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Elaboration of thin chromium silicide layers on P+ implanted silicon
Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
Elaboration of thin chromium silicide layers on P+ implanted silicon R. Labbani*, R. Halimi, A. Bouabellou, A. Bouguerra Laboratory of Thin Films and Interfaces, Department of Physics, Faculty of Sciences, Mentouri University, Route de Ain El Bey, 25000 Constantine, Algeria
Abstract The aim of the present work is to investigate the mechanism of growth of chromium silicides versus several parameters. The samples are constituted of a thin chromium layer (E80 nm) which is deposited under vacuum by electron beam evaporation/condensation onto P+ implanted Si(1 0 0) and Si(1 1 1) substrates. Phosphorus ion implantation, with a dose of 5 1014 or 5 1015 P+ cm 2, is performed at an energy of 40 keV. The samples are thermally annealed at 723–823 K (450–5501C) for various periods of time. The analysis is carried out by means of X-ray diffraction and H+ Rutherford backscattering spectrometry (RBS) techniques. The RUMP 3.30 software is used to simulate RBS spectra. It is shown that, after annealing treatment, the CrSi2 silicide is formed at Cr/Si(1 0 0) and Cr/Si(1 1 1) interfaces. For one temperature and time of annealing, the thickness of the chromium disilicide is lower for the high phosphorus dose (i.e., 5 1015 P+ cm 2) than for the low dose (i.e., 5 1014P+ cm 2). In other words, the presence of phosphorus in silicon substrates delays the formation of the CrSi2 phase but does not inhibit it. Finally, it is established that the growth of CrSi2 compound is more rapid on silicon substrates with /1 1 1S orientation than on the /1 0 0S orientation, which is not in agreement with the non-implanted specimens case. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 68.55.Ln Keywords: Silicide; Silicon; Chromium; Metal/silicon interface
1. Introduction Silicides of transition metals are intermediate compounds that are very useful in electronic circuit technology with Very Large Scale Integration (VLSI) [1,2]. Indeed, they can be used as a Schottky barrier, diffusion barrier, ohmic contact, in interconnections, etc. They present interesting *Corresponding author. Tel.: +213-4-614711; fax: +213-4923489. E-mail address: [email protected] (R. Labbani).
properties such as: good thermal stability during fabrication of integrated circuits, good compatibility with SiO2 as a substrate, etc. Silicide phases may be formed via different processes [3] like the co-deposition of silicon and metal by sputtering, evaporation or chemical-vapour deposition. The solid-phase reaction at the Metal/Silicon (M/Si) interface is also a method widely used for silicide formation, which requires a thin metal layer deposition on silicon substrate and heating at adequate temperature and/or time. In general, silicides can present either metallic or semiconductor
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 2 0 9 5 - 2
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
characteristics. The CrSi2 phase is a compound of particular interest because it is a semiconductor material with an energy gap of 0.35 eV, which is very suitable for opto-electronic technology [4,5]. In this work, we have studied the interaction phenomenon and the formation of chromium silicide at the interface between a thin chromium layer (80 nm) and a phosphorus-implanted silicon substrate: (Si(1 0 0) or Si(1 1 1)). These effects were investigated as functions of: silicon substrate orientation, applied dose of phosphorus ions and the temperature or time of annealing.
2. Experiment The substrates were commercialized silicon wafers (p type), with /1 0 0S or /1 1 1S orientation, a thickness of B400 mm and a resistivity of 60–80 O cm. Prior to P+ ion implantation, a thin film of SiO2 (25 nm) was deposited by dry oxidation in a gas mixture of O2+HCl. This operation was accomplished in 50 min at 1173 K (9001C). The oxide layer is considered necessary to avoid impurity contamination and any eventual loss of phosphorus during P+ implantation. The ion implantation was performed through the SiO2 oxide layer by the impact of P+ 2 primary ions in the gaseous phase. The energy of phosphorus ions was chosen to be 40 keV, whereas two values of the dose were used: 5 1014 P+ cm 2 or 5 1015 P+ cm 2. For P+ electrical activation, the implanted substrates were then heat treated at 1173 K (9001C) in an oxygen atmosphere for 60 min. Next, the SiO2 layers of implanted and nonimplanted silicon substrates were chemically etched at room temperature in a HF:H2O2=1:10 solution, which removed both the native and the grown SiO2 layers. Thus prepared, the substrates were transferred immediately to a vacuum metaldeposition system where they were maintained at 373 K (1001C) during application of a thin metallic layer (80 nm) of Cr by electron-beam evaporation in vacuum (10 5 Pa, 10 7 Torr). The final preparation procedure was to perform a conventional heat treatment at temperatures varying from 723 to 823 K (4501C to 5501C) in vacuum (10 3 Pa, 10 5 Torr).
