- Email: [email protected]
On the segregation of metals during low-energy oxygen bombardment of silicon
Applied Surface Science 135 Ž1998. 200–204
On the segregation of metals during low-energy oxygen bombardment of silicon Mladen Petravic´
)
Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National UniÕersity, Canberra, ACT 0200, Australia Received 6 February 1998; accepted 10 April 1998
Abstract Segregation of metal impurities in Si under oxygen ion bombardment has been studied using secondary ion mass spectrometry. A strong evidence has been found for the thermodynamic driving force for segregation. This includes segregation of metal atoms at both interfaces of a buried SiO 2 layer and the ‘antisegregation’ behaviour of metal atoms having lower heats of oxide formation than Si. Segregation is enhanced at elevated temperatures when the metal diffusivity in amorphous Si is higher. q 1998 Elsevier Science B.V. All rights reserved. PACS: 68.55 Ln; 79.20 Rf; 82.66 Dp; 82.80 Ms Keywords: Surface segregation; Compound formation; Secondary ion mass spectroscopy; Silicon oxides; Metal oxides
1. Introduction Interactions at semiconductor surfaces induced by low-energy oxygen ions constitute an interesting area for the investigation of fundamental ion-induced surface chemical reactions. For example, low-energy oxygen bombardment may produce stoichiometric oxides at the surfaces of many semiconductors even at room temperature w1x. The ion beam oxidation process proceeds along several athermal steps and can create some new compounds, such as Si xGe1yxO 2 , which cannot be grown thermally w1x. On the other hand, ion-induced oxidation of semi-
)
Tel.: q61-6-249-0355; Fax: q61-6-249-0511; E-mail: [email protected]
conductor surfaces has attracted considerable interest due to possible applications in semiconductor technology. For example, high-dose oxygen bombardment of Si is now a key method for producing high quality silicon-on-insulator ŽSOI. substrates for very large scale integration ŽVLSI. technology w2x. At the same time, low-energy oxygen ions are routinely used in Secondary Ion Mass Spectrometry ŽSIMS. for in-depth concentration profiling of impurities because they provide stable ion yields and enhance the ionization probability for positive secondary ions. It has been observed, however, that this method produces a large broadening of metal profiles in silicon under bombardment at near normal incidence w3–5x. Such anomalous broadening has been explained in terms of beam-induced segregation of impurity atoms towards the interface between the
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 7 4 - 8
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
SiO 2 surface film, formed during oxygen bombardment, and the Si substrate w6–8x. Several suggestions have been made to explain the driving force for such segregation during SIMS analysis. An early explanation considers bombardment-induced Gibbsian segregation, driven by the different heats of oxide formation, as the dominant driving force for the segregation of Cu and Ag in Si w9x. Further studies have shown a correlation between the segregation behaviour in Si and the electronegativity of some impurity atoms w10x, but several exceptions from that rule have been found. For example, Sb and Ag have similar electronegativities, but still show quite different segregation behaviours w11x. In later studies, a strong correlation has been established between the segregation and the heat of oxide formation w5,12,13x, but this time, inconsistency in normalising the formation enthalpies of oxides led to inconclusive results. A recent model w14x, also based on thermodynamic arguments, proposes that a large solubility difference between metals in amorphous Si and SiO 2 represents the strong driving force for segregation. Alternatively, the build-up of an electric field across the insulating SiO 2 layer during oxygen bombardment may provide the driving force for the ion migration through the oxide. For example, it is wellknown that the segregation behaviour of Na in SiO 2 under ion bombardment is dominated by the electric field-driven effects w15x. In addition, it has been shown that bombardment with negative oxygen ions, under conditions where surface oxide still forms, may suppress the migration of Na and reduce the segregation of some metals in Si w16x.
2. Experimental In this work, we provide additional evidence for the thermodynamically driven segregation of metal impurities in Si under oxygen ion bombardment. We present results which cannot be described by the field-induced effects, including migration of metal atoms out of a buried oxide layer, both towards the surface and into the bulk, and the antisegregation of some metals, such as Ti, Al and Mg, having lower heat of oxide formation than Si.
