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Some mechanisms of sputtered negative ion production
Vacuum/volume34/numbers 1-2/pages 153 to 156/1984 Printed in
0042-207X/84S3.00+ .00 Pergamon Press Ltd
Great Britain
Some mechanisms of sputtered negative ion production M C Underwood*,
Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford, UK
Sputtered negative ion spectra from a carbon target under 12 keV, H ; and D ; bombardment were investigated at 300 K and elevated temperature. A t 300 K the absolute yield of ' =C- was measured and the ionization probability determined. The ionization probability is shown to be in good agreement with that predicted from the calculated survival probability of an affinity level in an atom leaving the surface. The yield of ion clusters, 1=C; and 1=C; was determined and are consistent with the existence of clusters as continguous atoms on the surface prior to sputtering. A cluster formation process where by sputtered atoms re-combine away from the target surface is shown to be highly improbable. A t elevated temperature the yield of I=C- and ~aC; increased and this is attributed to two mechanisms, i.e. a changing surface adsorbate coverage and changes in surface topography. Both of these processes can result in a surface work function change and so influence ion yield.
Introduction Some fraction of the sputtered particles from a surface under ion bombardment are charged and are generally referred to as secondary ions. Secondary ion production has been extensively reviewed recently, but a number of aspects remain poorly investigated or unclear ~-3. Positive ion emission has been studied more thoroughly than negative ion emission. The temperature dependence of ionic yield is almost completely uninvestigated for negative ion yield 4 and incompletely studied for positive ions 5-9. The production mechanism(s) of negative ion clusters e.g. a2C~ is also unclear. In this paper the absolute yield of ~2C- has been measured, sputtered negative ion fraction determined and compared with calculation. The yield of negative ion clusters has also been measured and the formation mechanism is discussed. Finally, the yield of I ' C - , and a2C2 as a function of changing target temperature has been measured and the results considered in terms of work function and surface topography changes. The results presented here give some insights into the formation processes involved in sputtered negative ion production.
magnet of bending angle 48 °. A carbon target (known commercially as PAPYEX, Le Carbonne, Portslade, Sussex), as received from the manufacturer, was bombarded with 12 keV H i and D~ ions. For similar bombardment conditions and dose of 2 x I0 Is ions cm -2 the spectra obtained are shown in Figures 1 and 2. The negative ions produced may conveniently be classified into three categories: (1) ions characteristic of the substrate 1~Cand 1~C~; (2) hydrogen isotope bearing molecular ions e.g. l aCD- and (3) impurity ions e.g. ~9F-. The influence of hydrogen isotope concentration on ~ ' C D - and m'CH- yield t~ and lattice saturation processes by the incident ion beam ~' are discussed elsewhere. Only substrate ion yields are considered here.
;IZC2H2,'2CNYieLd (normaLized to =2C- yield ) t2c -
Experimenta] apparatus and results The apparatus has been described elsewhere 1°. Briefly, a mass analysed primary beam bombarded the target in a chamber at a pressure of 7.0x 10 -s torT, the target temperature capable of being varied in the range 77-800 K. Secondary ions are extracted by an electrostatic lens and mass analysed using a double focusing * Present address: Applied Physics Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex, UK.
~2CH-
,zC~
Figure 1. Negative ion spectrum from carbon under 12 keV, H~ bombardment at 300 K, Dose = 2 x l0 ts ions cm- '. 153
M C Underwood: Mechanisms of sputtered negative ion production Table 1. Absolute charged particle fraction
Sputtered ion
Incident ion
Sputtering yield (atoms/ion)
'zC-
12.5 keV H~
5.0x 10-~
I I;'C~H;-, '~C2D', +2CN-
,60 -
Yield {normaLized t.o '2C- yield )
Incident ion current (pA)
Corrected secondary ion current (pA)
Proportion of sputtered atoms negatively charged
1.4+0.1
1.9:f0.2 x 10~
2.7+0.3 x 10-~
The yield of 12C- and l ' C ~ was measured in the temperature range 77-800 K under 12 keV, D~" bombardment. The heating and cooling cycle is summarized in Figure 3 and the yields of 1 ' C and 1"C~ shown in Figures 4 and 5 respectively, the data being
e-- -- - q
800
I
J'~C-
7o0
mCi
/ 600
'60~,
S-
l
o
X
/
~400
H"
t
I
®
Jl
X
/
X
I
j
I
~
CD-
300e-
.t
I
200
I I
0
Table 2. Carbon cluster yield normalized to ~aC- yield for 12 keV, H~ and D~ bombardment at 300 K
Incident ion H~" D~ 154
Sputtered ion yield normalized to monomer intensity ,zC-
'2CI
1 1
0.48 0:83
'2C~ 0.017+0.006 0.04+0.01
,t /
/
/
IO0
Figure 2. Negative ion spectrum from carbon under 12keV, D~ bombardment at 300 K, Dose--2 x l0 ts ions cm-2.