To identify the formed phases and to record their growth for specific values of parameters such as the annealing time or temperature, Si crystallographic orientation, and/or a phosphorus dose, Rutherford Backscattering Spectrometry (RBS) and X-ray Diffraction (XRD) techniques were used. The XRD analyses were performed on a vertical diffractometer (Philips 1050/70) with KaCu radiation and a Ni filter. The RBS measurements were performed using H+ particles with 0.3 MeV energy and an electrostatic detector of 2 keV resolution. The angles j (between an incident beam of H+ particles and the detector) and y (between the normal to the sample and incident beam) were chosen to be 451 and 01, respectively. The detection solid angle was equal to 0.456 msr. The obtained RBS spectra were simulated using the software RUMP 3.30 (Rutherford Universal Manipulation Programme) which was developed at the Cornell university (New York) [6].
3. Results and discussion During chromium deposition, XRD analysis revealed that no silicide phases had been formed. Indeed, the diagram of Fig. 1a (corresponding to non-annealed sample) is formed by characteristic peaks of Si and Cr only. Furthermore, no peaks of Si (100)
(b)
CrSi2 (111)
Intensity (arb. Units)
224
CrSi2(110)
Cr (110)
Si (100)
(a)
Cr (110)
30
40
50
60
70
80
Bragg angle 2θ (°)
Fig. 1. XRD spectra of 5 1014 P+ cm 2 implanted Cr/Si samples. non-annealed, annealed at 748 K (4751C) during 30 min.
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
silicon or chromium oxides were present in this spectrum, suggesting a low level of contamination in the prepared specimens. The same results were confirmed by the RBS technique, as illustrated in Fig. 2. Indeed, the spectrum corresponding to a non-annealed sample (solid curve) is formed only by Si and Cr signals. Furthermore, the absence of a tail on the high-energy end of the Cr signal indicates that the external surface of the metallic layer (Cr) was uniform on the totality of the sample. After thermal annealing, both XRD and RBS techniques show the formation of the unique phase CrSi2 that occurs by solid-phase reaction at the Cr/ Si interface. On the XRD diagram of Fig. 1b, one can notice new characteristic peaks corresponding to this chromium disilicide. The formation of CrSi2 silicide is also illustrated in the spectrum (dashed curve) of Fig. 2 corresponding to a sample (5 1014 P+ cm 2 Si (1 1 1)) annealed at 748 K (4751C) for 20 min. In this spectrum, we note a significant overlapping of Si and Cr signals and the formation of a new one corresponding to the CrSi2 phase. This silicide is obtained for both /1 0 0S and /1 1 1S silicon substrate orientations; it is formed at the Cr/Si interface. Besides, if the spectra of Fig. 3a are examined further, one will notice that the thickness of this phase (i.e., CrSi2) increased with the rise of the annealing temperature until the chromium layer is totally consumed.
Fig. 2. RBS spectra of 5 1014 P+ cm 20 min (dashed curve).