201
Oxygen bombardment was carried out in a SIMS apparatus ŽRiber MIQ 256., using either 6–12 keV q beam at angles of inciOq 2 beam or 14.5 keV O dence between 08 and 608 to the surface normal, both to form the surface oxide and to analyse the samples. A resistance heating stage was employed in several experiments for sample heating during analysis. After heating, all samples were profiled again at room temperature to check for any thermal diffusion effects.
3. Materials In the present work, we used Cz Si wafers of n-type, Ž100. orientation and resistivity of 1–10 V cm. To study segregation phenomenon under oxygen bombardment, Si samples were ion-implanted with Mg, Al, Ti, Cu or Ta to a dose of 10 14 –10 15 atomsrcm2 and in an energy range of 40–100 keV.
4. Results and discussion The main features of the segregation behaviour of metals in Si during oxygen bombardment are illustrated in Fig. 1 for Cu and Ta implants. The Cu profiles ŽFig. 1a. obtained at angles between 608 and 338, show no unusual distortion and the broadening with angle of incidence can be attributed to the ion beam mixing which usually limits the depth resolution in SIMS w17x. At 258, however, when a stoichiometric surface SiO 2 is formed under bombardment w8x, Cu segregates at a moving Si–SiO 2 interface giving rise to an anomalously broad SIMS profile. The same effect has been observed in SIMS profiles of some other fast diffusing metal atoms implanted into Si, such as Au and Ni w5x. In the case of less mobile atoms, such as Ta ŽFig. 1b. or Cr and Ge w5x, the room temperature SIMS profiles exhibit no anomalous distortions even at angles where stoichiometric oxide forms Žsee Fig. 1b.. However, migration towards the Si–SiO 2 interface is possible if metal mobilities are enhanced by heating the sample during oxygen bombardment, as illustrated in Fig. 1b for Ta profile taken at 3908C. The heat of oxide formation Žnormalised to the number of oxygen atoms. for all metals mentioned
202
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
Fig. 1. Ža. Typical SIMS profiles of Cu implanted into Si obtained Ž . with 8 keV Oq 2 bombardment at different angles of incidence. b SIMS profiles of Ta from an ion-implanted Si sample, obtained at room temperature and 3908C with 8 keV Oq 2 bombardment at 208 incidence.
so far exceeds that of SiO 2 . Consequently, there is much stronger chemical driving force for the formation of SiO 2 than any metal oxide during oxygen bombardment. It is important to mention here that there is a large difference in the number of atoms in the metal oxides Žranging from two, as in CuO, up to seven, as in Ta 2 O5 ., while the formation enthalpies are listed per mole w18x. By normalising the formation enthalpies to the number of metal atoms w12x, one may wrongly predict the direction of segregation for several atoms Žfor example, segregation of Al out of the oxide and Ta into the oxide, in contradiction to the experimental results from Refs. w10,13x and this work.. Normalisation to the number of oxygen atoms appears to be more appropriate w13x.
A recent model for segregation at Si–SiO 2 interface w14x proposes that a low solubility of many metals in SiO 2 and their high solubility in disordered Si result in metal segregation at the Si side of the interface Žwhich is highly distorted due to the bombardment. if metals are mobile in amorphous Si during oxygen bombardment. Thus, the segregation of metal atoms at the amorphous–crystalline interface, just below the Si–SiO 2 interface, occurs as a result of a strong thermodynamic driving force. The lower diffusivity of some metals in amorphous Si may cause trapping of metal atoms in the moving SiO 2 layer during oxygen bombardment. These trapped atoms are sputtered away during irradiation and no anomalous distortion of SIMS profile occurs. Our data shown in Fig. 1 are fully consistent with the above model. Further evidence in favour of the thermodynamic driving force and the model from Ref. w14x is shown in Fig. 2. Here, the buried oxide layer was formed in a Si sample, implanted initially by Cu Ž100 keV, 5 = 10 14 cmy2 ., by the high-dose oxygen implantation. At 33 keV implantation energy, oxygen was ˚ corresponding to placed just at a depth of ; 700 A, the maximum concentration of implanted Cu. At a dose of ; 5 = 10 17 Orcm2 , a thin, buried SiO 2 layer has formed and Cu is observed to segregate out of the oxide at both Si–SiO 2 interfaces as it thermodynamically prefers to reside in disordered Si at both sides of the buried oxide layer. At higher oxygen doses, when the buried oxide layer extends all the
Fig. 2. Segregation of Cu at both interfaces of a buried SiO 2 layer formed by oxygen ion implantation ŽSIMS profiles obtained with . 7 keV Oq 2 beam at 508 incidence .
