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Comments on “Sputtering of dimers off a silicon surface” by M.I. Nietiadi et al.
Nuclear Instruments and Methods in Physics Research B 302 (2013) 55–56
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Discussion
Comments on ‘‘Sputtering of dimers off a silicon surface’’ by M.I. Nietiadi et al. Hans Oechsner Institute of Surface and Thin Film Analysis IFOS and Department of Physics, Technical University of Kaiserslautern, Trippstadter Str. 120, D-67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Received 19 March 2013 Received in revised form 11 March 2013 Accepted 28 March 2013 Available online 6 April 2013
a b s t r a c t The conclusions in a recent paper by Nitiadi et al. on the sputtering of neutral Si-dimers are critically commented. Ó 2013 Elsevier B.V. All rights reserved.
Keyword: Sputtered dimers
In a recent publication Nietiadi et al. [1] report on their simulation of sputter induced atom and dimer emission from a (100) Si surface under bombardment with 2 keV Ar+. They come to the conclusion that the so-called recombination (or atomic combination) model for dimer formation (cf part 2 of these comments) is ruled out by their results. This has to be contradicted for several reasons. (1) The authors compare the simulated kinetic energy distributions of sputtered neutral Si-atoms and Si2-dimers with the socalled Thompson-formula [2]
fðEÞ E=ðE þ UÞnþ1
ð1Þ
which has been experimentally well confirmed for energy distributions of sputtered neutral atoms with n-values around 2 (see e.g. Refs. [3–6]). Using n = 3, the authors apply Eq. (1) – probably employing the same U-value as for the atom distribution – to their dimer energy distribution and find a good fit from about 0.2 through about 50 eV. They compare such a ‘‘slow decay’’ with n = 3 with a power exponent of n = 5 which they mention to be predicted by the model calculations by Können et al. [7] based on the recombination model. From the discrepancy between the two nvalues they conclude that this model is not valid. When looking into Ref. [7] in more detail one finds immediately that a power of n = 4.5 is predicted only asymptotically for V ? 1, i.e. for high center-of- mass velocities V or kinetic energies E of a dimer being formed according to the recombination model. As seen from the calculation of a dimer energy distribution in Ref. [7], f(E) becomes more flat for lower energies E. This is well corroborated by corresponding experimental results [6,8] and agrees with that of the authors in Ref. [1]. In addition, the high energy fall-off of their dimer distribution in Fig. 2(b) of Ref. [1] follows well a power of n = 4.5 as predicted in Ref. [7] for the recombination model. E-mail address: [email protected] 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.048
Hence, the conclusion of Nietiadi et al. that this model has been ruled out is not justified even by their own findings. (2) The recombination – or atomic combination model (ACM) as a synonym – for the formation of sputtered neutral dimers (and also trimers) was first proposed by the author of these comments [9] and elaborated independently by Gerhard [10] and in Ref. [7]. According to ACM two (or three) not necessarily next neighboured surface atoms being knocked on individually within one single sputter cascade may agglomerate to a molecule during the emission process. By applying simple kinematic collision theory it can be shown that direct co- emission of neighboured surface atoms is possible only when the pair-bonding (or dissociation) energy D of the corresponding molecule is by a factor of about 1.8 higher than the atomic surface binding energy U [10]. This is in general in not fulfilled for metals and also not for Si with D/U 0.77 for D 3.3 eV [11] and U from Ref. [1]. From probability arguments [9,10,12] the yield of sputtered neutral dimers is expected to be proportional to the square of the atom sputter yield. For an experimental check Secondary Neutral Mass Spectrometry SNMS [9] which avoids the influence of the well known matrix effects on the formation of secondary ions appears to be the appropriate technique. A careful evaluation of the SNMS-data for ion bombardment of a series of elemental metals with Ar+-ions around 1 keV [13] displays a good proportionality between the dimer-to-atom ratio and the atomic sputter yield in accordance with the predictions from ACM. Even more convincingly, the ACM-based model calculations in Ref. [10] delivered quantitative dimer yields which agree almost completely with the experimental values. As a kind of an experimentum crucis the formation of sputtered dimers (and also trimers) has been studied with SNMS for binary NiW- and NiMo alloys of varying bulk concentrations cb [12]. After bombardment with 1.2 keV Ar+-ions until stationary conditions are achieved, Auger Electron Spectroscopy AES revealed always a
56
H. Oechsner / Nuclear Instruments and Methods in Physics Research B 302 (2013) 55–56
Fig. 1. (a) Ratio of the SNMS dimer signals I (NiW0) and I (W20) from NiW-alloys versus the ratio of the Ni and W bulk concentrations cb. (b) Same signal ratios plotted versus the ratio of the Ni and W surface concentrations cs being different from the bulk concentrations due to preferential Ni – sputtering under normal bombardment with 1.2 keV Ar+. (From Ref. [15]).
