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Sputtering of molecular gas solids by keV ions
Nuclear Instruments and Methods in Physics Research B13 (1986) 360-364 North-Holland, Amsterdam
360
SPUTTERING
OF MOLECULAR
D.B. CHRISEY,
J.W. BORING,
Department of Nuclear Engineering
GAS SOLIDS BY keV IONS
J.A. PHIPPS,
and Engineering
and R.E. JOHNSON
Physics, University of Virginia*. Charlottesville, VA 22901, USA
W.L. BROWN AT&T
Bell ~a~~r~t~ries, Murray Hi&, NJ 07974, USA
The sputtering of D,O and CO solids by keV ions has been studied by measuring the absolute sputtering yield and the masses of ejected particles. The yield results for several ions on D,O indicate that for the case where the energy deposited in nuclear motion is comparable to that deposited electronically, the sputtering yield is still determined to a large extent by the electronic energy. The mass spectra ejected from D,O by 30 keV Kr” ions have a behavior very much like those produced by MeV ions. The bombardment of CO by Krt results in the rapid production of a dark residue, stable at room temperature, and the ejection of CO, CO,, O,, 0, C,, and‘(CO),.
1. Introduction
Several molecular condensed gases have been shown to sputter efficiently when bombarded by MeV light ions, where the deposited energy is largely in the form of electronic excitation [l, 21. We endeavor in this paper to explore the behavior of solid D,O and CO when bombarded by keV ions. This is a region in which the magnitudes of the deposition of energy in the form of nuclear motion and electronic energy are comparable. The chemical changes effected in condensed gas solids by ion bombardment are produced by the breaking of chemical bonds, which can occur as a result of a disturbance of the electrons involved in the bonding (electronic deposition) or in the direct displacement of an atom from its normal position in the solid (nuclear deposition) [3,4]. We study here differences that might occur due to differences in these bond breaking schemes.
2. Experimental The techniques employed here were the same as those employed in the study of rare gas solids, as reported in these Proceedings [S]. The target films were formed by condensing CO gas or D,O vapor onto a gold substrate maintained at 15 K. The gas flow was regulated by means of a calibrated leak valve. The gas was introduced into the target chamber through a nozzle directed at the substrate and 3 cm away from it. The pressure in the target chamber during deposition * The work at the University of Virginia is supported by NSF grants DMR-82-11555 and AST-82-00477. 016g-583X/86/$03.50
(North-Holland
0
Elsevier
Physics Publishing
Science
Publishers
Division)
B.V.
would change from -3 x low9 Torr to -1 x 10m7Torr. A typical 2000nm film was grown in -15 min. The sputtering yields were determined by measuring changes in the film thickness using the energy loss in the film by alpha particles from a radioactive source on the substrate on which the film was condensed. The mass spectra of ejected particles was observed by the use of a quadrupole mass spectrometer which was separated in a vacuum sense from the target chamber by a section of cryogenic differential pumping.
3. Results and discussion 3.1. DzO The total sputtering yield values measured here for D,O targets at 1.5K are given in table 1, along with the appropriate values of electronic stopping cross section S, and the corresponding nuclear energy deposition at the surface CUS,. The values of S, were obtained from the Lindhard formula (61, while S, was found by using a fit to the Thomas-Fermi universal nuclear stopping cross section curve [7]. The values of (Ywere obtained from the curve given by Andersen and Bay [8] that includes an empirical correction for surface effects for large MJM, ratios. For D,O we have taken M, = 16, assuming that the initial cascade is dominated by the 0 recoils. In this table one is impressed by the fact that the yield increases with ion energy E for Ne’ and Ar+, as does S,, in spite of the fact that cuS, is decreasing and has roughly the same magnitude as S,. This seems to indicate that for D,O the electronic ejection is surprisingly efficient compared to the collision cascade initiated by the energy described by US,. In attempting
D. B. Chrisey et al. I Sputtering of molecular gas solids by keV ions
361
Table 1 Sputtering yields for D,O Ion
aa) (M, = 16)
He
0.50
Ne
0.24
Ar
0.20
Kr
0.17
a) a from ref. [8].
30 50 10 30 50 30 50 30 50 ‘) S, from ref. [6].
