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Analysis of extended defects in nitrogen annealed CZ silicon by optical and electron beam methods
Materials Science and Engineering B91– 92 (2002) 170– 173 www.elsevier.com/locate/mseb
Analysis of extended defects in nitrogen annealed CZ silicon by optical and electron beam methods C. Frigeri a,*, M. Ma b,c, T. Irisawa b, T. Ogawa c,1 a
CNR-MASPEC Institute, Parco Area delle Scienze 37 /A, Fontanini, 43010 Parma, Italy b Computer Center, Gakushuin Uni6ersity, Mejiro, Tokyo 171, Japan c Department of Physics, Gakushuin Uni6ersity, Mejiro, Tokyo 171, Japan
Abstract The effect of annealing in nitrogen atmosphere on the formation of crystal defects in the OSF-ring of Czochralski silicon has been studied by comparison with samples annealed in oxygen atmosphere by using optical and electron beam based methods. By annealing in nitrogen the formation of extrinsic stacking faults is prevented whereas oxygen precipitates form in nearly the same density as in the oxygen annealed sample. Additionally, loop-like microdefects were generated that were not observed for annealing in oxygen ambient. The results are explained by assuming that extra vacancies are introduced into Si from the nitrogen annealing atmosphere. They are expected to recombine with Si interstitials, thus preventing the growth of the stacking faults, and to create the observed microdefects. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Stacking faults; Oxygen precipitates; Vacancies; TEM; LST; OSF-ring; Si; Annealing
1. Introduction The study of the relationship between type and density of crystal defects and nitrogen in silicon is receiving considerable attention since it has been seen that the N doping of Si causes lattice hardening and increase of oxygen precipitates density under some specific conditions [1–4]. The most important extended defects in Si are dislocation loops and voids in the interstitial and vacancy rich part of the crystal, respectively, stacking faults located in the boundary region between those two regions [5] and oxygen precipitates. All these defects form during the annealing and processing steps, applied to the as-grown wafers for device production, due to the aggregation of self interstitials, or vacancies, or oxygen interstitials or combinations thereof. All these point defects generally exist in supersaturation in the as-grown crystals. * Corresponding author. Tel.: + 39-0521-269235; fax: + 39-0521269206. E-mail address: [email protected] (C. Frigeri). 1 Present address: Laboratory for Physics and Technology of Crystals, 2-14-1-202, Shin-sayama, Sayama, Saitama 350-1331, Japan.
In a Czochralski (CZ) silicon crystal the formation of dislocation loops and voids can be controlled by proper choice of the V/G ratio, where V and G are the pull rate and the temperature gradient at the growing interface, respectively [6]. High V/G values give vacancy rich crystals which are preferred, as voids are less detrimental than loops. Stacking faults and the related oxidation induced stacking fault (OSF) ring are not present in such crystals as the OSF-ring, border between inner vacancy region and outer interstitial region, can be pushed outside the crystal if high V/G values are used [5]. Fully vacancy rich Si crystals seem, however, to be hard to be obtained with the 300 mm, and larger, Si ingots to be grown in the very near future because such large crystals require low V/G ratios in order to favour dissipation of the latent heat and avoid cracking [6]. The harmful OSF-ring is therefore expected to be still present in the large diameter crystals. It is thus necessary to devise methods to eliminate the stacking faults other than the increase of the V/G ratio. In this paper we report on the effect of using a nitrogen annealing atmosphere on the formation of defects, especially stacking faults, in CZ-Si and the possible mechanisms behind it. Observations have been carried out with optical and electron beam based methods.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 8 5 - 0
C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
2. Experimental The as-grown CZ-Si crystal was 5 in. diameter, ptype, (100) oriented and with an interstitial oxygen atoms concentration of 1.4× 1018 cm − 3. The V/G ratio was such that it allowed the formation of the OSF-ring. Wafers cut from neighbouring regions in the ingot centre were submitted to annealing in oxygen atmosphere at 1150 °C or in nitrogen atmosphere at 1100 °C. Annealing time was 16 h in both cases. Transmission electron microscopy (TEM) and multichroic infrared light scattering tomography (MC-IRLST) were used for defect analysis. The plan view specimens for TEM bright and dark field observations, performed at 200 kV, were prepared by mechanical lapping followed by Ar+ ion beam bombardment. For taking the MC-IR-LST maps an Nd-YAG laser (u= 1.06 mm) was made to impinge into the sample along the [010] direction and continuously scanned along the [001] direction. An infrared objective lens set along the
171
[100] direction was used to collect the output light signal. Though the system allows to build photoluminescence maps, here only light scattering tomography (LST) maps are given which were taken by recording the elastic component of the output light. Further details and scheme of the MC-IR-LST technique can be found elsewhere [7–10].
