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Effect of irradiated-surface topography on spatial distributions of sputtered particles
566
Nuclear
EFFECT OF IRRADIATED-SURFACE OF SPUTIERED PARTICLES A.I. DODONOV, I.M. FAYAZOV, and Yu.N. ZHUKOVA
Instruments
TOPOGRAPHY
E.A. KRYLOVA,
and Methods
ON SPATIAL
in Physics Research
B48 (1990) 566-570 North-Holland
DISTRIBUTIONS
E.S. MASHKOVA,
V.A. MOLCHANOV
Institute of Nuclear Physics, Moscow State University, 119899 Moscow B-234, USSR
Spatial distributions of sputtered particles for the 30 keV Ar+ + Ag combination in a broad interval of ion incidence angles have been experimentally studied using the collector technique. It was found that these spatial distributions were much more complicated than for the Ar+-Cu combination studied earlier. The obtained results were compared with the Roosendaal-Sanders theory
predictions. The observed discrepancies are qualitatively discussed following the Littmark-Hofer topography on the sputtering regularities. 1.
Introduction
A number of experimental studies was recently performed (see, for example, refs. [l-5]), concerned with spatial distributions of sputtered particles. The studies show that to correctly describe the shape of spatial distributions of sputtered particles it is necessary to take into account two sputtering mechanisms, namely, the cascade mechanism and the direct-ejection mechanism. The role of direct ejection increases as the primary-ion incidence angle increases. However, in a number of cases [3,6] it fails to describe the sputteredparticle spatial distributions in terms of the two above mechanisms. In these cases the shape of the sputteredparticle spatial distribution is determined essentially by the surface topography developed under ion bombardment. It is hence of interest to investigate the evolution of sputtered-particle spatial distribution under intense ion bombardment over a broad interval of ion incidence angles and its relation with irradiated surface morphology. The present work is devoted to this problem.
treatment of the effect of surface
The topography of irradiated analysed using scanning electron perimental design is schematically
target surfaces was microscopy. The exshown in fig. 1.
3. Results
The spatial distributions of sputtered particles are shown in figs. l-4 and 6. They are presented as sets of contour lines of equal intensity (the topograms) depending on the polar (0) and azimuthal (+) ejection angles 30keV
Ai -
Ag (polv)
d
q
12”
2. Experimental The experiment was performed using the mass monochromator of the Institute of Nuclear Physics of Moscow State University [7]. The experimental procedure and data processing were described previously [3,4]. The targets were prepared of polycrystalline silver. Silver was chosen as the target material because many investigations of its surface morphology evolution under ion bombardment have been made [8-lo]. The samples were mechanically polished, chemically etched, and cleaned by 30 keV Ar+ ion bombardment for 10 min. The irradiation was performed with 30 keV energy Arf ions. The ion current density is about 0.5 mA/cm2, the irradiation time typically 30 min. 0168-583X/!30/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Fig. 1. A schematic of the experiment (n is the ion sliding angle, 0 and $I are the polar and azimuthal ejection angles); contour lines of equal sputtered-particle intensity (the topogram) and sections of the topogram in two mutually perpendicular planes for (I = 12 O.
A.I. Dodonov et al. / Effect
30keVAr'-
Ag (polyl
ofirradiated-surface topography d =45”
on spatial distributions of sputtered particles
30keVAr'lons
567
d=20"
L Fig. 2. The topogram and its sections for a = 45 ‘.
of the sputtered particles. In the figures, the angular distributions of sputtered particles in two mutually perpendicular planes, namely, in the ion incidence plane and in the plane perpendicular to it, have also been drawn. The curves were normalized in all cases, the intensity at 8 = O” (the center of a topogram) was taken to be 100%.
30 keV Ai -
An (poty)
d
=
Ie
sa
50'
C9--
Fig. 4. A comparison of the topograms for wpper [5] and silver targets at the same ion sliding angle. At the bottom, the section of the topogram for silver in the iok incidence plane is shown.
70"
Fig. 3. The topogram and its sections for QL = 70 *.
