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Silicon isotopic tracing with the 29Si(p, γ) narrow resonance near 415 keV
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 441±445
www.elsevier.nl/locate/nimb
Silicon isotopic tracing with the 29Si(p, c) narrow resonance near 415 keV q I.C. Vickridge
a,*
, O. Kaitasov b, R.J. Chater c, J.A. Kilner
c
a c
Groupe de Physique des Solides, UMR7588 du CNRS, Universit e de Paris 6 et 7, Tour 23, 2 Place Jussieu, Paris, France b Centre de Spectroscopie Nucl eaire et Spectrometrie de Masse, UMR 8609, Bat. 104 et 108, 91405 Orsay, France Materials Department, Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 2BP, UK
Abstract Atomic transport of silicon during the formation of modi®ed surface layers on silicon single crystal wafers has often been evoked in processes such as SIMOX or thermal oxidation but has never been directly observed. Recently, observations of substantial oxygen exchange at the SiO2 /Si interface during dry thermal oxidation indicate diusion of oxygen-containing species from the interface to the gas phase, one candidate being the molecule SiO. Silicon isotopic tracing studies could contribute decisive new information to help understand these and other processes. Preparation of single crystal Si enriched in one of the rare isotopes has so far proven to be dicult. In this paper we thick epitaxial layers enriched in 29 Si to over 30 at.% by ion implantation. report on the preparation of about 400 A After amorphisation and high dose implantation, the solid phase epitaxial regrowth rate is much reduced compared to low dose implants because of signi®cant oxygen contamination due to recoil implantation of oxygen, but channeling in the enriched layer shows a vmin of a few percent, indicating good epitaxial regrowth. Our ®rst application of silicon isotopic tracing is to the study of the dry thermal oxidation of silicon. We have found signi®cant silicon loss during this process. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.65.M Keywords: Oxidation; Silicon;
29
Si; Isotopic tracing; Atomic transport
1. Introduction Silicon atomic transport during processes such as oxidation, formation of silicides, and irradiation
q
Work supported by GDR86 of the CNRS. Corresponding author. Tel.: +33-1-44-27-46-86; fax: +331-43-54-28-78. E-mail address: [email protected] (I.C. Vickridge). *
have been dicult to observe directly. The use of inert marker layers can provide information about the movement of silicon, however correct interpretation of inert marker experiments can be very dicult, for example with contradictory results for dierent markers [1]. An alternative is isotopic tracing with isotopically enriched silicon layers. The quantities of enriched material required for direct epitaxy, for example by CVD, are too great to be practical, however ion beam methods, in
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 7 5 - 8
442
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
which a mass-selected 29 Si or 30 Si beam is sent onto a suitable substrate is a viable route allowing the use of source material of natural isotopic composition. Direct epitaxy by low energy (30± 300 eV) ion beam deposition has been successfully used [2], however UHV conditions are required since the presence of oxygen reduces the solid phase thermal epitaxial growth rate by about 2 orders of magnitude per at.% of oxygen in the layer [3], and the low rigidity of the low energy ion beam, which is typically decelerated from the some tens of keV required for mass separation, makes it very dicult to obtain epitaxial layers of uniform thickness over an area of the order of 1 cm2 . Very recently signi®cant progress has been made in the use of ion beam deposition by the Bochum group [4]. The purpose of this paper is to show that good quality single crystal silicon enriched in 29 Si can be obtained even from an implanter with modest vacuum, by high dose implantation and suitable thermal annealing, and to present our ®rst studies of silicon atomic transport. 2. Preparation of
29
Si enriched single crystal silicon
