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Position and shape of sputtering emission spots measured with hemispherical collectors and electron backscattering
Nuclear Instruments and Methods in Physics Research 813 (1986) 353-3.56 North-Holland, Ams:erdam
353
POSITION AND SHAPE OF SPUTTERING EMISSION SPOTS MEASURED WITH HEMISPHERICAL COLLECTORS AND ELECTRON BACKSCATTERING Michael ERDMANN, Optisches Institur, Sekr. Pll,
J&-g LINDERS,
Heinz NIEDRIG
and Martin STERNBERG
Technische Universitiit Berlin, StraJje des 17. Juni 13.5, 1000 Berlin 1.2, Fed. Rep. Germany
Two-dimensional intensity distributions and angular positions of the emission spots occurring in sputtering experiments with single crystals have been determined for gold and silver targets of different surface orientatjons under bombardment with argon and oxygen ions. The differential sputtering yield was measured in the whole angular range between 3” and 80” to the target normal using the collector method: after condensation of the sputtered target material on cooled hemispherical collectors the thickness distributions of the sputtered films were measured in a microprocessor controlled electron backscattering device with an angular
resolution of 0.2” and a thickness resolution of about 1% of the thickness measuring range. l?igital acquisition of the measured data and an image storage unit allow several representation and evaluation methods as e.g., diagrams of constant thickness lines or grey level representations of the emission distributions. Half widths of emission spots have been evaluated in different measuring planes and a strong variation of the maximum spot intensity with the polar emission angle was found.
Investigation of the emission spots occurring in sputtering experiments with single crystals with respect to the dependence on temperature, on surface orientation or on crystal lattice orientation is important for research on basic sputtering mechanisms. Emission distribution measurements have generally been performed up to now for high symmetric surface orientations [I, 2,9, 111. Positions and half widths of the spots can be obtained then with the collector method at least for one azimuth using simple collectors such as, for instance, plane or cylindrically bent foils. For the determination of the dependence of spot characteristics on polar angle of ejection a crystal surface without rotational symmetry is helpful. In this case the spots are ejected into directions with different polar angles. For a correct evaluation and interpretation of such asymmetric emission distributions, especially with respect to distortions or angular shifts of the spots, measurements in the single azimuthal or polar directions are not sufficient. The determination of the whole emission distribution is required to obtain the desired information about the spot position and shape for all spots occurring and for fitting of the non-preferential background. This purpose requires hemispherical collectors for deposition of the sputtered target material. The emission distributions then correspond to undistorted film thickness distributions which are measured by electron backscattering. To investigate the asymmetric emission distributions a surface orientation near (11 3 1) was selected. Six low index directions of the type (110) point into the 0168-583X/86/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
hemisphere above this surface. At these different polar angles spot ejection can be assumed. Furthermore this surface orientation is of special interest in sputtering experiments because of its topography development during ion exposure 131.
2. Sputtering experiments Single crystal surfaces with the desired orientation were obtained by cutting slices from crystal rods. The required cutting angles were determined by comparison of experimentally obtained and simulated electron channeling patterns (ECPs) of the existing and desired surfaces. The ECPs were recorded in a scanning electron microscope with double tilting stage. The method, which is simultaneously usefui for a check of crystal quality in the surface region, is described in detail in ref. [4]. Ag and Au single crystal surfaces with slight deviations from the (113 I)-orientation were exposed Table 1 Experimental data and results (T. target temperature; t?,,,, &ri2; half width in polar and azimuthai direction averaged over all ( IlO)-spots)
4 b) c) d) e) f)
24 keV Ar’ --, Ag 20 keV Ar’ + Ag 20 keV Ar’ -+ Ag 22 keV 0: + Au 22 keV 0; + Au 40keV Ar++Ag
T(K)
@I,,
4,x
162 440 700 230 170 130
18 21” 28” 16” 16” 11”
13 18” 30” 16” 13” 13”
VII. SPU’ITERING
354
M. Erdmann
et at. i Position and shape
to Ar”- and Oi-ions, in the case of Ag for different target temperatures. All experiments were performed with perpendicular ion incidence (ion beam diam. 0.5 mm) using a Penning-type ion gun [5]. The sputtered target atoms were gathered by condensation in liquid nitrogen cooled hemispherical Al-collectors of 40 mm diam. The characteristic data of the sputtering experiments are arranged in table 1.