The total efficiency of secondary ion collection and transmission, under the experimental conditions used here, was measured as 4.5% 1°. The ion current corresponding to 12Cintensity was measured using a suppressed Faraday cup and calibrated pico-ammeter. Using the sputtering yield data of Robertol 3 the sputtered ion fraction, i.e. the fraction of sputtered carbon atoms negatively charged, P~, can be calculated. This information is summarized in Table 1. The dimer/ion ratio i . e J 2 C ~ / 1 2 C - = 0.48 under H~" and 0.83 under D~ bombardment at 300 K. The dimer yield is therefore enhanced under D~" bombardment compared with H~" bombardment. Although the relative intensities of ~2C- and 12C~" changed with bombardment time initially, after an integrated dose of 2.0 x 10 is ions em -2 the dimer/ion ratio remained constant, and the differences under H I and D~" bombardment were a highly reproducible feature of the data- The carbon trimer i.e. lzC~ was less intense, and the relative intensities of ~2C~ and 12C~, compared with 1~C- under H~" and D~" bombardment are shown in Table 2.
/
I ! ~ i ,I t IO0 200 300 400 500600 Time (rains)
Figure 3. Target heating and cooling cycle under 12 keV, D; bombardment.
rl O D
1.0
o
oo
[] []
0
0
0
•8o o o°O° 0
Start
"o
O
°
05 o
E
o
t
ooo
o Z
D Decreasing temperature o Increasing temperature
0
L
I
I
t
J
I
1(30 200 300 400 500 600 Torget temperature (K)
I
700
I
800
Figure 4. ~2C- yield variation with changing target temperature under 12 keV, D~ bombardment. Data normalized to maximum value. normalized to the maximum value obtained. Monitoring of the chamber residual gas indicated a decrease in mass 18 (H~O) partial pressure as the target temperature decreased and a rise in deutero-methane partial pressure as the target was heated. Discussion
It is probably the case that no single mechanism underlies secondary ion production, although under certain circumstances
M C Underwood." Mechanisms of sputtered negative ion production
1.0 "0
oO0 o
_¢ St.l't o
) 0
0 0 0000
or3 0 0
% O0
nO000000
0
12,> I~'2.+2) and
05
E
0
Z
Oo
O
I
° °o
oOB~ ° End
~o
IOO
I
2OO
I
30O
I~2,+l)> I~2,+3~ o Decreosingtemperature o Increosing temperoture I
4OO
I
5OO
I
6O0
I
70O
I
8OO
Torget temperoture (K) Figure 5. IzCf yield variation with changing target temperature under 12 keV, D~ bombardment. Data normalized to maximum value.
one ion formation process might dominate the creation process 1-3. The physical basis of some models of secondary ion creation have been criticised and it is essential that the description of the formation process be physically sound. The agreement between model predictions and experimental data is not a sufficient condition for model validity. In this work the model of Nzrskov and Lundquist ~4 is adopted as they give a physically well founded description of the secondary ion creation process in terms of the survival probability of the initial ionisation, or affinity level, on a non-adiabatic passage from the surface. The formation probability for a negative ion, ,,- is given by _
neighbouring sites before ejection ~. Under caesium bombardment a regular sequence of carbon polymer peaks is often observed 15 and the sequence of clusters is usually described by,
2 [(e_,,c,,¢_A)/h=,.)(e_~,,o=~)] /t
where 0 is the work function (eV) A the election affinity (eV) C l ~ 0.024 C2 ~ 0.034 7 =2.3 A - I and V is the ion velocity normal to the target surface. Ifwe assume that the outgoing particle velocity normal to the target surface is given by the most probable ion velocity then V= 7.6 x 10 ~ m secfor 12C- ~5, giving =- =2.2 x 10 -2. The measurement of the fraction of sputtered atoms negatively charged, P~, may be considered to be a 'total' ionization probability which is an average of a - over some broad energy range. The agreement between the measured negatively charged panicle fraction, P~ = 2.7 +_0.3 x I 0 - 2 and the calculated value of ~,- indicates that the energy distribution of sputtered ~ C - ions is probably small. This is supported by the work of Doucas ~5.
