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Synthesis of cobalt silicide on porous silicon by high dose ion implantation
Nuclear Instruments and Methods in Physics Research B 178 (2001) 283±286
www.elsevier.nl/locate/nimb
Synthesis of cobalt silicide on porous silicon by high dose ion implantation A.R. Ramos a
V , F. P aszti c, E. Kotai c, E. azsonyi d, O. Conde b, M.R. da Silva a, M.F. da Silva a, J.C. Soares a
a,*
ITN ± Inst. Tecnol ogico e Nuclear, Estr. Nacional 10, P-2686-953 Sacav em, and Centro de Fõsica Nuclear da Universida de de Lisboa, Av. Prof. Gama Pinto 2, P-1649-003 Lisboa, Portugal b FCUL ± Fac. de Ci^ encias da Univ. de Lisboa, Dep. Fõsica, Ed C1, P-1749-016 Lisboa, Portugal c KFKI ± Res. Inst. for Particle and Nuclear Physics, P.O. Box 49, H-1525, Budapest, Hungary d MTA ± Res. Inst. for Technical Phys. and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary
Abstract High dose ion implantation was used to form cobalt silicide on porous Si containing dierent concentrations of light impurities (C and O). Samples were implanted with 100 keV Co ions to ¯uences of 2 1017 ions=cm2 at room temperature, 350°C and 450°C, and then annealed (600°C for 60 min + 1000°C for 30 min). The formed silicide compounds were studied by Rutherford backscattering spectrometry (RBS), glancing incidence X-ray diraction (GIXRD) and four point probe resistivity measurements. The quality and type of the formed silicide were found to depend on the original impurity level as well as on the implantation temperature and annealing. Best results were obtained at 350°C, where a nearly rectangular low resistivity layer was formed immediately after implantation. Annealing at 600°C improved the layer resistivity, but the higher temperature annealing destroyed the porous layer. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.10.-i; 61.18.Bn; 61.43.Gt; 68.55.Ln Keywords: Porous silicon; Silicide; Backscattering spectrometry; ion implantation; XRD
1. Introduction Porous silicon (PS) [1] is extensively investigated for its electro-luminescence property: its use may allow the integration of optical and electronic
*
Corresponding author. Tel.: +351-21-994-6084; fax: +35121-994-1525. E-mail address: [email protected] (A.R. Ramos).
units into a single device with conventional technology. A basic requirement for most applications is the formation of electric contacts on PS. Our previous experiments revealed that it is possible to form good quality silicide layers on PS through high dose Cr implantation [2]. Results showed that both O and C, native contaminants of the PS, were partly expelled from the implanted layer, which densi®ed during implantation [2]. However, Cr forms a comparatively high resistivity silicide
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 8 1 - X
284
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
(1500 lX cm). In order to form a lower resistivity silicide already after the implantation, thus avoiding the need for high temperature annealing which would destroy the PS [1], Co implantations were carried out and cobalt silicide formation on PS studied. 2. Experimental Three dierent kinds of 5 lm thick PS layers were prepared by anodic etching. Sample preparation details can be found in [2,3]. Chosen samples were pre-oxidized right after manufacturing by annealing at 300°C for 60 min in an N2 atmosphere with 1% O2 . Between preparation and implantation, the samples were kept in containers at normal and pentane atmosphere. In the latter they oxidized more quickly. All samples were 2 implanted with 100 keV Co ions, at 3 lA=cm current density and 15° tilt, up to a ¯uence of 2 2 1017 ions=cm at room temperature (RT), 350°C (LT) and 450°C (HT). All samples were annealed at 600°C for 60 min, in 10 5 Pa vacuum. The LT implanted samples suered an extra anneal at 1000°C for 30 min, also in vacuum. The in-depth composition of the various samples was determined by Rutherford backscattering spectrometry (RBS) (RBS at H 140° and 180° scattering angles, IBM geometry, 1.6 MeV He and 1.5/1.73 MeV H and RBS at H 165°, CORNELL geometry, 3.043/3.145/3.8 MeV He). RBS data were evaluated with RUMP [4], RBX [5] and NDF [6]. Glancing incidence X-ray diffraction (GIXRD) measurements were done with a Siemens D-5000 diractometer (1° incidence, Cu±Ka radiation). Phase identi®cation was based on [7]. 3. Results and discussion The simulated depth pro®les indicate that during the high temperature implantation C and O were partially expelled from the implanted layer, piling up behind it (see Fig. 1(a)). For initial O/Si ratios in the buried PS layer of 6 0.20, the retained oxygen content in the Co implanted layer is 0; it
Fig. 1. (a) Typical RBS spectra (3.045 MeV 4 He , H 165°, CORNELL geometry, 60° tilt) of a Si wafer and a spongy PS sample with 70% porosity, kept in normal atmosphere. Note the O resonance peak caused by the O contamination at the surface. For the PS sample, it is clearly seen that, during implantation, the O was expelled from both the forming silicide layer and the layer just behind it, piling up deeper in a buried SiOx layer. (b) RBS spectra (1.6 MeV 4 He , H 140°, IBM geometry, 35° tilt) of two columnar pre-oxidised PS samples of 70% porosity, kept in normal atmosphere previous to implantation at dierent temperatures. Dierent peak areas for the Co pro®le indicate that the retained dose decreases with increasing implantation temperature in the PS samples.