219 280 184 319 411 304 393 282 364 ‘) S, from ref. [7?.
to analyze yield data of this sort it is frequently instructive to say that the yield Y is composed of a part Y, caused by the nuclear deposition and a part Y, produced from electronic deposition, such that Y = Y, + Y,. In using this separation we can calculate Y, from the usual cascade formulation of Sigmund [9]. The results of these calculations are shown in table 1 as Y, = f (0.042c~S,iO.524), where the parentheses contain the simple sputter formula of Sigmund with S, in eV A*, and 0.524 eV is the sublimation energy for H,O. The factor f is inserted because the S, used here is characteristic of the molecule and we should use a collision cross section in the denominator that is also characteristic of molecules. By the use of f we have taken it to be 3 times the atomic cross section, a case we considered earlier in a study of SO, [lo]. We have then subtracted Y, from Y to get the tabulated Y,. One sees that the Y, and Y, are of comparable size for the situation where S, and CUS,are comparable. In fig. 1 we display plots of Y and Y, vs. S,, along with the Y, = k(S,)’ line obtained from MeV H’ and He’ data [ 111. One sees that the Y, results do not depart radically from the He+ values where Y, is small. Another way of attempting to separate Y, and Y, is to say that Y, is given by the MeV line in fig. 1, and then subtracting this from the measured Y to get Y,. This is clearly not reasonable for the present data since for the ions Net and Ar’ the values of Y, obtained in this way increase with increasing ion energy, while (YS, decreases. The principal conclusion from these results is that for S, and aS, about equal in magnitude, the corresponding values of Y, and Y, are also comparable. This is in contrast to the conclusion of Stevanovic et al. [12] for Xe where the Y, and Y, were stated to be comparable only if S, was roughly an order of magnitude larger than (YS,. We note that if the water molecule is simply treated as an atomic species [ll] the calculated yield would be a factor of three larger which would exceed the measured yields for five of the line values. We should also emphasize that our calculation of Y,
6.0 4.1 174 124 98 283 253 473 492
4.8 5.3 7.8 13.0 17.0 16.3 18.9 16.5 19.8
d, Y. = f(0.042)
0.2 0.1 4.6 3.3 2.6 7.6 6.8 12.6 13.1
4.6 5.2 3.2 9.7 14.4 8.7 12.1 3.9 6.7
cuSJO.524.
40 OPEN SYMBOLS FILLED
ARE
MEASURED
SYMBOLS
l
He+
.
Ne+
.
Ar+
.
Kr+
ARE
YIELD
Y
Ye = Y - Y,
-$
/,/ IO
I
se
kV-i2)
1
IIlll
1C
Fig. 1. Measured yield Y and electronic yield Y, = Y - Y, for D,O versus electronic stopping cross section S,. utilized a surface binding ation energy for water,
energy equal to the sublimU = 0.524eV. The binding VII. SPUTTERING
362
I). B. Chrisey et al. J Spluttering of molecular gas solids by keV ions
energy that we used [13] previously to obtain a reasonable fit to the energy distribution of ejected D,O molecules by 50 keV Ar’ was 0.055 eV If one simply used this lower binding energy to generate Y, values without changing the cross section further [lo], the results are completely unreasonable since Y,, would be roughly an order of magnitude larger than the measured values of Y. This suggests that the source of the rather broad energy distribution is not simply a lower effective binding energy for the solid during sputter ejection. The significant electronic contribution to the yield based on the present estimates results in a large contribution of low energy molecules ejected, resulting in the observed broad distribution [3]. The dependence of the absolute sputtering yield on target temperature is shown in fig. 2 for 30 keV Kr.” + D,O. The general character of this curve is the same as that published previously [14] for 1.5 MeV He’ + H,O, in that there is a region at low temperatures where the yield is independent of temperature, followed by a region at higher temperatures where the yield increases with increasing temperature. The difference in the two cases is that for the MeV ions the temperature-independent region extends to roughly 100 K, whereas for the present results the yield has already started to increase at 80K. Since this onset was associated with the production of 0, and D,, the difference is probably due to the much larger density of energy deposition for the 30 keV Kr’ case. MeV C’ bombarding H,O also had an onset at a lower temperature 11.51. The results of the dependence of the ejection on temperature for 0, and D, from a D,O target are
I
,
I
I
shown in fig. 3. The behavior of the curves for these two ejecta is the same as observed earlier [14] for 1.5 MeV He+ + D,O, in that the 0, signal increases continuously as the temperature increases, while for D, the signal is constant at low temperatures, after which it increases as well. In the region of increase they both rise an order of magnitude in about 60 K. These increases are indicative of the role of diffusion in the production and ejection of these species 131. 3.2.