Cluster emission
The emission of ion clusters has been subject to various interpretations. Ejected atoms can interact with each other in the near surface region to form a cluster by a recombination process 16. The atoms comprising the cluster may originate from the same collision cascade, but do not necessarily have to arise from contiguous sites on the surface. However, in the absence of long range ionic forces calculations indicate that most of them originate from a circular region of radius 5 A ~. Clusters may also be directly emitted, i.e. the constituents of a cluster were located at
where I - is the negative ion current, and n is an integer. F o r the case o f A r + bombardment of graphite, the observed intensities of successive ion clusters obey the following inequality 's
I~.> I~2.± i). However, under H I and D~" bombardment the intensity of 12C~ and 12Ci polymers falls offrapidly with increasing cluster size, i.e. I - > I ~ >>I~. F o r the incident beam currents used in these measurements of ~ 100/~A cm -~, i.e. 6 x 10 -2 ions A -2 sec- ~ the sputtered atom flux is -,-6x10 - ( a t o m s A -2 s e c - ' (assuming 12 keV, D~" bombardment). If we assume that the interaction of two sputtered atoms can occur over a range of 10 A for a distance of 1000 A from the surface, then for a 5 eV sputtered carbon atom the time to traverse 1000 A is 3 x 10-~2 sec. So, the chances of two particles from the same region of the target being in this volume at the same time is about 2 x 10-13. This simple calculation indicates that the recombination ofsputtered atoms to form a cluster appears to be unlikely. However, it is interesting to note that the maximum energy transfer, TMAX,tO a C 2 cluster by collision with a 12 keV, H~" ion is 3.36 keV and 5.88 keV for a 12 keV, D~" ion. The ratio TMAX (D~" bombardment)/TM^ x (]-I~" bombardment)= 1.75 and from Table 2 the ratio of normalized C~ yield under D~" and H~ bombardment is 1.75. F o r the case of a C 3 cluster the ratio of maximum energy transfer = 1.8 which compares with the ratio of normalized yield 2.8 + 1.2. These observations add weight to the view that clusters exist as contiguous atoms on the carbon surface.
Temperature variation effects The measurements of 12C- and 12C~ yields indicate that the production of these ions is dependent upon, (i) the instantaneous temperature, (ii) the past pattern of temperature rise, (iii) the time spent at a particular temperature. The target temperature influences the surface adsorbate coverage as the sticking probability and dwell time are both temperature dependent. As the target was cooled from 300-77 K water vapour molecules condensed on the target surface, as indicated by a decrease in mass 18 in the residual gas spectrum. The precise ways in which adsorbate coverage influences ion yield are not fully understood and a number of mechanisms may be at work. However, for the purposes of the following discussion it will be assumed that adsorbed atoms perturb the local work function of the surface around each adsorbed atom, and a change in coverage causes a change in the effective work function of the whole surface. Assuming a sticking coefficient of 1, an upper limit of coverage by H 2 0 molecules is 1 x 1015 molecules cm -2 McCracken ~9 has shown that dissociative desorption dominates the release of adsorbed H 2 0 under ion bombardment with a release rate of ~ 125 H 2 0 molecules/incident ion for 6 keV, D + bombardment. So, the coverage, 0, of the target bombarded by the beam is small (the mean residence time of adsorbed H 2 0 155
M C Underwood: Mechanisms of sputtered negative ion production
