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The effect of near-surface structure on sputtering of Cu atoms by Ar ions
526
Nuclear Instruments and Methods in Physics Research B33 (1988) 526-529 North-Holland, Amsterdam
THE EFFECT OF NEAR-SURFACE
STRUCTURE
ON SPUTTERING
OF Cu ATOMS BY Ar IONS
M. HAUTALA Department
of Physics, University of Helsinki, Siltavuorenpenger
20 D, SF-#I70
Helsinki, Finland
J. LIKONEN Technical Research Centre of Finland, Reactor Laboratory,
Otakaari 3 A, SF-02150 Espoo, Finland
Sputtering of Cu targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code COSIPO. The sensitivity of the angular distributions of sputtered atoms on the near-surface structure is demonstrated by haying a thin monorandom layer on a random/monocrystalline Cu target. Just two monolayers nearly destroy the angular distributions due to the underlying structure.
1. Introduction Studies of angular distributions of particles emitted from targets during a sputter process provide information for a better understanding of sputtering phenomena. From a theoretical viewpoint the random collision cascade theory [l] describes satisfactorily the sputtering process for amorphous and polycrystalline solids. For crystalline targets, the sputtering yields are influenced by the lattice structure especially for particle incidence and emergence in close packed directions. The first explanation for the ejection patterns of sputtered particles was proposed by Silsbee [2] who suggested a focusing collision chain mechanism. An
alternative to the Silsbee mechanism was developed by Lehmarm and Sigmund [3]. Their model requires that the target surface has an ordered structure and the collision cascade in the bulk is random. Neither model turns out to be fully satisfactory. In a recent computer simulation [4] it was found that both mechanisms take place in single crystal sputtering. In a real experiment the target surface may be dirty and due to ion-bombardment surface topography may change, i.e. facets, cliffs and ripples are developed. Szymonski et al. [5] have found the angular distributions of sputtered particles to be strongly dependent on the vacuum conditions. A number of computer simulations of sputtering
Fig. 1. The angular distributions of Cu atoms sputtered with normally incident 5 keV Ar ions when the target is single crystal with a (100) surface. The target has approximately one (a) and two (b) monolayers thick random layer on the top. 0 is the polar angle with respect to the surface normal and + is the azimuthal angle. 0168-583X/88/$03.50 (North-Holland
@ Elsevier Physics Publishing
Science
Publishers
Division)
B.V.
hf. Hautala and J. Likonen
exists, where the target is either amorphous (actually liquid like) [7], polycrystalline [8,9], random [lo] or monocrystal [4,9,11]. We have recently studied [12] the effect of the target structure by the binary collision cascade simulation code COSIPO [13]. The use of the same code in each case makes it easier to extract the real, parameter independent, deviations. Very recently Yamamura et al. [6] have investigated the bombarding angle-dependence of sputtering under various surface conditions. The present paper continues the study of the effect of target structure. We study how sensitive the angular
52-l
/ Sputtering of Cu atoms by Ar ions
distributions are on surface structure modifications, i.e. how sensitive the experimentally measured distributions are on the surface contamination or disorder. To our knowledge, this important topic has not been quantitatively calculated in the literature.
2. Parameters As in most earlier studies, the Moliere potential was applied and at the surface the planar surface barrier with the binding energy of E, = 3.5 eV was used. In
‘1
bl
s
d) 1
Fig. 2. The same as in fig. 1 but the target is random with one (a), two (b), three (c), four (d) and six (e) (100) monolayers on the top. VII. SPU’lTERING/SIMS
528
M. Hautala and J. Likonen / Sputtering of Cu atoms by Ar ions
this paper we treat two kinds of targets; a monocrystal
0
0
target with a thin random layer at the surface or vice versa. The random target is a single crystal which is randomly rotated in space after each collision. In the random case the position of the surface is somewhat arbitrary, since the shape of each microcrystal at the surface has to be chosen. In the present calculations the following method has been used. The surface area is determined by -outsurf < z < insurf. The influence of the lack of flattness of the surface can be studied by changing outsurf. Insurf and outsurf were chosen to be half a monolayer. One monolayer is a/2, where a is the lattice unit. If the target atom is in the surface area and no candidate is found on which the target atom could collide, it will be sputtered if it has enough energy to overcome the surface potential barrier. In the single crystal case the surface is flat because there are always atoms at the surface. Thus, insurf and outsurf do not have any influence on the yields. The interface between the surface layer and the substrate is also left somewhat arbitrary. The neighbouring structure is always determined by the position of the previous collider. Thus the position of the interface is arbitrary by about one monolayer. Further details of the calculations and a more full description of the parameters are given in ref.