One can see that the spatial distributions have a complicated shape. In the studied interval of the polar ejection angle B (0 O-50 o ) the sputtered-particle intensity depends on the ejection angles B and + much more weakly than in the case of the 30 keV Ar+-Cu combination, see e.g. ref. [5f and also fig_ 4. This is especially true at reasonably large ion sliding angles, see e.g. i-t=45O and a=70°. The intensity maxima are also observed, but in contrast to the 30 keV Ar+-Cu combination, they are located outside the primary-ion incidence plane and symmetrical to it. One can see that at small ion sliding angles (see (Y= 6 O, 12”) the maxima are observed at rather large azimuthal ejection angles + As the ion sliding angle increases, the maxima approach the ion incidence plane, see (Y= 45 O. At large LYan arc rather than two sharp maxima is observed. Irradiated target surfaces were examined using scanning electron microscopy. The SEM micrographs are presented in fig. 5. One can see that at all ion sliding angles the irradiated surfaces are completely covered VIII. SPUTTERING, DESORFTION
568
A.I. Dodonov et al. / Effect of irradiated-surface
topography on spatial distributions of sputtered particles
Fig. 5. SEM micrographs of polycrystalline silver targets at different ion sliding angles n: unexpose d target (a), after irradiation; (b,b’)a=6°,(c,c’)a=150,(d,d’)u=250,(e,e’)cY=700.
A.I. Dodonov et al. / Effect of irradiated-surface
with cones whose axes are parallel to the primary-ion incidence direction. At small a there are very narrow cones, whereas at large a there are wide supercones. The irradiated-surface morphology and its dependence on ion dose was comprehensively analysed in ref. [15]; see also the paper by Whitton in ref. [9]. In principle, the density of the cones for copper may be very high. However, for our experimental conditions for the copper targets the density of the cones was observed to be smaller than for the silver targets (compare fig. 5 in the present paper with fig. 4 in ref. [S]; see also fig. 3 in ref.
PI). 4. Discussion The calculations of sputtered-particle spatial distributions were performed using the Roosendaal-Sanders cascade theory [ll]. The calculated data are presented in fig. 6 (bottom). Comparing the calculated distributions with the experimental ones, one can conclude that they show neither quantitative nor qualitative agreement. It should be remembered that for the 30 keV Ar+-Cu combination, at least at large a, qualitative agreement between theory and experiment was obtained (see fig. 3 in ref. [5]). It should also be noted that for the 30 keV Ar+-Cu combination at small a the shape of the sputtered-particle spatial distribution was strongly influenced by direct ejection processes - a sharp peak in the forward direction located in the primary-ion incidence plane was observed. As seen from the experimental material presented above no similar maxima in the case of the silver target are observed.
30 keV Ar++ Ag(poLy)
topography on spatial distributions of sputtered particles
569
Fig. 5 shows that on the silver surface conical structures develop under ion bombardment and cones cover the whole irradiated area. The influence of surface structure on sputtering regularities was first theoretically investigated by Littmark and Hofer [12]. They demonstrated that the surface structure developing under intense ion irradiation had a strong effect on both the sputtering yield and the sputtered-particle angular distribution. This is a consequence of two effects: yield enhancement due to increased effective projectile incidence angle (relative to a flat target surface) and redeposition of ejected particles on the elements of the developed structures. They also emphasize that the general tendency of the developed structure was to enhance particle ejection at larger polar angles. Our experimental data are in qualitative agreement with this prediction. Indeed, the experimental spatial distributions show, in the studied interval of ejection angles 0 (0 o-5Oo), that the sputtered-particle intensity on average is practically independent of the polar ejection angle provided the ion sliding angle is not too small, whereas the improved Sigmund theory [13] predicts a cos f31.55dependence of the sputtered-particle intensity. The study of effects of surface morphology on sputtering regularities was recently started using the computer simulation technique [14]. Unfortunately, the models of surface morphology used in such simulations are not adequate enough to compare with real surface structures observed in sputtering experiments (see e.g. refs. [8-10,151 and fig. 5 of this paper). This circumstance, as well as the difference in the primary ion energies in our experiment and in the simulations, do not allow us to compare our experimental results with these simulations. The authors thank SD. Fedorovich for his help in performing the measurements. We are deeply obliged to N.A. Butylkina for SEM analysis of the targets.
References 111 R. Becerra-Acevedo, J. Bohdansky, W. Eckstein and J.
Fig. 6. The topograms and their sections in the ion incidence plane at different a. Top: the experiment: bottom: the calculations according to the Roosendaal-Sanders formula (see eq. (12) in ref. [ll]).