Two-inch wafers of n-type phosphorous-doped Siltronix Czochralski silicon of resitivity 3±6 X cm, oriented (1 0 0), were cleaned by degreasing in trichlorethylene followed by acetone, a 4% HF/ethanol dip, re-oxidation for a few minutes in 1:1 H2 SO4 /H2 O2 , re-etched in 4% HF/ethanol, and ®nally rinsed in deionised water. Typically between 1 and 4 h passed between the cleaning and mounting the wafers in the implanter `IRMA' of the Centre de Spectroscopie Nucleaire et Spectrometrie de Masse (CSNSM), Orsay. It has been of shown that under these conditions less than 5 A oxide will be present when the wafer is placed in the implanter [5]. Wafers were ®rst amorphised at room temperature with 80 keV 29 Si at a dose of 8 1015 cmÿ2 , which is double the dose of the amorphisation recipe of [6]. RBS-channeling con®rmed that the samples were amorphised from about 180 nm up to the surface. A high dose (up to 2 1017 cmÿ2 of 29 Si ) was then implanted at 25 keV (projected range 37 nm, range straggle
17 nm), so that all of the implanted ions came to rest in the amorphised layer. These as-implanted layers were characterised by RBS-channeling, by SIMS and by nuclear resonance pro®ling (NRP) [7±9] via the narrow resonance near 415 keV in the reaction 29 Si(p, c)30 P [10,11]. Experimental conditions are given in the caption to Fig. 1. The three measurements are in reasonable agreement concerning the total enrichment in 29 Si. The incident dose for the sample shown was 1 ´ 1017 29 Si cmÿ2 , and we see that the SIMS and NRP measurements, which use the natural abundance of the substrate as a calibration point are, respectively, 6% and 3% above the incident dose. This is surprising in view of the suggestion that up to 40% of the 29 Si beam may be 28 SiH during low energy deposition of 29 Si on silicon with a retarded beam from the same implanter [10]. Ion distributions from TRIM version 97.06 [12], although not strictly valid for such high doses in which we have substrate growth, suggest that the maximum concentration of 29 Si is near 40 at.%, which is also in reasonable agreement with the SIMS measurement (38 at.%) and the NRP measurement (37 at.%). We can only conclude that possible contamination of the 29 Si beam by 28 SiH must be sensitive to ion source parameters, and note that we have observed no evidence of high levels of beam contamination in the experiments described here. Also shown in Fig. 1 is the oxygen depth pro®le measured by SIMS from this same sample. The straight line indicates the expected logarithmic decay of the oxygen signal due to ion beam mixing of the native surface oxide during the sputter pro®ling process. The measured deviation from this shows that oxygen has been incorporated within the 29 Si enriched layer. Thermal annealing was carried out in 2 104 Pa electronic grade Ar in a quartz tube vacuum furnace with a base pressure better than 5 10ÿ5 Pa. Solid phase epitaxial growth (SPEG) of silicon amorphised by ion beams is a very rapid process ± for example at 700°C we can expect a regrowth rate of about 60 nm per minute [13]. In our case we found that regrowth rates were very much slower, and the amorphised ®lm was regrown at 1100°C with a 20 min anneal. This is
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
443
Fig. 2. 1.8 MeV (1 0 0) axial channeling spectra from annealed high dose 29 Si implant and virgin silicon, showing the EOR defects remaining from the amorphising implant and the good regrowth of the 29 Si enriched layer.
amount of residual end-of-range (EOR) damage from the amorphising implant, but the silicon between this EOR damage and the surface is indistinguishable from a virgin (1 0 0) silicon wafer by channelling. This is important since it shows that the marked layer of 29 Si is of good crystalline quality, opening the way for silicon atomic transport studies. 3. Silicon movement during oxidation of silicon
Fig. 1. Characterisation of a high dose 25 keV 29 Si implant into a pre-amorphised Si layer. (a) SIMS pro®ling conditions: Atomika 6500 instrument, 5 keV Xe primary beam, negative ions detected, depth scale from crater depth measurement. The right-hand scale is simply the ratio of the secondary ion yields, and not the ratio of oxygen to silicon in the substrate. (b) NRP pro®ling conditions: 29 Si(p, c)30 P resonance at 415 keV, 200 nA beam, 10 lC per point, 2 mm diameter beam spot, BGO detector at 0°. Symbols are measurements, lines are calculated curves as described in text. (c) 2.0 MeV RBS at 170° detection angle, channelled and random. The solid line is a RUMP simulation assuming the concentration pro®le found from the NRP measurement, with the stopping power simply scaled to correctly reproduce the Si plateau height.