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225
Furthermore, for the high temperatures (X748 K (4751C)), one will notice the formation of Cr2O3 chromium oxide (with a thickness of B10 nm) on the top surface of the samples. In addition, the thickness of CrSi2 silicide increased with longer annealing times (Fig. 3b). Up to now, only specimens implanted with 5 1014 P+ cm 2 have been considered. For the 5 1015 P+ cm 2 implanted samples, it has been observed that CrSi2 formation occurs only for high temperature (i.e., 823 K (5501C)). For samples annealed at 723 K (4501C), no silicide formation has been detected, although the samples were maintained at this temperature for 180 min. For anneals performed at 748 K (4751C), chromium disilicide begins to appear only after long time (i.e., 120 or 180 min). Hence, we deduce that the presence of phosphorus in silicon substrates delays the formation of CrSi2 silicide, but does not inhibit it. Besides, it is reported that during CrSi2 formation, phosphorus is transported from Si substrate, across the silicide layer, to the CrSi2/ Cr interface where it is accumulated [7]. Thus, we assume that the delay of the reaction observed for implanted samples is due to P+ redistribution, since CrSi2 formation is governed by the reaction at the CrSi2/Cr interface where phosphorus accumulation occurs. Interestingly, it has been noticed that the growth of chromium disilicide is more widespread on
implanted Cr/Si(1 1 1) samples, non-annealed (solid curve), and annealed at 748 K (4751C) for
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R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
Fig. 3. RBS spectra of 5 1014 P+ cm 2 Cr/Si(1 1 1) samples: (a) annealed, during 30 min, at different temperatures, (b) annealed at 748 K (4751C) for several annealing times.
/1 1 1S oriented silicon substrates than on /1 0 0S oriented ones. This result is verified for both 5 1014 P+ cm 2 (Fig. 4a) and 15 + 2 5 10 P cm (Fig. 4b) implanted specimens. However, for virgin specimens (we do not report the corresponding spectra), the opposite phenomenon is observed, i.e., if the sample is not implanted with phosphorus, the growth is rather more favourable on Si(1 0 0) than on Si(1 1 1). Thus, one can think that the presence of P+ ions in silicon substrates is responsible for this effect. In other words, we suppose that in the case of Si(1 0 0) substrates, during phosphorus redistribution the quantity of P atoms that moved from
silicon matrix to the Cr/Silicide interface was larger than in the Si(1 1 1) case. This is because each Si atom in Si(1 0 0) has fewer nearest neighbours than those in Si(1 1 1). Hence, P+ redistribution from silicon towards the CrSi2/Cr interface will be easier and more prevalent in Cr/ CrSi2/Si(1 0 0) specimens than in Cr/CrSi2/Si(1 1 1) ones. Consequently, the quantity of accumulated P+ ions at CrSi2/Cr interface will be large enough which delays the reaction and the formation of CrSi2 phase especially for Cr/CrSi2/Si(1 0 0) samples. In summary, we have shown that, among the different phases present in the phase diagram of the Cr/Si binary system [8], only the CrSi2 silicide
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
227
Fig. 4. RBS spectra of: (a) Cr/Si(1 0 0) sample (dotted line) implanted with 5 1014 P+ cm 2, annealed at 748 K (4751C) for 20 min; Cr/Si(1 1 1) sample (solid line) implanted and annealed as above, (b) Cr/Si(1 0 0) sample (dotted line) implanted with 5 1015 P+ cm 2, annealed at 823 K (5501C) for 30 min; Cr/Si(1 1 1) sample (solid line) implanted and annealed as above.
is formed. Of all the CrxSiy phases, chromium disilicide exhibits the lowest energy of formation [9] and was predicted by Pretorius [10]. Elsewhere, we have established that the thickness of CrSi2 silicide layers grows with the increase in the annealing time and/or temperature. We have also shown that the presence of phosphorus in a silicon matrix delays the formation of CrSi2, but does not inhibit it. For the high dose of 5 1015 P+ cm 2, we have noticed that the chromium disilicide occurs only for a high temperature (823 K (5501C)) and/or a long (X120 min) annealing time.
Interestingly, for both 5 1014 P+ cm 2 and 5 1015 P+ cm 2 implanted silicon matrix, we have established that the growth of a CrSi2 phase is more rapid on Si(1 1 1) than on Si(1 0 0) substrates. Finally, a very thin layer of Cr2O3 oxide has been detected when the annealing temperature has been high and/or the annealing time has been long. We speculate that this oxidation resulted from residual oxygen traces in the system where the heat treatments were performed. The same phenomenon has been reported in the literature by Martinez et al. [11];
228
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
it can be produced via chromium grain boundaries [12].