Roth, Nucl. Instr. and Meth. B2 (1984) 631. PI A.I. Dodonov, ES. Mashkova, V.A. Molchanov and V.B. Fleurov, Poverkhnost 9 (1986) 33. 131 A.I. Dodonov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova and V.A. Molchanov, Nucl. Instr. and Meth. B33 (1988) 534. 141 A.I. Dodonov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova and V.A. Mochanov, Radiat. Eff. 107 (1988) 15. 151 A.I. Dodonov, I.M. Fayazov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova, V.A. Molchanov and W. Eckstein, Appl. Phys. A49 (1989) 299. 161 A.I. Dodonov, E.A. Krylova, E.S. Mashkova, V.A. VIII. SPUTTERING,
DESORPTION
570
A.I. Dodonov et al. / Effect of irradiated-surface
Molchanov, SD. Fedorovich and I.M. Fayazov, Poverchnost 6 (1988) 133. [7] ES. Mashkova and V.A. Mochanov, Medium-Energy Ion Reflection from Solids (North-Holland, Amsterdam, 1985). [S] S. Kundu, D. Ghose, D. Basu and S.B. Karmohapatro, Nucl. Instr. and Meth. B12 (1985) 352. [9] G. Kiriakidis, G. Carter and J.L. Whitton (eds.), Erosion and Growth of Solids Stimulated by Atom and Ion Beams (Martinus Nijhoff, 1986).
topography on spatial distributions of sputtered particles [lo] [ll] [12] [13] [14] [15]
M. Nindi and D. Stulik, Vacuum 38 (1988) 1071. H.E. Roosendaal and J.B. Sanders, Radiat. Eff. 52 (1980) 137. U. Littmark and W.O. Hofer, J. Mater. 13 (1978) 2577. P. Sigmund, Nucl. Instr. and Meth. B27 (1987) 1. Y. Yamamura, C. Miissner and H. Oechsner, Radiat. Eff. 105 (1987) 31. G. Carter, B. NavinSek and J.L. Whitton, in: Sputtering by Particle Bombardment, vol. 2, ed. R. Behrisch (Springer, Berlin, Heidelberg, 1983) p. 231.
Nuclear
EFFECT OF IRRADIATED-SURFACE OF SPUTIERED PARTICLES A.I. DODONOV, I.M. FAYAZOV, and Yu.N. ZHUKOVA
Instruments
TOPOGRAPHY
E.A. KRYLOVA,
and Methods
ON SPATIAL
in Physics Research
B48 (1990) 566-570 North-Holland
DISTRIBUTIONS
E.S. MASHKOVA,
V.A. MOLCHANOV
Institute of Nuclear Physics, Moscow State University, 119899 Moscow B-234, USSR
Spatial distributions of sputtered particles for the 30 keV Ar+ + Ag combination in a broad interval of ion incidence angles have been experimentally studied using the collector technique. It was found that these spatial distributions were much more complicated than for the Ar+-Cu combination studied earlier. The obtained results were compared with the Roosendaal-Sanders theory
predictions. The observed discrepancies are qualitatively discussed following the Littmark-Hofer topography on the sputtering regularities. 1.
Introduction
A number of experimental studies was recently performed (see, for example, refs. [l-5]), concerned with spatial distributions of sputtered particles. The studies show that to correctly describe the shape of spatial distributions of sputtered particles it is necessary to take into account two sputtering mechanisms, namely, the cascade mechanism and the direct-ejection mechanism. The role of direct ejection increases as the primary-ion incidence angle increases. However, in a number of cases [3,6] it fails to describe the sputteredparticle spatial distributions in terms of the two above mechanisms. In these cases the shape of the sputteredparticle spatial distribution is determined essentially by the surface topography developed under ion bombardment. It is hence of interest to investigate the evolution of sputtered-particle spatial distribution under intense ion bombardment over a broad interval of ion incidence angles and its relation with irradiated surface morphology. The present work is devoted to this problem.
treatment of the effect of surface
The topography of irradiated analysed using scanning electron perimental design is schematically
target surfaces was microscopy. The exshown in fig. 1.