tw
3. Results and discussion In figs, 1 and 2 the angular distributions are given in various situations. In the monocrystalline case the (101) collision chains (the Wehner spots [14]) are clearly seen to be dominant. In fig. la one monolayer thick random layer has almost destroyed the Wehner spots and in fig. lb they have completely disappeared. Thus, the Wehner spots are seen to be very sensitive to the two top layers of the target. This is quite natural since the spots are due to the collision chains and if the topmost atom is not in the right position it will sputter in a wrong direction. That means that experimentally one has to be very careful in cleaning the surface if realistic distributions are wanted. The maxima of the Wehner spots are at 50 o k 3O, i.e. they are 5O + 3O off from the (101)~axes due to the surface refraction and deflection. The experimental result is 5 ’ f 4O [15] and the Marlowe result is 9 o k 2” [4]. From fig. 2 it can be seen that the Wehner spots become more pronounced when the number of (100) monolayers increases. It is quite evident that the four top layers mainly cause the Wehner spots. However, the spots in the case of totally monocrystalline target are still much more pronounced than in fig. 2e [12]. Actually we found that 15% of the sputtered atoms were of a generation larger than 8 in this case [12]. This fact
Fig. 3. The yield of Cu atoms sputtered with 5 keV Ar ions as a function of the sputtering angle. In the upper part the target is 3 (. . .) and 6 (- - -) (100) monorandom with 1( -), layers on the top. In the lower part of the figure the (100) or two monocrystalline target has approximately one ( -) (. . .) random layers on the top. 8 = 90 o corresponds to the direction aligned to the surface and 0 = 180 o to the surface normal.
points out the importance of long collision chains in monocrystalline sputtering. The angular distributions of sputtered atoms are nearly cosine if the target is random. The angular distributions for the single crystal target are very different from those of the random target. In fig. 3 the angular distributions of sputtered Cu atoms are presented in various cases. The distribution is nearly cosine in the case of one monolayer on the top of a random target and in the case of two random layers on the top of a monocrystalline target. Especially in the case of three and six monolayers the peak at 130 O, which is due to the (101) axes, is distinct. The same peak can also be
M. Hautala and J. Likonen
/ Sputtering
seen in the case of one random layer in the lower part of fig. 3. The refraction due to the planar binding potential moves the peak position So from the (101) direction towards the surface. It also broadens the left side of the peaks. If the target were totally monocrystalline, the peak would be more developed [12] again showing the role of collision chains. The role of chains can further be studied by looking at the generation of each sputtered particle. The calculations show that with increasing recoil generation the atoms increasingly sputter in the (101) direction, when the target is totally monocyrstalline [12]. This is due to the fact that in the beginning of the collision cascade, when the generation is small, the collisions occur near the surface and the atoms sputter rather randomly. When the generation increases, the collision chain starts deeper from the target and the momentum moves preferably in the (101) direction. In monocrystalline targets the collision chains contributing to sputtering are longer than in random targets due to the long-range order in the former structures. As a consequence the relative amount of the sputtered Cu atoms of various generations differed appreciably from that, in which the target was totally random [12]. On the contrary, in the present calculations even in the case of six monolayers on the top of a random target the relative amounts of various generations are not much different from those in random targets. This again gives support to the interpretation that the spots are due to both the collision chains and the Lehmann and Sigmund mechanism.