3. Results The wafer annealed in oxygen is used as reference sample. The presented results are taken from the OSFring regions of both the samples, i.e. annealed in either nitrogen or oxygen. Fig. 1a and b shows TEM images of the typical defects in the sample annealed in oxygen. They are extrinsic stacking faults (Fig. 1a) and polyhedral defects, 200–300 nm in size (Fig. 1b). The latter ones have sides parallel to the 011 directions and are characterised by an inner cross with arms parallel to the 010 directions: they are thus oxygen precipitates [11,12]. In the sample annealed in nitrogen the stacking faults were not observed. The detected defects were oxygen related polyhedral precipitates similar to those observed in the oxygen annealed sample (Fig. 2a and b) and very small ( 20–30 nm) dislocation loop-like strain centres whose features have not been fully established yet (Fig. 2a and c). They gave outside– inside contrast upon reversing the diffraction vector g. Fig. 3a and b shows the LST maps taken from the OSF-ring region in the samples annealed in oxygen and nitrogen atmosphere, respectively. The scattering centres are uniformly distributed with nearly the same density of 7 × 107 cm − 3 in both samples.
4. Discussion
Fig. 1. Defects in the sample annealed in oxygen. TEM bright field images of (a) a piece of a stacking fault (the bar is 1 mm), and (b) an oxygen polyhedral precipitate (the bar is 200 nm). Arrows are g = [022].
The results show that by annealing in nitrogen the formation of the stacking faults is prevented but not the one of the oxygen precipitates whereas new crystal microdefects (loop-like strain centres) are introduced. The extrinsic stacking faults, seen in the oxygen annealed wafer, are usually assumed to start at an oxygen precipitate and then grow by absorption of Si interstitials, mostly those emitted by the oxygen precipitates under formation (see below) since native interstitials in excess are not expected to exist in the OSF-ring region [5]. We suggest that the absence of stacking faults in the nitrogen annealed sample is due to the lack of interstitials which, in turn, has to be ascribed to the introduction of vacancies from the nitrogen annealing ambient. From the observation of the enhanced diffusion of Sb during annealing in nitrogen at 1000 °C, Kook and Jacodine concluded that vacancies are intro-
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C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
duced very likely into Si from the nitrogen atmosphere as Sb was known to diffuse mostly via vacancies [13], which was also confirmed by later investigations [14]. It is therefore assumed here that annealing in nitrogen further enriches the Si wafer with vacancies. During annealing in nitrogen it is thus highly probable that the excess Si interstitials emitted during the formation of the oxygen precipitates (see below) recombine with the newly created extra vacancies rather than contributing
Fig. 3. LST maps at the same magnification of CZ-Si samples annealed in (a) oxygen atmosphere and (b) nitrogen atmosphere. Bar represents 100 mm.
Fig. 2.
Fig. 2. Defects in the sample annealed in nitrogen. (a) TEM bright field image of an oxygen polyhedral precipitate (P) and two small microdefects on the bottom left corner (one is arrowed). Arrow is g = [022]. The bar is 200 nm. (b) Bright field image of an oxygen precipitate at higher magnification taken at the [100] zone axis with no g selected. The bar is 200 nm. (c) High magnification dark field image of a loop-like microdefect like the ones in (a). Arrow is g = [022]. The bar is 20 nm.