probably due to recoil implantation of oxygen, consistent with the SIMS observation above, and perhaps other impurities. Oxygen reduces the SPEG rate by 2 orders of magnitude per at.% of oxygen [3]. (1 0 0) 1.8 MeV channelling spectra for a typical annealed high dose implant, and a virgin silicon crystal are shown in Fig. 2. The slight dechannelling at 900 keV shows that there is a small
For our ®rst application of isotopic tracing with such material we chose the dry oxidation of silicon. The classical view of this process, largely based on the Deal and Grove model [14] is that molecular oxygen from the gas diuses interstitially through the growing oxide without exchange with the oxygen already in the oxide, and reacts at the SiO2 interface. When the oxide is thick enough, the growth rate is limited by diusion of the oxygen through the oxide, resulting in a parabolic dependence of oxide thickness with time. However, recent experiments in which oxide growth measured from gas uptake is compared to oxide growth as measured by weight increase suggest that matter is lost during dry oxidation at temperatures as low as 900°C [15]. In addition, recent results from 18 O isotopic tracing have shown that there is in fact exchange of oxygen in an interfacial region of some 2 nm at the SiO2 /Si interface. For oxidation at 1100°C, in the parabolic growth regime, over 25% of oxygen that arrives at the SiO2 / Si interface leaves it again [16]. Where does this oxygen go? It cannot diuse back to the surface
444
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
against the concentration gradient, so it is either dissolved in the silicon substrate or diuses back to the surface as a species other than the incoming oxidising species. One candidate for such a species is the molecule SiO, in which case, obviously, one should observe silicon loss as well. An implanted and annealed sample was thus oxidised at 1100°C for 10 h in 200 mbar of dry < 1 ppm H2 O O2 . The excitation curves of the 415 keV 29 Si(p, c) resonance measured before and after the oxidation are shown in Fig. 3. If the amount of 29 Si remains constant during the oxidation process, then the areas of the two curves should be equal (in the absence of the 4.7% of 29 Si naturally present), even though that corresponding to the oxidised sample will be stretched in the energy scale. It is clear that there has been loss of 29 Si during the oxidation. Also shown in Fig. 3 are calculated excitation curves corresponding to assumed 29 Si concentration pro®les, obtained with the SPACES code [9], which has recently been modi®ed to cater for a sample modelled as layers which may have arbitrary matrix composition. The solid lines are the best ®ts, whilst the dotted line is the expected excitation curve after oxidation if no 29 Si is lost during the oxidation. From these ®ts we deduce that 30% of the implanted 29 Si has been lost during the oxidation. We consider three possibilities:
(i) the silicon may be lost from the interface to the surface or (ii) to the volume or (iii) from the surface by evaporation. The conditions chosen for the oxidation are well inside the region of stability of SiO2 in O2 gas [17], so we believe that (iii) is unlikely. The thermal growth of SiO2 on silicon implies a supersaturation of Si atoms at the growth interface due to the volume increase from Si to SiO2 , and indeed the injection of silicon interstitials into the substrate during oxidation modi®es the diusion coecients of substitutional impurities [18], however the silicon self-interstitial is probably indirect [19] and so the rapid diusion of the interstitial should be seen as the rapid diusion of a network deformation rather than as the rapid transport of an individual atom. A check for 29 Si atoms on obvious traps such as the rough back surface of the wafer failed to reveal any. This suggests that during the dry thermal oxidation of silicon, silicon is lost from the SiO2 /Si interfacial region and transported to the surface of the sample where it is dissipated into the gas phase. We ®nally note that a very recent report has come to our attention of a similar experiment to that described here, but in which the authors do not comment on the implications of the dierence in area of their 29 Si excitation curves before and after oxidation [20].
4. Conclusion Good quality single crystal silicon enriched in Si may be routinely prepared by ion beam amorphisation, high dose ion implantation and thermal annealing. This, combined with isotopically sensitive concentration pro®ling methods such as NRP or SIMS, opens the way to silicon atomic transport studies of processes such as thermal layer growth, diusion, ion implantation, laser annealing and so on. The ®rst application of this new tool has shown that silicon atoms are lost, probably from the SiO2 /Si interface to the ambient gas, during the dry thermal oxidation of silicon. 29
Fig. 3. 29 Si(p, c)30 P excitation curves from an enriched silicon single crystal before and after thermal oxidation, showing loss of silicon during the oxidation process. Symbols are measured data and lines are calculated curves as described in the text. Experimental details are as for the excitation curves of Fig. 1.