4. Conclusion Among all the phases predicted by the phase diagram of the Cr/Si binary system, only the CrSi2 silicide is formed. The thickness layer of this phase increases as the temperature and/or the time of annealing increase. Furthermore, it is shown that the growth of CrSi2 silicide is delayed by the presence of phosphorus in silicon substrates, especially for 5 1015 P+ cm 2 dose. Finally, it is established that for both 5 1014 P+ cm 2 and 5 1015 P+ cm 2 implanted silicon samples, the CrSi2 growth is more rapid on Si(1 1 1) than on Si(1 0 0) substrates, a finding that contrasts with the results for non-implanted specimens.
References [1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. [2] K. Maex, Mater. Sci. Eng. R11 (2–3) (1993). [3] M.-A. Nicolet, S.S. Lau, in: N. Einspruch, G. Larabee (Eds.), VLSI Electronics: Microstructure Science, Academic Press, New York, 1983.(Chap. 6) [4] M.C. Bost, J.E. Mahan, J. Appl. Phys. 63 (1988) 839. [5] C. Heck, M. Kusaka, M. Hirai, M. Iwami, Y. Yokota, Thin Solid Films 281–282 (1996) 94. [6] L.R. Dolittle, Nucl. Inst. and Meth. B 9 (1985) 344. . [7] A. Rockett, J.E. Greene, H. Jiang, M. Ostling, C.S. Petersson, J. Appl. Phys. 64 (8) (1988) 4187. [8] F.A. Shunk, Constitution of Binary Alloys, 2nd Suppl, McGraw-Hill, New York, 1969, p. 280. [9] C.J. Smittels, Metals Reference Book, Butterworths, London, 1976. [10] R. Pretorius, Vacuum 41 (4–6) (1991) 1038. [11] A. Martinez, D. Estev, A. Guivarch, P. Auvray, P. Henoc, G. Pelous, Solid State Electron. 23 (1980) 55. [12] F.M. D’Heurle, P. Gas, J. Philibert, MRS, San Francisco, Spring, 1994.
Elaboration of thin chromium silicide layers on P+ implanted silicon R. Labbani*, R. Halimi, A. Bouabellou, A. Bouguerra Laboratory of Thin Films and Interfaces, Department of Physics, Faculty of Sciences, Mentouri University, Route de Ain El Bey, 25000 Constantine, Algeria
Abstract The aim of the present work is to investigate the mechanism of growth of chromium silicides versus several parameters. The samples are constituted of a thin chromium layer (E80 nm) which is deposited under vacuum by electron beam evaporation/condensation onto P+ implanted Si(1 0 0) and Si(1 1 1) substrates. Phosphorus ion implantation, with a dose of 5 1014 or 5 1015 P+ cm 2, is performed at an energy of 40 keV. The samples are thermally annealed at 723–823 K (450–5501C) for various periods of time. The analysis is carried out by means of X-ray diffraction and H+ Rutherford backscattering spectrometry (RBS) techniques. The RUMP 3.30 software is used to simulate RBS spectra. It is shown that, after annealing treatment, the CrSi2 silicide is formed at Cr/Si(1 0 0) and Cr/Si(1 1 1) interfaces. For one temperature and time of annealing, the thickness of the chromium disilicide is lower for the high phosphorus dose (i.e., 5 1015 P+ cm 2) than for the low dose (i.e., 5 1014P+ cm 2). In other words, the presence of phosphorus in silicon substrates delays the formation of the CrSi2 phase but does not inhibit it. Finally, it is established that the growth of CrSi2 compound is more rapid on silicon substrates with /1 1 1S orientation than on the /1 0 0S orientation, which is not in agreement with the non-implanted specimens case. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 68.55.Ln Keywords: Silicide; Silicon; Chromium; Metal/silicon interface
1. Introduction Silicides of transition metals are intermediate compounds that are very useful in electronic circuit technology with Very Large Scale Integration (VLSI) [1,2]. Indeed, they can be used as a Schottky barrier, diffusion barrier, ohmic contact, in interconnections, etc. They present interesting *Corresponding author. Tel.: +213-4-614711; fax: +213-4923489. E-mail address: [email protected] (R. Labbani).