CO
Upon bombardment of a 10 /*m CO film with a fluence of 4 X lOI ions/cm’ of 30 keVKr’ it was observed that a black residue was formed [16,17], presumably due to an increase in the C/O ratio in the films. This carbonaceous film remained after warming the target substrate to room temperature. The formation of this black residue clearly indicates that the chemical nature of the film changes upon bombardment, implying that the ejection of various species should also depend on fluence. We explore this by observing the mass spectrometer signal for various chosen masses as a function of fluence. Typical behaviour is shown in fig. 4, where the ejected CO is plotted vs. fluence. One observes, surprisingly, that the CO signal does not rise quickly to its maximum value, but, rather, requires a fluence of -4 x lOI ions/cmZ to do so. It is also seen that after the maximum is reached there is a gradual decline in CO signal toward the background (beam off) value, without a plateau indicative of a constant ejection rate, as seen in the rare gas
I
,
I
I
5C l-
30 keV Kr*+
D20
40
.-5 u; 5 30 E
E-1 w 20
F
10/
-
L
I
1
#
,
L
I
I
I
0
20
40
60
80
100
120
140
160
0
TEMPERATURE
Fig.
2.
Measured yield versus temperature
(K)
for 30 keV Kr” + D,O.
D.B.
30 ke”
Kr*+
Chrisey et al. I Sputtering of molecular gas solids by keV ions
363
the ionizer of the mass spectrometer. The measurements were made at a fluence -2 x 1014 ions/cm’. Since the film appears to become carbon-rich as the erosion proceeds, it is fully expected that the ejecta will tend to be oxygen-rich [17]. This is observed in the ejection of 0, O,, and CO,. We also observed small amounts of (CO), and C, ejected as did Haring et al.
D20
1161.
1ol 0
20
40
80
60
160
140
120
100
TEMPERATURE
The above observations can be roughly understood by noting that either nuclear or electronic energy loss can lead to the dissociation of CO. The former is by direct collisions and the latter following electronic recombination: CO’ + e+ C + 0, where the C or 0 or both can be in excited states. The latter is an exothermic process of the type thought to account for the electronic sputtering of condensed gas solids [2,20]. At low temperatures a small fraction of the 0 can immediately react in the solid with CO to produce CO,, leaving excess C. If the density of excitation in a single track is high, or the species made mobile (e.g., high temperature) then a reaction between dissociated species can occur, forming C, and 0, as well as CO. The observed dependence of CO ejection may be due to the interaction between the newly excited species and the products produced by a previous ion, as was shown to be the case in D,O [3]. We note that CO’ will rapidly charge exchange when produced near one of the previously formed products (i.e., CO’ + (0, C, CO,, O,)+CO + (0’, C+, CO:, Oi), with a release of energy in each case much larger than that required to directly eject CO if the reaction occurs near the surface. In this reaction the ion produced will generally not be lost from the surface due to its higher surface binding energy. Recombination then deposits additional energy, ejecting CO and other neutral surface
(K)
Fig. 3. Ejection rate of 0, and Dz for 30 keV Kr’ + D,O versus temperature. solids [5]. This is understandable since the chemical composition is changing continuously as the erosion proceeds. We next explored the molecular species ejected from a CO film by 30 keV Kr+ [4, 16, 18, 191. In table 2 we present the relative sizes of the signals observed for the pertinent masses, relative to CO which is taken to be 100%. We have included both signals as-observed and after correcting for the dissociation of CO and CO, in
I
I
1
I
I
1
I
I
I
1
1
I
I
3OkeV
I
Kr++
I
I
I
,
,
I
,
CO
CO EJECTION
m
0
I 2
4 4
1 6
I 8
( IO
I 12
I ! 14 16 FLIJENCE
Fig. 4. The fluence
dependence
I I 1820
I 22
I 24
I 26
I I , 2830 32
I I I 34363840
I
(I d4ions/cm2)
of the CO ejection
for 30 keV Kr’ + CO.
VII. SPUTTERING
364 Table 2 Mass spectrum Mass
28 44 16 56 32 24
D. B. Chrisey et al. I Sputtering of molecular gas solids by keV ions
for 30 keV Kr’
+ CO at 15 K. From
Molecule
co CO* &)Z 0, C,
quadrupole
mass spectrometer
Relative quad. signal 100 7.8 7.2 1.6 0.4 0.2
Because the C atoms and C, can form additional bonds (e.g., C, + C-C,, etc.), larger species are formed that are more difficult to eject. At high fluences such chains are thought to eventually dominate the film as is the case with the erosion of solid methane [21]. The formation of such carbon residues is well known to any experimentalist using vacuum systems containing hydrocarbon contaminants. species.