3. Results
The spatial distributions of sputtered particles are shown in figs. l-4 and 6. They are presented as sets of contour lines of equal intensity (the topograms) depending on the polar (0) and azimuthal (+) ejection angles 30keV
Ai -
Ag (polv)
d
q
12”
2. Experimental The experiment was performed using the mass monochromator of the Institute of Nuclear Physics of Moscow State University [7]. The experimental procedure and data processing were described previously [3,4]. The targets were prepared of polycrystalline silver. Silver was chosen as the target material because many investigations of its surface morphology evolution under ion bombardment have been made [8-lo]. The samples were mechanically polished, chemically etched, and cleaned by 30 keV Ar+ ion bombardment for 10 min. The irradiation was performed with 30 keV energy Arf ions. The ion current density is about 0.5 mA/cm2, the irradiation time typically 30 min. 0168-583X/!30/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Fig. 1. A schematic of the experiment (n is the ion sliding angle, 0 and $I are the polar and azimuthal ejection angles); contour lines of equal sputtered-particle intensity (the topogram) and sections of the topogram in two mutually perpendicular planes for (I = 12 O.
A.I. Dodonov et al. / Effect
30keVAr'-
Ag (polyl
ofirradiated-surface topography d =45”
on spatial distributions of sputtered particles
30keVAr'lons
567
d=20"
L Fig. 2. The topogram and its sections for a = 45 ‘.
of the sputtered particles. In the figures, the angular distributions of sputtered particles in two mutually perpendicular planes, namely, in the ion incidence plane and in the plane perpendicular to it, have also been drawn. The curves were normalized in all cases, the intensity at 8 = O” (the center of a topogram) was taken to be 100%.
30 keV Ai -
An (poty)
d
=
Ie
sa
50'
C9--
Fig. 4. A comparison of the topograms for wpper [5] and silver targets at the same ion sliding angle. At the bottom, the section of the topogram for silver in the iok incidence plane is shown.
70"
Fig. 3. The topogram and its sections for QL = 70 *.
One can see that the spatial distributions have a complicated shape. In the studied interval of the polar ejection angle B (0 O-50 o ) the sputtered-particle intensity depends on the ejection angles B and + much more weakly than in the case of the 30 keV Ar+-Cu combination, see e.g. ref. [5f and also fig_ 4. This is especially true at reasonably large ion sliding angles, see e.g. i-t=45O and a=70°. The intensity maxima are also observed, but in contrast to the 30 keV Ar+-Cu combination, they are located outside the primary-ion incidence plane and symmetrical to it. One can see that at small ion sliding angles (see (Y= 6 O, 12”) the maxima are observed at rather large azimuthal ejection angles + As the ion sliding angle increases, the maxima approach the ion incidence plane, see (Y= 45 O. At large LYan arc rather than two sharp maxima is observed. Irradiated target surfaces were examined using scanning electron microscopy. The SEM micrographs are presented in fig. 5. One can see that at all ion sliding angles the irradiated surfaces are completely covered VIII. SPUTTERING, DESORFTION
568
A.I. Dodonov et al. / Effect of irradiated-surface
topography on spatial distributions of sputtered particles
Fig. 5. SEM micrographs of polycrystalline silver targets at different ion sliding angles n: unexpose d target (a), after irradiation; (b,b’)a=6°,(c,c’)a=150,(d,d’)u=250,(e,e’)cY=700.
A.I. Dodonov et al. / Effect of irradiated-surface
with cones whose axes are parallel to the primary-ion incidence direction. At small a there are very narrow cones, whereas at large a there are wide supercones. The irradiated-surface morphology and its dependence on ion dose was comprehensively analysed in ref. [15]; see also the paper by Whitton in ref. [9]. In principle, the density of the cones for copper may be very high. However, for our experimental conditions for the copper targets the density of the cones was observed to be smaller than for the silver targets (compare fig. 5 in the present paper with fig. 4 in ref. [S]; see also fig. 3 in ref.
PI). 4. Discussion The calculations of sputtered-particle spatial distributions were performed using the Roosendaal-Sanders cascade theory [ll]. The calculated data are presented in fig. 6 (bottom). Comparing the calculated distributions with the experimental ones, one can conclude that they show neither quantitative nor qualitative agreement. It should be remembered that for the 30 keV Ar+-Cu combination, at least at large a, qualitative agreement between theory and experiment was obtained (see fig. 3 in ref. [5]). It should also be noted that for the 30 keV Ar+-Cu combination at small a the shape of the sputtered-particle spatial distribution was strongly influenced by direct ejection processes - a sharp peak in the forward direction located in the primary-ion incidence plane was observed. As seen from the experimental material presented above no similar maxima in the case of the silver target are observed.
