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Sputtering of frozen Xenon by keV heavy ions
315
Nuclear Instruments and Methods in Physics Research Bl (1984) 315-320 North-Holland, Amsterdam
SPUTIXRINC Doris
V.
Department
OF FROZEN XENON BY keV HEAW IONS
STEVANOVIC, D.A. THOMPSON and J.A DAVIES * 01 Engineering Physics, McMasier
Universiiy, Hamilton, Ontario, Canada ~58.9 4Ml
Sputtering measurements of frozen Xe have been made using 20-80 keV heavy ions (N, Ar, Sb and Bi), i.e. where nuclear stopping is the dominant energy loss process. Sputtering yields (Y) of up to 1.6 x lo4 Xe atoms/ion have been obtained. The dependence of Y on the surface deposited (nuclear) energy F,(O) was found to exhibit two distinct regimes: at low F,(O) values, Y increases linearly with deposited energy, as predicted by cascade sputtering theory, but above a threshold value of - 500 eV A2/atom the sputtering yield exhibits a much stronger (approximately cubic) dependence on F,(O). Comparison of the sputtering yields obtained by monatomic and diatomic equal velocity heavy ion bombardments (e.g. Sb/Sb2) further confirmed this non-linear behaviour at large F,(O). Similar monatomic and diatomic light ion bombardments (e.g. N/N,) showed no molecular enhancement, hence a linear behaviour at lower F,(O). The current results, which are obtained in the energy regime where nuclear stopping dominates, are compared to earlier measurements at MeV energies in which electronic stopping dominates. This comparison suggests that the observed sputtering behaviour of frozen Xe is complex, with significant contributions from both nuclear and electronic processes.
1. Introduction Previous experiments [1,2] on the sputtering of frozen inert gases (Xe,Kr,Ar) were performed with energetic light ions, such as MeV He+, where the dominant energy loss process is by electronic excitation and ionization. In each case, the measured sputtering yields, Y,,, were 2-3 orders of magnitude greater than predicted by collision cascade theory. By varying the incident ion energy (OS-2 MeV) and species (H+, He+, N+ and Ar+), Ollerhead et al. [l] tried to determine the mechanism responsible for these high sputtering yields. Within the energy range available to them, the measured sputtering yields for H+ and He+ bombardment scaled roughly with the square of the electronic stopping, S,(O). On the other hand, the sputtering yield for N+ and Ar+ bombardments did not follow the expected energy dependence of the electronic stopping power curve, but instead approached the functional dependence of the elastic stopping power; i.e. the yield decreased for increasing energy. This result seems rather surprising because, even for Ar+, the dominant energyloss process at such high energies is via electronic excitation. For example, for OS-2 MeV N+, less than 5% of the total energy loss is due to nuclear recoil contributions. The present work extends the sputtering yield measurements in frozen Xe into the low-energy regime where nuclear stopping becomes the dominant energy
loss process. For this purpose, 20-80 keV N+, Ar+, Sb+, Bi+ and 80 keV N: and Sb: were chosen as incident ions. In the case of N+, the measurements were subsequently extended to 2.0 MeV in order to check the anomalous energy dependence in the earlier data of Ollerhead et al. [l]. The various energy loss regimes considered in the present work and in the previous investigation of frozen Xe sputtering are summarized schematically in fig. 1, in terms of the reduced energy scaling parameter e introduced by Lindhard et al. [3]. Their universal nuclear stopping power curve (S,,) and a typical electronic stopping power curve (S,) are shown. Previous experimental results on the sputtering of Xe involved ions in the E = 4-400 region, where energy is
-
-
/.-
OLLERHEAD
- -
,
I
I,,
I
I
2
3”
IO
PRESENT
et al WORK
1
20
E 4
Fig. 1. Nuclear (S,) and electronic (S,) stopping powers vs. * Permanent address: Solid State Science Branch, AECL, Chalk River, Ontario, Canada. 0168-583X/84/$03.00 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Lindhard’s reduced energy parameter e. The appropriate regions covered in the present study and in the previous investigation by Ollerhead et al. [l] are shown by horizontal arrows. IV. CONDENSED
GASES
316
D. V. Stevanovic et al. / Sputtering of frozen xenon
lost almost entirely by electronic collisions. In the present work (E = O.Ol-l.S), the major contribution to energy loss is from nuclear collisions. Note however that, throughout this low E regime, (fig. 1) there is always a significant (i.e. > 20%) S, contribution to the total stopping power.
