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Angular distribution of Ag atoms sputtered with A 5 keV Ar+ ion beam
Nuclear Instruments and Methods North-Holland, Amsterdam
ANGULAR Marek
in Physics Research
DISTRIBUTION
SZYMONSKI
B14 (1986) 263-267
OF Ag ATOMS SPUTTERED
*, Weixing
HUANG
263
WITH A 5 keV Ar + ION BEAM
and Jens ONSGAARD
Fysisk Insritut, Odense Universitet, Campusvej 55, DK - 5230 Odense M, Denmark
Received
28 June 1985 and in revised form 1 November
1985
A polycrystalline, high-purity Ag target was sputtered with a normally incident, 5 keV Ar + ion beam under ultrahigh vacuum (UHV) conditions. Sputtered material was collected on a cylindrically shaped Al foil and subsequently analysed in an Auger electron spectrometer. The obtained results show that sputtering under UHV conditions leads to a cosine-like distribution. The distribution taken after prolonged ion bombardment contained a hump at angles of 30-45” superimposed on the general form which we ascribe to an ion beam-induced texturing effect. The results are compared with recent investigations of other authors. The influence of the surface conditions of both the target and the foil used for collection of the sputtered material on the angular distribution measurements has also been investigated.
1. Introduction Angular distributions of material sputtered from polycrystalline and amorphous targets have been the subject of experimental activities for several decades [l]. From the theoretical side, both relevant sputtering models, the linear collision cascade [2] and the spike evaporation [3], predicted nothing more than a cosine distribution for the sputtered flux. Only for projectile energies close to the threshold or for light ion bombardment the number of generated recoils might be too small for development of the isotropic cascade [4]. Recent computer simulations [5,6], on the other hand, have shown over-cosine distributions for high bombarding energies, but close to cosine for low keV bombardment. Most of the normal incidence measurements, usually performed under moderate vacuum conditions, showed over-cosine distributions. Okutani et al. [7], however, measured angular distributions of Si atoms sputtered by low keV Art ions in an UHV chamber. Their normal incidence data agreed fairly well with a cosine law as predicted by theory. On the other hand UHV measurements by Allas et al. [8] yielded several over-cosine distributions for selfsputtering of Al, Ni and Cu; the projectile energies in this experiment were much higher. Vozzo and Reynolds [9] found that As, sputtering of Au in ordinary vacuum produced a strongly over-cosine distribution, but under UHV conditions the distribution was much closer to a cosine one.
* Present address: Instytut Fizyki, Uniwersytet Reymonta 4, 30-059 Krakow, Poland.
Jagiellohski.
0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
These results indicate that in some cases, adsorbed impurity layers may cause an over-cosine form for the distribution. Target atoms originating from layers deeper than the adsorbate are preferentially ejected into a narrow cone around the surface normal [lo]. Experimental evidence of this effect was found in recent angu!ar distribution studies of sputtering for alloys (see for example ref. [17]). The same argument holds for heavily damaged surfaces due to prolonged ion bombardment. In view of this, further experimental data are needed, obtained under UHV conditions and with well controlled surface purity and topography. We have performed, therefore, such measurements using Auger electron spectroscopy as a tool both for monitoring surface cleanliness and analysing the composition of the collected sputtered material. As an example, a polycrystalline Ag sample was sputtered with a 5 keV Ar ion beam. Silver was chosen not only to facilitate AES measurements (silver is frequently used as a standard sample material), but it is also interesting for comparison with the previous, high dose measurements [ll-131. The old work by Perovii: and CobiC [ll] was done under poor vacuum conditions. Thus, the observed deviation from a cosine distribution is not surprising. Although measurements by Emmoth et al. [12] and Besocke et al. [13] were performed under much better vacuum, still a distinct influence of surface topography is seen in their Ag distributions. Inspection of the bombarded target by scanning electron microscopy [ 131 revealed pronounced cone formation, presumably causing a narrow, forwardpeaked distribution, via the above-described mechanism.
