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The effect of target structure on sputtering of Cu atoms by Ar ions
Volume 124, number 6,7
5 October 1987
PHYSICS LETTERS A
THE EFFECT OF TARGET STRUCTURE
ON SPUTTERING
OF Cu ATOMS BY AI IONS
M. HAUTALA Department of Physics, University of Helsinki, Siltavuorenpenger 20 D. SF-001 70 Helsinki, Finland
and J. LIKONEN Technical Research Centre ofFinland, Reactor Laboratory, Otakaari 3A, SF-02150 Espoo, Finland
Received 16 June 1987; accepted for publication 4 August 1987 Communicated by D. Bloch
Sputtering of Cu single-crystal, polycrystal and amorphous targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code Cosipo. The sputtering yields, angular distributions and the space distributions of the original positions of the sputtered Cu atoms have been calculated. The results are discussed within the framework of cascade generations.
A number of computer simulations on sputtering exist, where the target is either amorphous (liquid like) [ 11, polycrystalline [ 2,3], random [ 41 or monocrystal [ 3,5-71. The results obtained are compared with the experimental data of different structures. It remains unclear how much the differences between the experimental and calculated sputtering yields are affected by the differences in the target structures. Our aim is to study the effect of the target structure by the binary collision cascade simulation code Cosipo [ 81. The use of the same code for each structure makes it easier to extract the real, parameter independent, deviations. This letter will give a short summary of the main results. To make the comparison as good as possible with the existing data, sputtering of Cu atoms by 5 keV Ar ions was chosen for the detailed study. As in most of the earlier studies [ 2,451, the Moliere potential was applied and at the surface the planar potential with a binding energy of 3.5 eV was used. Further details of the calculations, a more full description of the calculations and of the results as well as a critical discussion on the parameters will be published elsewhere. The comparison of the yields is given in fig. 1 as a function of the incidence angle. In the upper part
the monocrystalline yields are compared with the Marlowe calculations [ 51 and with the experimental data [ 91. The agreement between the simulated and experimental results is quite satisfactory, the deviations between the simulated results being larger. The essential difference between the computer codes Cosipo and Marlowe is that in Cosipo the crystal is perfect during the whole cascade calculation, whereas in Marlowe the distortion of the crystal is included, although the exact time treatment is a problem in binary collision calculations. These two codes can be understood as being two approximations on what really is happening in the cascade. The Marlowe code is a more realistic description but the logic in Cosipo is faster. In the lower part of fig. 1 the effect of the structure is shown. We have earlier pointed out that the nuclear stopping of recoils from the substitutional lattice sites is 20% greater in polycrystalline than in amorphous targets [ 81 (and even greater if compared to stopping which a recoil from an interstitial lattice site undergoes [ 81). Therefore it is not surprising that also the yield is larger. This difference should, however, be always remembered in the comparisons. In fig. 2 the angular distributions are compared. In the monocrystalline case the ( 110) collision chains
0375-9601/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
383
Volume 124, number 6,7
PHYSICS LETTERS A ~ _
5 October 1987 E=
tlONOCRYjTRLLlNE _
Rr
5 he”
Cu
10011
.~~..._~
Rr
5 keV
Cu
IO011
.._.~~.
RNGULAR
RtlORPHOUS
D[STRIEUTlON
FII=
0.D” 0.0”
POLfCiRYSTRCLlNE
ONOEROELINOEN
RNGULRR
DISTRIBUTION
E= S.OkeV TtTR. 0.F’ f[I=
Fig. I. The yields of Cu atoms sputtered with 5 keV Ar ions as a function of the entrance angle with respect to the surface normal. In the upper part the results of the present calculations are compared with the Marlowe calculations [ 51 and with the experimental data [ 91. In the lower part the effect of the target structure is studied.
(the Wehner spots [ lo] are distinctly seen to be dominant. Further calculations show that the amorphous structure yields a distribution somewhat more peaked that the polycrystalline one. The distribution is slightly overcosine, whereas for he polycrystalline target it is undercosine. In the case of the polycrystalline target this may be due to the collision sequences that do not average out [ 111. There is also 384
5.OkeV
TETR.