4. Summary It has been established in previous experiments that for the situation where electronic deposition in an insulating solid is orders of magnitude larger than the nuclear deposition, the sputtering yield is dominated by processes that convert the electronic energy into the nuclear motion required for ejection of atoms or molecules from the surface. The present results for the sputtering of D,O by keV ions seem to indicate that even for the situation where the two types of energy deposition are roughly equal, the ejection of particles due to the electronic deposition is comparably efficient. The study of the dependence of CO ejection from a CO target on ion fluence shows evidence that some of the ejection results from chemical processes that depend on products formed by previous ions.
References [l] W.L Brown, in: Ion Implantation and Beam Processing, eds, J. Poate and J. Williams (Academic Press, New York, 1984) p. 99. [2] R.E. Johnson and W. L. Brown, Nucl. Instr. and Meth. 198 (1982) 103. [3] CT. Reimann, J.W. Boring, R.E. Johnson, J.W. Garrett, K.R. Farmer, W.L. Brown, K.J. Marcantonio and W.M. Augustyniak, Surface Sci. 147 (1984) 227. [4] R. Pedrys, D.J. Oostra and A.E. devries, in: Desorption
(%)
(fluence -2 x 10” ions/cm’) Corrected for cracking of CO and CO, 100 7.9 4.7 1.6 0.4 0.2
Induced by Electronic Transitions, DIET II. ed., N. Tolk (Springer-Verlag, Berlin, 1985). [5] D.J. O’Shaughnessy, J.W. Boring, J.A. Phipps, R.E. Johnson and W.L. Brown, these Proceedings (ICACS ‘85) Nucl. Instr. and Meth. B13 (1986) 304. [6] J. Lindhard, M. Scharff and H.E. Schiott, Kgl. Dansk. Vid. Selsk. Mat. Fys. Medd. 33 (1963) No. 14. [7] R.E. Johnson, Introduction to Atomic and Molecular Collisions (Plenum, New York, 1982) p. 270. [8] H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment I, ed., R. Behrisch (Springer-Verlag, Berlin, 1981). [9] P. Sigmund, Phys. Rev. 184 (1969) 383. [lo] J.W. Boring, J.W. Garrett, T.A. Cummings, R.E. Johnson and W.L. Brown, Nucl. Instr. and M&h. Bl (1984) 321. W.M. Augustyniak, E. Brody, B.H. [Ill W.L. Brown, Cooper, L.J. Lanzerotti, A.L. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. [=I D.V. Stevanovic, D.A. Thompson and J.A. Davies, Nucl. Instr. and Meth. Bl (1984) 315. [I31 J.W. Boring, R.E. Johnson, C.T. Reimann, J.W. Garrett, W.L. Brown and K.J. Marcantonio, Nucl. Instr. and Meth. 218 (1983) 707. K.J. Marcantonio, [I41 W.L. Brown, W.M. Augustyniak, E.H. Simmons, J.W. Boring, R.E. Johnson and C.T. Reimann, Nucl. Instr. and Meth. Bl (1984) 307. Physics, University [151 R. Evatt, MS Thesis in Engineering of Virginia (1980). [W R.A. Haring, R. Pedrys, D.J. Oostra, A. Haring and A.E. devries, Nucl. Instr. and Meth. BS (1984) 476. [I71 0. Ellegaard, P. Borgesen and H. Sorensen, in: DIET II, ed., N. Tolk (Springer-Verlag, Berlin, 1985). [I81 R.A. Haring, R. Pedrys, D.J. Oostra, A. Haring and A.E. devries, Nucl. Instr. and Meth. B5 (1984) 483. [I91 R.A. Haring, A.W. Kolfschoten and A.E. devries, Nucl. Instr. and Meth. B2 (1984) 544. R.E. Johnson and W.L. Brown, Phys. PO1 CT. Reimann, Rev. Lett. 53 (1984) 600; F.L. Rook, R.E. Johnson and W.L. Brown, Surf. Sci. 164 (1985) 625. W.L. Brown and R.E. Johnson, Proc. 1211 L.J. Lanzerotti, NATO Conf. on Ices in the Solar System, ed., J. Klinger (D. Reidel, New York, 1985); A.F. Chang and L.J. Lanzerotti, J. Geophys. Res. 83 (1978) 2596.