M. Sz.vmonski et al. / Angular distribution ofsputtered Ag atoms
264
2. Experimental
A polycrystalline, high purity Ag sample (99.999% Ag) was mechanically polished with an accuracy of better than 0.5 pm and cleaned in an ultrasonic bath before being mounted in an UHV chamber. Scanning electron micrographs taken in advance and after sputtering showed the target surface as rather flat at a magnification of 3.6 X 104. No specific check on the presence of texture before sputtering was made. The system was evacuated to the lo-” Torr range and the sample was sputter cleaned until no traces of impurities could be detected with an Auger electron spectrometer. The spectrometer was the commercial 4 grid system manufactured by Varian. The target was sputtered with a 5 keV Ar+ beam delivered by a differentially pumped Leybold ion gun. The content of impurities was estimated to be less than 3%. Although the beam was not analysed, neutrals were prevented from striking the surface by deflecting the collimated beam prior to the entrance of the final aperature. There still exists a possibility of multiply charged ions being present in the beam with higher bombarding energies. According to both the computer simulations [5,6] and the experimental work [15] higher
Electron
energy ions would cause the distribution to be overcosine. Since we have observed nearly cosine distributions, as presented in the following section, we can exclude this effect from our consideration. The beam intensities were measured with a removable Faraday cup. Typical values were 0.4 PA impinging through a hole of 1 mm in diameter. The pressure in the chamber rose to 3 X lo-’ Torr during sputtering, mainly due to an increase in the partial pressure of Ar, while other rest gases were continuously pumped by ion-getter and turbomolecular pumps. Doses up to 4 x 10” ions/cm2 were typically used. This assures that the thicknesses of sputter-deposited films did not exceed the mean free path of Auger electrons. Repeated measurements with smaller doses have been performed in order to assure a linear dependence of the thickness of collected material on the dose within the whole investigated range of ejection angles. From the point of view of ion beam-induced morphology changes and texture effects, a small dose is preferable too, minimising an influence of those effects on the angular spectra. Regularly, the target, after a fluence of 2-3 x 10” Ar+ ions/cm2, was moved to a new position without changing the measurement geometry, whereupon a hitherto unirradiated area was exposed to the beam. Sputtered atoms were collected on aluminium or
Gun ( integral
Mass
)
Spectrometer
Vacuum
Leybold
Ion
Gun
meter
Y&indoy Electron
¶tus
Gun ( integral
P
Ar gas
inlet ransmlssion
Fig. 1. A schematic
view of the experimental
setup. For details
see text.
shaft
)
265
M. Szymonski et al. / Angular distribution o/sputtered Ag atoms
ejection
aluminium covered stainless steel ‘catcher foils. Cylindrical collectors with a radius of 23 mm were used and could be analysed directly before and after sputtering without breaking of the vacuum. The deposit was subsequently analysed with an AES analyser. The peak-topeak heights of the characteristic Auger Ag-MNN transitions, positioned at 347.5 and 353.4 eV, were measured as a function of the position x on the foil. Since x is directly related to the ejection angle 0 measured with respect to the surface normal, this function described the angular distribution of the sputtered material: 1(x(B)). A schematic view of the experimental setup is shown in fig. 1. Finally, a few words should be said on the possibility of resputtering of collected material, Resputtering is caused mainly by reflected Ar atoms in view of their high energy as compared to that of sputtered Ag atoms [3]. With a reflection coefficient of [18] - 0.10, an upper limit of half the primary energy for the mean of reflected Ar, and the established variation of the sputter yield with energy [3], the flux of resputtered Ag atoms reaches 5 3 pet. of the primary sputtered Ag flux, once the collector is homogeneously covered with silver, starting from zero at lower fluences. Therefore, the influence of resputtering on the measurement is smaller than that of other sources of error.
angle
(
degree
)
6.
4.