C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
to the growth of the stacking faults which therefore do not form. The driving force for the formation of the oxygen precipitates is considered to be the supersaturation of vacancies [15,16]. A supersaturation of vacancies, in fact, reduces the free energy for nucleation of the precipitates [15,16]. It also enhances the growth of the precipitates by absorption of vacancies at the growing precipitates and the recombination with emitted selfinterstitials which lowers the stress around them [15]. The formation of oxygen precipitates in the OSF-ring takes place because the OSF-ring develops close to the Si interstitial/vacancy regions crossover, but slightly on the vacancy rich side [5]. A vacancy supersaturation is thus available that will promote oxygen precipitation. This process causes the emission of those Si interstitials, that would be necessary for the growth of the stacking faults, mentioned before [15,16]. The additional supply of vacancies generated by the annealing in nitrogen, as hypothesised above, thus will not stop the formation of the oxygen precipitates. It should rather favour it. This would agree with the findings of Aihara et al. who ascribed the increase of the oxygen precipitate density in N-doped CZ-Si with respect to non N-doped Si, i.e. in the case when nitrogen is added to Si as a dopant, to the creation of extra vacancies associated with the presence of nitrogen in the Si lattice [2]. The detailed characteristics of the loop-like microdefects are not yet known. Further work is in progress to this aim. They might be due to the condensation of the excess vacancies introduced from the nitrogen atmosphere that did not recombine with the interstitials.
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Acknowledgements Work supported by JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics and CNR, DRI/Reparto I. References [1] K. Nakai, Y. Inoue, H. Yokota, A. Ikari, J. Takahashi, A. Tachikawa, K. Kitahara, Y. Ohta, W. Ohashi, J. Appl. Phys. 89 (2001) 4301. [2] K. Aihara, H. Takeno, Y. Hayamizu, M. Tamatsuka, T. Masui, J. Appl. Phys. 88 (2000) 3705. [3] Q. Sun, K.H. Yao, H.C. Gatos, J. Lagowski, J. Appl. Phys. 71 (1992) 3760. [4] F. Shimura, R.S. Hockett, Appl. Phys. Lett. 48 (1986) 224. [5] V.V. Voronkov, R. Falster, J.C. Holzer, Electrochem. Soc. Proc. 97 (22) (1997) 3. [6] W. von Ammon, E. Dornberger, P.O. Hansson, J. Cryst. Growth 198/199 (1999) 390. [7] T. Ogawa, in: J.P. Fillard (Ed.), Defect Recognition and Image Processing in III – V Compounds, Elsevier, Amsterdam, 1985, p. 1. [8] N. Nango, S. Iida, T. Ogawa, J. Appl. Phys. 86 (1999) 6000. [9] M. Ma, T. Ogawa, M. Watanabe, M. Eguchi, J. Cryst. Growth 205 (1999) 50. [10] M. Ma, T. Irisawa, T. Ogawa, C. Frigeri, Jpn. J. Appl. Phys. 40 (2001) 4153. [11] K. Sueoka, N. Ikeda, T. Yamamoto, Appl. Phys. Lett. 65 (1994) 1686. [12] M. Ma, T. Irisawa, T. Ogawa, C. Frigeri, Jpn. J. Appl. Phys. 40 (2001) 4149. [13] T. Kook, R.J. Jacodine, Mater. Res. Soc. Symp. Proc. 36 (1985) 83. [14] M. Jacob, P. Pichler, H. Ryssel, R. Falster, M. Cornara, D. Gambaro, M. Olmo, M. Pagani, Solid State Phenomena 57 –58 (1997) 349. [15] M. Hourai, T. Nagashima, E. Kajita, S. Miki, T. Shigematsu, J. Electrochem. Soc. 142 (1995) 3193. [16] J. Vanhellemont, Appl. Phys. Lett. 68 (1996) 3413.