molecules being < 1 0 - s see). If it is assumed that the small coverage by H 2 0 molecules locally increases the surface work function, cooling of the target would give rise to decreases in the yield of substrate ions, e.g. 12C- and ~2C~ (as observed). As the target temperature was increased from 77 K the yield of these substrate ions increased and this is assumed to be due to decreasing l-I20 coverage, giving rise to an increasing work function. This decrease in the coverage in the area bombarded by the beam can plausibly be explained by the temperature dependence of the sticking coefficient. At low coverage the sticking probability decreases as the temperature of the surface is increased, but data on the variation of the sticking coefficient of H 2 0 molecules on a carbon surface, with increasing temperature, are not known to this author. The yields of 1"C- and ~2C~" decreased approximately linearly with decreasing temperature below ,,, 800 K, the gradient of the curve decreasing for temperatures below ~400 K. This observation is consistent with changing adsorbate coverage with temperature. At small adsorbate coverage (0,~ 1) every adsorbed atom influences those substrate atoms localized around it, and the average distance between adsorbed atoms is large compared with the diameter of every influenced area. However, at higher coverage (as the target was cooled) all substrate atoms are influenced by adsorbate atoms and the secondary ion current should become relatively independent of coverage. At elevated temperature, above 750 K (for the increasing temperature cycle) 12C- and 12C~ yields increased, perhaps suggesting the operation of another process influencing ion yield. The erosion rate of carbon atoms is increased dramatically at these temperatures due to CD4 production and rapid changes in surface topography would be expected to occur, perhaps giving rise to a decrease in effective work function. This is speculative as the processes involved are complex, but there is evidence from the work of Hofer et al 2° that changes in surface topography influence ion yield. The hysteresis in ~2C- and 12C~ yields with temperature might have been a result of perturbing the dynamic equilibrium between the number of molecules arriving at the surface and the number leaving. An integrated ion close of --, 10 ~s ions cm -2 was required to obtain reproducible spectral yields, implying (for the beam current used here) a time greater than an order of magnitude more than the average time spent at a particular temperature. This suggests that the average time spent at a particular temperature was considerably less than that required to establish thermodynamic equilibrium, resulting in the hysteresis effects in ion yield. Conclusions
The absolute yield of 12C- ions under 12 keV, H~ bombardment has been measured, the negative ion fraction determined and compared with the predicted ionization probability, calculated from the survival probability of an affinity level. Within the constraints of assumptions made, the agreement is good. The
156
yields of t2C~" and 12C~ under 12 keY, H ] and D ] bombardment has been shown to be consistent with the ejection of clusters directly from the surface and the recombination of atoms near the target surface is unlikely. Both t ' C - and t2C~ yield increases with increasing temperature between 300-800 K and decreases for decreasing temperature between 300-77 K. The dynamic change in ion yield between 77-800 K is about a factor of 10. These changes in ion yield are consistent with a work function change due to residual gas (primarily H 2 0 ) adsorption and surface topography changes due to deutero-methane formation at elevated temperature. A hysteresis effect in negative ion yield was evident for the increasing and decreasing temperature cycle and was attributed to the failure to establish a dynamic equilibrium at the target surface during temperature cycling. These results give new insights into sputtered negative ion formation processes.
Acknowledgements The financial support of the Science and Engineering Research Council, and Culham Laboratory is gratefully acknowledged. It is a pleasure to record the interest of Dr G Doucas, Mr H R Mck Hyder and Dr G M McCracken in this work. I would like to thank British Petroleum pie for their financial support during the preparation of this paper.