4. Concluding remarks The present paper shows the feasibility of studying experimentally relevant parameters in sputtering using the binary collision approximation in lattice simulations. The effect of the surface structure is studied
of Cu atoms by Ar ions
529
extensively and is shown to be critical. The results show that sputtering measurements ultimately need a high vacuum in order to be able to extract reliable data. One should bear in mind that there are various approximations and uncertainties, like the interatomic potential and the surface binding model, in the calculations which would affect the absolute yields but should not change the conclusions of the present calculations. Further, the detailed structure of the random surface changes the results as well as the various parameters. If the surface is damaged, the use of the planar surface barrier is questionable.
References 111 P. Sigmund, Phys. Rev. 184 (1969) 383. PI R.H. Silsbee, J. Appl. Phys. 28 (1957) 1246.
131 C. Lehmann and P. Sigmund, Phys. Status Sohdi 16 (1966) 507. [41 M. Hou and W. Eckstein, Nucl. Instr. and Meth. B13 (1986) 324. PI M. Szymonski, W. Huang and J. Onsgaard, Nucl. Instr. and Meth. B14 (1986) 263. WI Y. Yamamura, C. Mossner and H. Oechsner, Radiat. Eff. 103 (1987) 25. [71 W. Eckstein and J.P. Biersack, Appl. Phys. A34 (1984) 73. 181 D.S. Karpuzov, Nucl. Instr. and Meth. B19/20 (1987) 109. 191 V.I. Shulga, Radiat. Effects 70 (1983) 65; 82 169 (1984); 84 (1985) 1. WI M. Hou and M.T. Robinson, Appl. Phys. 18 (1979) 381. 1111 M.T. Robinson: Sputtering by Particle Bombardment I, ed. R. Behrisch (Springer, Berlin, 1981) pp. 73-144. [12] J. Likonen and M. HautaIa, Appl. Phys. A45 (1988) 137. (131 M. Hautala: Phys. Rev. B30 (1984) 5010. [14] G.K. Wehner: J. Appl. Phys. 26 (1955) 1056; Phys. Rev. 102 (1956) 690. [15] C.H. Weijsenfeld: thesis, Utrecht (1966); Philips Research Reports Supplements 1967, no. 2.
VII. SPUTTERING/SIMS
Nuclear Instruments and Methods in Physics Research B33 (1988) 526-529 North-Holland, Amsterdam
THE EFFECT OF NEAR-SURFACE
STRUCTURE
ON SPUTTERING
OF Cu ATOMS BY Ar IONS
M. HAUTALA Department
of Physics, University of Helsinki, Siltavuorenpenger
20 D, SF-#I70
Helsinki, Finland
J. LIKONEN Technical Research Centre of Finland, Reactor Laboratory,
Otakaari 3 A, SF-02150 Espoo, Finland
Sputtering of Cu targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code COSIPO. The sensitivity of the angular distributions of sputtered atoms on the near-surface structure is demonstrated by haying a thin monorandom layer on a random/monocrystalline Cu target. Just two monolayers nearly destroy the angular distributions due to the underlying structure.
1. Introduction Studies of angular distributions of particles emitted from targets during a sputter process provide information for a better understanding of sputtering phenomena. From a theoretical viewpoint the random collision cascade theory [l] describes satisfactorily the sputtering process for amorphous and polycrystalline solids. For crystalline targets, the sputtering yields are influenced by the lattice structure especially for particle incidence and emergence in close packed directions. The first explanation for the ejection patterns of sputtered particles was proposed by Silsbee [2] who suggested a focusing collision chain mechanism. An
alternative to the Silsbee mechanism was developed by Lehmarm and Sigmund [3]. Their model requires that the target surface has an ordered structure and the collision cascade in the bulk is random. Neither model turns out to be fully satisfactory. In a recent computer simulation [4] it was found that both mechanisms take place in single crystal sputtering. In a real experiment the target surface may be dirty and due to ion-bombardment surface topography may change, i.e. facets, cliffs and ripples are developed. Szymonski et al. [5] have found the angular distributions of sputtered particles to be strongly dependent on the vacuum conditions. A number of computer simulations of sputtering
Fig. 1. The angular distributions of Cu atoms sputtered with normally incident 5 keV Ar ions when the target is single crystal with a (100) surface. The target has approximately one (a) and two (b) monolayers thick random layer on the top. 0 is the polar angle with respect to the surface normal and + is the azimuthal angle. 0168-583X/88/$03.50 (North-Holland
@ Elsevier Physics Publishing
Science
Publishers
Division)
B.V.