2. Experimental technique
l
I
I
I
1
400-
,
1 I I , , I , I
ZOO-
Films of Xe [(0.5-2) X 10” atoms/cm2 in thickness] were condensed on a cold silicon substrate upon which a thin film of Ti ( - 200 A) had been previously deposited. The silicon was cryogenically cooled to 40 K and was surrounded by a Cu cryoshield maintained at 30 K. A schematic of the target arrangement is given in fig. 2. A 5 W ohmic heater attached to the back of the target holder enabled the residual Xe films to be removed rapidly after each sputtering measurement: i.e. before condensing a fresh Xe film on the substrate. Typical ion bombardment doses were 10’3-1015 ions/cm2, with the incident beam swept horizontally and vertically across a 4 mm aperture in front of the target to ensure uniform bombardment. The thickness of the Xe films was measured by Rutherford backscattering of 2 MeV 4He+ ions, with a Si surface barrier detector being used to detect the particles scattered through an angle of 150’. The incident current density was always less than 10m9 A/mm2 in order to avoid possible heating effects, either by the analysing beam (which was collimated to a diameter of 2 mm) or by the implantation beam. At 80 keV, the measured sputtering yield was observed to be independent of current density over the range (0.1-4)~ 10e9 A/mm2. In each RBS analysis, a total 4He+ dose of 0.5 pC was required in order to obtain reasonable counting statistics. Measurements at considerably larger 4He+ doses showed that the sputtering loss/f.~C during RBS analysis was only - 2 X 1015 Xc/cm’; this loss is negligible compared to the large sputtering yields obtained for the heavy ions. Several successive bombardments and RBS analyses
m
r
, 0
Kt 0
00
200
CHANNEL
, 300
_:
\ 400
NVMBER
Fig. 3. Typical RBS spectrum for a frozen Xe film ( - 1 X 10” Xe atoms/cm2) on the Si(Ti) substrate. Analysing beam was 2.0 MeV 4He+.
were carried out on each Xe film in order to establish a reliable sputtering yield value. Any dependence of sputtering yield on film thickness was minimized by using much thicker frozen Xe films than the incident heavy ion range. A typical RBS spectrum is shown in fig. 3. The thin Ti layer on the Si substrate provided an independent calibration marker for monitoring the Xe film thickness; it also served to indicate whether the Xe film had become nonuniform after prolonged sputtering.
3. Results and discussion 3.1. LOW-Eregime (20-80
keV heavy
ions)
Data from a typical sputtering experiment are shown in fig. 4. The frozen Xe film thickness, as determined from the RBS yield following each Sb+ bombardment, is seen to decrease linearly with increasing Sb+ dose, indicating a constant sputtering yield Y,, = 1905 Xe
CRYOCOWR AAolAnoN
SHIELD
SLOPE
= 1905
Xe otoms/Sb+
ion
Fig. 4. Xe film thickness (derived from the 2.0 MeV 4He RBS Fig. 2. Schematic representation of the target assembly for frozen-gas sputtering measurements.
yield ) vs Sb+ ion dose. The linear dependence constant sputtering yield of 1905 atoms/ion.
indicates
a
D. V. Sievanovic et al. / Sputtering of frozen xenon
311
Table 1 Sputtering yields in the low-c regime. Ion
E
se69 (eV A2/atom)
F,,(O) (eV A2/atom)
WV)
Yohs
YeIa
N+
40 80
240 176
460 650
1 2
101 70
N:
80
480
920
4
217
Ar+
40 80
884 843
640 900
2 4
784 805
Sb+
40 80
1523 1762
810 1150
3 I
1905 2854
Sb;
80
3040
1620
15
16100
Bi+
20 40 60 80
1506 1802 2029 2180
600 850 1040 1200
2 3 6 8
3190 3160 5370 6500
a Predicted from fig. 6. atoms/ion. A similar linear decrease of thickness was observed in almost all cases. However, in the cascades where surface deposited nuclear energy, F,(O), is highest a significant (80 keV Sb: and Bi+ bombardments), decrease in slope sometimes occurred at bombardment the cause of doses in excess of - 5 x 1013 ions/cm*; this gradual change in sputtering yield at high dose is not yet understood. The observed heavy-ion sputtering yields (Yobs) are summarized in table 1. In the case of Bi+ and Sb:
bombardment, the initial linear region at bombardment doses ( 5 x 1013 ions/cm* was used to evaluate the sputtering yield. Included in table 1 are theoretical estimates of the surface deposited energies, I;,(O) and S,(O), resulting from the nuclear stopping and electronic excitation processes, respectively, as each bombarding ion penetrates the frozen Xe film. The values of F,(O) were obtained from the Winterbon tables [4] and the S,.(O) values were calculated from Lindhard et al. [3]. Column 5 gives the predicted sputtering yield Y, associ-
Table 2 Sputtering yields in the high-~ regime. IOIl
Present data: He+ N+
Ollerhead et al. data [l]: H+
E (keYI 2000 600 1000 2000
F,(O)
se(0) a
(eV AZ/atom)
(eV A2/atom)
0.3 29 15 7
Ycast
b
1100
- 0.1
1783 2302 3256
8 4 2
Yohs
7 79 114 203
250 500 1000
0.1 0.07 0.04
400 275 180
- 0.01 0.01 0.01
He+
600 1000 1500 2000
0.8 0.6 0.4 0.3
1500 1380 1250 1100
- 0.1 0.1 0.1 0.1
10 9 8 7
N+
600 1000 2000
1783 2302 3256
8 4 2
45 35 19
29 15 I
0.6 0.3 0.08
a S,(O) data were determined from ref. 7 for H+ and He+ and from ref. 8 for N+ bombardment. b Predicted from fig. 5. IV. CONDENSED
GASES
D. V. Stevanovic et al. / Sputtering of frozen xenon
318
BI . Sb
0
o Ar
energy between frozen Xe and the metal targets. Further evidence for the existence of these two markedly different sputtering regimes can be seen in table 1 by comparing the behaviour of the diatomic ions, 80 keV NC and Sb: , with that of the corresponding monatomic ions of identical velocity, i.e. 40 keV N+ and Sb+. In the nitrogen comparison, Yobs for the 80 keV NC bombardment is almost exactly twice the 40 keV N+ value, as would be predicted for the two-fold increase in F,(O), since both ions fall within the linear cascade regime. On the other hand, in the Sb case Y,, for the 80 keV Sb: bombardment is almost a factor of ten larger than the 40 keV Sb+ value, thus indicating clearly the existence of a much stronger than linear dependence on F,,(O) for cascades above the - 500 eV A2/atom threshold. 3.2. High-e regime (0.5-2.0
Fig. 5. Observed sputtering yield Y,, vs. F,(O) for 20-80 keV N+, Ar+, Sb+ and Bi+ bombardments. Note that F,(O) is the estimated surface-deposited energy due to nuclear stopping alone. The dashed curve represents the predicted values (Y,) given by linear cascade theory.