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
Acknowledgements We would like to thank J. Chaumont and S. Gautrot for overseeing the implantations, J.M. Guigner and T. Akermark for valuable assistance during annealing and oxidation experiments, G. Bentini for most valuable discussions concerning thermal annealing of ion-beam amorphised silicon, and E. Breelle, E. Girard and M. Vidal for eciently keeping the GPS accelerator operating well. References [1] C.M. Comrie, Nucl. Instr. and Meth. B 118 (1996) 119. [2] B.R. Appleton, S.J. Pennycook, R.A. Zuhr, N. Herbots, T.S. Noggle, Nucl. Instr. and Meth. B 19/20 (1987) 975. [3] E.F. Kennedy, C.L.J.W. Mayer, T.W. Sigmon, J. Appl. Phys. 48 (10) (1977) 4241. [4] F. Gorris, C. Krug, S. Kubsky, I.J.R. Baumvol, W.H. Schulte, C. Rolfs, Phys. Stat. Sol. (A) 173 (1999) 167. [5] J.J. Ganem, S. Rigo, I. Trimaille, G.N. Lu, Nucl. Instr. and Meth. B 64 (1992) 784.
445
[6] J. Thornton, P.L.F. Hemment, I.H. Wilson, Nucl. Instr. and Meth. B 19/20 (1987) 307. [7] B. Maurel, Thesis, Universite de Paris 7, 1980. [8] B. Maurel, G. Amsel, J.P. Nadai, Nucl. Instr. and Meth. 197 (1982) 1. [9] I.C. Vickridge, G. Amsel, Nucl. Instr. and Meth. B 45 (1990) 6. [10] I.J.R. Baumvol, L. Borucki, J. Chaumont, J.J. Ganem, O. Kaitsov, N. Piel, S. Rigo, W.H. Schulte, F.C. Stedile, I. Trimaille, Nucl. Instr. and Meth. B 118 (1996) 499. [11] G. Amsel, E. d'Artemare, G. Battistig, E. Girard, L.G. Gosset, P. Revesz, Nucl. Instr. and Meth. B 136 (1998) 545. [12] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, Oxford, 1985. [13] G.L. Olson, J.A. Roth, Mater. Sci. Rep. 3 (1) (1988) 1. [14] B.E. Deal, A.S. Grove, J. Appl. Phys. 36 (12) (1965) 3770. [15] T. Akermark, G. Hultquist, J. Electrochem. Soc. 144 (1997) 1456. [16] T. Akermark, L.G. Gosset, J.J. Ganem, I. Trimaille, I.C. Vickridge, S. Rigo, J. Appl. Phys. 86 (2) (1999) 1153. [17] F.W. Smith, G. Ghidini, J. Electrochem. Soc. 129 (6) (1982) 1300. [18] G.N. Wills, Sol. State Electron. 12 (1969) 133. [19] I.C. Vickridge, Thesis, Universite de Paris 7, 1990. [20] I.J.R. Baumvol, C. Krug, F.C. Stedile, F. Gorris, W.H. Schulte, Phys. Rev. B 60 (3) (1999) 1492.