0.00
, Fig. 2. The angular distributions of Cu atoms sputtered with 5 keV Ar ions when the target structure is polycrystalline (upper part) or single crystal with a (100) surface (lower part). f3is the polar angle with respect to the surface normal and @is the azimuthal angle.
some experimental support for an undercosine distribution in the case of the polycrystalline target [ 121 although it was interpreted to be due to texture. The role of the target structure is further studied in fig. 3 where the angular distributions are given for various generations of recoils. It is clearly seen that the
5 October I987
PHYSICS LETTERS A
MONOCRYSTALLINE GENERRT!ON;
-
POLYCRYSTALLINE
~----~~- flMORPti0~5
GENERRTION
Fig. 4. The relative yields of Cu atoms of various generations sputtered with S keV Ar ions.
Fig. 3. The yields of Cu atoms sputtered with 5000 5 keV Ar ions as a function of the sputtering angle for generations, 1, 4 and 7. The target is either amorphous (top) polycrystalline (middle) or single crystal with (100) surface (down).
target structure plays a major role at high generations. The maximum at around 135”, when the target is monocrystalline, is due to the ( 110) collision
chains. From fig. 4 it can be seen that the structure renders longer collision chains possible. It also shows that the major contribution to sputtering comes from generation 3, as well as that both crystalline targets yield surprisingly similar curves. In fig. 3 there was already slight indication about the similarity of the cascades in crystalline targets. Ignorance of the crystal structure distortion during the cascade development makes the code faster. How good is the approximation? The origins of the sputtered atoms are given in figs. 5 and 6 for monocrystalline and polycrystalline targets, respectively. It is seen that (i) the yield in the single crystal comes from a smaller area and nearly all the yield comes from the two top layers. This is due to the strong shadowing by the surface atoms. (ii) The highest probability for a specific target atom to be sputtered is on the average less than 0.1. So, normally the crystal is pretty well perfect and the approximation is good. As shown in fig. 1 the higher yields are an exception. If the sputtering yield is high, the surface is locally eroded and therefore the atoms from the not uppermost layers may sputter and consequently the yield is higher in the Marlowe calculations. Also some individual cascades may yield exceptionally 385
5 October 1987
PHYSICS LETTERS A
Volume 124, number 6,7
1.0
R
z-
1.0
'1
Fig. 5. The distribution of the original positions of the Cu atoms sputtered with 5 keV Ar ions, when the target is monocrystalline with a (100) surface. The x, y and z-scales are given in lattice units.
high sputtering, if the entrance position of the Ar-ion is suitable. One should bear in mind that there are various approximations in the calculations. The Moliere potential is certainly too strong, we have assumed the planar surface binding model to be valid and 386
Fig. 6. As in fig. 5, but the target is polycrystalline.
assumed the probability of escaping the surface to be either 0 or 1. The detailed structure of the polycrystalline surface changes the results as well as the various parameters. These all affect the absolute yields but not so much the relative values. All these approximations will be discussed in a subsequent paper. The goal of this letter was to show that one should always remember the role of the structure in sputtering studies.
Volume 124, number 6,7
PHYSICS LETTERS A
References [ 1 ] W. Eckstein and J.P. Biersack, Appl. Phys. A 34 (1984) 73.
[ 2 ] D.S. Karpuzov, Nucl. Instrum. Methods B 19/20 (1987) 109. [3] V.I. Shulga, Radiat. Eff. 70 (1983) 65; 82 (1984) 169; 85 (1985) 1. [4]M.HouandM.T.Robinson,Appl.Phys. 18 (1979) 381. [S] M. Hou and W. Eckstein, Nucl. Instrum. Methods B 13 (1986) 324.
S October 1987
[6] M.T. Robinson, in: Sputtering by particle bombardment, Vol. I, ed. R. Behrisch (Springer, New York 198 1) p. 73. [ 7 ] D.E. Harrison Jr., Radiat. Eff 70 (1983) 1. [8] M. Hautala, Phys. Rev. B 30 (1984) 5010. [ 91 B. Onderdelinden, Can. J. Phys. 46 (1986) 739. [lo] G.K. Wehner, J. Appl. Phys. 26 (1955) 1056; Phys. Rev. 102 (1956) 690. [ 111 P. Sigmund, Phys. Rev. 184 (1969) 383. [ 121 M. Szymonski, W. Huang and J. Onsgaard, Nucl. Instrum. MethodsB 14 (1986) 263.