ated with each S,(O) value; these were derived by interpolating the YobBdata in the high-s regime (table 2 and fig. 6) where electronic excitation is the dominant sputtering mechanism. Hence for all cases in table 1, Y,, (by itself) would be a negligible fraction of Yobs, indicating that nuclear stopping is the dominant sputtering mechanism throughout the low-s regime. Nevertheless, it is interesting to note that in this region the S,(O) and F,,(O) values are comparable in magnitude. The dependence of Y,, on F,,(O) can be subdivided clearly into two completely different zones, shown graphically in fig. 5. At low surface deposited energies, a linear dependence on F,(O) is observed, as predicted by theory [5]; furthermore, in this region, the absolute magnitude of Yobsis given quite well (i.e. within - 50%) by the Sigmund linear cascade treatment, based on a surface binding energy of 0.16 eV/atom for xenon. On the other hand, at F,(O) values > 500 eV A2/atom, Y,, increases much more rapidly and exhibits an approximate cubic dependence on F,(O). This enhanced sputtering behaviour in high density cascades is remarkably similar to that reported earlier in Ag, Au and Pt targets by Thompson [6], where again a sharp threshold was observed between a linear and almost cubic dependence on F,(O). In the case of the metals, this threshold occurs at significantly higher F,(O) values, (i.e. - lOOt-3500 eV A2/atom) and involves considerably smaller sputtering yields, as would be expected due to the large difference in surface binding
MeV
H+,
He+,
N ‘)
In the earlier study by Ollerhead et al. [I], where electronic stopping was expected to predominate (cf. fig. l), the sputtering of frozen Xe by 0.5-2.0 MeV N+ had exhibited a rather surprising energy dependency as E increased, implying that where Yobs decreased nuclear stopping was the predominant sputtering mechanism. On the other hand, the magnitude of Y,, at 500 keV was almost a factor of 10 larger than would be predicted by linear cascade theory for this F,(O) value of 36 eV A2/atom. We therefore decided to extend our present N+ sputtering study up to MeV energies in order to overlap with the previous work. These measurements (and also a 2 MeV He+ bombardment to check reproducibility) were carried out at Chalk River, using the same accelerator and target chamber system as in ref. 1, and with the frozen Xe film at 20 K. The results of our MeV sputtering runs are summarized in table 2, together with the earlier data. Note that both sets of 2.0 MeV He data agree quite well, as do the 600 keV N+ results. However, our high energy N+ runs exhibit a completely different energy dependence from that reported earlier and, by 2.0 MeV, our Y,, value differs from the Ollerhead data by more than an order of magnitude. The present set of N+ bombardments exhibit at least qualitatively the expected dependence on electronic stopping power. Furthermore, the sputtering rate at each energy was checked several times, with an overall reproducibility of flO%. No satisfactory explanation has yet been found for the anomalously low sputtering yield reported [l] in the previous N+ bombardment study. Fig. 6 shows the dependence of the observed sputtering yields at MeV energies on the incident electronic stopping power, S,(O). For the N+ bombardments, only the data obtained in the present study are used. In each case we have first subtracted the estimated linear cascade
319
D. V. Stevanovic et al. / Sputtering of frozen xenon
dence in our frozen Xe sputtering studies is similar to that found in ice and other insulators: namely, the H+ and He+ results obey an m = 2 slope almost exactly, but that the extension to heavier ions (N+) produces a significant increase in the S,(O) dependence. The m = 2 curve passing through the H+ and He+ data in fig. 6 was used to derive the approximate Y,, contributions listed in table 1 for each of the low E heavy-ion bombardments. These Ye, contributions are seen to represent a negligible fraction of the observed sputtering yields in the 20-80 keV heavy-ion bombardments involved in the present study. 3.3. Intermediate-&
lo”
IO2
I2
S,(O) (eV 82/atotn) Fig. 6. Estimated yield due to electronic stopping vs. S,(O) MeV H+, He+ and NC bombardments. Values of S,(O), surface deposited energy due to electronic stopping, were tained from the Ziegler stopping power tabulations [7] for
for the ob-
H+ and He+ and from the recent stopping power measurements of Ward et al. [8] in the N+ case.