3. Determination
and
evaiuation
uf sputtering emission spots
22 keV At-+ t
-
A!4
y 2
-” loo--
2 !.! 5 f t-
-.
of the differential
sputtering yields
The thickness distributions of the sputter deposited films were measured in the electron backscattering device described in ref. [6]. For measurement of azimuthal thickness profiles (azimuthal scans) the electron probe was scanned over the film surface by rotation of the collector. The azimuthal thickness profiles were measured with an angular resolution of 0.2” and a film thickness resolution of 1%. Together with the experimental conditions of the sputtering experiments this leads to an angular resolution of 1.5” and a differential sputtering yield accuracy of 4%. The whole thickness distribution was obtained by registration of the azimuthal scans for different polar angles in the range from 3” to 80” relative to the collector axis with a polar distance of 1”. The desired angular resolution and the number of scans for mapping the whole thickness distribution result in a large number of data, which have to be recorded and evaluated. Therefore the experimental arrangement was connected to a microprocessor system. The con~guration for experiment control, data acquisition and evaluation including the image processing unit is described in ref. [7]. The following methods were used for evaluation and data representation: to eliminate the non-preferential background the generally assumed cosine-like dependence on the polar angle was fitted to the polar thickness profiles for several different azimuths. For this purpose the measured azimuthal scan data were combined to polar scans. A polar scan and the background fit of the corresponding emission distribution are shown in fig. 1. It should be emphasized, that the attempt to fit the background by evaluation of only one polar scan through the emission dist~bution is dangerous. The difference left between fit slope and distribution slope is caused by neighboured spots situated at other polar angles and could be misinterpreted as background contribution. This would result in a wrong fit causing errors in determination of spot maxima and half widths, To obtain a clearly arranged and quantitative overview over the emission distribution the azimuthal scans are transformed into constant thickness lines displayed
90
60.
-
.iO’
b POLAR
ANGLE
30 i
$0
90
“1
Fig. 1. Emission distribution in polar direction. The assumed non-preferential background is indicated. as polar coordinate diagram (fig, 2). In combination with a 16-bit grey level image processing unit a grey level representation of the measured data is possible (fig. 3). Bright dots stand for high emissiont dark dots for low emission into the corresponding angular direction. The picture gives only a qualitative impression of the emission distribution, but the correct quantitative information about the whole emission distribution is stored in the image memory, and the differential sputtering yields can be read for every dot of the display by simple cursor positioning. In order to check and evaluate the emission distributions in respect to crystal orientation and spot shifts the polar angles of the spot maxima were fitted to the low index directions
Fig. 2. Lines of constant differential sputtering yield after background subtraction (22 keV 0; + Ag near (113 1)). #, 8: Azimuthal resp. polar angle.
M. Erdmann
et al.
355
I Position and shape of sputtering emission spots I
I
I
22 keV Oi_rAu
l
7 c 5 21 e z 2
T=l70K T=UOK
0 20 keV Af*Ag \ -
Y
\
.
\
\ ‘4 *
\ 4 +,
i z
\
* \
‘. * ‘1 g 0.5 Qi, z \ 8 T-230K ++\ + 22 keVO;+Ag -v + 2L keV ATlAg T=162 K N” c T=700K +2OkeVAT+Ag $ c I I 0 30 60 polar angle Co1
\
l
Fig. 3. Emission distribution displayed as grey level representation (22 keV 0: +Ag near (11 3 1)). Bright dots: high differential sputtering yield; dark dots: low differential sputtering yield.
by computer lattice.
aided
rotation
of the simulated
crystal
4. Results 4.1. Spot positions Except for the Ag( 111) emission distribution, where the well known (1 lo)- and the weaker (10 O)-spots were observed, only spots of the (1 1 1)-type could be detected. For most emission distributions the positions of the spots agree very well with the low index (1 1 O)lattice directions within a deviation of less than 1”. A considerable polar shift of all spot maxima away from the target normal of 4” and 5” respectively was observed only in the cases of the 22 keV O,-bombardment of Au and the 24 keV Ar-irradiation of an Ag-surface at 160 K (near (113 l)-orientation). The azimuthal angles correspond to the calculated values. 4.2. Spot height In the case of the asymmetric surfaces the relative height of the spot maxima depends upon the polar angle. An increasing polar ejection angle leads to a decrease of the spot maximum (fig. 4). The highest spot of each emission distribution is fitted to the dashed line and the other spots are normalized correspondingly. It was found, that there is an approximately linear dependence of the spot maximum upon the polar angle, except for the result at 700 K.