strong increase of the W- and Mo-surface concentrations cs over their bulk concentrations due to preferential sputtering of the Nicomponent [14,15]. According to ACM the dimer sputtering yields of a target with constituents A and B will be described by [13,15].
YA2 Y2A
YAB YA YB
YB2 Y2B
ð2Þ
From mass conservation and for atomic sputtering the partial atomic sputter yields are given by Y X ¼ cbX Y tot under stationary P sputter conditions (Ytot = YX is the total sputter yield). Because the SNMS- signals I are proportional to the individual particle yields, the dimer ratios I(AB)/I(B2), e.g., are expected to be proportional to cbA =cbB from the atomic combination model ACM. Such a behaviour is, for instance, well demonstrated by Fig. 1a for the neutral dimers NiW and W2. Identical results are found in Ref. [12] for other ratios of sputtered neutral dimers (and also trimers) from the NiW- and NiMo-samples. Because a direct emission of diatomic pairs would be controlled by the surface concentrations cs which differ strongly from the
bulk concentrations cb for both sets of alloy samples, the corresponding dimer ratios should be proportional to the ratios of the respective cs-values. Fig. 1b demonstrates that this is not the case. Fig. 1a and b and corresponding results [12,15] provide thus a direct proof that atomic combination is responsible for the formation of sputtered diatomic (or triatomic) molecules for metals, and with regard of its D/U-value (see part 1)) also for Si. Because of the arguments in part 1) and the clear evidence for ACM which has been ignored by the authors of Ref. [1], their conclusion that the recombination or atomic combination model is ruled out by their results can certainly not be maintained. Acknowledgement The author appreciates stimulating discussions with Michael Kopnarski, IFOS. References [1] M.L. Nietiadi, Y. Rosandi, M. Kopnarski, H.M. Urbassek, Nucl. Instr. Meth. B 289 (2012) 97. [2] M.W. Thompson, Philos. Mag. 18 (1968) 377. [3] H. Oechsner, Z. Phys. 238 (1970) 433. [4] F. Bernhardt, H. Oechsner, E. Stumpe, Nucl. Instr. Meth. 132 (1976) 329. [5] R. Brizzolara, C.B. Cooper, T.K. Olson, Nucl. Instr. Meth. B 35 (1988) 36. [6] W. Husinsky, G. Nicolussi, G. Betz, Nucl. Instr. Meth. B 82 (1993) 323. [7] G.P. Können, A. Tip, A.E. de Vries, Radiat. Effects 21 (1974) 269. [8] R. Brizzolara, C.B. Cooper, Nucl. Instr. Meth. B 43 (1989) 136. [9] H. Oechsner, W. Gerhard, Surface Sci. 44 (1974) 480. [10] W. Gerhard, Z. Phys. B 22 (1975) 31. [11] J.A. Kerr, A.F. Trotman-Dickenson, in: R.C. Weast, M.J. Astle, (Eds.), CRC Handbook of Chemistry and Physics, 63rd ed., Boca Raton, 1983, p. F-185. [12] H. Oechsner, Int. J. Mass Spectrom. Ion Proc. 103 (1990) 31. [13] W. Gerhard, H. Oechsner, Z. Phys. B 22 (1975) 41. [14] J. Bartella, H. Oechsner, Surface Sci. 126 (1983) 581. [15] H. Oechsner, in: M.M. Popovic, P. Krstic (Eds.), The Physics of Ionized Gases, World Scientific Publ., Singapore, 1985, p. 571.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Discussion
Comments on ‘‘Sputtering of dimers off a silicon surface’’ by M.I. Nietiadi et al. Hans Oechsner Institute of Surface and Thin Film Analysis IFOS and Department of Physics, Technical University of Kaiserslautern, Trippstadter Str. 120, D-67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Received 19 March 2013 Received in revised form 11 March 2013 Accepted 28 March 2013 Available online 6 April 2013
a b s t r a c t The conclusions in a recent paper by Nitiadi et al. on the sputtering of neutral Si-dimers are critically commented. Ó 2013 Elsevier B.V. All rights reserved.