References
J K Wittmaack, Inelastic Ion Surface Collisions. (Edited by H H Tolk, T C Tully, W Heiland and C W White), p 153. Academic Press, New York (1976). 2 p Williams, SurfSci, 90, 588 (1979). a R B Wright and D M Gruen, J Chem Phys, 72, 147 (1980). " M Ya Fogel, R P Slabospitskii, A B Rastrepin, Soviet Phys-Tech Phys, 5, 58 (1960). R Laurent and G SIodzian, Ion Surface Interaction, Sputtering and Related Phenomena. (Edited by R Behrisch, W Heiland, W Poschenreider, P Staib and H Verbeek), p. 153. Gordon and Breach Ltd, London (1973). 6 R C Bradley, J App Phys, 30, I (1959). 7 R C Bradley and E Reudl, J App Phys, 33, 880 (1962). 8 B A Tsipinylik and V ] Veksler, Vacuum, 29, 155 (1979). 9 M Mohri, S Kato, S Shinado, K Watanabe, T Yamashina, J Nucl Mater, 93 & 94, 692 (1980). ~o M C Underwood, D Phil Oxford University (1982). ,1 M C Underwood, J Nucl Mater, 111 & 112, 584 (1982). 3: M C Underwood, S K Erents and E S Hotston, J Nucl Mater, 93 & 94, 575 (1980). 3a j B Roberto, Prov Workshop on Sputtering caused by Plasma (Neutral Beam) Surface Interaction (1979). Argonne Nat Lab Rep CONF 790775 (1980). 34j K Norskov and B 1 Lundquist, Phys Rec B, 19, 5661 (1979). 3s G Doucas, lnt J Mass Spertrom Ion Phys, 25, 71 (1977). 16 B J Garrison, N Winograd and D E Harrison Jr, Phys Rec B, 18, 6000 (1978). 37 B J Garrison and N Winograd, Science, 216, 805 (1982). ~s M Leleyter and P Joyes, as ref 5, p 185. ~9 G M McCracken, Vacuum, 24, 464 (1974). 2o W O Hofer, H L Bay, P J Martin, J Nucl Mater, 76 & 77, 156 (1978).
0042-207X/84S3.00+ .00 Pergamon Press Ltd
Great Britain
Some mechanisms of sputtered negative ion production M C Underwood*,
Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford, UK
Sputtered negative ion spectra from a carbon target under 12 keV, H ; and D ; bombardment were investigated at 300 K and elevated temperature. A t 300 K the absolute yield of ' =C- was measured and the ionization probability determined. The ionization probability is shown to be in good agreement with that predicted from the calculated survival probability of an affinity level in an atom leaving the surface. The yield of ion clusters, 1=C; and 1=C; was determined and are consistent with the existence of clusters as continguous atoms on the surface prior to sputtering. A cluster formation process where by sputtered atoms re-combine away from the target surface is shown to be highly improbable. A t elevated temperature the yield of I=C- and ~aC; increased and this is attributed to two mechanisms, i.e. a changing surface adsorbate coverage and changes in surface topography. Both of these processes can result in a surface work function change and so influence ion yield.
Introduction Some fraction of the sputtered particles from a surface under ion bombardment are charged and are generally referred to as secondary ions. Secondary ion production has been extensively reviewed recently, but a number of aspects remain poorly investigated or unclear ~-3. Positive ion emission has been studied more thoroughly than negative ion emission. The temperature dependence of ionic yield is almost completely uninvestigated for negative ion yield 4 and incompletely studied for positive ions 5-9. The production mechanism(s) of negative ion clusters e.g. a2C~ is also unclear. In this paper the absolute yield of ~2C- has been measured, sputtered negative ion fraction determined and compared with calculation. The yield of negative ion clusters has also been measured and the formation mechanism is discussed. Finally, the yield of I ' C - , and a2C2 as a function of changing target temperature has been measured and the results considered in terms of work function and surface topography changes. The results presented here give some insights into the formation processes involved in sputtered negative ion production.
magnet of bending angle 48 °. A carbon target (known commercially as PAPYEX, Le Carbonne, Portslade, Sussex), as received from the manufacturer, was bombarded with 12 keV H i and D~ ions. For similar bombardment conditions and dose of 2 x I0 Is ions cm -2 the spectra obtained are shown in Figures 1 and 2. The negative ions produced may conveniently be classified into three categories: (1) ions characteristic of the substrate 1~Cand 1~C~; (2) hydrogen isotope bearing molecular ions e.g. l aCD- and (3) impurity ions e.g. ~9F-. The influence of hydrogen isotope concentration on ~ ' C D - and m'CH- yield t~ and lattice saturation processes by the incident ion beam ~' are discussed elsewhere. Only substrate ion yields are considered here.
;IZC2H2,'2CNYieLd (normaLized to =2C- yield ) t2c -
Experimenta] apparatus and results The apparatus has been described elsewhere 1°. Briefly, a mass analysed primary beam bombarded the target in a chamber at a pressure of 7.0x 10 -s torT, the target temperature capable of being varied in the range 77-800 K. Secondary ions are extracted by an electrostatic lens and mass analysed using a double focusing * Present address: Applied Physics Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex, UK.