then increases with the O content. This expelling process is even more pronounced at room temperature: for identical PS substrates, the concentration of retained O increases with the implantation temperature. Such behavior may indicate oxygen redistribution in the implanted layer during the high temperature implantations. The as-implanted Co pro®les are nearly rectangular, A diusion with an average thickness of 600 A. like tail is visible, increasing with implantation temperature (see Fig. 1(b)). In the case of mono-
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
crystalline Si, the retained Co dose increases with implantation temperature; for PS, it decreases (see Fig. 1(b)). For the high temperature implantations (HT and LT), the retained dose decreases with the O content, with higher values in monocrystalline Si and lower values in heavily contaminated samples. Conversely, for the RT implantations, the Co dose increases with the O content. This points to an oxygen enhanced Co diusion mechanism towards the surface during high temperature implantation. Sheet resistance increases with O/Si ratio in the implanted layer (i.e., with the original O content),
285
but dierently for the various temperatures. For lower ratios, the high-temperature implanted samples have lower resistivity; above 0.22 O/Si ratio, the sheet resistance increases faster for the high temperature samples than for the RT ones (see Fig. 2(a)). This may be the result of coarser SiO2 precipitates in the high temperature samples. The X-ray diraction data taken for batch LT indicate that the intensity of the diraction lines and the average grain size of the CoSi2 precipitates decrease as the O content grows (see Fig. 2(b)). At about 0.22 O/Si ratio another silicide phase appears: CoSi. There seems to be some evidence of CoSi also disappearing at even higher O contents. X-ray diraction measurements (not shown) revealed that, up to 0.70 O/Si ratio, annealing promotes the transformation of CoSi into CoSi2 , if the ®rst was present, or the growth of the CoSi2 . For even higher O/Si ratios, CoSi also starts growing in the annealed sample. In the more contaminated samples, it is the only phase present. For the high temperature implantations, annealing at 600°C results in lower resistivity for all samples. Alternatively, in the case of RT implantations, the same annealing results in a resistivity increase for samples of >0.25 O/Si ratio. The additional 1000°C annealing results in increased resistivity and diusion like Co pro®les in all PS samples, possibly because of structure degradation in the underlying PS [1]. 4. Conclusions
Fig. 2. Sheet resistance values for LT and RT samples (a) and average grain size for LT samples (b) versus O/Si ratio in the buried porous layer.
RBS methods combined with X-ray diraction and sheet resistance measurements oer good control over the ion beam synthesis of cobalt silicide on PS layers. Under the experimental conditions used, in PS with an O/Si ratio <0.30 it is possible to form a good quality cobalt disilicide layer immediately after implantation. The light impurities present in the original substrate partly escape from the implanted layer, piling up behind it. In more contaminated substrates, another more Co rich phase appears (CoSi). Above 0.70 O/Si ratio no silicide can be detected by GIXRD. Best electrical properties are obtained with LT-implanted samples, though their resistivity is still 3
286
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
times higher than their monocrystalline counterpart (15 versus 45 lX cm).
Acknowledgements This work was supported by the Hungarian± Portuguese bilateral intergovernmental cooperation TeT P-14/97 and ICCTI 423/OMFB, as well as by a Ph.D. grant FCT, Portugal, BD/9242/96 (A.R.R.) and Hungarian OTKA grants T-019147 and T-023547.
References [1] J.C. Vial, J. Derrien (Eds.), Porous Silicon Science and Technology, Springer, Berlin, 1995. Vazsonyi, [2] A.R. Ramos, O. Conde, F. Paszti, G. Battistig, E. M.R. da Silva, M.F. da Silva, J.C. Soares, Nucl. Instr. and Meth. B 161±163 (2000) 926. Vazsonyi, M. Ko [3] E. os, G. Jalsovszky, I. P ocsik, J. Luminescence 57 (1993) 121. [4] L.R. Doolittle, Nucl. Instr. and Meth. B 5 (1985) 344. [5] E. K otai, Nucl. Instr. and Meth. B 85 (1994) 588. [6] N.P. Barradas, C. Jeynes, R.P. Webb, Appl. Phys. Lett. 71 (1997) 291. [7] Joint Committee on Powder Diraction Standards, Powder Diraction File, ASTM, Philadelphia, 1992.