Keyword: Sputtered dimers
In a recent publication Nietiadi et al. [1] report on their simulation of sputter induced atom and dimer emission from a (100) Si surface under bombardment with 2 keV Ar+. They come to the conclusion that the so-called recombination (or atomic combination) model for dimer formation (cf part 2 of these comments) is ruled out by their results. This has to be contradicted for several reasons. (1) The authors compare the simulated kinetic energy distributions of sputtered neutral Si-atoms and Si2-dimers with the socalled Thompson-formula [2]
fðEÞ E=ðE þ UÞnþ1
ð1Þ
which has been experimentally well confirmed for energy distributions of sputtered neutral atoms with n-values around 2 (see e.g. Refs. [3–6]). Using n = 3, the authors apply Eq. (1) – probably employing the same U-value as for the atom distribution – to their dimer energy distribution and find a good fit from about 0.2 through about 50 eV. They compare such a ‘‘slow decay’’ with n = 3 with a power exponent of n = 5 which they mention to be predicted by the model calculations by Können et al. [7] based on the recombination model. From the discrepancy between the two nvalues they conclude that this model is not valid. When looking into Ref. [7] in more detail one finds immediately that a power of n = 4.5 is predicted only asymptotically for V ? 1, i.e. for high center-of- mass velocities V or kinetic energies E of a dimer being formed according to the recombination model. As seen from the calculation of a dimer energy distribution in Ref. [7], f(E) becomes more flat for lower energies E. This is well corroborated by corresponding experimental results [6,8] and agrees with that of the authors in Ref. [1]. In addition, the high energy fall-off of their dimer distribution in Fig. 2(b) of Ref. [1] follows well a power of n = 4.5 as predicted in Ref. [7] for the recombination model. E-mail address: [email protected] 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.048
Hence, the conclusion of Nietiadi et al. that this model has been ruled out is not justified even by their own findings. (2) The recombination – or atomic combination model (ACM) as a synonym – for the formation of sputtered neutral dimers (and also trimers) was first proposed by the author of these comments [9] and elaborated independently by Gerhard [10] and in Ref. [7]. According to ACM two (or three) not necessarily next neighboured surface atoms being knocked on individually within one single sputter cascade may agglomerate to a molecule during the emission process. By applying simple kinematic collision theory it can be shown that direct co- emission of neighboured surface atoms is possible only when the pair-bonding (or dissociation) energy D of the corresponding molecule is by a factor of about 1.8 higher than the atomic surface binding energy U [10]. This is in general in not fulfilled for metals and also not for Si with D/U 0.77 for D 3.3 eV [11] and U from Ref. [1]. From probability arguments [9,10,12] the yield of sputtered neutral dimers is expected to be proportional to the square of the atom sputter yield. For an experimental check Secondary Neutral Mass Spectrometry SNMS [9] which avoids the influence of the well known matrix effects on the formation of secondary ions appears to be the appropriate technique. A careful evaluation of the SNMS-data for ion bombardment of a series of elemental metals with Ar+-ions around 1 keV [13] displays a good proportionality between the dimer-to-atom ratio and the atomic sputter yield in accordance with the predictions from ACM. Even more convincingly, the ACM-based model calculations in Ref. [10] delivered quantitative dimer yields which agree almost completely with the experimental values. As a kind of an experimentum crucis the formation of sputtered dimers (and also trimers) has been studied with SNMS for binary NiW- and NiMo alloys of varying bulk concentrations cb [12]. After bombardment with 1.2 keV Ar+-ions until stationary conditions are achieved, Auger Electron Spectroscopy AES revealed always a
56
H. Oechsner / Nuclear Instruments and Methods in Physics Research B 302 (2013) 55–56
Fig. 1. (a) Ratio of the SNMS dimer signals I (NiW0) and I (W20) from NiW-alloys versus the ratio of the Ni and W bulk concentrations cb. (b) Same signal ratios plotted versus the ratio of the Ni and W surface concentrations cs being different from the bulk concentrations due to preferential Ni – sputtering under normal bombardment with 1.2 keV Ar+. (From Ref. [15]).