Fig. 4. (llO)-spot height each emission distribution dashed line.
L.
\
versus polar ejection angle. For the highest spot was fitted to the
4.3. Half widths The mean half widths of the (IlO)-spots in polar (0,,,) and azimuthal directions ($,,,) after subtraction of the cosine-like background (fig. 1) are listed in table 1 for some experiments. The temperature of an asymmetric Ag surface bombarded with Ar’-ions was varied (table 1, a-c). As expected, the half widths become larger with increasing target temperature. At 160 K and 440 K the azimuthal half widths are smaller than the polar ones whereas it is the opposite in the case of 700 K target temperature. The spots of the Ag(1 1 l)emission distribution (table 1, f) are more extended in azimuthal than in polar direction in contrast to the spots of the Ag-surface near (113 1) at a comparable temperature (table 1, a).
5. Discussion The negligible angular deviation of most (1 1 0)-spot maxima from the crystal directions has been confirmed frequently [l, 8,9]. However, in the cases of the Auand Ag-surface near (113 1) at low temperatures (table 1, a, e), the nearly constant shift for all spots towards the surface combined with an excellent agreement in the azimuthal angle must be taken into account. It may be due to the effect of surface diffraction [lo], but since the polar shift occurred only for some experimental conditions (target temperature, beam current density), an influence of the developed surface topography under ion bombardment might be suspected: the Ag-surface near (11 3 1) is covered with densely standing cones [3]. However, it is unknown for the present experiments, VII.
SPUTTERING
356
M. Erdmann
et al. I Position and shape of sputtering emission spot.
after which ion dose this topography appears and how it influences the emission angle. The surface potential barrier, influencing preferably the atoms ejected at small angles to the surface, could cause the decrease of the spot maxima with increasing polar angle. In addition the mean number of collisions until a collision sequence reaches the surface increases with increasing polar angle. As the length of a collision cascade is limited, this may also reduce the intensity of the spots situated at large polar angles. Although the spot maxima differ in the asymmetric emission distributions, a similar behaviour of the temperature dependence was observed. The half widths in polar as well as those in azimuthal directions show the characteristic raise at higher temperatures related to increased atomic oscillations. The asymmetric shapes of the spots, resulting in different half widths for different measuring planes, were discussed generally in an earlier work [6]. The relation between polar and azimuthal half widths and its dependence upon the target temperature and the crystal orientation give some interesting aspects for further investigations. This work has been supported Deutsche Forschungsgemeinschaft.
by a grant
of the
References
PI F. Schulz, Thesis, TH Miinchen (1967). PI W.O. Hofer, in: Sputtering by Particle Bombardment III, ed., R. Behrisch, Topics in Applied Physics (Springer, Berlin, Heidelberg, New York) to be published. [31 J.L. Whitton and G. Carter, Proc. Symp. on Sputtering, Perchtoldsdorf, Vienna (1980), eds., P. Varga, G. Betz and F.P. Viehbiick (Inst. f. Allg. Phys., TU Wien, Wien, 1980) p. 552. [41 W. Hylla, H.-J. Kohl, H. Niedrig and D. Wendtland, Proc. 3rd Pfefferkorn Conf., Ocean City, Maryland (1984)
(SEM
Inc., AMF O’Hare, Chicago) p. 237. J. Linders, H.-J. Lippold, H. Niedrig and T. Sebald, Radiat. Effects 59 (1982) 183. Nucl. Instr. [61 J. Linders, H. Niedrig and M. Sternberg, and Meth. B2 (1984) 649. [71 M. Erdmann, J. Linders, H. Niedrig and M. Sternberg, Proc. Eighth European Congress on Electron Microscopy, Budapest, Hungary, Vol. 1 (1984) p. 121. PI P. Erlenwein, Thesis, TU Berlin (1977) D 83. [91 C.H. Weijsenfeld, Thesis, University of Utrecht (1966); Philips Res. Repts Suppl. No. 2 (1967). WI M.T. Robinson, in: Sputtering by Particle Bombardment I, ed., R. Behrisch, Topics in Applied Physics (Springer, Berlin, Heidelberg, New York, 1981). [Ill P. Erlenwein and H. Niedrig, Proc. VII Int. Conf. on Atomic Collisions in Solids, Moscow (1977) p. 65.