contribution (Y,,,), shown in table 2; this correction was derived by appropriate extrapolation of the low-~ data in fig. 5. The resulting data points for H+ and He+ bombardments (fig. 6) show a reasonably close fit to a quadratic (i.e. m = 2) dependence, extending over approximately two order of magnitude in sputtering yield. Such a dependence on [S,(O)]*, had been observed previously [9] in bombarding ice with MeV H+ and He+ ions and an even stronger dependence, i.e. approaching [ S,(0)]4, was seen [lO,ll] in the bombardment of various insulators, including ice, with heavier ions such as 2-20 MeV fluorine. Careful examination of fig. 6 shows that the observed stopping power depen-
regime
(O.S-2.0
MeV
Ar ‘)
At intermediate E values, both the nuclear cascade and the electronic sputtering mechanisms would be expected to contribute significantly to the observed sputtering yield. The MeV Ar+ bombardments of Ollerhead et al. [l] fall within this energy regime. As the data in table 3 indicate, their Yobs values are quite similar in magnitude to our (Ye, + Y,,) terms predicted from figs. 5 and 6. This agreement is although not quantitative, a surprisingly good prediction for such a complex energy loss regime, especially where F,(O) falls well within the linear cascade regime of fig. 5. However, when F,,(O) approaches the threshold value - 500 eV A2/atom, this additivity of electronic and cascade contributions becomes less reliable, as seen for the 600 keV ArC bombardment in table 3.
4. Summary In the low-~ region (table 1) where S,(O) and F,(O) are comparable in magnitude, clearly the nuclear stopping is the dominant mechanism contributing to the sputtering yield. At somewhat higher energies (table 3), where S,(O) exceeds F,(O) by l-2 orders of magnitude, the observed sputtering involves significant contributions from both nuclear and electronic energy loss mechanisms. These two mechanisms appear to be approximately additive. However deviation from this additivity increases as the F,(O) values approach 500 eV A*/atom, suggesting that
Table 3 Sputtering yields in the intermediate-e regime. Ion
Ar+
E
F,(O) (eV A*/atom)
S,(O) B (eV A2/atom)
Lsc +
r, =
(keV)
(fig. 5)
(fig. 6)
600 1000 1500 2000
340 227 159 125
2460 3200 3900 4500
130 87 60 46
70 105 180 240
Ywita,
Yohs
200 192 240 286
320 260 250 280
a Determined from ref. 8. IV. CONDENSED
GASES
320
D. V. Stevanovic et al. / Sputtering of /men
cooperative effects’ between both forms of energy deposition may result in an enh ancement of YabSover that expected from a simply additive process. At still higher energies (table 2) where S,(O) exceeds F,(O) by at least 3 orders of magnitude, electronic stopping clearly becomes the dominant sputtering mechanism. We are especially indebted to T. Jackman for assisting with the MeV N+ measurements reported in table 2 and for helpful discussions on the manuscript. Partial funding for this work was supplied by the Natural Sciences and Engineering Council of Canada.
References (11 R.W. Ollerhead, J. tittiger,
J.A. Davies, J. L’Ecuyer, H.K. Haugen and N. Matsunami, Rad. Eff. 49 (1980) 203.
xenon
[2] F. Besenbacher, J. Bettiger, 0. Gravensen, J.L. Hansen and H. Sorensen, Nucl. Instr. and Meth. 191 (1981) 221. M. Scharff and H.E. S&i@, Kgl. Dansk. 131 J. Lindhard, Vid. Selsk. Mat. Fys. Medd. 33 (1963) No. 14. Ion implantation range and energy de141 K.B. Winterbon, position distribution (Plenum, New York, 1975) vol. 2. 151 P. Sigmund, Phys. Rev. 184 (1969) 393. PI D.A. Thompson, J. Appl. Phys. 52 (1982) 982. [71 J.F. Ziegler, The stopping and ranges of ions in matter, ~01s. 3 and 4 (Pergamon, New York, 1977). LV Mitchell, W.N. Lennard, PI D. Ward, H.R. Andrews, R.B. Walker and N. Rud, Can. J. Phys. 57 (1979) 645. E. Brody, B. Cooper, 191 W.L. Brown, W.M. Augustyniak, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. Rad. WI L.E. Seiberling, J.E. Griffith and T.A. Tombrello, Eff. 52 (1980) 201. and R.E. Johnson, Science 1111 W.L. Brown, L.J. Lanzerotti 218 (1982) 525.