9 Q
s
0.z-
Fig. 2. Polar plot of the angular distribution of Ag sputtered from a polycrystalline silver sample by 5 keV Ar+ bombardment under UHV conditions. The fluence was 1.5X 10” ions/cm2. The distribution has been normalized to give the same absolute yield as a cosine distribution, represented by a solid semicircle.
3. Results and discussion A polar plot of the Auger peak to peak height of sputter-deposited Ag is presented in fig. 2. The experimental points have been normalized in order to give the same integrated yield (27r/Y( 0) sin 8 d0) as a cosine distribution represented in fig. 2 as the solid semicircle. From the figure it is seen clearly that the angular distribution of Ag atoms sputtered with a 5 keV Ar+ beam is very close to cosine in contrast to the over-cosine distributions found in previous work [ll-131. A prolonged ion bombardment of our Ag polycrystalline target resulted in the distribution shown in fig. 3. In addition to the general form of the distribution, a hump appears between 30 and 45”. Although we cannot exclude that the sample was textured prior to the ion bombardment, we have definitely seen that the fully developed hump in the spectrum (as seen in fig. 3) appeared only after a fluence exceeding 3-5 x 10” ions/cm2. Thus, by comparison of fig. 2 (fluence of 1.5 x 10” ions/cm2) and fig. 3 (fluence larger than 4 X 10” ions/cm2) we can ascribe the above effect to beam-induced texturing of the sample surface. Similar effects have frequently been observed for polycrystalline metals bombarded with fluences of a few times 10” ions/cm2 or more [14]. It is very likely that the irradiated silver sample was textured with (111) directions
ejection
angle (degree)
I 40'
. .
.
/ 50’
.
Fig. from ment 10”
3. Polar plot of the angular distribution of Ag sputtered a polycrystalline silver sample by 5 keV Art bombardunder UHV conditions. The total fluence exceeded 5X ions/cm2. The distribution has been normalized as in fig.
M. Szymonski
266
et al. / Angular
largely oriented perpendicular to the surface [15]. Preferential ejection around close-packed (110) directions is consistent with the observed angular position of the hump. Such beam-induced texture effects have recently been proposed as the explanation of similar humps in the angular distribution of sputtered Cu atoms [15] and were very likely responsible for many experimental observations of heart-shaped distributions [8,16]. If we assume that a certain fraction of the sputtered atoms is ejected from the textured surface within a narrow solid angle, centered at (110) directions, we might subtract this fraction from the experimental distribution. Describing a probability for this directional ejection by a Gaussian function centered at the (110) direction with a half width of 15”, we have obtained the fraction f= 15% for the best agreement of the remaining distribution with the cosine function. Note that only the width and amplitude of the Gaussian were fitted but not the position of the maximum which was taken from the texture model considerations. The result is presented in fig. 4. We assert, therefore, that the angular distribution of sputtered Ag is cosine in shape, except for the texture effect giving rise to the additional hump in the distribution. Such an isotropic distribution of ejection angles was predicted by the linear collision cascade theory of sputtering [2] with the assumption of planar surface
1.0
.\. -
‘-J+ l\
t-
-I t/l
&
.-
.
0.8
5
_
.
\
~-
_~
.
.
. 6-O
0
eject& Fig. 4. points, curve), curve), text.
A!2
AT'
ai$e
( degree
.
.
80
>
The same distribution as in fig. 3. The experimental corrected for directional (110) ejection (lower solid are compared with a cosine distribution (upper solid normalized to the same integrated yield. For details see
distrihutron
ofsputtered Ag utoms
ejection
angle
(degree)
Fig. 5. Polar plot of the angular distribution of material sputtered from polycrystalline silver sample with a 5 keV Ar+ beam under the pressure of 2 x 10m5 Torr. The total fluence was 2 X 10” ions/cm2. The normalization is the same as in fig. 2. A cosine distribution is represented by a solid semicircle.