hf. Hautala and J. Likonen
exists, where the target is either amorphous (actually liquid like) [7], polycrystalline [8,9], random [lo] or monocrystal [4,9,11]. We have recently studied [12] the effect of the target structure by the binary collision cascade simulation code COSIPO [13]. The use of the same code in each case makes it easier to extract the real, parameter independent, deviations. Very recently Yamamura et al. [6] have investigated the bombarding angle-dependence of sputtering under various surface conditions. The present paper continues the study of the effect of target structure. We study how sensitive the angular
52-l
/ Sputtering of Cu atoms by Ar ions
distributions are on surface structure modifications, i.e. how sensitive the experimentally measured distributions are on the surface contamination or disorder. To our knowledge, this important topic has not been quantitatively calculated in the literature.
2. Parameters As in most earlier studies, the Moliere potential was applied and at the surface the planar surface barrier with the binding energy of E, = 3.5 eV was used. In
‘1
bl
s
d) 1
Fig. 2. The same as in fig. 1 but the target is random with one (a), two (b), three (c), four (d) and six (e) (100) monolayers on the top. VII. SPU’lTERING/SIMS
528
M. Hautala and J. Likonen / Sputtering of Cu atoms by Ar ions
this paper we treat two kinds of targets; a monocrystal
0
0
target with a thin random layer at the surface or vice versa. The random target is a single crystal which is randomly rotated in space after each collision. In the random case the position of the surface is somewhat arbitrary, since the shape of each microcrystal at the surface has to be chosen. In the present calculations the following method has been used. The surface area is determined by -outsurf < z < insurf. The influence of the lack of flattness of the surface can be studied by changing outsurf. Insurf and outsurf were chosen to be half a monolayer. One monolayer is a/2, where a is the lattice unit. If the target atom is in the surface area and no candidate is found on which the target atom could collide, it will be sputtered if it has enough energy to overcome the surface potential barrier. In the single crystal case the surface is flat because there are always atoms at the surface. Thus, insurf and outsurf do not have any influence on the yields. The interface between the surface layer and the substrate is also left somewhat arbitrary. The neighbouring structure is always determined by the position of the previous collider. Thus the position of the interface is arbitrary by about one monolayer. Further details of the calculations and a more full description of the parameters are given in ref.
tw
3. Results and discussion In figs, 1 and 2 the angular distributions are given in various situations. In the monocrystalline case the (101) collision chains (the Wehner spots [14]) are clearly seen to be dominant. In fig. la one monolayer thick random layer has almost destroyed the Wehner spots and in fig. lb they have completely disappeared. Thus, the Wehner spots are seen to be very sensitive to the two top layers of the target. This is quite natural since the spots are due to the collision chains and if the topmost atom is not in the right position it will sputter in a wrong direction. That means that experimentally one has to be very careful in cleaning the surface if realistic distributions are wanted. The maxima of the Wehner spots are at 50 o k 3O, i.e. they are 5O + 3O off from the (101)~axes due to the surface refraction and deflection. The experimental result is 5 ’ f 4O [15] and the Marlowe result is 9 o k 2” [4]. From fig. 2 it can be seen that the Wehner spots become more pronounced when the number of (100) monolayers increases. It is quite evident that the four top layers mainly cause the Wehner spots. However, the spots in the case of totally monocrystalline target are still much more pronounced than in fig. 2e [12]. Actually we found that 15% of the sputtered atoms were of a generation larger than 8 in this case [12]. This fact
Fig. 3. The yield of Cu atoms sputtered with 5 keV Ar ions as a function of the sputtering angle. In the upper part the target is 3 (. . .) and 6 (- - -) (100) monorandom with 1( -), layers on the top. In the lower part of the figure the (100) or two monocrystalline target has approximately one ( -) (. . .) random layers on the top. 8 = 90 o corresponds to the direction aligned to the surface and 0 = 180 o to the surface normal.