Nuclear Instruments and Methods in Physics Research Bl (1984) 315-320 North-Holland, Amsterdam
SPUTIXRINC Doris
V.
Department
OF FROZEN XENON BY keV HEAW IONS
STEVANOVIC, D.A. THOMPSON and J.A DAVIES * 01 Engineering Physics, McMasier
Universiiy, Hamilton, Ontario, Canada ~58.9 4Ml
Sputtering measurements of frozen Xe have been made using 20-80 keV heavy ions (N, Ar, Sb and Bi), i.e. where nuclear stopping is the dominant energy loss process. Sputtering yields (Y) of up to 1.6 x lo4 Xe atoms/ion have been obtained. The dependence of Y on the surface deposited (nuclear) energy F,(O) was found to exhibit two distinct regimes: at low F,(O) values, Y increases linearly with deposited energy, as predicted by cascade sputtering theory, but above a threshold value of - 500 eV A2/atom the sputtering yield exhibits a much stronger (approximately cubic) dependence on F,(O). Comparison of the sputtering yields obtained by monatomic and diatomic equal velocity heavy ion bombardments (e.g. Sb/Sb2) further confirmed this non-linear behaviour at large F,(O). Similar monatomic and diatomic light ion bombardments (e.g. N/N,) showed no molecular enhancement, hence a linear behaviour at lower F,(O). The current results, which are obtained in the energy regime where nuclear stopping dominates, are compared to earlier measurements at MeV energies in which electronic stopping dominates. This comparison suggests that the observed sputtering behaviour of frozen Xe is complex, with significant contributions from both nuclear and electronic processes.
1. Introduction Previous experiments [1,2] on the sputtering of frozen inert gases (Xe,Kr,Ar) were performed with energetic light ions, such as MeV He+, where the dominant energy loss process is by electronic excitation and ionization. In each case, the measured sputtering yields, Y,,, were 2-3 orders of magnitude greater than predicted by collision cascade theory. By varying the incident ion energy (OS-2 MeV) and species (H+, He+, N+ and Ar+), Ollerhead et al. [l] tried to determine the mechanism responsible for these high sputtering yields. Within the energy range available to them, the measured sputtering yields for H+ and He+ bombardment scaled roughly with the square of the electronic stopping, S,(O). On the other hand, the sputtering yield for N+ and Ar+ bombardments did not follow the expected energy dependence of the electronic stopping power curve, but instead approached the functional dependence of the elastic stopping power; i.e. the yield decreased for increasing energy. This result seems rather surprising because, even for Ar+, the dominant energyloss process at such high energies is via electronic excitation. For example, for OS-2 MeV N+, less than 5% of the total energy loss is due to nuclear recoil contributions. The present work extends the sputtering yield measurements in frozen Xe into the low-energy regime where nuclear stopping becomes the dominant energy
loss process. For this purpose, 20-80 keV N+, Ar+, Sb+, Bi+ and 80 keV N: and Sb: were chosen as incident ions. In the case of N+, the measurements were subsequently extended to 2.0 MeV in order to check the anomalous energy dependence in the earlier data of Ollerhead et al. [l]. The various energy loss regimes considered in the present work and in the previous investigation of frozen Xe sputtering are summarized schematically in fig. 1, in terms of the reduced energy scaling parameter e introduced by Lindhard et al. [3]. Their universal nuclear stopping power curve (S,,) and a typical electronic stopping power curve (S,) are shown. Previous experimental results on the sputtering of Xe involved ions in the E = 4-400 region, where energy is
-
-
/.-
OLLERHEAD
- -
,
I
I,,
I
I
2
3”
IO
PRESENT
et al WORK
1
20
E 4
Fig. 1. Nuclear (S,) and electronic (S,) stopping powers vs. * Permanent address: Solid State Science Branch, AECL, Chalk River, Ontario, Canada. 0168-583X/84/$03.00 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Lindhard’s reduced energy parameter e. The appropriate regions covered in the present study and in the previous investigation by Ollerhead et al. [l] are shown by horizontal arrows. IV. CONDENSED
GASES
316
D. V. Stevanovic et al. / Sputtering of frozen xenon
lost almost entirely by electronic collisions. In the present work (E = O.Ol-l.S), the major contribution to energy loss is from nuclear collisions. Note however that, throughout this low E regime, (fig. 1) there is always a significant (i.e. > 20%) S, contribution to the total stopping power.