www.elsevier.nl/locate/nimb
Synthesis of cobalt silicide on porous silicon by high dose ion implantation A.R. Ramos a
V , F. P aszti c, E. Kotai c, E. azsonyi d, O. Conde b, M.R. da Silva a, M.F. da Silva a, J.C. Soares a
a,*
ITN ± Inst. Tecnol ogico e Nuclear, Estr. Nacional 10, P-2686-953 Sacav em, and Centro de Fõsica Nuclear da Universida de de Lisboa, Av. Prof. Gama Pinto 2, P-1649-003 Lisboa, Portugal b FCUL ± Fac. de Ci^ encias da Univ. de Lisboa, Dep. Fõsica, Ed C1, P-1749-016 Lisboa, Portugal c KFKI ± Res. Inst. for Particle and Nuclear Physics, P.O. Box 49, H-1525, Budapest, Hungary d MTA ± Res. Inst. for Technical Phys. and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary
Abstract High dose ion implantation was used to form cobalt silicide on porous Si containing dierent concentrations of light impurities (C and O). Samples were implanted with 100 keV Co ions to ¯uences of 2 1017 ions=cm2 at room temperature, 350°C and 450°C, and then annealed (600°C for 60 min + 1000°C for 30 min). The formed silicide compounds were studied by Rutherford backscattering spectrometry (RBS), glancing incidence X-ray diraction (GIXRD) and four point probe resistivity measurements. The quality and type of the formed silicide were found to depend on the original impurity level as well as on the implantation temperature and annealing. Best results were obtained at 350°C, where a nearly rectangular low resistivity layer was formed immediately after implantation. Annealing at 600°C improved the layer resistivity, but the higher temperature annealing destroyed the porous layer. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.10.-i; 61.18.Bn; 61.43.Gt; 68.55.Ln Keywords: Porous silicon; Silicide; Backscattering spectrometry; ion implantation; XRD
1. Introduction Porous silicon (PS) [1] is extensively investigated for its electro-luminescence property: its use may allow the integration of optical and electronic
*
Corresponding author. Tel.: +351-21-994-6084; fax: +35121-994-1525. E-mail address: [email protected] (A.R. Ramos).
units into a single device with conventional technology. A basic requirement for most applications is the formation of electric contacts on PS. Our previous experiments revealed that it is possible to form good quality silicide layers on PS through high dose Cr implantation [2]. Results showed that both O and C, native contaminants of the PS, were partly expelled from the implanted layer, which densi®ed during implantation [2]. However, Cr forms a comparatively high resistivity silicide
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 8 1 - X
284
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
(1500 lX cm). In order to form a lower resistivity silicide already after the implantation, thus avoiding the need for high temperature annealing which would destroy the PS [1], Co implantations were carried out and cobalt silicide formation on PS studied. 2. Experimental Three dierent kinds of 5 lm thick PS layers were prepared by anodic etching. Sample preparation details can be found in [2,3]. Chosen samples were pre-oxidized right after manufacturing by annealing at 300°C for 60 min in an N2 atmosphere with 1% O2 . Between preparation and implantation, the samples were kept in containers at normal and pentane atmosphere. In the latter they oxidized more quickly. All samples were 2 implanted with 100 keV Co ions, at 3 lA=cm current density and 15° tilt, up to a ¯uence of 2 2 1017 ions=cm at room temperature (RT), 350°C (LT) and 450°C (HT). All samples were annealed at 600°C for 60 min, in 10 5 Pa vacuum. The LT implanted samples suered an extra anneal at 1000°C for 30 min, also in vacuum. The in-depth composition of the various samples was determined by Rutherford backscattering spectrometry (RBS) (RBS at H 140° and 180° scattering angles, IBM geometry, 1.6 MeV He and 1.5/1.73 MeV H and RBS at H 165°, CORNELL geometry, 3.043/3.145/3.8 MeV He). RBS data were evaluated with RUMP [4], RBX [5] and NDF [6]. Glancing incidence X-ray diffraction (GIXRD) measurements were done with a Siemens D-5000 diractometer (1° incidence, Cu±Ka radiation). Phase identi®cation was based on [7]. 3. Results and discussion The simulated depth pro®les indicate that during the high temperature implantation C and O were partially expelled from the implanted layer, piling up behind it (see Fig. 1(a)). For initial O/Si ratios in the buried PS layer of 6 0.20, the retained oxygen content in the Co implanted layer is 0; it
Fig. 1. (a) Typical RBS spectra (3.045 MeV 4 He , H 165°, CORNELL geometry, 60° tilt) of a Si wafer and a spongy PS sample with 70% porosity, kept in normal atmosphere. Note the O resonance peak caused by the O contamination at the surface. For the PS sample, it is clearly seen that, during implantation, the O was expelled from both the forming silicide layer and the layer just behind it, piling up deeper in a buried SiOx layer. (b) RBS spectra (1.6 MeV 4 He , H 140°, IBM geometry, 35° tilt) of two columnar pre-oxidised PS samples of 70% porosity, kept in normal atmosphere previous to implantation at dierent temperatures. Dierent peak areas for the Co pro®le indicate that the retained dose decreases with increasing implantation temperature in the PS samples.