Analysis of extended defects in nitrogen annealed CZ silicon by optical and electron beam methods C. Frigeri a,*, M. Ma b,c, T. Irisawa b, T. Ogawa c,1 a
CNR-MASPEC Institute, Parco Area delle Scienze 37 /A, Fontanini, 43010 Parma, Italy b Computer Center, Gakushuin Uni6ersity, Mejiro, Tokyo 171, Japan c Department of Physics, Gakushuin Uni6ersity, Mejiro, Tokyo 171, Japan
Abstract The effect of annealing in nitrogen atmosphere on the formation of crystal defects in the OSF-ring of Czochralski silicon has been studied by comparison with samples annealed in oxygen atmosphere by using optical and electron beam based methods. By annealing in nitrogen the formation of extrinsic stacking faults is prevented whereas oxygen precipitates form in nearly the same density as in the oxygen annealed sample. Additionally, loop-like microdefects were generated that were not observed for annealing in oxygen ambient. The results are explained by assuming that extra vacancies are introduced into Si from the nitrogen annealing atmosphere. They are expected to recombine with Si interstitials, thus preventing the growth of the stacking faults, and to create the observed microdefects. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Stacking faults; Oxygen precipitates; Vacancies; TEM; LST; OSF-ring; Si; Annealing
1. Introduction The study of the relationship between type and density of crystal defects and nitrogen in silicon is receiving considerable attention since it has been seen that the N doping of Si causes lattice hardening and increase of oxygen precipitates density under some specific conditions [1–4]. The most important extended defects in Si are dislocation loops and voids in the interstitial and vacancy rich part of the crystal, respectively, stacking faults located in the boundary region between those two regions [5] and oxygen precipitates. All these defects form during the annealing and processing steps, applied to the as-grown wafers for device production, due to the aggregation of self interstitials, or vacancies, or oxygen interstitials or combinations thereof. All these point defects generally exist in supersaturation in the as-grown crystals. * Corresponding author. Tel.: + 39-0521-269235; fax: + 39-0521269206. E-mail address: [email protected] (C. Frigeri). 1 Present address: Laboratory for Physics and Technology of Crystals, 2-14-1-202, Shin-sayama, Sayama, Saitama 350-1331, Japan.
In a Czochralski (CZ) silicon crystal the formation of dislocation loops and voids can be controlled by proper choice of the V/G ratio, where V and G are the pull rate and the temperature gradient at the growing interface, respectively [6]. High V/G values give vacancy rich crystals which are preferred, as voids are less detrimental than loops. Stacking faults and the related oxidation induced stacking fault (OSF) ring are not present in such crystals as the OSF-ring, border between inner vacancy region and outer interstitial region, can be pushed outside the crystal if high V/G values are used [5]. Fully vacancy rich Si crystals seem, however, to be hard to be obtained with the 300 mm, and larger, Si ingots to be grown in the very near future because such large crystals require low V/G ratios in order to favour dissipation of the latent heat and avoid cracking [6]. The harmful OSF-ring is therefore expected to be still present in the large diameter crystals. It is thus necessary to devise methods to eliminate the stacking faults other than the increase of the V/G ratio. In this paper we report on the effect of using a nitrogen annealing atmosphere on the formation of defects, especially stacking faults, in CZ-Si and the possible mechanisms behind it. Observations have been carried out with optical and electron beam based methods.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 8 5 - 0
C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
2. Experimental The as-grown CZ-Si crystal was 5 in. diameter, ptype, (100) oriented and with an interstitial oxygen atoms concentration of 1.4× 1018 cm − 3. The V/G ratio was such that it allowed the formation of the OSF-ring. Wafers cut from neighbouring regions in the ingot centre were submitted to annealing in oxygen atmosphere at 1150 °C or in nitrogen atmosphere at 1100 °C. Annealing time was 16 h in both cases. Transmission electron microscopy (TEM) and multichroic infrared light scattering tomography (MC-IRLST) were used for defect analysis. The plan view specimens for TEM bright and dark field observations, performed at 200 kV, were prepared by mechanical lapping followed by Ar+ ion beam bombardment. For taking the MC-IR-LST maps an Nd-YAG laser (u= 1.06 mm) was made to impinge into the sample along the [010] direction and continuously scanned along the [001] direction. An infrared objective lens set along the
171
[100] direction was used to collect the output light signal. Though the system allows to build photoluminescence maps, here only light scattering tomography (LST) maps are given which were taken by recording the elastic component of the output light. Further details and scheme of the MC-IR-LST technique can be found elsewhere [7–10].