2. Experimental technique
l
I
I
I
1
400-
,
1 I I , , I , I
ZOO-
Films of Xe [(0.5-2) X 10” atoms/cm2 in thickness] were condensed on a cold silicon substrate upon which a thin film of Ti ( - 200 A) had been previously deposited. The silicon was cryogenically cooled to 40 K and was surrounded by a Cu cryoshield maintained at 30 K. A schematic of the target arrangement is given in fig. 2. A 5 W ohmic heater attached to the back of the target holder enabled the residual Xe films to be removed rapidly after each sputtering measurement: i.e. before condensing a fresh Xe film on the substrate. Typical ion bombardment doses were 10’3-1015 ions/cm2, with the incident beam swept horizontally and vertically across a 4 mm aperture in front of the target to ensure uniform bombardment. The thickness of the Xe films was measured by Rutherford backscattering of 2 MeV 4He+ ions, with a Si surface barrier detector being used to detect the particles scattered through an angle of 150’. The incident current density was always less than 10m9 A/mm2 in order to avoid possible heating effects, either by the analysing beam (which was collimated to a diameter of 2 mm) or by the implantation beam. At 80 keV, the measured sputtering yield was observed to be independent of current density over the range (0.1-4)~ 10e9 A/mm2. In each RBS analysis, a total 4He+ dose of 0.5 pC was required in order to obtain reasonable counting statistics. Measurements at considerably larger 4He+ doses showed that the sputtering loss/f.~C during RBS analysis was only - 2 X 1015 Xc/cm’; this loss is negligible compared to the large sputtering yields obtained for the heavy ions. Several successive bombardments and RBS analyses
m
r
, 0
Kt 0
00
200
CHANNEL
, 300
_:
\ 400
NVMBER
Fig. 3. Typical RBS spectrum for a frozen Xe film ( - 1 X 10” Xe atoms/cm2) on the Si(Ti) substrate. Analysing beam was 2.0 MeV 4He+.
were carried out on each Xe film in order to establish a reliable sputtering yield value. Any dependence of sputtering yield on film thickness was minimized by using much thicker frozen Xe films than the incident heavy ion range. A typical RBS spectrum is shown in fig. 3. The thin Ti layer on the Si substrate provided an independent calibration marker for monitoring the Xe film thickness; it also served to indicate whether the Xe film had become nonuniform after prolonged sputtering.
3. Results and discussion 3.1. LOW-Eregime (20-80
keV heavy
ions)
Data from a typical sputtering experiment are shown in fig. 4. The frozen Xe film thickness, as determined from the RBS yield following each Sb+ bombardment, is seen to decrease linearly with increasing Sb+ dose, indicating a constant sputtering yield Y,, = 1905 Xe
CRYOCOWR AAolAnoN
SHIELD
SLOPE
= 1905
Xe otoms/Sb+
ion
Fig. 4. Xe film thickness (derived from the 2.0 MeV 4He RBS Fig. 2. Schematic representation of the target assembly for frozen-gas sputtering measurements.
yield ) vs Sb+ ion dose. The linear dependence constant sputtering yield of 1905 atoms/ion.
indicates
a
D. V. Sievanovic et al. / Sputtering of frozen xenon
311
Table 1 Sputtering yields in the low-c regime. Ion
E
se69 (eV A2/atom)
F,,(O) (eV A2/atom)
WV)
Yohs
YeIa
N+
40 80
240 176
460 650
1 2
101 70
N:
80
480
920
4
217
Ar+
40 80
884 843
640 900
2 4
784 805
Sb+
40 80
1523 1762
810 1150
3 I
1905 2854
Sb;
80
3040
1620
15
16100
Bi+
20 40 60 80
1506 1802 2029 2180
600 850 1040 1200
2 3 6 8
3190 3160 5370 6500
a Predicted from fig. 6. atoms/ion. A similar linear decrease of thickness was observed in almost all cases. However, in the cascades where surface deposited nuclear energy, F,(O), is highest a significant (80 keV Sb: and Bi+ bombardments), decrease in slope sometimes occurred at bombardment the cause of doses in excess of - 5 x 1013 ions/cm*; this gradual change in sputtering yield at high dose is not yet understood. The observed heavy-ion sputtering yields (Yobs) are summarized in table 1. In the case of Bi+ and Sb:
bombardment, the initial linear region at bombardment doses ( 5 x 1013 ions/cm* was used to evaluate the sputtering yield. Included in table 1 are theoretical estimates of the surface deposited energies, I;,(O) and S,(O), resulting from the nuclear stopping and electronic excitation processes, respectively, as each bombarding ion penetrates the frozen Xe film. The values of F,(O) were obtained from the Winterbon tables [4] and the S,.(O) values were calculated from Lindhard et al. [3]. Column 5 gives the predicted sputtering yield Y, associ-
Table 2 Sputtering yields in the high-~ regime. IOIl
Present data: He+ N+
Ollerhead et al. data [l]: H+
E (keYI 2000 600 1000 2000
F,(O)
se(0) a
(eV AZ/atom)
(eV A2/atom)
0.3 29 15 7
Ycast
b
1100
- 0.1
1783 2302 3256
8 4 2
Yohs
7 79 114 203
250 500 1000
0.1 0.07 0.04
400 275 180
- 0.01 0.01 0.01
He+
600 1000 1500 2000
0.8 0.6 0.4 0.3
1500 1380 1250 1100
- 0.1 0.1 0.1 0.1
10 9 8 7
N+
600 1000 2000
1783 2302 3256
8 4 2
45 35 19
29 15 I
0.6 0.3 0.08
a S,(O) data were determined from ref. 7 for H+ and He+ and from ref. 8 for N+ bombardment. b Predicted from fig. 5. IV. CONDENSED
GASES
D. V. Stevanovic et al. / Sputtering of frozen xenon
318
BI . Sb
0
o Ar
energy between frozen Xe and the metal targets. Further evidence for the existence of these two markedly different sputtering regimes can be seen in table 1 by comparing the behaviour of the diatomic ions, 80 keV NC and Sb: , with that of the corresponding monatomic ions of identical velocity, i.e. 40 keV N+ and Sb+. In the nitrogen comparison, Yobs for the 80 keV NC bombardment is almost exactly twice the 40 keV N+ value, as would be predicted for the two-fold increase in F,(O), since both ions fall within the linear cascade regime. On the other hand, in the Sb case Y,, for the 80 keV Sb: bombardment is almost a factor of ten larger than the 40 keV Sb+ value, thus indicating clearly the existence of a much stronger than linear dependence on F,,(O) for cascades above the - 500 eV A2/atom threshold. 3.2. High-e regime (0.5-2.0
Fig. 5. Observed sputtering yield Y,, vs. F,(O) for 20-80 keV N+, Ar+, Sb+ and Bi+ bombardments. Note that F,(O) is the estimated surface-deposited energy due to nuclear stopping alone. The dashed curve represents the predicted values (Y,) given by linear cascade theory.