properties such as: good thermal stability during fabrication of integrated circuits, good compatibility with SiO2 as a substrate, etc. Silicide phases may be formed via different processes [3] like the co-deposition of silicon and metal by sputtering, evaporation or chemical-vapour deposition. The solid-phase reaction at the Metal/Silicon (M/Si) interface is also a method widely used for silicide formation, which requires a thin metal layer deposition on silicon substrate and heating at adequate temperature and/or time. In general, silicides can present either metallic or semiconductor
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 2 0 9 5 - 2
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
characteristics. The CrSi2 phase is a compound of particular interest because it is a semiconductor material with an energy gap of 0.35 eV, which is very suitable for opto-electronic technology [4,5]. In this work, we have studied the interaction phenomenon and the formation of chromium silicide at the interface between a thin chromium layer (80 nm) and a phosphorus-implanted silicon substrate: (Si(1 0 0) or Si(1 1 1)). These effects were investigated as functions of: silicon substrate orientation, applied dose of phosphorus ions and the temperature or time of annealing.
2. Experiment The substrates were commercialized silicon wafers (p type), with /1 0 0S or /1 1 1S orientation, a thickness of B400 mm and a resistivity of 60–80 O cm. Prior to P+ ion implantation, a thin film of SiO2 (25 nm) was deposited by dry oxidation in a gas mixture of O2+HCl. This operation was accomplished in 50 min at 1173 K (9001C). The oxide layer is considered necessary to avoid impurity contamination and any eventual loss of phosphorus during P+ implantation. The ion implantation was performed through the SiO2 oxide layer by the impact of P+ 2 primary ions in the gaseous phase. The energy of phosphorus ions was chosen to be 40 keV, whereas two values of the dose were used: 5 1014 P+ cm 2 or 5 1015 P+ cm 2. For P+ electrical activation, the implanted substrates were then heat treated at 1173 K (9001C) in an oxygen atmosphere for 60 min. Next, the SiO2 layers of implanted and nonimplanted silicon substrates were chemically etched at room temperature in a HF:H2O2=1:10 solution, which removed both the native and the grown SiO2 layers. Thus prepared, the substrates were transferred immediately to a vacuum metaldeposition system where they were maintained at 373 K (1001C) during application of a thin metallic layer (80 nm) of Cr by electron-beam evaporation in vacuum (10 5 Pa, 10 7 Torr). The final preparation procedure was to perform a conventional heat treatment at temperatures varying from 723 to 823 K (4501C to 5501C) in vacuum (10 3 Pa, 10 5 Torr).
To identify the formed phases and to record their growth for specific values of parameters such as the annealing time or temperature, Si crystallographic orientation, and/or a phosphorus dose, Rutherford Backscattering Spectrometry (RBS) and X-ray Diffraction (XRD) techniques were used. The XRD analyses were performed on a vertical diffractometer (Philips 1050/70) with KaCu radiation and a Ni filter. The RBS measurements were performed using H+ particles with 0.3 MeV energy and an electrostatic detector of 2 keV resolution. The angles j (between an incident beam of H+ particles and the detector) and y (between the normal to the sample and incident beam) were chosen to be 451 and 01, respectively. The detection solid angle was equal to 0.456 msr. The obtained RBS spectra were simulated using the software RUMP 3.30 (Rutherford Universal Manipulation Programme) which was developed at the Cornell university (New York) [6].
3. Results and discussion During chromium deposition, XRD analysis revealed that no silicide phases had been formed. Indeed, the diagram of Fig. 1a (corresponding to non-annealed sample) is formed by characteristic peaks of Si and Cr only. Furthermore, no peaks of Si (100)
(b)
CrSi2 (111)
Intensity (arb. Units)
224
CrSi2(110)
Cr (110)
Si (100)
(a)
Cr (110)
30
40
50
60
70
80
Bragg angle 2θ (°)
Fig. 1. XRD spectra of 5 1014 P+ cm 2 implanted Cr/Si samples. non-annealed, annealed at 748 K (4751C) during 30 min.