strong increase of the W- and Mo-surface concentrations cs over their bulk concentrations due to preferential sputtering of the Nicomponent [14,15]. According to ACM the dimer sputtering yields of a target with constituents A and B will be described by [13,15].
YA2 Y2A
YAB YA YB
YB2 Y2B
ð2Þ
From mass conservation and for atomic sputtering the partial atomic sputter yields are given by Y X ¼ cbX Y tot under stationary P sputter conditions (Ytot = YX is the total sputter yield). Because the SNMS- signals I are proportional to the individual particle yields, the dimer ratios I(AB)/I(B2), e.g., are expected to be proportional to cbA =cbB from the atomic combination model ACM. Such a behaviour is, for instance, well demonstrated by Fig. 1a for the neutral dimers NiW and W2. Identical results are found in Ref. [12] for other ratios of sputtered neutral dimers (and also trimers) from the NiW- and NiMo-samples. Because a direct emission of diatomic pairs would be controlled by the surface concentrations cs which differ strongly from the
bulk concentrations cb for both sets of alloy samples, the corresponding dimer ratios should be proportional to the ratios of the respective cs-values. Fig. 1b demonstrates that this is not the case. Fig. 1a and b and corresponding results [12,15] provide thus a direct proof that atomic combination is responsible for the formation of sputtered diatomic (or triatomic) molecules for metals, and with regard of its D/U-value (see part 1)) also for Si. Because of the arguments in part 1) and the clear evidence for ACM which has been ignored by the authors of Ref. [1], their conclusion that the recombination or atomic combination model is ruled out by their results can certainly not be maintained. Acknowledgement The author appreciates stimulating discussions with Michael Kopnarski, IFOS. References [1] M.L. Nietiadi, Y. Rosandi, M. Kopnarski, H.M. Urbassek, Nucl. Instr. Meth. B 289 (2012) 97. [2] M.W. Thompson, Philos. Mag. 18 (1968) 377. [3] H. Oechsner, Z. Phys. 238 (1970) 433. [4] F. Bernhardt, H. Oechsner, E. Stumpe, Nucl. Instr. Meth. 132 (1976) 329. [5] R. Brizzolara, C.B. Cooper, T.K. Olson, Nucl. Instr. Meth. B 35 (1988) 36. [6] W. Husinsky, G. Nicolussi, G. Betz, Nucl. Instr. Meth. B 82 (1993) 323. [7] G.P. Können, A. Tip, A.E. de Vries, Radiat. Effects 21 (1974) 269. [8] R. Brizzolara, C.B. Cooper, Nucl. Instr. Meth. B 43 (1989) 136. [9] H. Oechsner, W. Gerhard, Surface Sci. 44 (1974) 480. [10] W. Gerhard, Z. Phys. B 22 (1975) 31. [11] J.A. Kerr, A.F. Trotman-Dickenson, in: R.C. Weast, M.J. Astle, (Eds.), CRC Handbook of Chemistry and Physics, 63rd ed., Boca Raton, 1983, p. F-185. [12] H. Oechsner, Int. J. Mass Spectrom. Ion Proc. 103 (1990) 31. [13] W. Gerhard, H. Oechsner, Z. Phys. B 22 (1975) 41. [14] J. Bartella, H. Oechsner, Surface Sci. 126 (1983) 581. [15] H. Oechsner, in: M.M. Popovic, P. Krstic (Eds.), The Physics of Ionized Gases, World Scientific Publ., Singapore, 1985, p. 571.