ated with each S,(O) value; these were derived by interpolating the YobBdata in the high-s regime (table 2 and fig. 6) where electronic excitation is the dominant sputtering mechanism. Hence for all cases in table 1, Y,, (by itself) would be a negligible fraction of Yobs, indicating that nuclear stopping is the dominant sputtering mechanism throughout the low-s regime. Nevertheless, it is interesting to note that in this region the S,(O) and F,,(O) values are comparable in magnitude. The dependence of Y,, on F,,(O) can be subdivided clearly into two completely different zones, shown graphically in fig. 5. At low surface deposited energies, a linear dependence on F,(O) is observed, as predicted by theory [5]; furthermore, in this region, the absolute magnitude of Yobsis given quite well (i.e. within - 50%) by the Sigmund linear cascade treatment, based on a surface binding energy of 0.16 eV/atom for xenon. On the other hand, at F,(O) values > 500 eV A2/atom, Y,, increases much more rapidly and exhibits an approximate cubic dependence on F,(O). This enhanced sputtering behaviour in high density cascades is remarkably similar to that reported earlier in Ag, Au and Pt targets by Thompson [6], where again a sharp threshold was observed between a linear and almost cubic dependence on F,(O). In the case of the metals, this threshold occurs at significantly higher F,(O) values, (i.e. - lOOt-3500 eV A2/atom) and involves considerably smaller sputtering yields, as would be expected due to the large difference in surface binding
MeV
H+,
He+,
N ‘)
In the earlier study by Ollerhead et al. [I], where electronic stopping was expected to predominate (cf. fig. l), the sputtering of frozen Xe by 0.5-2.0 MeV N+ had exhibited a rather surprising energy dependency as E increased, implying that where Yobs decreased nuclear stopping was the predominant sputtering mechanism. On the other hand, the magnitude of Y,, at 500 keV was almost a factor of 10 larger than would be predicted by linear cascade theory for this F,(O) value of 36 eV A2/atom. We therefore decided to extend our present N+ sputtering study up to MeV energies in order to overlap with the previous work. These measurements (and also a 2 MeV He+ bombardment to check reproducibility) were carried out at Chalk River, using the same accelerator and target chamber system as in ref. 1, and with the frozen Xe film at 20 K. The results of our MeV sputtering runs are summarized in table 2, together with the earlier data. Note that both sets of 2.0 MeV He data agree quite well, as do the 600 keV N+ results. However, our high energy N+ runs exhibit a completely different energy dependence from that reported earlier and, by 2.0 MeV, our Y,, value differs from the Ollerhead data by more than an order of magnitude. The present set of N+ bombardments exhibit at least qualitatively the expected dependence on electronic stopping power. Furthermore, the sputtering rate at each energy was checked several times, with an overall reproducibility of flO%. No satisfactory explanation has yet been found for the anomalously low sputtering yield reported [l] in the previous N+ bombardment study. Fig. 6 shows the dependence of the observed sputtering yields at MeV energies on the incident electronic stopping power, S,(O). For the N+ bombardments, only the data obtained in the present study are used. In each case we have first subtracted the estimated linear cascade
319
D. V. Stevanovic et al. / Sputtering of frozen xenon
dence in our frozen Xe sputtering studies is similar to that found in ice and other insulators: namely, the H+ and He+ results obey an m = 2 slope almost exactly, but that the extension to heavier ions (N+) produces a significant increase in the S,(O) dependence. The m = 2 curve passing through the H+ and He+ data in fig. 6 was used to derive the approximate Y,, contributions listed in table 1 for each of the low E heavy-ion bombardments. These Ye, contributions are seen to represent a negligible fraction of the observed sputtering yields in the 20-80 keV heavy-ion bombardments involved in the present study. 3.3. Intermediate-&
lo”
IO2
I2
S,(O) (eV 82/atotn) Fig. 6. Estimated yield due to electronic stopping vs. S,(O) MeV H+, He+ and NC bombardments. Values of S,(O), surface deposited energy due to electronic stopping, were tained from the Ziegler stopping power tabulations [7] for
for the ob-
H+ and He+ and from the recent stopping power measurements of Ward et al. [8] in the N+ case.
