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Thermal stability of cobalt and nickel silicides
PERGAMON
Microelectronics Reliability 38 (1998) 1495±1498
Thermal stability of cobalt and nickel silicides M.C. Poon a, *, C.H. Ho a, F. Deng b, S.S. Lau b, H. Wong c a
Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong b Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093-0407, U.S.A. c Department of Electronic Engineering, City University of Hong Kong, Kowloontong, Hong Kong Received 9 October 1997; in revised form 16 March 1998
Abstract Thermal stability of cobalt and nickel silicides on crystalline Si (c-Si) and amorphous Si (a-Si) has been investigated. We have found that CoSi2 is thermally stable on a-Si and c-Si substrates up to 9508C for 30 min. NiSi is stable and shows low resistivity on c-Si at around 7008C for 30 min, but is unstable on a-Si substrate even after annealing at 4008C. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Metal silicides have been widely used in microelectronic applications, particularly as contact material to reduce the series resistance of source, drain and gate regions in MOSFETs. Among all silicides, cobalt-disilicide (CoSi2) and nickel-monosilicide (NiSi) have been demonstrated to be two of the most promising silicide materials for future ULSI, thin ®lm transistor (TFT) and novel devices [1±5]. They have the advantages of having the lowest resistivities (015 mO.cm), good thermal stability (up to 700±9008C), low formation temperature (0400±6008C), and little or no resistivity degradation on narrow lines/gates. Moreover, for CoSi2, it has less lateral gate-source/drain silicide overgrowth, good resistance to HF and plasma etching, and does not react with oxide below 9008C. For NiSi, it has the advantages of less Si consumption (important for device applications on ultra-thin Si layers of 0500 AÊ or less), no reaction with N2, and a simple single-step annealing. This paper aims to provide a ®rst study to explore and compare the thermal stability and process windows of NiSi and CoSi2 in amorphous
* To whom correspondence should be addressed.
Si (a-Si) and single-crystalline Si (c-Si) substrates after 30 min long annealing.
2. Experimental The a-Si layers of 1900 AÊ thick were CVD-deposited at 4608C onto thermally grown SiO2 of 1500 AÊ thick on h100i Si wafers (20 O.cm, p-type, B-doped). About 300 AÊ of Ni ®lm and 210 AÊ of Co ®lm were deposited separately by electron beam evaporation onto the surface of the a-Si and c-Si samples. The evaporation rate was about 5 AÊ/s in a vacuum greater than 5 10 ÿ 7 Torr (base pressure was 05 10 ÿ 8 Torr). The ®lm thickness was measured by a crystal thickness monitor which was calibrated by Rutherford backscattering spectrometry (RBS). After evaporation, the thermal stability test of NiSi on a-Si and c-Si substrates was carried out by furnace annealing of 400±8008C (in steps of 508C) for 30 min in a nitrogen and forming gas ambient. CoSi2 on a-Si and c-Si substrates were annealed at 500±10008C (in steps of 508C) for 30 min. RBS measurements with 2.3 MeV He++ ions with a scattering angle of 1708 were used for the compositional investigations. The sheet resistance was measured with the Van der Pauw con®guration.
0026-2714/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 8 ) 0 0 0 4 5 - 6
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Fig. 1. Sheet resistance vs annealing temperature for cobalt silicide on single crystalline and amorphous silicon substrates.
3. Results and discussion Fig. 1 shows the sheet resistance of cobalt silicide on a-Si and c-Si substrates after annealing at 450±10008C for 30 min. After the 4508C annealing, the sheet resistances of a-Si and c-Si are around 140 and 114 O/sq, respectively. For a-Si, the resistance decreases to 9.0 and 3.0 O/sq after 500 and 5508C annealing, and remains almost constant with the temperature after 550±9508C annealing. After 10008C annealing, the resistance increases to about 8.2 O/sq. For c-Si, the resistance decreases to about 54, 3.0, 2.6 and 2.2 O/sq after 500, 550, 600 and 6508C annealing, respectively, and remains almost constant with temperature for 650±9508C annealing. After 10008C annealing, the resistance increases slightly to 2.6 O/sq. Fig. 2 compares the RBS spectra for cobalt silicide on a-Si and c-Si substrates after 5508C and 30 min annealing. For samples with a-Si layers [Fig. 2(a)], from the RBS computer ®t (depth uncertainty 05%), the layer structure is found to be CoSi2±Si±SiO2 and the thicknesses are around 750±1050±1500 AÊ. For c-Si samples [Fig. 2(b)], the silicide is still a mixture of CoSi and CoSi2. CoSi has not been fully transformed into CoSi2. Nevertheless, the entire silicide layer becomes CoSi2 after annealing at 6008C. Both samples on a-Si and c-Si then retain similar RBS spectra and the CoSi2 structure remains after annealing at 600±9508C. The annealing results show that CoSi2 is thermally stable on both c-Si and a-Si even after 9508C for 30 min; these results are in agreement with the reported experimental resistivity (015 mO.cm) and thermal stability of CoSi2 on c-Si [1, 2]. It is known that for Co ®lm on c-Si, Co reacts with Si at approximately 200±3008C to form Co2Si. At around 5008C,
CoSi forms by consuming more Si. CoSi is stable on c-Si up to around 5508C and CoSi2, the low resistivity phase, forms at around 6008C. At around 10008C, the resistance increases due to oxidation of the CoSi2 layer and agglomeration. The formation of the CoSi2 layer for a-Si at 5508C is in agreement with the reported result by Xiao et al. [4], where CoSi2 can be formed at lower temperature (by about 508C) on a-Si compared with c-Si, due to the enhanced nucleation of CoSi2 on a-Si [5]. Fig. 3 compares the sheet resistance of nickel silicide on a-Si and c-Si substrates after annealing at 250±8008C for 30 min. After the 2508C annealing, the resistance of a-Si and c-Si are around 12.5 and 8.6
Fig. 2. RBS spectra for cobalt silicide on (a) amorphous silicon (o curve) and (b) crystalline silicon (- curve) after 5508C and 30 min annealing.