~2CH-
,zC~
Figure 1. Negative ion spectrum from carbon under 12 keV, H~ bombardment at 300 K, Dose = 2 x l0 ts ions cm- '. 153
M C Underwood: Mechanisms of sputtered negative ion production Table 1. Absolute charged particle fraction
Sputtered ion
Incident ion
Sputtering yield (atoms/ion)
'zC-
12.5 keV H~
5.0x 10-~
I I;'C~H;-, '~C2D', +2CN-
,60 -
Yield {normaLized t.o '2C- yield )
Incident ion current (pA)
Corrected secondary ion current (pA)
Proportion of sputtered atoms negatively charged
1.4+0.1
1.9:f0.2 x 10~
2.7+0.3 x 10-~
The yield of 12C- and l ' C ~ was measured in the temperature range 77-800 K under 12 keV, D~" bombardment. The heating and cooling cycle is summarized in Figure 3 and the yields of 1 ' C and 1"C~ shown in Figures 4 and 5 respectively, the data being
e-- -- - q
800
I
J'~C-
7o0
mCi
/ 600
'60~,
S-
l
o
X
/
~400
H"
t
I
®
Jl
X
/
X
I
j
I
~
CD-
300e-
.t
I
200
I I
0
Table 2. Carbon cluster yield normalized to ~aC- yield for 12 keV, H~ and D~ bombardment at 300 K
Incident ion H~" D~ 154
Sputtered ion yield normalized to monomer intensity ,zC-
'2CI
1 1
0.48 0:83
'2C~ 0.017+0.006 0.04+0.01
,t /
/
/
IO0
Figure 2. Negative ion spectrum from carbon under 12keV, D~ bombardment at 300 K, Dose--2 x l0 ts ions cm-2.
The total efficiency of secondary ion collection and transmission, under the experimental conditions used here, was measured as 4.5% 1°. The ion current corresponding to 12Cintensity was measured using a suppressed Faraday cup and calibrated pico-ammeter. Using the sputtering yield data of Robertol 3 the sputtered ion fraction, i.e. the fraction of sputtered carbon atoms negatively charged, P~, can be calculated. This information is summarized in Table 1. The dimer/ion ratio i . e J 2 C ~ / 1 2 C - = 0.48 under H~" and 0.83 under D~ bombardment at 300 K. The dimer yield is therefore enhanced under D~" bombardment compared with H~" bombardment. Although the relative intensities of ~2C- and 12C~" changed with bombardment time initially, after an integrated dose of 2.0 x 10 is ions em -2 the dimer/ion ratio remained constant, and the differences under H I and D~" bombardment were a highly reproducible feature of the data- The carbon trimer i.e. lzC~ was less intense, and the relative intensities of ~2C~ and 12C~, compared with 1~C- under H~" and D~" bombardment are shown in Table 2.
/
I ! ~ i ,I t IO0 200 300 400 500600 Time (rains)
Figure 3. Target heating and cooling cycle under 12 keV, D; bombardment.
rl O D
1.0
o
oo
[] []
0
0
0
•8o o o°O° 0
Start
"o
O
°
05 o
E
o
t
ooo
o Z
D Decreasing temperature o Increasing temperature
0
L
I
I
t
J
I
1(30 200 300 400 500 600 Torget temperature (K)
I
700
I
800
Figure 4. ~2C- yield variation with changing target temperature under 12 keV, D~ bombardment. Data normalized to maximum value. normalized to the maximum value obtained. Monitoring of the chamber residual gas indicated a decrease in mass 18 (H~O) partial pressure as the target temperature decreased and a rise in deutero-methane partial pressure as the target was heated. Discussion
It is probably the case that no single mechanism underlies secondary ion production, although under certain circumstances
M C Underwood." Mechanisms of sputtered negative ion production
1.0 "0
oO0 o
_¢ St.l't o
) 0
0 0 0000
or3 0 0
% O0
nO000000
0
12,> I~'2.+2) and
05
E
0
Z
Oo
O
I
° °o
oOB~ ° End
~o
IOO
I
2OO
I
30O
I~2,+l)> I~2,+3~ o Decreosingtemperature o Increosing temperoture I
4OO
I
5OO
I
6O0
I
70O
I
8OO
Torget temperoture (K) Figure 5. IzCf yield variation with changing target temperature under 12 keV, D~ bombardment. Data normalized to maximum value.