387
5 October 1987
PHYSICS LETTERS A
THE EFFECT OF TARGET STRUCTURE
ON SPUTTERING
OF Cu ATOMS BY AI IONS
M. HAUTALA Department of Physics, University of Helsinki, Siltavuorenpenger 20 D. SF-001 70 Helsinki, Finland
and J. LIKONEN Technical Research Centre ofFinland, Reactor Laboratory, Otakaari 3A, SF-02150 Espoo, Finland
Received 16 June 1987; accepted for publication 4 August 1987 Communicated by D. Bloch
Sputtering of Cu single-crystal, polycrystal and amorphous targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code Cosipo. The sputtering yields, angular distributions and the space distributions of the original positions of the sputtered Cu atoms have been calculated. The results are discussed within the framework of cascade generations.
A number of computer simulations on sputtering exist, where the target is either amorphous (liquid like) [ 11, polycrystalline [ 2,3], random [ 41 or monocrystal [ 3,5-71. The results obtained are compared with the experimental data of different structures. It remains unclear how much the differences between the experimental and calculated sputtering yields are affected by the differences in the target structures. Our aim is to study the effect of the target structure by the binary collision cascade simulation code Cosipo [ 81. The use of the same code for each structure makes it easier to extract the real, parameter independent, deviations. This letter will give a short summary of the main results. To make the comparison as good as possible with the existing data, sputtering of Cu atoms by 5 keV Ar ions was chosen for the detailed study. As in most of the earlier studies [ 2,451, the Moliere potential was applied and at the surface the planar potential with a binding energy of 3.5 eV was used. Further details of the calculations, a more full description of the calculations and of the results as well as a critical discussion on the parameters will be published elsewhere. The comparison of the yields is given in fig. 1 as a function of the incidence angle. In the upper part
the monocrystalline yields are compared with the Marlowe calculations [ 51 and with the experimental data [ 91. The agreement between the simulated and experimental results is quite satisfactory, the deviations between the simulated results being larger. The essential difference between the computer codes Cosipo and Marlowe is that in Cosipo the crystal is perfect during the whole cascade calculation, whereas in Marlowe the distortion of the crystal is included, although the exact time treatment is a problem in binary collision calculations. These two codes can be understood as being two approximations on what really is happening in the cascade. The Marlowe code is a more realistic description but the logic in Cosipo is faster. In the lower part of fig. 1 the effect of the structure is shown. We have earlier pointed out that the nuclear stopping of recoils from the substitutional lattice sites is 20% greater in polycrystalline than in amorphous targets [ 81 (and even greater if compared to stopping which a recoil from an interstitial lattice site undergoes [ 81). Therefore it is not surprising that also the yield is larger. This difference should, however, be always remembered in the comparisons. In fig. 2 the angular distributions are compared. In the monocrystalline case the ( 110) collision chains
0375-9601/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
383
Volume 124, number 6,7
PHYSICS LETTERS A ~ _
5 October 1987 E=
tlONOCRYjTRLLlNE _
Rr
5 he”
Cu
10011
.~~..._~
Rr
5 keV
Cu
IO011
.._.~~.
RNGULAR
RtlORPHOUS
D[STRIEUTlON
FII=
0.D” 0.0”
POLfCiRYSTRCLlNE
ONOEROELINOEN
RNGULRR
DISTRIBUTION
E= S.OkeV TtTR. 0.F’ f[I=
Fig. I. The yields of Cu atoms sputtered with 5 keV Ar ions as a function of the entrance angle with respect to the surface normal. In the upper part the results of the present calculations are compared with the Marlowe calculations [ 51 and with the experimental data [ 91. In the lower part the effect of the target structure is studied.
(the Wehner spots [ lo] are distinctly seen to be dominant. Further calculations show that the amorphous structure yields a distribution somewhat more peaked that the polycrystalline one. The distribution is slightly overcosine, whereas for he polycrystalline target it is undercosine. In the case of the polycrystalline target this may be due to the collision sequences that do not average out [ 111. There is also 384
5.OkeV
TETR.