360
SPUTTERING
OF MOLECULAR
D.B. CHRISEY,
J.W. BORING,
Department of Nuclear Engineering
GAS SOLIDS BY keV IONS
J.A. PHIPPS,
and Engineering
and R.E. JOHNSON
Physics, University of Virginia*. Charlottesville, VA 22901, USA
W.L. BROWN AT&T
Bell ~a~~r~t~ries, Murray Hi&, NJ 07974, USA
The sputtering of D,O and CO solids by keV ions has been studied by measuring the absolute sputtering yield and the masses of ejected particles. The yield results for several ions on D,O indicate that for the case where the energy deposited in nuclear motion is comparable to that deposited electronically, the sputtering yield is still determined to a large extent by the electronic energy. The mass spectra ejected from D,O by 30 keV Kr” ions have a behavior very much like those produced by MeV ions. The bombardment of CO by Krt results in the rapid production of a dark residue, stable at room temperature, and the ejection of CO, CO,, O,, 0, C,, and‘(CO),.
1. Introduction
Several molecular condensed gases have been shown to sputter efficiently when bombarded by MeV light ions, where the deposited energy is largely in the form of electronic excitation [l, 21. We endeavor in this paper to explore the behavior of solid D,O and CO when bombarded by keV ions. This is a region in which the magnitudes of the deposition of energy in the form of nuclear motion and electronic energy are comparable. The chemical changes effected in condensed gas solids by ion bombardment are produced by the breaking of chemical bonds, which can occur as a result of a disturbance of the electrons involved in the bonding (electronic deposition) or in the direct displacement of an atom from its normal position in the solid (nuclear deposition) [3,4]. We study here differences that might occur due to differences in these bond breaking schemes.
2. Experimental The techniques employed here were the same as those employed in the study of rare gas solids, as reported in these Proceedings [S]. The target films were formed by condensing CO gas or D,O vapor onto a gold substrate maintained at 15 K. The gas flow was regulated by means of a calibrated leak valve. The gas was introduced into the target chamber through a nozzle directed at the substrate and 3 cm away from it. The pressure in the target chamber during deposition * The work at the University of Virginia is supported by NSF grants DMR-82-11555 and AST-82-00477. 016g-583X/86/$03.50
(North-Holland
0
Elsevier
Physics Publishing
Science
Publishers
Division)
B.V.
would change from -3 x low9 Torr to -1 x 10m7Torr. A typical 2000nm film was grown in -15 min. The sputtering yields were determined by measuring changes in the film thickness using the energy loss in the film by alpha particles from a radioactive source on the substrate on which the film was condensed. The mass spectra of ejected particles was observed by the use of a quadrupole mass spectrometer which was separated in a vacuum sense from the target chamber by a section of cryogenic differential pumping.
3. Results and discussion 3.1. DzO The total sputtering yield values measured here for D,O targets at 1.5K are given in table 1, along with the appropriate values of electronic stopping cross section S, and the corresponding nuclear energy deposition at the surface CUS,. The values of S, were obtained from the Lindhard formula (61, while S, was found by using a fit to the Thomas-Fermi universal nuclear stopping cross section curve [7]. The values of (Ywere obtained from the curve given by Andersen and Bay [8] that includes an empirical correction for surface effects for large MJM, ratios. For D,O we have taken M, = 16, assuming that the initial cascade is dominated by the 0 recoils. In this table one is impressed by the fact that the yield increases with ion energy E for Ne’ and Ar+, as does S,, in spite of the fact that cuS, is decreasing and has roughly the same magnitude as S,. This seems to indicate that for D,O the electronic ejection is surprisingly efficient compared to the collision cascade initiated by the energy described by US,. In attempting
D. B. Chrisey et al. I Sputtering of molecular gas solids by keV ions
361
Table 1 Sputtering yields for D,O Ion
aa) (M, = 16)
He
0.50
Ne
0.24
Ar
0.20
Kr
0.17
a) a from ref. [8].
30 50 10 30 50 30 50 30 50 ‘) S, from ref. [6].