[51 V. Alexander,
353
POSITION AND SHAPE OF SPUTTERING EMISSION SPOTS MEASURED WITH HEMISPHERICAL COLLECTORS AND ELECTRON BACKSCATTERING Michael ERDMANN, Optisches Institur, Sekr. Pll,
J&-g LINDERS,
Heinz NIEDRIG
and Martin STERNBERG
Technische Universitiit Berlin, StraJje des 17. Juni 13.5, 1000 Berlin 1.2, Fed. Rep. Germany
Two-dimensional intensity distributions and angular positions of the emission spots occurring in sputtering experiments with single crystals have been determined for gold and silver targets of different surface orientatjons under bombardment with argon and oxygen ions. The differential sputtering yield was measured in the whole angular range between 3” and 80” to the target normal using the collector method: after condensation of the sputtered target material on cooled hemispherical collectors the thickness distributions of the sputtered films were measured in a microprocessor controlled electron backscattering device with an angular
resolution of 0.2” and a thickness resolution of about 1% of the thickness measuring range. l?igital acquisition of the measured data and an image storage unit allow several representation and evaluation methods as e.g., diagrams of constant thickness lines or grey level representations of the emission distributions. Half widths of emission spots have been evaluated in different measuring planes and a strong variation of the maximum spot intensity with the polar emission angle was found.
Investigation of the emission spots occurring in sputtering experiments with single crystals with respect to the dependence on temperature, on surface orientation or on crystal lattice orientation is important for research on basic sputtering mechanisms. Emission distribution measurements have generally been performed up to now for high symmetric surface orientations [I, 2,9, 111. Positions and half widths of the spots can be obtained then with the collector method at least for one azimuth using simple collectors such as, for instance, plane or cylindrically bent foils. For the determination of the dependence of spot characteristics on polar angle of ejection a crystal surface without rotational symmetry is helpful. In this case the spots are ejected into directions with different polar angles. For a correct evaluation and interpretation of such asymmetric emission distributions, especially with respect to distortions or angular shifts of the spots, measurements in the single azimuthal or polar directions are not sufficient. The determination of the whole emission distribution is required to obtain the desired information about the spot position and shape for all spots occurring and for fitting of the non-preferential background. This purpose requires hemispherical collectors for deposition of the sputtered target material. The emission distributions then correspond to undistorted film thickness distributions which are measured by electron backscattering. To investigate the asymmetric emission distributions a surface orientation near (11 3 1) was selected. Six low index directions of the type (110) point into the 0168-583X/86/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
hemisphere above this surface. At these different polar angles spot ejection can be assumed. Furthermore this surface orientation is of special interest in sputtering experiments because of its topography development during ion exposure 131.
2. Sputtering experiments Single crystal surfaces with the desired orientation were obtained by cutting slices from crystal rods. The required cutting angles were determined by comparison of experimentally obtained and simulated electron channeling patterns (ECPs) of the existing and desired surfaces. The ECPs were recorded in a scanning electron microscope with double tilting stage. The method, which is simultaneously usefui for a check of crystal quality in the surface region, is described in detail in ref. [4]. Ag and Au single crystal surfaces with slight deviations from the (113 I)-orientation were exposed Table 1 Experimental data and results (T. target temperature; t?,,,, &ri2; half width in polar and azimuthai direction averaged over all ( IlO)-spots)
4 b) c) d) e) f)
24 keV Ar’ --, Ag 20 keV Ar’ + Ag 20 keV Ar’ -+ Ag 22 keV 0: + Au 22 keV 0; + Au 40keV Ar++Ag
T(K)
@I,,
4,x
162 440 700 230 170 130
18 21” 28” 16” 16” 11”
13 18” 30” 16” 13” 13”
VII. SPU’ITERING
354
M. Erdmann
et at. i Position and shape
to Ar”- and Oi-ions, in the case of Ag for different target temperatures. All experiments were performed with perpendicular ion incidence (ion beam diam. 0.5 mm) using a Penning-type ion gun [5]. The sputtered target atoms were gathered by condensation in liquid nitrogen cooled hemispherical Al-collectors of 40 mm diam. The characteristic data of the sputtering experiments are arranged in table 1.