3. Results The wafer annealed in oxygen is used as reference sample. The presented results are taken from the OSFring regions of both the samples, i.e. annealed in either nitrogen or oxygen. Fig. 1a and b shows TEM images of the typical defects in the sample annealed in oxygen. They are extrinsic stacking faults (Fig. 1a) and polyhedral defects, 200–300 nm in size (Fig. 1b). The latter ones have sides parallel to the 011 directions and are characterised by an inner cross with arms parallel to the 010 directions: they are thus oxygen precipitates [11,12]. In the sample annealed in nitrogen the stacking faults were not observed. The detected defects were oxygen related polyhedral precipitates similar to those observed in the oxygen annealed sample (Fig. 2a and b) and very small ( 20–30 nm) dislocation loop-like strain centres whose features have not been fully established yet (Fig. 2a and c). They gave outside– inside contrast upon reversing the diffraction vector g. Fig. 3a and b shows the LST maps taken from the OSF-ring region in the samples annealed in oxygen and nitrogen atmosphere, respectively. The scattering centres are uniformly distributed with nearly the same density of 7 × 107 cm − 3 in both samples.
4. Discussion
Fig. 1. Defects in the sample annealed in oxygen. TEM bright field images of (a) a piece of a stacking fault (the bar is 1 mm), and (b) an oxygen polyhedral precipitate (the bar is 200 nm). Arrows are g = [022].
The results show that by annealing in nitrogen the formation of the stacking faults is prevented but not the one of the oxygen precipitates whereas new crystal microdefects (loop-like strain centres) are introduced. The extrinsic stacking faults, seen in the oxygen annealed wafer, are usually assumed to start at an oxygen precipitate and then grow by absorption of Si interstitials, mostly those emitted by the oxygen precipitates under formation (see below) since native interstitials in excess are not expected to exist in the OSF-ring region [5]. We suggest that the absence of stacking faults in the nitrogen annealed sample is due to the lack of interstitials which, in turn, has to be ascribed to the introduction of vacancies from the nitrogen annealing ambient. From the observation of the enhanced diffusion of Sb during annealing in nitrogen at 1000 °C, Kook and Jacodine concluded that vacancies are intro-
172
C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
duced very likely into Si from the nitrogen atmosphere as Sb was known to diffuse mostly via vacancies [13], which was also confirmed by later investigations [14]. It is therefore assumed here that annealing in nitrogen further enriches the Si wafer with vacancies. During annealing in nitrogen it is thus highly probable that the excess Si interstitials emitted during the formation of the oxygen precipitates (see below) recombine with the newly created extra vacancies rather than contributing
Fig. 3. LST maps at the same magnification of CZ-Si samples annealed in (a) oxygen atmosphere and (b) nitrogen atmosphere. Bar represents 100 mm.
Fig. 2.
Fig. 2. Defects in the sample annealed in nitrogen. (a) TEM bright field image of an oxygen polyhedral precipitate (P) and two small microdefects on the bottom left corner (one is arrowed). Arrow is g = [022]. The bar is 200 nm. (b) Bright field image of an oxygen precipitate at higher magnification taken at the [100] zone axis with no g selected. The bar is 200 nm. (c) High magnification dark field image of a loop-like microdefect like the ones in (a). Arrow is g = [022]. The bar is 20 nm.