219 280 184 319 411 304 393 282 364 ‘) S, from ref. [7?.
to analyze yield data of this sort it is frequently instructive to say that the yield Y is composed of a part Y, caused by the nuclear deposition and a part Y, produced from electronic deposition, such that Y = Y, + Y,. In using this separation we can calculate Y, from the usual cascade formulation of Sigmund [9]. The results of these calculations are shown in table 1 as Y, = f (0.042c~S,iO.524), where the parentheses contain the simple sputter formula of Sigmund with S, in eV A*, and 0.524 eV is the sublimation energy for H,O. The factor f is inserted because the S, used here is characteristic of the molecule and we should use a collision cross section in the denominator that is also characteristic of molecules. By the use of f we have taken it to be 3 times the atomic cross section, a case we considered earlier in a study of SO, [lo]. We have then subtracted Y, from Y to get the tabulated Y,. One sees that the Y, and Y, are of comparable size for the situation where S, and CUS,are comparable. In fig. 1 we display plots of Y and Y, vs. S,, along with the Y, = k(S,)’ line obtained from MeV H’ and He’ data [ 111. One sees that the Y, results do not depart radically from the He+ values where Y, is small. Another way of attempting to separate Y, and Y, is to say that Y, is given by the MeV line in fig. 1, and then subtracting this from the measured Y to get Y,. This is clearly not reasonable for the present data since for the ions Net and Ar’ the values of Y, obtained in this way increase with increasing ion energy, while (YS, decreases. The principal conclusion from these results is that for S, and aS, about equal in magnitude, the corresponding values of Y, and Y, are also comparable. This is in contrast to the conclusion of Stevanovic et al. [12] for Xe where the Y, and Y, were stated to be comparable only if S, was roughly an order of magnitude larger than (YS,. We note that if the water molecule is simply treated as an atomic species [ll] the calculated yield would be a factor of three larger which would exceed the measured yields for five of the line values. We should also emphasize that our calculation of Y,
6.0 4.1 174 124 98 283 253 473 492
4.8 5.3 7.8 13.0 17.0 16.3 18.9 16.5 19.8
d, Y. = f(0.042)
0.2 0.1 4.6 3.3 2.6 7.6 6.8 12.6 13.1
4.6 5.2 3.2 9.7 14.4 8.7 12.1 3.9 6.7
cuSJO.524.
40 OPEN SYMBOLS FILLED
ARE
MEASURED
SYMBOLS
l
He+
.
Ne+
.
Ar+
.
Kr+
ARE
YIELD
Y
Ye = Y - Y,
-$
/,/ IO
I
se
kV-i2)
1
IIlll
1C
Fig. 1. Measured yield Y and electronic yield Y, = Y - Y, for D,O versus electronic stopping cross section S,. utilized a surface binding ation energy for water,
energy equal to the sublimU = 0.524eV. The binding VII. SPUTTERING
362
I). B. Chrisey et al. J Spluttering of molecular gas solids by keV ions
energy that we used [13] previously to obtain a reasonable fit to the energy distribution of ejected D,O molecules by 50 keV Ar’ was 0.055 eV If one simply used this lower binding energy to generate Y, values without changing the cross section further [lo], the results are completely unreasonable since Y,, would be roughly an order of magnitude larger than the measured values of Y. This suggests that the source of the rather broad energy distribution is not simply a lower effective binding energy for the solid during sputter ejection. The significant electronic contribution to the yield based on the present estimates results in a large contribution of low energy molecules ejected, resulting in the observed broad distribution [3]. The dependence of the absolute sputtering yield on target temperature is shown in fig. 2 for 30 keV Kr.” + D,O. The general character of this curve is the same as that published previously [14] for 1.5 MeV He’ + H,O, in that there is a region at low temperatures where the yield is independent of temperature, followed by a region at higher temperatures where the yield increases with increasing temperature. The difference in the two cases is that for the MeV ions the temperature-independent region extends to roughly 100 K, whereas for the present results the yield has already started to increase at 80K. Since this onset was associated with the production of 0, and D,, the difference is probably due to the much larger density of energy deposition for the 30 keV Kr’ case. MeV C’ bombarding H,O also had an onset at a lower temperature 11.51. The results of the dependence of the ejection on temperature for 0, and D, from a D,O target are
I
,
I
I
shown in fig. 3. The behavior of the curves for these two ejecta is the same as observed earlier [14] for 1.5 MeV He+ + D,O, in that the 0, signal increases continuously as the temperature increases, while for D, the signal is constant at low temperatures, after which it increases as well. In the region of increase they both rise an order of magnitude in about 60 K. These increases are indicative of the role of diffusion in the production and ejection of these species 131. 3.2.