one ion formation process might dominate the creation process 1-3. The physical basis of some models of secondary ion creation have been criticised and it is essential that the description of the formation process be physically sound. The agreement between model predictions and experimental data is not a sufficient condition for model validity. In this work the model of Nzrskov and Lundquist ~4 is adopted as they give a physically well founded description of the secondary ion creation process in terms of the survival probability of the initial ionisation, or affinity level, on a non-adiabatic passage from the surface. The formation probability for a negative ion, ,,- is given by _
neighbouring sites before ejection ~. Under caesium bombardment a regular sequence of carbon polymer peaks is often observed 15 and the sequence of clusters is usually described by,
2 [(e_,,c,,¢_A)/h=,.)(e_~,,o=~)] /t
where 0 is the work function (eV) A the election affinity (eV) C l ~ 0.024 C2 ~ 0.034 7 =2.3 A - I and V is the ion velocity normal to the target surface. Ifwe assume that the outgoing particle velocity normal to the target surface is given by the most probable ion velocity then V= 7.6 x 10 ~ m secfor 12C- ~5, giving =- =2.2 x 10 -2. The measurement of the fraction of sputtered atoms negatively charged, P~, may be considered to be a 'total' ionization probability which is an average of a - over some broad energy range. The agreement between the measured negatively charged panicle fraction, P~ = 2.7 +_0.3 x I 0 - 2 and the calculated value of ~,- indicates that the energy distribution of sputtered ~ C - ions is probably small. This is supported by the work of Doucas ~5.
Cluster emission
The emission of ion clusters has been subject to various interpretations. Ejected atoms can interact with each other in the near surface region to form a cluster by a recombination process 16. The atoms comprising the cluster may originate from the same collision cascade, but do not necessarily have to arise from contiguous sites on the surface. However, in the absence of long range ionic forces calculations indicate that most of them originate from a circular region of radius 5 A ~. Clusters may also be directly emitted, i.e. the constituents of a cluster were located at
where I - is the negative ion current, and n is an integer. F o r the case o f A r + bombardment of graphite, the observed intensities of successive ion clusters obey the following inequality 's
I~.> I~2.± i). However, under H I and D~" bombardment the intensity of 12C~ and 12Ci polymers falls offrapidly with increasing cluster size, i.e. I - > I ~ >>I~. F o r the incident beam currents used in these measurements of ~ 100/~A cm -~, i.e. 6 x 10 -2 ions A -2 sec- ~ the sputtered atom flux is -,-6x10 - ( a t o m s A -2 s e c - ' (assuming 12 keV, D~" bombardment). If we assume that the interaction of two sputtered atoms can occur over a range of 10 A for a distance of 1000 A from the surface, then for a 5 eV sputtered carbon atom the time to traverse 1000 A is 3 x 10-~2 sec. So, the chances of two particles from the same region of the target being in this volume at the same time is about 2 x 10-13. This simple calculation indicates that the recombination ofsputtered atoms to form a cluster appears to be unlikely. However, it is interesting to note that the maximum energy transfer, TMAX,tO a C 2 cluster by collision with a 12 keV, H~" ion is 3.36 keV and 5.88 keV for a 12 keV, D~" ion. The ratio TMAX (D~" bombardment)/TM^ x (]-I~" bombardment)= 1.75 and from Table 2 the ratio of normalized C~ yield under D~" and H~ bombardment is 1.75. F o r the case of a C 3 cluster the ratio of maximum energy transfer = 1.8 which compares with the ratio of normalized yield 2.8 + 1.2. These observations add weight to the view that clusters exist as contiguous atoms on the carbon surface.