binding. Over-cosine distributions, frequently observed previously, might be related, to some extent, to poor vacuum conditions of those experiments as discussed in the introduction. This is visualized in fig. 5, where the angular distribution of Ag atoms sputtered in a vacuum of 2 X 10e5 Torr is plotted. A striking difference between this distribution and that taken under UHV conditions (fig. 2) is clearly visible in agreement with the theoretical predictions [lo]. AES inspection of the material collected during this last measurement showed a significant content of sulfur, oxygen and carbon impurities. The above analysis of the collected material indicates still another important factor, which may influence angular distribution measurements performed under poor vacuum conditions. A large content of impurities present in the catcher foil before and/or during collection of the sputtered material can certainly affect the sticking probability. Consequently, positions on the foil near normal ejection, with a thicker layer of deposited material, might have a sticking ratio different from position at glancing ejection. We have seen evidence for such an artefact by comparing the data obtained with and without sputter cleaning of the collector foil prior
M. Szymonski
et al. / Angular distribution of sputtered Ag utoms
to measurements. The distributions obtained sputter cleaned collector were more cosine those collected on an impurity-covered foil, vacuum conditions during the deposition were
with the like than although the same.
Inspiring discussions with Prof. Peter Sigmund, Dr. Per Morgen and the Danish sputtering club are gratefully acknowledged. We are most grateful to Dr. Hans A. Boye for taking scanning electron micrographs. Two of us (MS. and W.H.) acknowledge support from the Danish Natural Science Research Council for the time period during which these measurements were carried out. W.H. would also like to acknowledge support from Ingeborg og Leo Dannins Legat for Videnskabelig Forskning.
References [l] Among the first reports were: R. Seeliger and K. Sommermeyer, 2. Physik 93 (1935) 692; G.K. Wehner and D. Rosenberg, J. Appl. Phys. 31 (1969) 177. [2] P. Sigmund, Phys. Rev. 184 (1969) 383. [3] P. Sigmund, in: Sputtering by Particle Bombardment I, ed., R. Behrisch (Springer-Heidelberg-New York, 1981) p. 9. [4] U. Littmark and P. Sigmund, J. Phys. D8 (1975) 241.
261
[51 J.P. Biersack and W. Eckstein. Appl. Phys. A34 (1984) 73. [61 M. Hautala and H.J. Whitlow, Nucl. Instr. and Meth. B6 (1985) 466. [71 T. Okutani, M. Shikata. S. Ichimura and R. Shimizu, J. Appl. Phys. 51 (1980) 2884. J.M. Lambert. P.H. Tread0 PI R.G. Allas. A.R. Knudson, and G.W. Reynolds, Nucl. Instr. and Meth. 194 (1982) 615. [91 F.R. Vozzo and G.W. Reynolds. Nucl. Instr. and Meth. 209/210 (1983) 555. PO1 P. Sigmund, A. Oliva and G. Falcone. Nucl. Instr. and Meth. 194 (1982) 541. Phenomena in 1111 B. PeroviC and B. eobic, in: Ionization Gases 11, ed., H. Maecker (North-Holland, Amsterdam, 1962) p. 1165. P21 B. Emmoth, Th. Fried and M. Braun, J. Nucl. Mater. 76/77 (1978) 129. I131 K. Besocke, S. Berger, W.O. Hofer and U. Littmark. Radiat. Eff. 66 (1982) 35. P41 G. Garter, B. Navinsek and J.L. Whitton, in: Sputtering by Particle Bombardment II, ed.. R. Behrisch (Springer, Berlin-Heidelberg-New York-Tokyo, 1983) p. 231. P51 H.H. Andersen, B. Stenum, T. Sorensen and H.J. Whitlow. Nucl. Instr. and Meth. B6 (1985) 459. U61 J.L. Vossen, J. Vat. Sci. Technol. 11 (1974) 875. u71 H.H. Andersen, B. Stenum. T. Sorensen and H.J. Whitlow. Nucl. Instr. and Meth. 209/210 (1983) 487. 1181 J. Bottiger, J.A. Davies, P. Sigmund and K.B. Winterbon, Radiat. Eff. 11 (1971) 69.