CO
Upon bombardment of a 10 /*m CO film with a fluence of 4 X lOI ions/cm’ of 30 keVKr’ it was observed that a black residue was formed [16,17], presumably due to an increase in the C/O ratio in the films. This carbonaceous film remained after warming the target substrate to room temperature. The formation of this black residue clearly indicates that the chemical nature of the film changes upon bombardment, implying that the ejection of various species should also depend on fluence. We explore this by observing the mass spectrometer signal for various chosen masses as a function of fluence. Typical behaviour is shown in fig. 4, where the ejected CO is plotted vs. fluence. One observes, surprisingly, that the CO signal does not rise quickly to its maximum value, but, rather, requires a fluence of -4 x lOI ions/cmZ to do so. It is also seen that after the maximum is reached there is a gradual decline in CO signal toward the background (beam off) value, without a plateau indicative of a constant ejection rate, as seen in the rare gas
I
,
I
I
5C l-
30 keV Kr*+
D20
40
.-5 u; 5 30 E
E-1 w 20
F
10/
-
L
I
1
#
,
L
I
I
I
0
20
40
60
80
100
120
140
160
0
TEMPERATURE
Fig.
2.
Measured yield versus temperature
(K)
for 30 keV Kr” + D,O.
D.B.
30 ke”
Kr*+
Chrisey et al. I Sputtering of molecular gas solids by keV ions
363
the ionizer of the mass spectrometer. The measurements were made at a fluence -2 x 1014 ions/cm’. Since the film appears to become carbon-rich as the erosion proceeds, it is fully expected that the ejecta will tend to be oxygen-rich [17]. This is observed in the ejection of 0, O,, and CO,. We also observed small amounts of (CO), and C, ejected as did Haring et al.
D20
1161.
1ol 0
20
40
80
60
160
140
120
100
TEMPERATURE
The above observations can be roughly understood by noting that either nuclear or electronic energy loss can lead to the dissociation of CO. The former is by direct collisions and the latter following electronic recombination: CO’ + e+ C + 0, where the C or 0 or both can be in excited states. The latter is an exothermic process of the type thought to account for the electronic sputtering of condensed gas solids [2,20]. At low temperatures a small fraction of the 0 can immediately react in the solid with CO to produce CO,, leaving excess C. If the density of excitation in a single track is high, or the species made mobile (e.g., high temperature) then a reaction between dissociated species can occur, forming C, and 0, as well as CO. The observed dependence of CO ejection may be due to the interaction between the newly excited species and the products produced by a previous ion, as was shown to be the case in D,O [3]. We note that CO’ will rapidly charge exchange when produced near one of the previously formed products (i.e., CO’ + (0, C, CO,, O,)+CO + (0’, C+, CO:, Oi), with a release of energy in each case much larger than that required to directly eject CO if the reaction occurs near the surface. In this reaction the ion produced will generally not be lost from the surface due to its higher surface binding energy. Recombination then deposits additional energy, ejecting CO and other neutral surface
(K)
Fig. 3. Ejection rate of 0, and Dz for 30 keV Kr’ + D,O versus temperature. solids [5]. This is understandable since the chemical composition is changing continuously as the erosion proceeds. We next explored the molecular species ejected from a CO film by 30 keV Kr+ [4, 16, 18, 191. In table 2 we present the relative sizes of the signals observed for the pertinent masses, relative to CO which is taken to be 100%. We have included both signals as-observed and after correcting for the dissociation of CO and CO, in
I
I
1
I
I
1
I
I
I
1
1
I
I
3OkeV
I
Kr++
I
I
I
,
,
I
,
CO
CO EJECTION
m
0
I 2
4 4
1 6
I 8
( IO
I 12
I ! 14 16 FLIJENCE
Fig. 4. The fluence
dependence
I I 1820
I 22
I 24
I 26
I I , 2830 32
I I I 34363840
I
(I d4ions/cm2)
of the CO ejection
for 30 keV Kr’ + CO.
VII. SPUTTERING
364 Table 2 Mass spectrum Mass
28 44 16 56 32 24
D. B. Chrisey et al. I Sputtering of molecular gas solids by keV ions
for 30 keV Kr’
+ CO at 15 K. From
Molecule
co CO* &)Z 0, C,
quadrupole
mass spectrometer
Relative quad. signal 100 7.8 7.2 1.6 0.4 0.2
Because the C atoms and C, can form additional bonds (e.g., C, + C-C,, etc.), larger species are formed that are more difficult to eject. At high fluences such chains are thought to eventually dominate the film as is the case with the erosion of solid methane [21]. The formation of such carbon residues is well known to any experimentalist using vacuum systems containing hydrocarbon contaminants. species.