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
way to the surface, the near surface Cu peak is sputtered away. We observed the similar effect for a shallow Au implant in Si. Following the thermodynamic arguments, one would expect quite different segregation behaviour in Si for metals, such as Al, Mg or Ti, in comparison with Cu or Ta. Al, Mg and Ti, having lower heats of oxide formation Žper mole of oxygen. than SiO 2 , are expected to segregate towards the moving oxide interface as the oxidation of metal atoms, not Si, is now a thermodynamically favourable reaction. If impurity atoms are mobile enough in Si, a strong thermodynamic force may overcome the ion beam mixing effects and produce concentration profiles much sharper than expected from the ordinary ballistic mixing w17x. Indeed, Fig. 3 clearly shows that Al and Mg atoms, ions implanted into Si, migrate toward the surface and incoming oxygen ions during
203
Fig. 4. Depth profiles of Ti from an ion-implanted Si sample taken at room temperature and 3508C for different impact angles. Antisegregation at 158 impact is greatly enhanced at an elevated temperature.
Oq bombardment at angles where stoichiometric 2 surface oxide forms Ž158 in Fig. 3.. Thus, the resulting SIMS profiles are sharper than those obtained at 358. At angles above 308 to the normal, where oxygen bombardment does not result in a stoichiometric SiO 2 w8x, broadening is dominated by the ion beam mixing. Even stronger antisegregation behaviour is illustrated in Fig. 4 for Ti atoms implanted into Si. Profile obtained at 158 Žwhen a stoichiometric oxide forms at the surface. is much sharper than the profile obtained at 308 Žwhen only a substoichiometric oxide forms at the surface.. At elevated temperatures, providing higher mobility for Ti atoms, sharpening becomes even more pronounced, as shown in Fig. 4 for a profile taken at 3508C. It would be hard to explain our results, shown in Figs. 2–4, by any electric field-induced effect. Therefore, we believe that, in our experiment, chemical reactions and thermodynamics dominate the segregation behaviour of metals in Si under bombardment conditions where stoichiometric oxide forms.
5. Conclusion
Fig. 3. Antisegregation behaviour of Al Ža. and Mg Žb. during bombardment of ion-implanted samples with 10 keV Oq 2 at 158 to the normal.
In conclusion, we have shown that a thermodynamic model, which takes into account different heats of formation for SiO 2 and the oxide of the segregated metal, a large solubility difference be-
204
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
tween metals in amorphous Si and SiO 2 and different mobilities of metal atoms in amorphous Si, can account for the observed segregation behaviour of a large number of metals in Si under oxygen bombardment.