ANGULAR Marek
in Physics Research
DISTRIBUTION
SZYMONSKI
B14 (1986) 263-267
OF Ag ATOMS SPUTTERED
*, Weixing
HUANG
263
WITH A 5 keV Ar + ION BEAM
and Jens ONSGAARD
Fysisk Insritut, Odense Universitet, Campusvej 55, DK - 5230 Odense M, Denmark
Received
28 June 1985 and in revised form 1 November
1985
A polycrystalline, high-purity Ag target was sputtered with a normally incident, 5 keV Ar + ion beam under ultrahigh vacuum (UHV) conditions. Sputtered material was collected on a cylindrically shaped Al foil and subsequently analysed in an Auger electron spectrometer. The obtained results show that sputtering under UHV conditions leads to a cosine-like distribution. The distribution taken after prolonged ion bombardment contained a hump at angles of 30-45” superimposed on the general form which we ascribe to an ion beam-induced texturing effect. The results are compared with recent investigations of other authors. The influence of the surface conditions of both the target and the foil used for collection of the sputtered material on the angular distribution measurements has also been investigated.
1. Introduction Angular distributions of material sputtered from polycrystalline and amorphous targets have been the subject of experimental activities for several decades [l]. From the theoretical side, both relevant sputtering models, the linear collision cascade [2] and the spike evaporation [3], predicted nothing more than a cosine distribution for the sputtered flux. Only for projectile energies close to the threshold or for light ion bombardment the number of generated recoils might be too small for development of the isotropic cascade [4]. Recent computer simulations [5,6], on the other hand, have shown over-cosine distributions for high bombarding energies, but close to cosine for low keV bombardment. Most of the normal incidence measurements, usually performed under moderate vacuum conditions, showed over-cosine distributions. Okutani et al. [7], however, measured angular distributions of Si atoms sputtered by low keV Art ions in an UHV chamber. Their normal incidence data agreed fairly well with a cosine law as predicted by theory. On the other hand UHV measurements by Allas et al. [8] yielded several over-cosine distributions for selfsputtering of Al, Ni and Cu; the projectile energies in this experiment were much higher. Vozzo and Reynolds [9] found that As, sputtering of Au in ordinary vacuum produced a strongly over-cosine distribution, but under UHV conditions the distribution was much closer to a cosine one.
* Present address: Instytut Fizyki, Uniwersytet Reymonta 4, 30-059 Krakow, Poland.
Jagiellohski.
0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
These results indicate that in some cases, adsorbed impurity layers may cause an over-cosine form for the distribution. Target atoms originating from layers deeper than the adsorbate are preferentially ejected into a narrow cone around the surface normal [lo]. Experimental evidence of this effect was found in recent angu!ar distribution studies of sputtering for alloys (see for example ref. [17]). The same argument holds for heavily damaged surfaces due to prolonged ion bombardment. In view of this, further experimental data are needed, obtained under UHV conditions and with well controlled surface purity and topography. We have performed, therefore, such measurements using Auger electron spectroscopy as a tool both for monitoring surface cleanliness and analysing the composition of the collected sputtered material. As an example, a polycrystalline Ag sample was sputtered with a 5 keV Ar ion beam. Silver was chosen not only to facilitate AES measurements (silver is frequently used as a standard sample material), but it is also interesting for comparison with the previous, high dose measurements [ll-131. The old work by Perovii: and CobiC [ll] was done under poor vacuum conditions. Thus, the observed deviation from a cosine distribution is not surprising. Although measurements by Emmoth et al. [12] and Besocke et al. [13] were performed under much better vacuum, still a distinct influence of surface topography is seen in their Ag distributions. Inspection of the bombarded target by scanning electron microscopy [ 131 revealed pronounced cone formation, presumably causing a narrow, forwardpeaked distribution, via the above-described mechanism.