Temperature variation effects The measurements of 12C- and 12C~ yields indicate that the production of these ions is dependent upon, (i) the instantaneous temperature, (ii) the past pattern of temperature rise, (iii) the time spent at a particular temperature. The target temperature influences the surface adsorbate coverage as the sticking probability and dwell time are both temperature dependent. As the target was cooled from 300-77 K water vapour molecules condensed on the target surface, as indicated by a decrease in mass 18 in the residual gas spectrum. The precise ways in which adsorbate coverage influences ion yield are not fully understood and a number of mechanisms may be at work. However, for the purposes of the following discussion it will be assumed that adsorbed atoms perturb the local work function of the surface around each adsorbed atom, and a change in coverage causes a change in the effective work function of the whole surface. Assuming a sticking coefficient of 1, an upper limit of coverage by H 2 0 molecules is 1 x 1015 molecules cm -2 McCracken ~9 has shown that dissociative desorption dominates the release of adsorbed H 2 0 under ion bombardment with a release rate of ~ 125 H 2 0 molecules/incident ion for 6 keV, D + bombardment. So, the coverage, 0, of the target bombarded by the beam is small (the mean residence time of adsorbed H 2 0 155
M C Underwood: Mechanisms of sputtered negative ion production
molecules being < 1 0 - s see). If it is assumed that the small coverage by H 2 0 molecules locally increases the surface work function, cooling of the target would give rise to decreases in the yield of substrate ions, e.g. 12C- and ~2C~ (as observed). As the target temperature was increased from 77 K the yield of these substrate ions increased and this is assumed to be due to decreasing l-I20 coverage, giving rise to an increasing work function. This decrease in the coverage in the area bombarded by the beam can plausibly be explained by the temperature dependence of the sticking coefficient. At low coverage the sticking probability decreases as the temperature of the surface is increased, but data on the variation of the sticking coefficient of H 2 0 molecules on a carbon surface, with increasing temperature, are not known to this author. The yields of 1"C- and ~2C~" decreased approximately linearly with decreasing temperature below ,,, 800 K, the gradient of the curve decreasing for temperatures below ~400 K. This observation is consistent with changing adsorbate coverage with temperature. At small adsorbate coverage (0,~ 1) every adsorbed atom influences those substrate atoms localized around it, and the average distance between adsorbed atoms is large compared with the diameter of every influenced area. However, at higher coverage (as the target was cooled) all substrate atoms are influenced by adsorbate atoms and the secondary ion current should become relatively independent of coverage. At elevated temperature, above 750 K (for the increasing temperature cycle) 12C- and 12C~ yields increased, perhaps suggesting the operation of another process influencing ion yield. The erosion rate of carbon atoms is increased dramatically at these temperatures due to CD4 production and rapid changes in surface topography would be expected to occur, perhaps giving rise to a decrease in effective work function. This is speculative as the processes involved are complex, but there is evidence from the work of Hofer et al 2° that changes in surface topography influence ion yield. The hysteresis in ~2C- and 12C~ yields with temperature might have been a result of perturbing the dynamic equilibrium between the number of molecules arriving at the surface and the number leaving. An integrated ion close of --, 10 ~s ions cm -2 was required to obtain reproducible spectral yields, implying (for the beam current used here) a time greater than an order of magnitude more than the average time spent at a particular temperature. This suggests that the average time spent at a particular temperature was considerably less than that required to establish thermodynamic equilibrium, resulting in the hysteresis effects in ion yield. Conclusions
The absolute yield of 12C- ions under 12 keV, H~ bombardment has been measured, the negative ion fraction determined and compared with the predicted ionization probability, calculated from the survival probability of an affinity level. Within the constraints of assumptions made, the agreement is good. The
156
yields of t2C~" and 12C~ under 12 keY, H ] and D ] bombardment has been shown to be consistent with the ejection of clusters directly from the surface and the recombination of atoms near the target surface is unlikely. Both t ' C - and t2C~ yield increases with increasing temperature between 300-800 K and decreases for decreasing temperature between 300-77 K. The dynamic change in ion yield between 77-800 K is about a factor of 10. These changes in ion yield are consistent with a work function change due to residual gas (primarily H 2 0 ) adsorption and surface topography changes due to deutero-methane formation at elevated temperature. A hysteresis effect in negative ion yield was evident for the increasing and decreasing temperature cycle and was attributed to the failure to establish a dynamic equilibrium at the target surface during temperature cycling. These results give new insights into sputtered negative ion formation processes.
Acknowledgements The financial support of the Science and Engineering Research Council, and Culham Laboratory is gratefully acknowledged. It is a pleasure to record the interest of Dr G Doucas, Mr H R Mck Hyder and Dr G M McCracken in this work. I would like to thank British Petroleum pie for their financial support during the preparation of this paper.
References
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