of t h e b u b b l e s a n d the rate of precipitation also increases. T h e s u b s e q u e n t m i g r a t i o n o f these bubbles u n d e r a driving force such as a t e m p e r a t u r e gradient results in release o f bursts of gas at the target surface. It is p r o b a b l e t h a t n o t all t r a p p e d gas resides in such clusters, a n d indeed a prerequisite o f this f o r m a t i o n m a y b e h i g h ion dose rates a n d energy to ensure nucleation, a n d a h i g h target t e m p e r a t u r e to e n s u r e m i g r a t i o n a n d coalescence. M u c h t r a p p e d gas will be dissolved as single a t o m s t r a p p e d substitutionally or interstitially a n d b u b b l e s o f various sizes, t o g e t h e r with regions o f d a m a g e d target. T h e a n n e a l i n g o u t of this gas will therefore be complex, a n d indeed Kelly a n d Ruedl6~ h a v e o b t a i n e d direct visual i n f o r m a t i o n with a n electron microscope s h o w i n g gas release via emission f r o m d a m a g e d regions (with the distributed activation energies expected for such a process), b u b b l e g r o w t h a n d emission, a n d release processes w h i c h were g o v e r n e d by p h a s e changes, dislocation m i g r a t i o n , a n d surface c o n t a m i n a n t effects. Kelly49b suggests t h a t gas release m a y often b e characterised by three stages if oxide dissolution or p h a s e changes are u n i m p o r t a n t . I n the first stage, gas is released f r o m d a m a g e d regions close to the surface with m i g r a t i o n energies d e t e r m i n e d by the d a m a g e configurations or surface p r o x i m i t y considerations. This release process will, as o b s e r v e d in m a n y o f the e x p e r i m e n t s we h a v e discussed, be d o m i n a n t a t low particle energies a n d involve energies o f vacancy m i g r a t i o n c o n d i t i o n e d by the surface proximity. A t h i g h e r b o m b a r d i n g energies a n d deep ion penetrations, gas release will result after a n o r m a l diffusional m i g r a t i o n t h r o u g h the target a n d will generally require a higher t e m p e r a t u r e r a n g e for operation, ie stage 1I. A t large ion doses, t r a p p e d a t o m s m a y be able to capture vacancies a n d coalesce to f o r m b u b b l e s before r e a c h i n g the surface d u r i n g stage II m i g r a t i o n a n d the gas is finally released due to stage III b u b b l e migration. T h e relative i m p o r t a n c e of e a c h process will d e p e n d u p o n the ion energy a n d type a n d the target m a t e r i a l a n d t e m p e r a t u r e .

Conclusions It is a b u n d a n t l y clear t h a t whilst a c o n s i d e r a b l e literature h a s a c c u m u l a t e d o n the m a c r o s c o p i c effects associated w i t h ion t r a p p i n g a n d gas release, only a vague picture of t h e physical p h e n o m e n a associated w i t h these processes is available. Progress h a s been m a d e in u n d e r s t a n d i n g the m e c h a n i s m o f ion p e n e t r a t i o n into a n d t h r o u g h a solid, b u t the fate o f the t r a p p e d a t o m s is still largely unresolved. I n the present review we h a v e

speculated o n the p r o b a b l e , a n d in some cases the observed, m i c r o s c o p i c n a t u r e o f trapping, a n d s h o w n h o w these speculations c a n lead to microscopic e x p l a n a t i o n s of gas release p h e n o mena. E x p e r i m e n t s c o n d u c t e d o n the a t o m i c scale are urgently required to clarify these concepts, and, w i t h the increasing s o p h i s t i c a t i o n of techniques such as electron t r a n s m i s s i o n m i c r o s c o p y a n d field ion microscopy, in w h i c h events o n the a t o m i c scale are directly observable, there a p p e a r s to be every r e a s o n to expect a n early i m p r o v e m e n t of o u r knowledge.