M. Sz.vmonski et al. / Angular distribution ofsputtered Ag atoms
264
2. Experimental
A polycrystalline, high purity Ag sample (99.999% Ag) was mechanically polished with an accuracy of better than 0.5 pm and cleaned in an ultrasonic bath before being mounted in an UHV chamber. Scanning electron micrographs taken in advance and after sputtering showed the target surface as rather flat at a magnification of 3.6 X 104. No specific check on the presence of texture before sputtering was made. The system was evacuated to the lo-” Torr range and the sample was sputter cleaned until no traces of impurities could be detected with an Auger electron spectrometer. The spectrometer was the commercial 4 grid system manufactured by Varian. The target was sputtered with a 5 keV Ar+ beam delivered by a differentially pumped Leybold ion gun. The content of impurities was estimated to be less than 3%. Although the beam was not analysed, neutrals were prevented from striking the surface by deflecting the collimated beam prior to the entrance of the final aperature. There still exists a possibility of multiply charged ions being present in the beam with higher bombarding energies. According to both the computer simulations [5,6] and the experimental work [15] higher
Electron
energy ions would cause the distribution to be overcosine. Since we have observed nearly cosine distributions, as presented in the following section, we can exclude this effect from our consideration. The beam intensities were measured with a removable Faraday cup. Typical values were 0.4 PA impinging through a hole of 1 mm in diameter. The pressure in the chamber rose to 3 X lo-’ Torr during sputtering, mainly due to an increase in the partial pressure of Ar, while other rest gases were continuously pumped by ion-getter and turbomolecular pumps. Doses up to 4 x 10” ions/cm2 were typically used. This assures that the thicknesses of sputter-deposited films did not exceed the mean free path of Auger electrons. Repeated measurements with smaller doses have been performed in order to assure a linear dependence of the thickness of collected material on the dose within the whole investigated range of ejection angles. From the point of view of ion beam-induced morphology changes and texture effects, a small dose is preferable too, minimising an influence of those effects on the angular spectra. Regularly, the target, after a fluence of 2-3 x 10” Ar+ ions/cm2, was moved to a new position without changing the measurement geometry, whereupon a hitherto unirradiated area was exposed to the beam. Sputtered atoms were collected on aluminium or
Gun ( integral
Mass
)
Spectrometer
Vacuum
Leybold
Ion
Gun
meter
Y&indoy Electron
¶tus
Gun ( integral
P
Ar gas
inlet ransmlssion
Fig. 1. A schematic
view of the experimental
setup. For details
see text.
shaft
)
265
M. Szymonski et al. / Angular distribution o/sputtered Ag atoms
ejection
aluminium covered stainless steel ‘catcher foils. Cylindrical collectors with a radius of 23 mm were used and could be analysed directly before and after sputtering without breaking of the vacuum. The deposit was subsequently analysed with an AES analyser. The peak-topeak heights of the characteristic Auger Ag-MNN transitions, positioned at 347.5 and 353.4 eV, were measured as a function of the position x on the foil. Since x is directly related to the ejection angle 0 measured with respect to the surface normal, this function described the angular distribution of the sputtered material: 1(x(B)). A schematic view of the experimental setup is shown in fig. 1. Finally, a few words should be said on the possibility of resputtering of collected material, Resputtering is caused mainly by reflected Ar atoms in view of their high energy as compared to that of sputtered Ag atoms [3]. With a reflection coefficient of [18] - 0.10, an upper limit of half the primary energy for the mean of reflected Ar, and the established variation of the sputter yield with energy [3], the flux of resputtered Ag atoms reaches 5 3 pet. of the primary sputtered Ag flux, once the collector is homogeneously covered with silver, starting from zero at lower fluences. Therefore, the influence of resputtering on the measurement is smaller than that of other sources of error.
angle
(
degree
)
6.
4.