www.elsevier.nl/locate/nimb
Silicon isotopic tracing with the 29Si(p, c) narrow resonance near 415 keV q I.C. Vickridge
a,*
, O. Kaitasov b, R.J. Chater c, J.A. Kilner
c
a c
Groupe de Physique des Solides, UMR7588 du CNRS, Universit e de Paris 6 et 7, Tour 23, 2 Place Jussieu, Paris, France b Centre de Spectroscopie Nucl eaire et Spectrometrie de Masse, UMR 8609, Bat. 104 et 108, 91405 Orsay, France Materials Department, Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 2BP, UK
Abstract Atomic transport of silicon during the formation of modi®ed surface layers on silicon single crystal wafers has often been evoked in processes such as SIMOX or thermal oxidation but has never been directly observed. Recently, observations of substantial oxygen exchange at the SiO2 /Si interface during dry thermal oxidation indicate diusion of oxygen-containing species from the interface to the gas phase, one candidate being the molecule SiO. Silicon isotopic tracing studies could contribute decisive new information to help understand these and other processes. Preparation of single crystal Si enriched in one of the rare isotopes has so far proven to be dicult. In this paper we thick epitaxial layers enriched in 29 Si to over 30 at.% by ion implantation. report on the preparation of about 400 A After amorphisation and high dose implantation, the solid phase epitaxial regrowth rate is much reduced compared to low dose implants because of signi®cant oxygen contamination due to recoil implantation of oxygen, but channeling in the enriched layer shows a vmin of a few percent, indicating good epitaxial regrowth. Our ®rst application of silicon isotopic tracing is to the study of the dry thermal oxidation of silicon. We have found signi®cant silicon loss during this process. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.65.M Keywords: Oxidation; Silicon;
29
Si; Isotopic tracing; Atomic transport
1. Introduction Silicon atomic transport during processes such as oxidation, formation of silicides, and irradiation
q
Work supported by GDR86 of the CNRS. Corresponding author. Tel.: +33-1-44-27-46-86; fax: +331-43-54-28-78. E-mail address: [email protected] (I.C. Vickridge). *
have been dicult to observe directly. The use of inert marker layers can provide information about the movement of silicon, however correct interpretation of inert marker experiments can be very dicult, for example with contradictory results for dierent markers [1]. An alternative is isotopic tracing with isotopically enriched silicon layers. The quantities of enriched material required for direct epitaxy, for example by CVD, are too great to be practical, however ion beam methods, in
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 7 5 - 8
442
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
which a mass-selected 29 Si or 30 Si beam is sent onto a suitable substrate is a viable route allowing the use of source material of natural isotopic composition. Direct epitaxy by low energy (30± 300 eV) ion beam deposition has been successfully used [2], however UHV conditions are required since the presence of oxygen reduces the solid phase thermal epitaxial growth rate by about 2 orders of magnitude per at.% of oxygen in the layer [3], and the low rigidity of the low energy ion beam, which is typically decelerated from the some tens of keV required for mass separation, makes it very dicult to obtain epitaxial layers of uniform thickness over an area of the order of 1 cm2 . Very recently signi®cant progress has been made in the use of ion beam deposition by the Bochum group [4]. The purpose of this paper is to show that good quality single crystal silicon enriched in 29 Si can be obtained even from an implanter with modest vacuum, by high dose implantation and suitable thermal annealing, and to present our ®rst studies of silicon atomic transport. 2. Preparation of
29
Si enriched single crystal silicon
Two-inch wafers of n-type phosphorous-doped Siltronix Czochralski silicon of resitivity 3±6 X cm, oriented (1 0 0), were cleaned by degreasing in trichlorethylene followed by acetone, a 4% HF/ethanol dip, re-oxidation for a few minutes in 1:1 H2 SO4 /H2 O2 , re-etched in 4% HF/ethanol, and ®nally rinsed in deionised water. Typically between 1 and 4 h passed between the cleaning and mounting the wafers in the implanter `IRMA' of the Centre de Spectroscopie Nucleaire et Spectrometrie de Masse (CSNSM), Orsay. It has been of shown that under these conditions less than 5 A oxide will be present when the wafer is placed in the implanter [5]. Wafers were ®rst amorphised at room temperature with 80 keV 29 Si at a dose of 8 1015 cmÿ2 , which is double the dose of the amorphisation recipe of [6]. RBS-channeling con®rmed that the samples were amorphised from about 180 nm up to the surface. A high dose (up to 2 1017 cmÿ2 of 29 Si ) was then implanted at 25 keV (projected range 37 nm, range straggle