References w1x N. Herbots, O.C. Hellman, P. Ye, X. Wang, O. Vancauwenberghe, in: J.W. Rabalais ŽEd.., Low Energy Ion–Surface Interactions, Wiley, Chichester, 1994, p. 389. w2x A.K. Robinson, C.D. Marsh, U. Bussmann, J.A. Kilner, Y. Li, J. Vanhellemont, K.J. Reeson, P.L.F. Hemment, G.R. Booker, Nucl. Instr. Meth. B 55 Ž1991. 555. w3x P. Williams, J.E. Baker, Nucl. Instr. Meth. 182r183 Ž1981. 15. w4x K. Wittmaack, N. Menzel, Appl. Phys. Lett. 50 Ž1987. 815. w5x M. Petravic, ´ B.G. Svensson, J.S. Williams, J.M. Glasko, Nucl. Instr. Meth. B 118 Ž1996. 151.
w6x W. Reuter, Nucl. Instr. Meth. B 15 Ž1986. 173. w7x N. Menzel, K. Wittmaack, Mat. Sci. Eng. B 12 Ž1992. 91. w8x J.S. Williams, M. Petravic, ´ B.G. Svensson, M. Conway, J. Appl. Phys. 76 Ž1994. 1840. w9x V.R. Deline, W. Reuter, R. Kelly, in: A. Benninghoven, R.J. Colton, D.S. Simons, H.W. Werner ŽEds.., Secondary Ion Mass Spectrometry, SIMS V, Springer Series in Chem. Phys. 44, Springer, Berlin, 1986, p. 99. w10x S.M. Hues, P. Williams, Nucl. Instr. Meth. B 15 Ž1986. 206. w11x K. Wittmaack, Nucl. Instr. Meth. B 19r20 Ž1987. 484. w12x Y. Homma, K. Wittmaack, Appl. Phys. A 50 Ž1990. 417. w13x R.G. Willson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, Section 2.2, Wiley, New York, 1989. w14x J.S. Williams, K.T. Short, M. Petravic, ´ B.G. Svensson, Nucl. Instr. Meth. B 121 Ž1997. 24. w15x C.W. Magee, W.L. Harrington, Appl. Phys. Lett. 33 Ž1978. 193. w16x C.J. Vriezema, P.C. Zalm, Surf. Interf. Anal. 17 Ž1991. 875. w17x M. Petravic, ´ B.G. Svensson, J.S. Williams, Appl. Phys. Lett. 62 Ž1993. 278. w18x D.R. Linde ŽEd.., Handbook of Chemistry and Physics, 74th edn., Section 5, 1995–1996.
On the segregation of metals during low-energy oxygen bombardment of silicon Mladen Petravic´
)
Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National UniÕersity, Canberra, ACT 0200, Australia Received 6 February 1998; accepted 10 April 1998
Abstract Segregation of metal impurities in Si under oxygen ion bombardment has been studied using secondary ion mass spectrometry. A strong evidence has been found for the thermodynamic driving force for segregation. This includes segregation of metal atoms at both interfaces of a buried SiO 2 layer and the ‘antisegregation’ behaviour of metal atoms having lower heats of oxide formation than Si. Segregation is enhanced at elevated temperatures when the metal diffusivity in amorphous Si is higher. q 1998 Elsevier Science B.V. All rights reserved. PACS: 68.55 Ln; 79.20 Rf; 82.66 Dp; 82.80 Ms Keywords: Surface segregation; Compound formation; Secondary ion mass spectroscopy; Silicon oxides; Metal oxides
1. Introduction Interactions at semiconductor surfaces induced by low-energy oxygen ions constitute an interesting area for the investigation of fundamental ion-induced surface chemical reactions. For example, low-energy oxygen bombardment may produce stoichiometric oxides at the surfaces of many semiconductors even at room temperature w1x. The ion beam oxidation process proceeds along several athermal steps and can create some new compounds, such as Si xGe1yxO 2 , which cannot be grown thermally w1x. On the other hand, ion-induced oxidation of semi-
)
Tel.: q61-6-249-0355; Fax: q61-6-249-0511; E-mail: [email protected]
conductor surfaces has attracted considerable interest due to possible applications in semiconductor technology. For example, high-dose oxygen bombardment of Si is now a key method for producing high quality silicon-on-insulator ŽSOI. substrates for very large scale integration ŽVLSI. technology w2x. At the same time, low-energy oxygen ions are routinely used in Secondary Ion Mass Spectrometry ŽSIMS. for in-depth concentration profiling of impurities because they provide stable ion yields and enhance the ionization probability for positive secondary ions. It has been observed, however, that this method produces a large broadening of metal profiles in silicon under bombardment at near normal incidence w3–5x. Such anomalous broadening has been explained in terms of beam-induced segregation of impurity atoms towards the interface between the
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 7 4 - 8
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