9 Q
s
0.z-
Fig. 2. Polar plot of the angular distribution of Ag sputtered from a polycrystalline silver sample by 5 keV Ar+ bombardment under UHV conditions. The fluence was 1.5X 10” ions/cm2. The distribution has been normalized to give the same absolute yield as a cosine distribution, represented by a solid semicircle.
3. Results and discussion A polar plot of the Auger peak to peak height of sputter-deposited Ag is presented in fig. 2. The experimental points have been normalized in order to give the same integrated yield (27r/Y( 0) sin 8 d0) as a cosine distribution represented in fig. 2 as the solid semicircle. From the figure it is seen clearly that the angular distribution of Ag atoms sputtered with a 5 keV Ar+ beam is very close to cosine in contrast to the over-cosine distributions found in previous work [ll-131. A prolonged ion bombardment of our Ag polycrystalline target resulted in the distribution shown in fig. 3. In addition to the general form of the distribution, a hump appears between 30 and 45”. Although we cannot exclude that the sample was textured prior to the ion bombardment, we have definitely seen that the fully developed hump in the spectrum (as seen in fig. 3) appeared only after a fluence exceeding 3-5 x 10” ions/cm2. Thus, by comparison of fig. 2 (fluence of 1.5 x 10” ions/cm2) and fig. 3 (fluence larger than 4 X 10” ions/cm2) we can ascribe the above effect to beam-induced texturing of the sample surface. Similar effects have frequently been observed for polycrystalline metals bombarded with fluences of a few times 10” ions/cm2 or more [14]. It is very likely that the irradiated silver sample was textured with (111) directions
ejection
angle (degree)
I 40'
. .
.
/ 50’
.
Fig. from ment 10”
3. Polar plot of the angular distribution of Ag sputtered a polycrystalline silver sample by 5 keV Art bombardunder UHV conditions. The total fluence exceeded 5X ions/cm2. The distribution has been normalized as in fig.
M. Szymonski
266
et al. / Angular
largely oriented perpendicular to the surface [15]. Preferential ejection around close-packed (110) directions is consistent with the observed angular position of the hump. Such beam-induced texture effects have recently been proposed as the explanation of similar humps in the angular distribution of sputtered Cu atoms [15] and were very likely responsible for many experimental observations of heart-shaped distributions [8,16]. If we assume that a certain fraction of the sputtered atoms is ejected from the textured surface within a narrow solid angle, centered at (110) directions, we might subtract this fraction from the experimental distribution. Describing a probability for this directional ejection by a Gaussian function centered at the (110) direction with a half width of 15”, we have obtained the fraction f= 15% for the best agreement of the remaining distribution with the cosine function. Note that only the width and amplitude of the Gaussian were fitted but not the position of the maximum which was taken from the texture model considerations. The result is presented in fig. 4. We assert, therefore, that the angular distribution of sputtered Ag is cosine in shape, except for the texture effect giving rise to the additional hump in the distribution. Such an isotropic distribution of ejection angles was predicted by the linear collision cascade theory of sputtering [2] with the assumption of planar surface
1.0
.\. -
‘-J+ l\
t-
-I t/l
&
.-
.
0.8
5
_
.
\
~-
_~
.
.
. 6-O
0
eject& Fig. 4. points, curve), curve), text.
A!2
AT'
ai$e
( degree
.
.
80
>
The same distribution as in fig. 3. The experimental corrected for directional (110) ejection (lower solid are compared with a cosine distribution (upper solid normalized to the same integrated yield. For details see
distrihutron
ofsputtered Ag utoms
ejection
angle
(degree)
Fig. 5. Polar plot of the angular distribution of material sputtered from polycrystalline silver sample with a 5 keV Ar+ beam under the pressure of 2 x 10m5 Torr. The total fluence was 2 X 10” ions/cm2. The normalization is the same as in fig. 2. A cosine distribution is represented by a solid semicircle.