contribution (Y,,,), shown in table 2; this correction was derived by appropriate extrapolation of the low-~ data in fig. 5. The resulting data points for H+ and He+ bombardments (fig. 6) show a reasonably close fit to a quadratic (i.e. m = 2) dependence, extending over approximately two order of magnitude in sputtering yield. Such a dependence on [S,(O)]*, had been observed previously [9] in bombarding ice with MeV H+ and He+ ions and an even stronger dependence, i.e. approaching [ S,(0)]4, was seen [lO,ll] in the bombardment of various insulators, including ice, with heavier ions such as 2-20 MeV fluorine. Careful examination of fig. 6 shows that the observed stopping power depen-
regime
(O.S-2.0
MeV
Ar ‘)
At intermediate E values, both the nuclear cascade and the electronic sputtering mechanisms would be expected to contribute significantly to the observed sputtering yield. The MeV Ar+ bombardments of Ollerhead et al. [l] fall within this energy regime. As the data in table 3 indicate, their Yobs values are quite similar in magnitude to our (Ye, + Y,,) terms predicted from figs. 5 and 6. This agreement is although not quantitative, a surprisingly good prediction for such a complex energy loss regime, especially where F,(O) falls well within the linear cascade regime of fig. 5. However, when F,,(O) approaches the threshold value - 500 eV A2/atom, this additivity of electronic and cascade contributions becomes less reliable, as seen for the 600 keV ArC bombardment in table 3.
4. Summary In the low-~ region (table 1) where S,(O) and F,(O) are comparable in magnitude, clearly the nuclear stopping is the dominant mechanism contributing to the sputtering yield. At somewhat higher energies (table 3), where S,(O) exceeds F,(O) by l-2 orders of magnitude, the observed sputtering involves significant contributions from both nuclear and electronic energy loss mechanisms. These two mechanisms appear to be approximately additive. However deviation from this additivity increases as the F,(O) values approach 500 eV A*/atom, suggesting that
Table 3 Sputtering yields in the intermediate-e regime. Ion
Ar+
E
F,(O) (eV A*/atom)
S,(O) B (eV A2/atom)
Lsc +
r, =
(keV)
(fig. 5)
(fig. 6)
600 1000 1500 2000
340 227 159 125
2460 3200 3900 4500
130 87 60 46
70 105 180 240
Ywita,
Yohs
200 192 240 286
320 260 250 280
a Determined from ref. 8. IV. CONDENSED
GASES
320
D. V. Stevanovic et al. / Sputtering of /men
cooperative effects’ between both forms of energy deposition may result in an enh ancement of YabSover that expected from a simply additive process. At still higher energies (table 2) where S,(O) exceeds F,(O) by at least 3 orders of magnitude, electronic stopping clearly becomes the dominant sputtering mechanism. We are especially indebted to T. Jackman for assisting with the MeV N+ measurements reported in table 2 and for helpful discussions on the manuscript. Partial funding for this work was supplied by the Natural Sciences and Engineering Council of Canada.
References (11 R.W. Ollerhead, J. tittiger,
J.A. Davies, J. L’Ecuyer, H.K. Haugen and N. Matsunami, Rad. Eff. 49 (1980) 203.
xenon
[2] F. Besenbacher, J. Bettiger, 0. Gravensen, J.L. Hansen and H. Sorensen, Nucl. Instr. and Meth. 191 (1981) 221. M. Scharff and H.E. S&i@, Kgl. Dansk. 131 J. Lindhard, Vid. Selsk. Mat. Fys. Medd. 33 (1963) No. 14. Ion implantation range and energy de141 K.B. Winterbon, position distribution (Plenum, New York, 1975) vol. 2. 151 P. Sigmund, Phys. Rev. 184 (1969) 393. PI D.A. Thompson, J. Appl. Phys. 52 (1982) 982. [71 J.F. Ziegler, The stopping and ranges of ions in matter, ~01s. 3 and 4 (Pergamon, New York, 1977). LV Mitchell, W.N. Lennard, PI D. Ward, H.R. Andrews, R.B. Walker and N. Rud, Can. J. Phys. 57 (1979) 645. E. Brody, B. Cooper, 191 W.L. Brown, W.M. Augustyniak, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. Rad. WI L.E. Seiberling, J.E. Griffith and T.A. Tombrello, Eff. 52 (1980) 201. and R.E. Johnson, Science 1111 W.L. Brown, L.J. Lanzerotti 218 (1982) 525.