3. Determination
and
evaiuation
uf sputtering emission spots
22 keV At-+ t
-
A!4
y 2
-” loo--
2 !.! 5 f t-
-.
of the differential
sputtering yields
The thickness distributions of the sputter deposited films were measured in the electron backscattering device described in ref. [6]. For measurement of azimuthal thickness profiles (azimuthal scans) the electron probe was scanned over the film surface by rotation of the collector. The azimuthal thickness profiles were measured with an angular resolution of 0.2” and a film thickness resolution of 1%. Together with the experimental conditions of the sputtering experiments this leads to an angular resolution of 1.5” and a differential sputtering yield accuracy of 4%. The whole thickness distribution was obtained by registration of the azimuthal scans for different polar angles in the range from 3” to 80” relative to the collector axis with a polar distance of 1”. The desired angular resolution and the number of scans for mapping the whole thickness distribution result in a large number of data, which have to be recorded and evaluated. Therefore the experimental arrangement was connected to a microprocessor system. The con~guration for experiment control, data acquisition and evaluation including the image processing unit is described in ref. [7]. The following methods were used for evaluation and data representation: to eliminate the non-preferential background the generally assumed cosine-like dependence on the polar angle was fitted to the polar thickness profiles for several different azimuths. For this purpose the measured azimuthal scan data were combined to polar scans. A polar scan and the background fit of the corresponding emission distribution are shown in fig. 1. It should be emphasized, that the attempt to fit the background by evaluation of only one polar scan through the emission dist~bution is dangerous. The difference left between fit slope and distribution slope is caused by neighboured spots situated at other polar angles and could be misinterpreted as background contribution. This would result in a wrong fit causing errors in determination of spot maxima and half widths, To obtain a clearly arranged and quantitative overview over the emission distribution the azimuthal scans are transformed into constant thickness lines displayed
90
60.
-
.iO’
b POLAR
ANGLE
30 i
$0
90
“1
Fig. 1. Emission distribution in polar direction. The assumed non-preferential background is indicated. as polar coordinate diagram (fig, 2). In combination with a 16-bit grey level image processing unit a grey level representation of the measured data is possible (fig. 3). Bright dots stand for high emissiont dark dots for low emission into the corresponding angular direction. The picture gives only a qualitative impression of the emission distribution, but the correct quantitative information about the whole emission distribution is stored in the image memory, and the differential sputtering yields can be read for every dot of the display by simple cursor positioning. In order to check and evaluate the emission distributions in respect to crystal orientation and spot shifts the polar angles of the spot maxima were fitted to the low index directions
Fig. 2. Lines of constant differential sputtering yield after background subtraction (22 keV 0; + Ag near (113 1)). #, 8: Azimuthal resp. polar angle.
M. Erdmann
et al.
355
I Position and shape of sputtering emission spots I
I
I
22 keV Oi_rAu
l
7 c 5 21 e z 2
T=l70K T=UOK
0 20 keV Af*Ag \ -
Y
\
.
\
\ ‘4 *
\ 4 +,
i z
\
* \
‘. * ‘1 g 0.5 Qi, z \ 8 T-230K ++\ + 22 keVO;+Ag -v + 2L keV ATlAg T=162 K N” c T=700K +2OkeVAT+Ag $ c I I 0 30 60 polar angle Co1
\
l
Fig. 3. Emission distribution displayed as grey level representation (22 keV 0: +Ag near (11 3 1)). Bright dots: high differential sputtering yield; dark dots: low differential sputtering yield.
by computer lattice.
aided
rotation
of the simulated
crystal
4. Results 4.1. Spot positions Except for the Ag( 111) emission distribution, where the well known (1 lo)- and the weaker (10 O)-spots were observed, only spots of the (1 1 1)-type could be detected. For most emission distributions the positions of the spots agree very well with the low index (1 1 O)lattice directions within a deviation of less than 1”. A considerable polar shift of all spot maxima away from the target normal of 4” and 5” respectively was observed only in the cases of the 22 keV O,-bombardment of Au and the 24 keV Ar-irradiation of an Ag-surface at 160 K (near (113 l)-orientation). The azimuthal angles correspond to the calculated values. 4.2. Spot height In the case of the asymmetric surfaces the relative height of the spot maxima depends upon the polar angle. An increasing polar ejection angle leads to a decrease of the spot maximum (fig. 4). The highest spot of each emission distribution is fitted to the dashed line and the other spots are normalized correspondingly. It was found, that there is an approximately linear dependence of the spot maximum upon the polar angle, except for the result at 700 K.