SiO 2 surface film, formed during oxygen bombardment, and the Si substrate w6–8x. Several suggestions have been made to explain the driving force for such segregation during SIMS analysis. An early explanation considers bombardment-induced Gibbsian segregation, driven by the different heats of oxide formation, as the dominant driving force for the segregation of Cu and Ag in Si w9x. Further studies have shown a correlation between the segregation behaviour in Si and the electronegativity of some impurity atoms w10x, but several exceptions from that rule have been found. For example, Sb and Ag have similar electronegativities, but still show quite different segregation behaviours w11x. In later studies, a strong correlation has been established between the segregation and the heat of oxide formation w5,12,13x, but this time, inconsistency in normalising the formation enthalpies of oxides led to inconclusive results. A recent model w14x, also based on thermodynamic arguments, proposes that a large solubility difference between metals in amorphous Si and SiO 2 represents the strong driving force for segregation. Alternatively, the build-up of an electric field across the insulating SiO 2 layer during oxygen bombardment may provide the driving force for the ion migration through the oxide. For example, it is wellknown that the segregation behaviour of Na in SiO 2 under ion bombardment is dominated by the electric field-driven effects w15x. In addition, it has been shown that bombardment with negative oxygen ions, under conditions where surface oxide still forms, may suppress the migration of Na and reduce the segregation of some metals in Si w16x.
2. Experimental In this work, we provide additional evidence for the thermodynamically driven segregation of metal impurities in Si under oxygen ion bombardment. We present results which cannot be described by the field-induced effects, including migration of metal atoms out of a buried oxide layer, both towards the surface and into the bulk, and the antisegregation of some metals, such as Ti, Al and Mg, having lower heat of oxide formation than Si.
201
Oxygen bombardment was carried out in a SIMS apparatus ŽRiber MIQ 256., using either 6–12 keV q beam at angles of inciOq 2 beam or 14.5 keV O dence between 08 and 608 to the surface normal, both to form the surface oxide and to analyse the samples. A resistance heating stage was employed in several experiments for sample heating during analysis. After heating, all samples were profiled again at room temperature to check for any thermal diffusion effects.
3. Materials In the present work, we used Cz Si wafers of n-type, Ž100. orientation and resistivity of 1–10 V cm. To study segregation phenomenon under oxygen bombardment, Si samples were ion-implanted with Mg, Al, Ti, Cu or Ta to a dose of 10 14 –10 15 atomsrcm2 and in an energy range of 40–100 keV.
4. Results and discussion The main features of the segregation behaviour of metals in Si during oxygen bombardment are illustrated in Fig. 1 for Cu and Ta implants. The Cu profiles ŽFig. 1a. obtained at angles between 608 and 338, show no unusual distortion and the broadening with angle of incidence can be attributed to the ion beam mixing which usually limits the depth resolution in SIMS w17x. At 258, however, when a stoichiometric surface SiO 2 is formed under bombardment w8x, Cu segregates at a moving Si–SiO 2 interface giving rise to an anomalously broad SIMS profile. The same effect has been observed in SIMS profiles of some other fast diffusing metal atoms implanted into Si, such as Au and Ni w5x. In the case of less mobile atoms, such as Ta ŽFig. 1b. or Cr and Ge w5x, the room temperature SIMS profiles exhibit no anomalous distortions even at angles where stoichiometric oxide forms Žsee Fig. 1b.. However, migration towards the Si–SiO 2 interface is possible if metal mobilities are enhanced by heating the sample during oxygen bombardment, as illustrated in Fig. 1b for Ta profile taken at 3908C. The heat of oxide formation Žnormalised to the number of oxygen atoms. for all metals mentioned
202
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
Fig. 1. Ža. Typical SIMS profiles of Cu implanted into Si obtained Ž . with 8 keV Oq 2 bombardment at different angles of incidence. b SIMS profiles of Ta from an ion-implanted Si sample, obtained at room temperature and 3908C with 8 keV Oq 2 bombardment at 208 incidence.