30 keV Ar++ Ag(poLy)
topography on spatial distributions of sputtered particles
569
Fig. 5 shows that on the silver surface conical structures develop under ion bombardment and cones cover the whole irradiated area. The influence of surface structure on sputtering regularities was first theoretically investigated by Littmark and Hofer [12]. They demonstrated that the surface structure developing under intense ion irradiation had a strong effect on both the sputtering yield and the sputtered-particle angular distribution. This is a consequence of two effects: yield enhancement due to increased effective projectile incidence angle (relative to a flat target surface) and redeposition of ejected particles on the elements of the developed structures. They also emphasize that the general tendency of the developed structure was to enhance particle ejection at larger polar angles. Our experimental data are in qualitative agreement with this prediction. Indeed, the experimental spatial distributions show, in the studied interval of ejection angles 0 (0 o-5Oo), that the sputtered-particle intensity on average is practically independent of the polar ejection angle provided the ion sliding angle is not too small, whereas the improved Sigmund theory [13] predicts a cos f31.55dependence of the sputtered-particle intensity. The study of effects of surface morphology on sputtering regularities was recently started using the computer simulation technique [14]. Unfortunately, the models of surface morphology used in such simulations are not adequate enough to compare with real surface structures observed in sputtering experiments (see e.g. refs. [8-10,151 and fig. 5 of this paper). This circumstance, as well as the difference in the primary ion energies in our experiment and in the simulations, do not allow us to compare our experimental results with these simulations. The authors thank SD. Fedorovich for his help in performing the measurements. We are deeply obliged to N.A. Butylkina for SEM analysis of the targets.
References 111 R. Becerra-Acevedo, J. Bohdansky, W. Eckstein and J.
Fig. 6. The topograms and their sections in the ion incidence plane at different a. Top: the experiment: bottom: the calculations according to the Roosendaal-Sanders formula (see eq. (12) in ref. [ll]).
Roth, Nucl. Instr. and Meth. B2 (1984) 631. PI A.I. Dodonov, ES. Mashkova, V.A. Molchanov and V.B. Fleurov, Poverkhnost 9 (1986) 33. 131 A.I. Dodonov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova and V.A. Molchanov, Nucl. Instr. and Meth. B33 (1988) 534. 141 A.I. Dodonov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova and V.A. Mochanov, Radiat. Eff. 107 (1988) 15. 151 A.I. Dodonov, I.M. Fayazov, S.D. Fedorovich, E.A. Krylova, E.S. Mashkova, V.A. Molchanov and W. Eckstein, Appl. Phys. A49 (1989) 299. 161 A.I. Dodonov, E.A. Krylova, E.S. Mashkova, V.A. VIII. SPUTTERING,
DESORPTION
570
A.I. Dodonov et al. / Effect of irradiated-surface
Molchanov, SD. Fedorovich and I.M. Fayazov, Poverchnost 6 (1988) 133. [7] ES. Mashkova and V.A. Mochanov, Medium-Energy Ion Reflection from Solids (North-Holland, Amsterdam, 1985). [S] S. Kundu, D. Ghose, D. Basu and S.B. Karmohapatro, Nucl. Instr. and Meth. B12 (1985) 352. [9] G. Kiriakidis, G. Carter and J.L. Whitton (eds.), Erosion and Growth of Solids Stimulated by Atom and Ion Beams (Martinus Nijhoff, 1986).
topography on spatial distributions of sputtered particles [lo] [ll] [12] [13] [14] [15]
M. Nindi and D. Stulik, Vacuum 38 (1988) 1071. H.E. Roosendaal and J.B. Sanders, Radiat. Eff. 52 (1980) 137. U. Littmark and W.O. Hofer, J. Mater. 13 (1978) 2577. P. Sigmund, Nucl. Instr. and Meth. B27 (1987) 1. Y. Yamamura, C. Miissner and H. Oechsner, Radiat. Eff. 105 (1987) 31. G. Carter, B. NavinSek and J.L. Whitton, in: Sputtering by Particle Bombardment, vol. 2, ed. R. Behrisch (Springer, Berlin, Heidelberg, 1983) p. 231.