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
silicon or chromium oxides were present in this spectrum, suggesting a low level of contamination in the prepared specimens. The same results were confirmed by the RBS technique, as illustrated in Fig. 2. Indeed, the spectrum corresponding to a non-annealed sample (solid curve) is formed only by Si and Cr signals. Furthermore, the absence of a tail on the high-energy end of the Cr signal indicates that the external surface of the metallic layer (Cr) was uniform on the totality of the sample. After thermal annealing, both XRD and RBS techniques show the formation of the unique phase CrSi2 that occurs by solid-phase reaction at the Cr/ Si interface. On the XRD diagram of Fig. 1b, one can notice new characteristic peaks corresponding to this chromium disilicide. The formation of CrSi2 silicide is also illustrated in the spectrum (dashed curve) of Fig. 2 corresponding to a sample (5 1014 P+ cm 2 Si (1 1 1)) annealed at 748 K (4751C) for 20 min. In this spectrum, we note a significant overlapping of Si and Cr signals and the formation of a new one corresponding to the CrSi2 phase. This silicide is obtained for both /1 0 0S and /1 1 1S silicon substrate orientations; it is formed at the Cr/Si interface. Besides, if the spectra of Fig. 3a are examined further, one will notice that the thickness of this phase (i.e., CrSi2) increased with the rise of the annealing temperature until the chromium layer is totally consumed.
Fig. 2. RBS spectra of 5 1014 P+ cm 20 min (dashed curve).
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225
Furthermore, for the high temperatures (X748 K (4751C)), one will notice the formation of Cr2O3 chromium oxide (with a thickness of B10 nm) on the top surface of the samples. In addition, the thickness of CrSi2 silicide increased with longer annealing times (Fig. 3b). Up to now, only specimens implanted with 5 1014 P+ cm 2 have been considered. For the 5 1015 P+ cm 2 implanted samples, it has been observed that CrSi2 formation occurs only for high temperature (i.e., 823 K (5501C)). For samples annealed at 723 K (4501C), no silicide formation has been detected, although the samples were maintained at this temperature for 180 min. For anneals performed at 748 K (4751C), chromium disilicide begins to appear only after long time (i.e., 120 or 180 min). Hence, we deduce that the presence of phosphorus in silicon substrates delays the formation of CrSi2 silicide, but does not inhibit it. Besides, it is reported that during CrSi2 formation, phosphorus is transported from Si substrate, across the silicide layer, to the CrSi2/ Cr interface where it is accumulated [7]. Thus, we assume that the delay of the reaction observed for implanted samples is due to P+ redistribution, since CrSi2 formation is governed by the reaction at the CrSi2/Cr interface where phosphorus accumulation occurs. Interestingly, it has been noticed that the growth of chromium disilicide is more widespread on
implanted Cr/Si(1 1 1) samples, non-annealed (solid curve), and annealed at 748 K (4751C) for
226
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Fig. 3. RBS spectra of 5 1014 P+ cm 2 Cr/Si(1 1 1) samples: (a) annealed, during 30 min, at different temperatures, (b) annealed at 748 K (4751C) for several annealing times.
/1 1 1S oriented silicon substrates than on /1 0 0S oriented ones. This result is verified for both 5 1014 P+ cm 2 (Fig. 4a) and 15 + 2 5 10 P cm (Fig. 4b) implanted specimens. However, for virgin specimens (we do not report the corresponding spectra), the opposite phenomenon is observed, i.e., if the sample is not implanted with phosphorus, the growth is rather more favourable on Si(1 0 0) than on Si(1 1 1). Thus, one can think that the presence of P+ ions in silicon substrates is responsible for this effect. In other words, we suppose that in the case of Si(1 0 0) substrates, during phosphorus redistribution the quantity of P atoms that moved from
silicon matrix to the Cr/Silicide interface was larger than in the Si(1 1 1) case. This is because each Si atom in Si(1 0 0) has fewer nearest neighbours than those in Si(1 1 1). Hence, P+ redistribution from silicon towards the CrSi2/Cr interface will be easier and more prevalent in Cr/ CrSi2/Si(1 0 0) specimens than in Cr/CrSi2/Si(1 1 1) ones. Consequently, the quantity of accumulated P+ ions at CrSi2/Cr interface will be large enough which delays the reaction and the formation of CrSi2 phase especially for Cr/CrSi2/Si(1 0 0) samples. In summary, we have shown that, among the different phases present in the phase diagram of the Cr/Si binary system [8], only the CrSi2 silicide
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
227
Fig. 4. RBS spectra of: (a) Cr/Si(1 0 0) sample (dotted line) implanted with 5 1014 P+ cm 2, annealed at 748 K (4751C) for 20 min; Cr/Si(1 1 1) sample (solid line) implanted and annealed as above, (b) Cr/Si(1 0 0) sample (dotted line) implanted with 5 1015 P+ cm 2, annealed at 823 K (5501C) for 30 min; Cr/Si(1 1 1) sample (solid line) implanted and annealed as above.