binding. Over-cosine distributions, frequently observed previously, might be related, to some extent, to poor vacuum conditions of those experiments as discussed in the introduction. This is visualized in fig. 5, where the angular distribution of Ag atoms sputtered in a vacuum of 2 X 10e5 Torr is plotted. A striking difference between this distribution and that taken under UHV conditions (fig. 2) is clearly visible in agreement with the theoretical predictions [lo]. AES inspection of the material collected during this last measurement showed a significant content of sulfur, oxygen and carbon impurities. The above analysis of the collected material indicates still another important factor, which may influence angular distribution measurements performed under poor vacuum conditions. A large content of impurities present in the catcher foil before and/or during collection of the sputtered material can certainly affect the sticking probability. Consequently, positions on the foil near normal ejection, with a thicker layer of deposited material, might have a sticking ratio different from position at glancing ejection. We have seen evidence for such an artefact by comparing the data obtained with and without sputter cleaning of the collector foil prior
M. Szymonski
et al. / Angular distribution of sputtered Ag utoms
to measurements. The distributions obtained sputter cleaned collector were more cosine those collected on an impurity-covered foil, vacuum conditions during the deposition were
with the like than although the same.
Inspiring discussions with Prof. Peter Sigmund, Dr. Per Morgen and the Danish sputtering club are gratefully acknowledged. We are most grateful to Dr. Hans A. Boye for taking scanning electron micrographs. Two of us (MS. and W.H.) acknowledge support from the Danish Natural Science Research Council for the time period during which these measurements were carried out. W.H. would also like to acknowledge support from Ingeborg og Leo Dannins Legat for Videnskabelig Forskning.
References [l] Among the first reports were: R. Seeliger and K. Sommermeyer, 2. Physik 93 (1935) 692; G.K. Wehner and D. Rosenberg, J. Appl. Phys. 31 (1969) 177. [2] P. Sigmund, Phys. Rev. 184 (1969) 383. [3] P. Sigmund, in: Sputtering by Particle Bombardment I, ed., R. Behrisch (Springer-Heidelberg-New York, 1981) p. 9. [4] U. Littmark and P. Sigmund, J. Phys. D8 (1975) 241.
261
[51 J.P. Biersack and W. Eckstein. Appl. Phys. A34 (1984) 73. [61 M. Hautala and H.J. Whitlow, Nucl. Instr. and Meth. B6 (1985) 466. [71 T. Okutani, M. Shikata. S. Ichimura and R. Shimizu, J. Appl. Phys. 51 (1980) 2884. J.M. Lambert. P.H. Tread0 PI R.G. Allas. A.R. Knudson, and G.W. Reynolds, Nucl. Instr. and Meth. 194 (1982) 615. [91 F.R. Vozzo and G.W. Reynolds. Nucl. Instr. and Meth. 209/210 (1983) 555. PO1 P. Sigmund, A. Oliva and G. Falcone. Nucl. Instr. and Meth. 194 (1982) 541. Phenomena in 1111 B. PeroviC and B. eobic, in: Ionization Gases 11, ed., H. Maecker (North-Holland, Amsterdam, 1962) p. 1165. P21 B. Emmoth, Th. Fried and M. Braun, J. Nucl. Mater. 76/77 (1978) 129. I131 K. Besocke, S. Berger, W.O. Hofer and U. Littmark. Radiat. Eff. 66 (1982) 35. P41 G. Garter, B. Navinsek and J.L. Whitton, in: Sputtering by Particle Bombardment II, ed.. R. Behrisch (Springer, Berlin-Heidelberg-New York-Tokyo, 1983) p. 231. P51 H.H. Andersen, B. Stenum, T. Sorensen and H.J. Whitlow. Nucl. Instr. and Meth. B6 (1985) 459. U61 J.L. Vossen, J. Vat. Sci. Technol. 11 (1974) 875. u71 H.H. Andersen, B. Stenum. T. Sorensen and H.J. Whitlow. Nucl. Instr. and Meth. 209/210 (1983) 487. 1181 J. Bottiger, J.A. Davies, P. Sigmund and K.B. Winterbon, Radiat. Eff. 11 (1971) 69.