so far exceeds that of SiO 2 . Consequently, there is much stronger chemical driving force for the formation of SiO 2 than any metal oxide during oxygen bombardment. It is important to mention here that there is a large difference in the number of atoms in the metal oxides Žranging from two, as in CuO, up to seven, as in Ta 2 O5 ., while the formation enthalpies are listed per mole w18x. By normalising the formation enthalpies to the number of metal atoms w12x, one may wrongly predict the direction of segregation for several atoms Žfor example, segregation of Al out of the oxide and Ta into the oxide, in contradiction to the experimental results from Refs. w10,13x and this work.. Normalisation to the number of oxygen atoms appears to be more appropriate w13x.
A recent model for segregation at Si–SiO 2 interface w14x proposes that a low solubility of many metals in SiO 2 and their high solubility in disordered Si result in metal segregation at the Si side of the interface Žwhich is highly distorted due to the bombardment. if metals are mobile in amorphous Si during oxygen bombardment. Thus, the segregation of metal atoms at the amorphous–crystalline interface, just below the Si–SiO 2 interface, occurs as a result of a strong thermodynamic driving force. The lower diffusivity of some metals in amorphous Si may cause trapping of metal atoms in the moving SiO 2 layer during oxygen bombardment. These trapped atoms are sputtered away during irradiation and no anomalous distortion of SIMS profile occurs. Our data shown in Fig. 1 are fully consistent with the above model. Further evidence in favour of the thermodynamic driving force and the model from Ref. w14x is shown in Fig. 2. Here, the buried oxide layer was formed in a Si sample, implanted initially by Cu Ž100 keV, 5 = 10 14 cmy2 ., by the high-dose oxygen implantation. At 33 keV implantation energy, oxygen was ˚ corresponding to placed just at a depth of ; 700 A, the maximum concentration of implanted Cu. At a dose of ; 5 = 10 17 Orcm2 , a thin, buried SiO 2 layer has formed and Cu is observed to segregate out of the oxide at both Si–SiO 2 interfaces as it thermodynamically prefers to reside in disordered Si at both sides of the buried oxide layer. At higher oxygen doses, when the buried oxide layer extends all the
Fig. 2. Segregation of Cu at both interfaces of a buried SiO 2 layer formed by oxygen ion implantation ŽSIMS profiles obtained with . 7 keV Oq 2 beam at 508 incidence .
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
way to the surface, the near surface Cu peak is sputtered away. We observed the similar effect for a shallow Au implant in Si. Following the thermodynamic arguments, one would expect quite different segregation behaviour in Si for metals, such as Al, Mg or Ti, in comparison with Cu or Ta. Al, Mg and Ti, having lower heats of oxide formation Žper mole of oxygen. than SiO 2 , are expected to segregate towards the moving oxide interface as the oxidation of metal atoms, not Si, is now a thermodynamically favourable reaction. If impurity atoms are mobile enough in Si, a strong thermodynamic force may overcome the ion beam mixing effects and produce concentration profiles much sharper than expected from the ordinary ballistic mixing w17x. Indeed, Fig. 3 clearly shows that Al and Mg atoms, ions implanted into Si, migrate toward the surface and incoming oxygen ions during
203
Fig. 4. Depth profiles of Ti from an ion-implanted Si sample taken at room temperature and 3508C for different impact angles. Antisegregation at 158 impact is greatly enhanced at an elevated temperature.