17 nm), so that all of the implanted ions came to rest in the amorphised layer. These as-implanted layers were characterised by RBS-channeling, by SIMS and by nuclear resonance pro®ling (NRP) [7±9] via the narrow resonance near 415 keV in the reaction 29 Si(p, c)30 P [10,11]. Experimental conditions are given in the caption to Fig. 1. The three measurements are in reasonable agreement concerning the total enrichment in 29 Si. The incident dose for the sample shown was 1 ´ 1017 29 Si cmÿ2 , and we see that the SIMS and NRP measurements, which use the natural abundance of the substrate as a calibration point are, respectively, 6% and 3% above the incident dose. This is surprising in view of the suggestion that up to 40% of the 29 Si beam may be 28 SiH during low energy deposition of 29 Si on silicon with a retarded beam from the same implanter [10]. Ion distributions from TRIM version 97.06 [12], although not strictly valid for such high doses in which we have substrate growth, suggest that the maximum concentration of 29 Si is near 40 at.%, which is also in reasonable agreement with the SIMS measurement (38 at.%) and the NRP measurement (37 at.%). We can only conclude that possible contamination of the 29 Si beam by 28 SiH must be sensitive to ion source parameters, and note that we have observed no evidence of high levels of beam contamination in the experiments described here. Also shown in Fig. 1 is the oxygen depth pro®le measured by SIMS from this same sample. The straight line indicates the expected logarithmic decay of the oxygen signal due to ion beam mixing of the native surface oxide during the sputter pro®ling process. The measured deviation from this shows that oxygen has been incorporated within the 29 Si enriched layer. Thermal annealing was carried out in 2 104 Pa electronic grade Ar in a quartz tube vacuum furnace with a base pressure better than 5 10ÿ5 Pa. Solid phase epitaxial growth (SPEG) of silicon amorphised by ion beams is a very rapid process ± for example at 700°C we can expect a regrowth rate of about 60 nm per minute [13]. In our case we found that regrowth rates were very much slower, and the amorphised ®lm was regrown at 1100°C with a 20 min anneal. This is
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
443
Fig. 2. 1.8 MeV (1 0 0) axial channeling spectra from annealed high dose 29 Si implant and virgin silicon, showing the EOR defects remaining from the amorphising implant and the good regrowth of the 29 Si enriched layer.
amount of residual end-of-range (EOR) damage from the amorphising implant, but the silicon between this EOR damage and the surface is indistinguishable from a virgin (1 0 0) silicon wafer by channelling. This is important since it shows that the marked layer of 29 Si is of good crystalline quality, opening the way for silicon atomic transport studies. 3. Silicon movement during oxidation of silicon
Fig. 1. Characterisation of a high dose 25 keV 29 Si implant into a pre-amorphised Si layer. (a) SIMS pro®ling conditions: Atomika 6500 instrument, 5 keV Xe primary beam, negative ions detected, depth scale from crater depth measurement. The right-hand scale is simply the ratio of the secondary ion yields, and not the ratio of oxygen to silicon in the substrate. (b) NRP pro®ling conditions: 29 Si(p, c)30 P resonance at 415 keV, 200 nA beam, 10 lC per point, 2 mm diameter beam spot, BGO detector at 0°. Symbols are measurements, lines are calculated curves as described in text. (c) 2.0 MeV RBS at 170° detection angle, channelled and random. The solid line is a RUMP simulation assuming the concentration pro®le found from the NRP measurement, with the stopping power simply scaled to correctly reproduce the Si plateau height.
probably due to recoil implantation of oxygen, consistent with the SIMS observation above, and perhaps other impurities. Oxygen reduces the SPEG rate by 2 orders of magnitude per at.% of oxygen [3]. (1 0 0) 1.8 MeV channelling spectra for a typical annealed high dose implant, and a virgin silicon crystal are shown in Fig. 2. The slight dechannelling at 900 keV shows that there is a small
For our ®rst application of isotopic tracing with such material we chose the dry oxidation of silicon. The classical view of this process, largely based on the Deal and Grove model [14] is that molecular oxygen from the gas diuses interstitially through the growing oxide without exchange with the oxygen already in the oxide, and reacts at the SiO2 interface. When the oxide is thick enough, the growth rate is limited by diusion of the oxygen through the oxide, resulting in a parabolic dependence of oxide thickness with time. However, recent experiments in which oxide growth measured from gas uptake is compared to oxide growth as measured by weight increase suggest that matter is lost during dry oxidation at temperatures as low as 900°C [15]. In addition, recent results from 18 O isotopic tracing have shown that there is in fact exchange of oxygen in an interfacial region of some 2 nm at the SiO2 /Si interface. For oxidation at 1100°C, in the parabolic growth regime, over 25% of oxygen that arrives at the SiO2 / Si interface leaves it again [16]. Where does this oxygen go? It cannot diuse back to the surface