References I H D Hagstrum, Phys Rev, 104, 1956, 672. 2 j Plficker, Pogg Ann, 105, 1858, 84. 3 R T Bayard and D Alpert, Rev Scient lnstrum, 21, 1950, 571. 4 I~ Alpert, Handbuch derPhysik, 12, 1958, 609. 5 G Carter, Vacuum, 9, 1959, 190. 0 (a) G Strotzer, Zeitschrift fiir Angewandte Physik, 10, 1958, 27; (b) G Strotzer, Zeitschriftf~r Angewandt Physik, 11, 1959, 223. 7 I Langmuir, J A m e r Chem Soc, 1913, 35, 93. 8 T W Hickmott, JApplPhys, 31, 1960, 128. 9 E E Donaldson, M F Winters and D E Horne, Conference on Adsorption Properties of Evaporated Metal Films, University o f Liverpool, 1963, (Unpublished). J0 G Carter, L H James and J H Leek, Vacuum, 12, 1962, 213. 11 R Jaeckel and E Teloy, Trans 8 A V S Nat Vac Symp, 1961, Vol 1, Pergamon Press (Oxford), p 406. 12 G Comsa and G Musa, Jscient lnstrum, 31, 1952, 291. 13 J Varnerin and J H Carmichael, JApplPhys, 26, 1955, 782. 14 E A Brown and J H Leek, Brit JApplPhys, 6, 1955, 161. 15 G Carter and J H Leck, Brit JApplPhys, 10, 1959, 364. 16 B Cobic, G Carter and J H Leek, Vacuum, 11, 1961, 247. 17 L H James and G Carter, JElec & Control, 12, 1962, 63. Is G Comsa and G Musa, Stud Cercet de Fizica, 8 (2), 1957, 119. 19 L V Kornelsen, Trans 8 A VS Nat Vac Syrup, 1961, Pergamon Press (Oxford), 281. 20 J S Colligon and J H Leek, Trans 8 A VS Nat Vac Symp, 1961, Pergamon Press (Oxford), p 275. 21 R B Burtt, J S Colligon and J H Leek, Brit J A p p l P h y s , 12, 1961, 396. 22 D P Corkhill and G Carter, Proc Conf on Electromagnetic Isotope Separators, Related lon Accelerators and their Application to Physics, Aarhus, Denmark, 1965, (to be published). 23 H Maddix and M A Allen, Trans 10 A VS Nat Vac Symp, 1963, Pergamon Press (Oxford), p 197. 24 R Kelly, Can J Chem, 39, 1961, 664. 25 (a) W von Meyern, Zeitschrift fiir Angewandte Physik, 84, 1933, 531; (b) W yon Meyern, Zeitschrift fiir Angewandt Physik, 91, 1934, 727. 26 J R Young, J Appl Phys, 27, 1956, 926. 27 B Cobic, G Carter and J H Leck, Brit JApplPhys, 12, 1961, 288. 28 L J Varneriu and J H Carmichael, J ApplPhys, 28, 1957, 913. 29 E W Kornelsen, Vakuum Technik, 13, 1964, 6. 30 F Brown and J A Davies, Can JPhys, 41, 1963, 844. 31 O Almen and G Bruce, J Nucl Inst and Methods, 11, 1961, 25~. 32 E V Kornelsen, Can J Phys, 42, 1964, 364. 33 J H Carmichael and F A Trendelenburg, J Appl Phys, 29, 1958, 1570. 34 (a) J A Davies, J D Mclntyre, R L Cushing and M Lounsbury, Can J Chem, 38, 1960, 1535; (b) M McCargo, F Brown and J A Davies, Can J Chem, 41, 1963, 2309; (c)J A Davies, B Domeij and J Uhler, Arkiv f Fysik, 24, 1963, 377; (d) M McCargo, F Brown and J A Davies, Can J Phys, 41, 1963, 1231; (e) J A Davies, F Brown and M McCargo, Can ar Phys, 41, 1963, 829; (f) B Domeij, F Brown, J A Davies and M McCargo, Can J P h y s , 42, 1964, 1624; (g) B Domeij, I Bergstrom, J A Davies and J Uhler, A r k i v f F y s i k , 24, 1963, 399; (h)R L Graham, F Brown, J A Davies and J P S Pringle, Can JPhys, 41, 1963, 1686. 35 j Lindhard, M Scharff and H E Schiott, Dan Vid Selsk Mat Fys Medd, 33, 1963, 3. 36 G Leibfried, J ApplPhys, 33, 1962, 1933. 37 (a) G R Piercy, M McCargo, F Brown and J A Davies, Can J Phys, 42, 1964, 1116; (b) D A Channing and J L Whitton, Phys Letters, 13, 1964, 27; (c) J A Davies, G C Ball, F Brown and B Domeij, Can J P h y s , 42, 1964, 1070; (d) E V Kornelsen, F Brown, J A Davies, B Domeij and G R Piercy, Phys Rev, 136, 1964, A849. 38 (a) M T Robinson and O S Oen, Phys Rev, 132, 1963, 2385; (b) C Lehmann and G Leibfried, J Appl Phys, 34, 1963, 2821. 39 N W Robinson and F Berz, Vacuum, 9, 1959, 48. 40 F A Baker and T A Giorgi, Brit J Appl Phys, 11, 1960, 433. 41 K B Blodgett and T A Vanderslice, Trans 8 A VS Nat Vac Symp, 1961, Pergamon Press (Oxford), 1962, p 26. 42 G P Smeaton, G Carter and J H Leek, Trans 9 A VS Nat Vac Syrup, 1962, Pergamon Press (Oxford), 1963, p 491. 43 R E Fox a n d T S Knoll, Trans 7 A VS Nat Vac Syrup, 1960, Pergamon

Press (Oxford), 1961, p 364.