C. Frigeri et al. / Materials Science and Engineering B91–92 (2002) 170–173
to the growth of the stacking faults which therefore do not form. The driving force for the formation of the oxygen precipitates is considered to be the supersaturation of vacancies [15,16]. A supersaturation of vacancies, in fact, reduces the free energy for nucleation of the precipitates [15,16]. It also enhances the growth of the precipitates by absorption of vacancies at the growing precipitates and the recombination with emitted selfinterstitials which lowers the stress around them [15]. The formation of oxygen precipitates in the OSF-ring takes place because the OSF-ring develops close to the Si interstitial/vacancy regions crossover, but slightly on the vacancy rich side [5]. A vacancy supersaturation is thus available that will promote oxygen precipitation. This process causes the emission of those Si interstitials, that would be necessary for the growth of the stacking faults, mentioned before [15,16]. The additional supply of vacancies generated by the annealing in nitrogen, as hypothesised above, thus will not stop the formation of the oxygen precipitates. It should rather favour it. This would agree with the findings of Aihara et al. who ascribed the increase of the oxygen precipitate density in N-doped CZ-Si with respect to non N-doped Si, i.e. in the case when nitrogen is added to Si as a dopant, to the creation of extra vacancies associated with the presence of nitrogen in the Si lattice [2]. The detailed characteristics of the loop-like microdefects are not yet known. Further work is in progress to this aim. They might be due to the condensation of the excess vacancies introduced from the nitrogen atmosphere that did not recombine with the interstitials.
173
Acknowledgements Work supported by JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics and CNR, DRI/Reparto I. References [1] K. Nakai, Y. Inoue, H. Yokota, A. Ikari, J. Takahashi, A. Tachikawa, K. Kitahara, Y. Ohta, W. Ohashi, J. Appl. Phys. 89 (2001) 4301. [2] K. Aihara, H. Takeno, Y. Hayamizu, M. Tamatsuka, T. Masui, J. Appl. Phys. 88 (2000) 3705. [3] Q. Sun, K.H. Yao, H.C. Gatos, J. Lagowski, J. Appl. Phys. 71 (1992) 3760. [4] F. Shimura, R.S. Hockett, Appl. Phys. Lett. 48 (1986) 224. [5] V.V. Voronkov, R. Falster, J.C. Holzer, Electrochem. Soc. Proc. 97 (22) (1997) 3. [6] W. von Ammon, E. Dornberger, P.O. Hansson, J. Cryst. Growth 198/199 (1999) 390. [7] T. Ogawa, in: J.P. Fillard (Ed.), Defect Recognition and Image Processing in III – V Compounds, Elsevier, Amsterdam, 1985, p. 1. [8] N. Nango, S. Iida, T. Ogawa, J. Appl. Phys. 86 (1999) 6000. [9] M. Ma, T. Ogawa, M. Watanabe, M. Eguchi, J. Cryst. Growth 205 (1999) 50. [10] M. Ma, T. Irisawa, T. Ogawa, C. Frigeri, Jpn. J. Appl. Phys. 40 (2001) 4153. [11] K. Sueoka, N. Ikeda, T. Yamamoto, Appl. Phys. Lett. 65 (1994) 1686. [12] M. Ma, T. Irisawa, T. Ogawa, C. Frigeri, Jpn. J. Appl. Phys. 40 (2001) 4149. [13] T. Kook, R.J. Jacodine, Mater. Res. Soc. Symp. Proc. 36 (1985) 83. [14] M. Jacob, P. Pichler, H. Ryssel, R. Falster, M. Cornara, D. Gambaro, M. Olmo, M. Pagani, Solid State Phenomena 57 –58 (1997) 349. [15] M. Hourai, T. Nagashima, E. Kajita, S. Miki, T. Shigematsu, J. Electrochem. Soc. 142 (1995) 3193. [16] J. Vanhellemont, Appl. Phys. Lett. 68 (1996) 3413.