0.00
, Fig. 2. The angular distributions of Cu atoms sputtered with 5 keV Ar ions when the target structure is polycrystalline (upper part) or single crystal with a (100) surface (lower part). f3is the polar angle with respect to the surface normal and @is the azimuthal angle.
some experimental support for an undercosine distribution in the case of the polycrystalline target [ 121 although it was interpreted to be due to texture. The role of the target structure is further studied in fig. 3 where the angular distributions are given for various generations of recoils. It is clearly seen that the
5 October I987
PHYSICS LETTERS A
MONOCRYSTALLINE GENERRT!ON;
-
POLYCRYSTALLINE
~----~~- flMORPti0~5
GENERRTION
Fig. 4. The relative yields of Cu atoms of various generations sputtered with S keV Ar ions.
Fig. 3. The yields of Cu atoms sputtered with 5000 5 keV Ar ions as a function of the sputtering angle for generations, 1, 4 and 7. The target is either amorphous (top) polycrystalline (middle) or single crystal with (100) surface (down).
target structure plays a major role at high generations. The maximum at around 135”, when the target is monocrystalline, is due to the ( 110) collision
chains. From fig. 4 it can be seen that the structure renders longer collision chains possible. It also shows that the major contribution to sputtering comes from generation 3, as well as that both crystalline targets yield surprisingly similar curves. In fig. 3 there was already slight indication about the similarity of the cascades in crystalline targets. Ignorance of the crystal structure distortion during the cascade development makes the code faster. How good is the approximation? The origins of the sputtered atoms are given in figs. 5 and 6 for monocrystalline and polycrystalline targets, respectively. It is seen that (i) the yield in the single crystal comes from a smaller area and nearly all the yield comes from the two top layers. This is due to the strong shadowing by the surface atoms. (ii) The highest probability for a specific target atom to be sputtered is on the average less than 0.1. So, normally the crystal is pretty well perfect and the approximation is good. As shown in fig. 1 the higher yields are an exception. If the sputtering yield is high, the surface is locally eroded and therefore the atoms from the not uppermost layers may sputter and consequently the yield is higher in the Marlowe calculations. Also some individual cascades may yield exceptionally 385
5 October 1987
PHYSICS LETTERS A
Volume 124, number 6,7
1.0
R
z-
1.0
'1
Fig. 5. The distribution of the original positions of the Cu atoms sputtered with 5 keV Ar ions, when the target is monocrystalline with a (100) surface. The x, y and z-scales are given in lattice units.
high sputtering, if the entrance position of the Ar-ion is suitable. One should bear in mind that there are various approximations in the calculations. The Moliere potential is certainly too strong, we have assumed the planar surface binding model to be valid and 386
Fig. 6. As in fig. 5, but the target is polycrystalline.
assumed the probability of escaping the surface to be either 0 or 1. The detailed structure of the polycrystalline surface changes the results as well as the various parameters. These all affect the absolute yields but not so much the relative values. All these approximations will be discussed in a subsequent paper. The goal of this letter was to show that one should always remember the role of the structure in sputtering studies.
Volume 124, number 6,7
PHYSICS LETTERS A
References [ 1 ] W. Eckstein and J.P. Biersack, Appl. Phys. A 34 (1984) 73.
[ 2 ] D.S. Karpuzov, Nucl. Instrum. Methods B 19/20 (1987) 109. [3] V.I. Shulga, Radiat. Eff. 70 (1983) 65; 82 (1984) 169; 85 (1985) 1. [4]M.HouandM.T.Robinson,Appl.Phys. 18 (1979) 381. [S] M. Hou and W. Eckstein, Nucl. Instrum. Methods B 13 (1986) 324.
S October 1987
[6] M.T. Robinson, in: Sputtering by particle bombardment, Vol. I, ed. R. Behrisch (Springer, New York 198 1) p. 73. [ 7 ] D.E. Harrison Jr., Radiat. Eff 70 (1983) 1. [8] M. Hautala, Phys. Rev. B 30 (1984) 5010. [ 91 B. Onderdelinden, Can. J. Phys. 46 (1986) 739. [lo] G.K. Wehner, J. Appl. Phys. 26 (1955) 1056; Phys. Rev. 102 (1956) 690. [ 111 P. Sigmund, Phys. Rev. 184 (1969) 383. [ 121 M. Szymonski, W. Huang and J. Onsgaard, Nucl. Instrum. MethodsB 14 (1986) 263.
387