is formed. Of all the CrxSiy phases, chromium disilicide exhibits the lowest energy of formation [9] and was predicted by Pretorius [10]. Elsewhere, we have established that the thickness of CrSi2 silicide layers grows with the increase in the annealing time and/or temperature. We have also shown that the presence of phosphorus in a silicon matrix delays the formation of CrSi2, but does not inhibit it. For the high dose of 5 1015 P+ cm 2, we have noticed that the chromium disilicide occurs only for a high temperature (823 K (5501C)) and/or a long (X120 min) annealing time.
Interestingly, for both 5 1014 P+ cm 2 and 5 1015 P+ cm 2 implanted silicon matrix, we have established that the growth of a CrSi2 phase is more rapid on Si(1 1 1) than on Si(1 0 0) substrates. Finally, a very thin layer of Cr2O3 oxide has been detected when the annealing temperature has been high and/or the annealing time has been long. We speculate that this oxidation resulted from residual oxygen traces in the system where the heat treatments were performed. The same phenomenon has been reported in the literature by Martinez et al. [11];
228
R. Labbani et al. / Nuclear Instruments and Methods in Physics Research A 480 (2002) 223–228
it can be produced via chromium grain boundaries [12].
4. Conclusion Among all the phases predicted by the phase diagram of the Cr/Si binary system, only the CrSi2 silicide is formed. The thickness layer of this phase increases as the temperature and/or the time of annealing increase. Furthermore, it is shown that the growth of CrSi2 silicide is delayed by the presence of phosphorus in silicon substrates, especially for 5 1015 P+ cm 2 dose. Finally, it is established that for both 5 1014 P+ cm 2 and 5 1015 P+ cm 2 implanted silicon samples, the CrSi2 growth is more rapid on Si(1 1 1) than on Si(1 0 0) substrates, a finding that contrasts with the results for non-implanted specimens.
References [1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. [2] K. Maex, Mater. Sci. Eng. R11 (2–3) (1993). [3] M.-A. Nicolet, S.S. Lau, in: N. Einspruch, G. Larabee (Eds.), VLSI Electronics: Microstructure Science, Academic Press, New York, 1983.(Chap. 6) [4] M.C. Bost, J.E. Mahan, J. Appl. Phys. 63 (1988) 839. [5] C. Heck, M. Kusaka, M. Hirai, M. Iwami, Y. Yokota, Thin Solid Films 281–282 (1996) 94. [6] L.R. Dolittle, Nucl. Inst. and Meth. B 9 (1985) 344. . [7] A. Rockett, J.E. Greene, H. Jiang, M. Ostling, C.S. Petersson, J. Appl. Phys. 64 (8) (1988) 4187. [8] F.A. Shunk, Constitution of Binary Alloys, 2nd Suppl, McGraw-Hill, New York, 1969, p. 280. [9] C.J. Smittels, Metals Reference Book, Butterworths, London, 1976. [10] R. Pretorius, Vacuum 41 (4–6) (1991) 1038. [11] A. Martinez, D. Estev, A. Guivarch, P. Auvray, P. Henoc, G. Pelous, Solid State Electron. 23 (1980) 55. [12] F.M. D’Heurle, P. Gas, J. Philibert, MRS, San Francisco, Spring, 1994.