points out the importance of long collision chains in monocrystalline sputtering. The angular distributions of sputtered atoms are nearly cosine if the target is random. The angular distributions for the single crystal target are very different from those of the random target. In fig. 3 the angular distributions of sputtered Cu atoms are presented in various cases. The distribution is nearly cosine in the case of one monolayer on the top of a random target and in the case of two random layers on the top of a monocrystalline target. Especially in the case of three and six monolayers the peak at 130 O, which is due to the (101) axes, is distinct. The same peak can also be
M. Hautala and J. Likonen
/ Sputtering
seen in the case of one random layer in the lower part of fig. 3. The refraction due to the planar binding potential moves the peak position So from the (101) direction towards the surface. It also broadens the left side of the peaks. If the target were totally monocrystalline, the peak would be more developed [12] again showing the role of collision chains. The role of chains can further be studied by looking at the generation of each sputtered particle. The calculations show that with increasing recoil generation the atoms increasingly sputter in the (101) direction, when the target is totally monocyrstalline [12]. This is due to the fact that in the beginning of the collision cascade, when the generation is small, the collisions occur near the surface and the atoms sputter rather randomly. When the generation increases, the collision chain starts deeper from the target and the momentum moves preferably in the (101) direction. In monocrystalline targets the collision chains contributing to sputtering are longer than in random targets due to the long-range order in the former structures. As a consequence the relative amount of the sputtered Cu atoms of various generations differed appreciably from that, in which the target was totally random [12]. On the contrary, in the present calculations even in the case of six monolayers on the top of a random target the relative amounts of various generations are not much different from those in random targets. This again gives support to the interpretation that the spots are due to both the collision chains and the Lehmann and Sigmund mechanism.
4. Concluding remarks The present paper shows the feasibility of studying experimentally relevant parameters in sputtering using the binary collision approximation in lattice simulations. The effect of the surface structure is studied
of Cu atoms by Ar ions
529
extensively and is shown to be critical. The results show that sputtering measurements ultimately need a high vacuum in order to be able to extract reliable data. One should bear in mind that there are various approximations and uncertainties, like the interatomic potential and the surface binding model, in the calculations which would affect the absolute yields but should not change the conclusions of the present calculations. Further, the detailed structure of the random surface changes the results as well as the various parameters. If the surface is damaged, the use of the planar surface barrier is questionable.
References 111 P. Sigmund, Phys. Rev. 184 (1969) 383. PI R.H. Silsbee, J. Appl. Phys. 28 (1957) 1246.
131 C. Lehmann and P. Sigmund, Phys. Status Sohdi 16 (1966) 507. [41 M. Hou and W. Eckstein, Nucl. Instr. and Meth. B13 (1986) 324. PI M. Szymonski, W. Huang and J. Onsgaard, Nucl. Instr. and Meth. B14 (1986) 263. WI Y. Yamamura, C. Mossner and H. Oechsner, Radiat. Eff. 103 (1987) 25. [71 W. Eckstein and J.P. Biersack, Appl. Phys. A34 (1984) 73. 181 D.S. Karpuzov, Nucl. Instr. and Meth. B19/20 (1987) 109. 191 V.I. Shulga, Radiat. Effects 70 (1983) 65; 82 169 (1984); 84 (1985) 1. WI M. Hou and M.T. Robinson, Appl. Phys. 18 (1979) 381. 1111 M.T. Robinson: Sputtering by Particle Bombardment I, ed. R. Behrisch (Springer, Berlin, 1981) pp. 73-144. [12] J. Likonen and M. HautaIa, Appl. Phys. A45 (1988) 137. (131 M. Hautala: Phys. Rev. B30 (1984) 5010. [14] G.K. Wehner: J. Appl. Phys. 26 (1955) 1056; Phys. Rev. 102 (1956) 690. [15] C.H. Weijsenfeld: thesis, Utrecht (1966); Philips Research Reports Supplements 1967, no. 2.
VII. SPUTTERING/SIMS