then increases with the O content. This expelling process is even more pronounced at room temperature: for identical PS substrates, the concentration of retained O increases with the implantation temperature. Such behavior may indicate oxygen redistribution in the implanted layer during the high temperature implantations. The as-implanted Co pro®les are nearly rectangular, A diusion with an average thickness of 600 A. like tail is visible, increasing with implantation temperature (see Fig. 1(b)). In the case of mono-
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
crystalline Si, the retained Co dose increases with implantation temperature; for PS, it decreases (see Fig. 1(b)). For the high temperature implantations (HT and LT), the retained dose decreases with the O content, with higher values in monocrystalline Si and lower values in heavily contaminated samples. Conversely, for the RT implantations, the Co dose increases with the O content. This points to an oxygen enhanced Co diusion mechanism towards the surface during high temperature implantation. Sheet resistance increases with O/Si ratio in the implanted layer (i.e., with the original O content),
285
but dierently for the various temperatures. For lower ratios, the high-temperature implanted samples have lower resistivity; above 0.22 O/Si ratio, the sheet resistance increases faster for the high temperature samples than for the RT ones (see Fig. 2(a)). This may be the result of coarser SiO2 precipitates in the high temperature samples. The X-ray diraction data taken for batch LT indicate that the intensity of the diraction lines and the average grain size of the CoSi2 precipitates decrease as the O content grows (see Fig. 2(b)). At about 0.22 O/Si ratio another silicide phase appears: CoSi. There seems to be some evidence of CoSi also disappearing at even higher O contents. X-ray diraction measurements (not shown) revealed that, up to 0.70 O/Si ratio, annealing promotes the transformation of CoSi into CoSi2 , if the ®rst was present, or the growth of the CoSi2 . For even higher O/Si ratios, CoSi also starts growing in the annealed sample. In the more contaminated samples, it is the only phase present. For the high temperature implantations, annealing at 600°C results in lower resistivity for all samples. Alternatively, in the case of RT implantations, the same annealing results in a resistivity increase for samples of >0.25 O/Si ratio. The additional 1000°C annealing results in increased resistivity and diusion like Co pro®les in all PS samples, possibly because of structure degradation in the underlying PS [1]. 4. Conclusions
Fig. 2. Sheet resistance values for LT and RT samples (a) and average grain size for LT samples (b) versus O/Si ratio in the buried porous layer.
RBS methods combined with X-ray diraction and sheet resistance measurements oer good control over the ion beam synthesis of cobalt silicide on PS layers. Under the experimental conditions used, in PS with an O/Si ratio <0.30 it is possible to form a good quality cobalt disilicide layer immediately after implantation. The light impurities present in the original substrate partly escape from the implanted layer, piling up behind it. In more contaminated substrates, another more Co rich phase appears (CoSi). Above 0.70 O/Si ratio no silicide can be detected by GIXRD. Best electrical properties are obtained with LT-implanted samples, though their resistivity is still 3
286
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 283±286
times higher than their monocrystalline counterpart (15 versus 45 lX cm).
Acknowledgements This work was supported by the Hungarian± Portuguese bilateral intergovernmental cooperation TeT P-14/97 and ICCTI 423/OMFB, as well as by a Ph.D. grant FCT, Portugal, BD/9242/96 (A.R.R.) and Hungarian OTKA grants T-019147 and T-023547.
References [1] J.C. Vial, J. Derrien (Eds.), Porous Silicon Science and Technology, Springer, Berlin, 1995. Vazsonyi, [2] A.R. Ramos, O. Conde, F. Paszti, G. Battistig, E. M.R. da Silva, M.F. da Silva, J.C. Soares, Nucl. Instr. and Meth. B 161±163 (2000) 926. Vazsonyi, M. Ko [3] E. os, G. Jalsovszky, I. P ocsik, J. Luminescence 57 (1993) 121. [4] L.R. Doolittle, Nucl. Instr. and Meth. B 5 (1985) 344. [5] E. K otai, Nucl. Instr. and Meth. B 85 (1994) 588. [6] N.P. Barradas, C. Jeynes, R.P. Webb, Appl. Phys. Lett. 71 (1997) 291. [7] Joint Committee on Powder Diraction Standards, Powder Diraction File, ASTM, Philadelphia, 1992.