Fig. 4. (llO)-spot height each emission distribution dashed line.
L.
\
versus polar ejection angle. For the highest spot was fitted to the
4.3. Half widths The mean half widths of the (IlO)-spots in polar (0,,,) and azimuthal directions ($,,,) after subtraction of the cosine-like background (fig. 1) are listed in table 1 for some experiments. The temperature of an asymmetric Ag surface bombarded with Ar’-ions was varied (table 1, a-c). As expected, the half widths become larger with increasing target temperature. At 160 K and 440 K the azimuthal half widths are smaller than the polar ones whereas it is the opposite in the case of 700 K target temperature. The spots of the Ag(1 1 l)emission distribution (table 1, f) are more extended in azimuthal than in polar direction in contrast to the spots of the Ag-surface near (113 1) at a comparable temperature (table 1, a).
5. Discussion The negligible angular deviation of most (1 1 0)-spot maxima from the crystal directions has been confirmed frequently [l, 8,9]. However, in the cases of the Auand Ag-surface near (113 1) at low temperatures (table 1, a, e), the nearly constant shift for all spots towards the surface combined with an excellent agreement in the azimuthal angle must be taken into account. It may be due to the effect of surface diffraction [lo], but since the polar shift occurred only for some experimental conditions (target temperature, beam current density), an influence of the developed surface topography under ion bombardment might be suspected: the Ag-surface near (11 3 1) is covered with densely standing cones [3]. However, it is unknown for the present experiments, VII.
SPUTTERING
356
M. Erdmann
et al. I Position and shape of sputtering emission spot.
after which ion dose this topography appears and how it influences the emission angle. The surface potential barrier, influencing preferably the atoms ejected at small angles to the surface, could cause the decrease of the spot maxima with increasing polar angle. In addition the mean number of collisions until a collision sequence reaches the surface increases with increasing polar angle. As the length of a collision cascade is limited, this may also reduce the intensity of the spots situated at large polar angles. Although the spot maxima differ in the asymmetric emission distributions, a similar behaviour of the temperature dependence was observed. The half widths in polar as well as those in azimuthal directions show the characteristic raise at higher temperatures related to increased atomic oscillations. The asymmetric shapes of the spots, resulting in different half widths for different measuring planes, were discussed generally in an earlier work [6]. The relation between polar and azimuthal half widths and its dependence upon the target temperature and the crystal orientation give some interesting aspects for further investigations. This work has been supported Deutsche Forschungsgemeinschaft.
by a grant
of the
References
PI F. Schulz, Thesis, TH Miinchen (1967). PI W.O. Hofer, in: Sputtering by Particle Bombardment III, ed., R. Behrisch, Topics in Applied Physics (Springer, Berlin, Heidelberg, New York) to be published. [31 J.L. Whitton and G. Carter, Proc. Symp. on Sputtering, Perchtoldsdorf, Vienna (1980), eds., P. Varga, G. Betz and F.P. Viehbiick (Inst. f. Allg. Phys., TU Wien, Wien, 1980) p. 552. [41 W. Hylla, H.-J. Kohl, H. Niedrig and D. Wendtland, Proc. 3rd Pfefferkorn Conf., Ocean City, Maryland (1984)
(SEM
Inc., AMF O’Hare, Chicago) p. 237. J. Linders, H.-J. Lippold, H. Niedrig and T. Sebald, Radiat. Effects 59 (1982) 183. Nucl. Instr. [61 J. Linders, H. Niedrig and M. Sternberg, and Meth. B2 (1984) 649. [71 M. Erdmann, J. Linders, H. Niedrig and M. Sternberg, Proc. Eighth European Congress on Electron Microscopy, Budapest, Hungary, Vol. 1 (1984) p. 121. PI P. Erlenwein, Thesis, TU Berlin (1977) D 83. [91 C.H. Weijsenfeld, Thesis, University of Utrecht (1966); Philips Res. Repts Suppl. No. 2 (1967). WI M.T. Robinson, in: Sputtering by Particle Bombardment I, ed., R. Behrisch, Topics in Applied Physics (Springer, Berlin, Heidelberg, New York, 1981). [Ill P. Erlenwein and H. Niedrig, Proc. VII Int. Conf. on Atomic Collisions in Solids, Moscow (1977) p. 65.
[51 V. Alexander,