489

W A Grant and G Carter: Ion trapping and gas release phenomena 44 G P S m e a t o n , G C a r t e r a n d J H L e c k , Suppl al Nuovo Cimento, Ser l,

Vol 1, 1963, 548. 45 A C a v a l e r u , G C o m s a a n d B iosifescu, Brit J Appl Phys, 15, 1964, 161. 46 R Kelly, Acta, Met, 12, 1964, 123. 47 G C a r t e r a n d J H Leck, Proc Roy Soc, A261, 1961, 303. 48 (a) L H J a m e s a n d G C a r t e r , Brit J Appl Phys, 13, 1962, 3; (b) L H J a m e s a n d G C a r t e r , Brit J Appl Phys, 14, 1963, 614; (c) L H J a m e s a n d G C a r t e r , Brit J ApplPhys, 14, 1963, 147; (d) L H J a m e s , J H L e c k a n d G C a r t e r , Brit J AppIPhys, 15, 1964, 681. 49 (a) R Kelly a n d F B r o w n , Acta, Met, 13, 1965, 169; (b) R Kelly a n d H J M a t z k e , J Nucl Materials, 1965 (to be published) ; (c) F B r o w n a n d H J M a t z k e , Conference on Electromagnetic Isotope Separators, Related

Ion Accelerators and Applications to Physics, Aarhus, Denmark, 1965. 5o A A u s k e r n , J Amer Ceram Soc, 47, 1964, 390. 51 D L M o r r i s o n , T S E l l e r m a n a n d D N S u n d e r m a n , J Appl Phys, 35,

1964, 1616. 52 (a) C Jech, Int .I of Appl Rad & Isotopes, 8, 1960, 179; (b) C Jech, Phys Stat Solidi, 2, 1962, 1299; (c) C Jech, Phys Stat Solidi, 4, 1964, 499. 53 G C a r t e r , PhD Thesis, 1959, University of Liverpool. 54 G P S m e a t o n , G C a r t e r a n d J H Leck, Brit J Appl Phys, 15, 1963, 205. 5s j p H o b s o n a n d T E d m o n d s , Can JPhys, 41, 1963, 827. 56 H S c h w a r z , Zeitschrifi fiir Angewandte Physik, 117, 1940, 23. 57 J H C a r m i c h a e l a n d P M W a t e r s , J ApplPhys, 33, 1962, 1470. 58 G C a r t e r , J S Colligon a n d J H Leck, Proc Phys Soc, 79, 1962, 299. 59 D S Billington a n d J H C r a w f o r d , Radiation Damage in Solids, Princeton Univ. Press (Princeton). 1961. 6o F Seitz a n d J S K o e h l e r , Progress in Solid State, Phys II, Academic Press (New York), 1956, p 305. 6t R S Nelson, Phil Mag, 8, 1963, 643. 62 M Navez, C Sella a n d D C h a p e r o t , Le Bombardement lonique CNRS, Bellevue, Paris, 1962, p 233. 63 D E R i m m e r a n d A H Cottrell, Phil Mag, 2, 1957, 1345. 64 C W T u c k e r a n d F J N o r t o n , J Nucl Materials, 2, 1960, 329. 65 R S B a r n e s a n d D J Mazey, Proc Roy Soc, 275, 1963, 47. 66 B Jouffrey, J de Microscopie, 2, 1963, 45. 67 G J Ogilvie a n d J B Sanders, A A T h o m s o n , J Phys Chem Solids, 24, 1963, 247. 68 R Kelly a n d E Ruedl, The 3rd European Conf on Elect Microscopy, Prague, 1964. 69 J H C a r m i c h a e l a n d J S Knoll, Trans 5 A VS Nat Vac Syrup, 1958, Pergamon Press (Oxford), 1961, p 18. 70 R O J e n k i n s a n d W G T r o d d e n , Vacuum, 10, 1960, 319.

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