4. Summary It has been established in previous experiments that for the situation where electronic deposition in an insulating solid is orders of magnitude larger than the nuclear deposition, the sputtering yield is dominated by processes that convert the electronic energy into the nuclear motion required for ejection of atoms or molecules from the surface. The present results for the sputtering of D,O by keV ions seem to indicate that even for the situation where the two types of energy deposition are roughly equal, the ejection of particles due to the electronic deposition is comparably efficient. The study of the dependence of CO ejection from a CO target on ion fluence shows evidence that some of the ejection results from chemical processes that depend on products formed by previous ions.
References [l] W.L Brown, in: Ion Implantation and Beam Processing, eds, J. Poate and J. Williams (Academic Press, New York, 1984) p. 99. [2] R.E. Johnson and W. L. Brown, Nucl. Instr. and Meth. 198 (1982) 103. [3] CT. Reimann, J.W. Boring, R.E. Johnson, J.W. Garrett, K.R. Farmer, W.L. Brown, K.J. Marcantonio and W.M. Augustyniak, Surface Sci. 147 (1984) 227. [4] R. Pedrys, D.J. Oostra and A.E. devries, in: Desorption
(%)
(fluence -2 x 10” ions/cm’) Corrected for cracking of CO and CO, 100 7.9 4.7 1.6 0.4 0.2
Induced by Electronic Transitions, DIET II. ed., N. Tolk (Springer-Verlag, Berlin, 1985). [5] D.J. O’Shaughnessy, J.W. Boring, J.A. Phipps, R.E. Johnson and W.L. Brown, these Proceedings (ICACS ‘85) Nucl. Instr. and Meth. B13 (1986) 304. [6] J. Lindhard, M. Scharff and H.E. Schiott, Kgl. Dansk. Vid. Selsk. Mat. Fys. Medd. 33 (1963) No. 14. [7] R.E. Johnson, Introduction to Atomic and Molecular Collisions (Plenum, New York, 1982) p. 270. [8] H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment I, ed., R. Behrisch (Springer-Verlag, Berlin, 1981). [9] P. Sigmund, Phys. Rev. 184 (1969) 383. [lo] J.W. Boring, J.W. Garrett, T.A. Cummings, R.E. Johnson and W.L. Brown, Nucl. Instr. and M&h. Bl (1984) 321. W.M. Augustyniak, E. Brody, B.H. [Ill W.L. Brown, Cooper, L.J. Lanzerotti, A.L. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. [=I D.V. Stevanovic, D.A. Thompson and J.A. Davies, Nucl. Instr. and Meth. Bl (1984) 315. [I31 J.W. Boring, R.E. Johnson, C.T. Reimann, J.W. Garrett, W.L. Brown and K.J. Marcantonio, Nucl. Instr. and Meth. 218 (1983) 707. K.J. Marcantonio, [I41 W.L. Brown, W.M. Augustyniak, E.H. Simmons, J.W. Boring, R.E. Johnson and C.T. Reimann, Nucl. Instr. and Meth. Bl (1984) 307. Physics, University [151 R. Evatt, MS Thesis in Engineering of Virginia (1980). [W R.A. Haring, R. Pedrys, D.J. Oostra, A. Haring and A.E. devries, Nucl. Instr. and Meth. BS (1984) 476. [I71 0. Ellegaard, P. Borgesen and H. Sorensen, in: DIET II, ed., N. Tolk (Springer-Verlag, Berlin, 1985). [I81 R.A. Haring, R. Pedrys, D.J. Oostra, A. Haring and A.E. devries, Nucl. Instr. and Meth. B5 (1984) 483. [I91 R.A. Haring, A.W. Kolfschoten and A.E. devries, Nucl. Instr. and Meth. B2 (1984) 544. R.E. Johnson and W.L. Brown, Phys. PO1 CT. Reimann, Rev. Lett. 53 (1984) 600; F.L. Rook, R.E. Johnson and W.L. Brown, Surf. Sci. 164 (1985) 625. W.L. Brown and R.E. Johnson, Proc. 1211 L.J. Lanzerotti, NATO Conf. on Ices in the Solar System, ed., J. Klinger (D. Reidel, New York, 1985); A.F. Chang and L.J. Lanzerotti, J. Geophys. Res. 83 (1978) 2596.