444
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
against the concentration gradient, so it is either dissolved in the silicon substrate or diuses back to the surface as a species other than the incoming oxidising species. One candidate for such a species is the molecule SiO, in which case, obviously, one should observe silicon loss as well. An implanted and annealed sample was thus oxidised at 1100°C for 10 h in 200 mbar of dry < 1 ppm H2 O O2 . The excitation curves of the 415 keV 29 Si(p, c) resonance measured before and after the oxidation are shown in Fig. 3. If the amount of 29 Si remains constant during the oxidation process, then the areas of the two curves should be equal (in the absence of the 4.7% of 29 Si naturally present), even though that corresponding to the oxidised sample will be stretched in the energy scale. It is clear that there has been loss of 29 Si during the oxidation. Also shown in Fig. 3 are calculated excitation curves corresponding to assumed 29 Si concentration pro®les, obtained with the SPACES code [9], which has recently been modi®ed to cater for a sample modelled as layers which may have arbitrary matrix composition. The solid lines are the best ®ts, whilst the dotted line is the expected excitation curve after oxidation if no 29 Si is lost during the oxidation. From these ®ts we deduce that 30% of the implanted 29 Si has been lost during the oxidation. We consider three possibilities:
(i) the silicon may be lost from the interface to the surface or (ii) to the volume or (iii) from the surface by evaporation. The conditions chosen for the oxidation are well inside the region of stability of SiO2 in O2 gas [17], so we believe that (iii) is unlikely. The thermal growth of SiO2 on silicon implies a supersaturation of Si atoms at the growth interface due to the volume increase from Si to SiO2 , and indeed the injection of silicon interstitials into the substrate during oxidation modi®es the diusion coecients of substitutional impurities [18], however the silicon self-interstitial is probably indirect [19] and so the rapid diusion of the interstitial should be seen as the rapid diusion of a network deformation rather than as the rapid transport of an individual atom. A check for 29 Si atoms on obvious traps such as the rough back surface of the wafer failed to reveal any. This suggests that during the dry thermal oxidation of silicon, silicon is lost from the SiO2 /Si interfacial region and transported to the surface of the sample where it is dissipated into the gas phase. We ®nally note that a very recent report has come to our attention of a similar experiment to that described here, but in which the authors do not comment on the implications of the dierence in area of their 29 Si excitation curves before and after oxidation [20].
4. Conclusion Good quality single crystal silicon enriched in Si may be routinely prepared by ion beam amorphisation, high dose ion implantation and thermal annealing. This, combined with isotopically sensitive concentration pro®ling methods such as NRP or SIMS, opens the way to silicon atomic transport studies of processes such as thermal layer growth, diusion, ion implantation, laser annealing and so on. The ®rst application of this new tool has shown that silicon atoms are lost, probably from the SiO2 /Si interface to the ambient gas, during the dry thermal oxidation of silicon. 29
Fig. 3. 29 Si(p, c)30 P excitation curves from an enriched silicon single crystal before and after thermal oxidation, showing loss of silicon during the oxidation process. Symbols are measured data and lines are calculated curves as described in the text. Experimental details are as for the excitation curves of Fig. 1.
I.C. Vickridge et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 441±445
Acknowledgements We would like to thank J. Chaumont and S. Gautrot for overseeing the implantations, J.M. Guigner and T. Akermark for valuable assistance during annealing and oxidation experiments, G. Bentini for most valuable discussions concerning thermal annealing of ion-beam amorphised silicon, and E. Breelle, E. Girard and M. Vidal for eciently keeping the GPS accelerator operating well. References [1] C.M. Comrie, Nucl. Instr. and Meth. B 118 (1996) 119. [2] B.R. Appleton, S.J. Pennycook, R.A. Zuhr, N. Herbots, T.S. Noggle, Nucl. Instr. and Meth. B 19/20 (1987) 975. [3] E.F. Kennedy, C.L.J.W. Mayer, T.W. Sigmon, J. Appl. Phys. 48 (10) (1977) 4241. [4] F. Gorris, C. Krug, S. Kubsky, I.J.R. Baumvol, W.H. Schulte, C. Rolfs, Phys. Stat. Sol. (A) 173 (1999) 167. [5] J.J. Ganem, S. Rigo, I. Trimaille, G.N. Lu, Nucl. Instr. and Meth. B 64 (1992) 784.
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