Oq bombardment at angles where stoichiometric 2 surface oxide forms Ž158 in Fig. 3.. Thus, the resulting SIMS profiles are sharper than those obtained at 358. At angles above 308 to the normal, where oxygen bombardment does not result in a stoichiometric SiO 2 w8x, broadening is dominated by the ion beam mixing. Even stronger antisegregation behaviour is illustrated in Fig. 4 for Ti atoms implanted into Si. Profile obtained at 158 Žwhen a stoichiometric oxide forms at the surface. is much sharper than the profile obtained at 308 Žwhen only a substoichiometric oxide forms at the surface.. At elevated temperatures, providing higher mobility for Ti atoms, sharpening becomes even more pronounced, as shown in Fig. 4 for a profile taken at 3508C. It would be hard to explain our results, shown in Figs. 2–4, by any electric field-induced effect. Therefore, we believe that, in our experiment, chemical reactions and thermodynamics dominate the segregation behaviour of metals in Si under bombardment conditions where stoichiometric oxide forms.
5. Conclusion
Fig. 3. Antisegregation behaviour of Al Ža. and Mg Žb. during bombardment of ion-implanted samples with 10 keV Oq 2 at 158 to the normal.
In conclusion, we have shown that a thermodynamic model, which takes into account different heats of formation for SiO 2 and the oxide of the segregated metal, a large solubility difference be-
204
M. PetraÕicr ´ Applied Surface Science 135 (1998) 200–204
tween metals in amorphous Si and SiO 2 and different mobilities of metal atoms in amorphous Si, can account for the observed segregation behaviour of a large number of metals in Si under oxygen bombardment.
References w1x N. Herbots, O.C. Hellman, P. Ye, X. Wang, O. Vancauwenberghe, in: J.W. Rabalais ŽEd.., Low Energy Ion–Surface Interactions, Wiley, Chichester, 1994, p. 389. w2x A.K. Robinson, C.D. Marsh, U. Bussmann, J.A. Kilner, Y. Li, J. Vanhellemont, K.J. Reeson, P.L.F. Hemment, G.R. Booker, Nucl. Instr. Meth. B 55 Ž1991. 555. w3x P. Williams, J.E. Baker, Nucl. Instr. Meth. 182r183 Ž1981. 15. w4x K. Wittmaack, N. Menzel, Appl. Phys. Lett. 50 Ž1987. 815. w5x M. Petravic, ´ B.G. Svensson, J.S. Williams, J.M. Glasko, Nucl. Instr. Meth. B 118 Ž1996. 151.
w6x W. Reuter, Nucl. Instr. Meth. B 15 Ž1986. 173. w7x N. Menzel, K. Wittmaack, Mat. Sci. Eng. B 12 Ž1992. 91. w8x J.S. Williams, M. Petravic, ´ B.G. Svensson, M. Conway, J. Appl. Phys. 76 Ž1994. 1840. w9x V.R. Deline, W. Reuter, R. Kelly, in: A. Benninghoven, R.J. Colton, D.S. Simons, H.W. Werner ŽEds.., Secondary Ion Mass Spectrometry, SIMS V, Springer Series in Chem. Phys. 44, Springer, Berlin, 1986, p. 99. w10x S.M. Hues, P. Williams, Nucl. Instr. Meth. B 15 Ž1986. 206. w11x K. Wittmaack, Nucl. Instr. Meth. B 19r20 Ž1987. 484. w12x Y. Homma, K. Wittmaack, Appl. Phys. A 50 Ž1990. 417. w13x R.G. Willson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, Section 2.2, Wiley, New York, 1989. w14x J.S. Williams, K.T. Short, M. Petravic, ´ B.G. Svensson, Nucl. Instr. Meth. B 121 Ž1997. 24. w15x C.W. Magee, W.L. Harrington, Appl. Phys. Lett. 33 Ž1978. 193. w16x C.J. Vriezema, P.C. Zalm, Surf. Interf. Anal. 17 Ž1991. 875. w17x M. Petravic, ´ B.G. Svensson, J.S. Williams, Appl. Phys. Lett. 62 Ž1993. 278. w18x D.R. Linde ŽEd.., Handbook of Chemistry and Physics, 74th edn., Section 5, 1995–1996.