M.C. Poon et al. / Microelectronics Reliability 38 (1998) 1495±1498
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Fig. 3. Sheet resistance vs annealing temperature for nickel silicide on single crystalline and amorphous silicon substrates.
O/sq, respectively. For a-Si, the resistance decreases with the temperature and is around 5.7, 3.5 and 3.5 O/sq after 300, 350 and 4008C annealing, respectively, and increases to 6.7, 6.3, 13.2 and 16 O/sq after 500, 600, 700 and 8008C annealing, respectively. For c-Si, the sheet resistance decreases to 4.9, 3.1, 2.4 after 300, 350, 4008C annealing, respectively, and remains almost constant with temperature at around 2.4 O/sq after 450±6508C annealing. The resistance then increases gradually with temperature and is around 3.7 O/sq after 700±8008C annealing. Fig. 4 compares the RBS spectra for nickel silicide on a-Si and c-Si substrates after 5008C annealing. For c-Si [Fig. 4(a)], using computer ®t, the silicide is NiSi and the thickness is around 700 AÊ. The spectrum is similar to the spectra after annealing at 400±7508C. For a-Si [Fig. 4(b)], however, the layer is found to be a mixture of NiSi,
NiSi2 and poly-Si instead of a distinct NiSi/Si structure. The experimental results show that NiSi is thermally stable on c-Si up to 7508C for 30 min. No report has been found on the thermal stability of NiSi on c-Si after prolonged annealing. It is known that for Ni ®lm on c-Si, Ni reacts with Si at about 2508C to form Ni2Si. At around 3508C, NiSi forms by consuming more Si. NiSi is stable on c-Si up to around 7008C, and NiSi2, the high resistivity phase, forms at around 7508C [2, 3]. However, NiSi is unstable on a-Si even after a 4008C/30 min annealing. Formation of NiSi2 at low temperature has been reported by Erokhin and co-workers in their study of Ni in ion implanted amorphized Si [6], but no report has been found on Ni in CVD a-Si and its thermal stability after long time annealing. Recently, NiSi has been applied using metal induced lateral crystallization technology to obtain high mobility a-Si TFT after 4508C/30 h annealing [7]. The instability of NiSi in a-Si might suggest possible diusion of Ni in the TFT channel, and might aect the performance of TFT and the use of NiSi in a-Si applications.
4. Conclusion
Fig. 4. RBS spectra for nickel silicide on (a) crystalline silicon (- curve) and (b) amorphous silicon (o curve) after 5008C and 30 min annealing.
In summary, we have found that CoSi2 is stable on a-Si and c-Si substrates, and shows low resistivity after 9508C and 30 min annealing. NiSi is stable and shows low resistivity on c-Si after around 7008C and 30 min annealing, but is unstable on a-Si substrate even after 4008C annealing. No distinct NiSi/Si structure is formed and both NiSi and NiSi2 are formed and mixed with the Si substrate. Results might suggest that CoSi2 is a better silicide material for a-Si device applications.
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M.C. Poon et al. / Microelectronics Reliability 38 (1998) 1495±1498
Acknowledgements This work was sponsored by the research grant (grant number HKUST 564/94E) of the Research Grant Council of Hong Kong. The work performed at UCSD was sponsored by the US National Science Foundation. The authors would like to thank J. K. O. Sin of the Hong Kong University of Science and Technology, and D. J. Qiao of the University of California of San Diego, for their technical help and valuable discussions.
References [1] Philips JR, Revesz P, Olowolafe JO, Mayer JW. Thermal stability of CoSi2 on single crystal and polycrystalline silicon. In: Materials Research Society Proceedings, vol. 182. 1990:57±63.
[2] Colgan EG, Gambino JP, Gambino P. Formation and stability of silicides on polycrystalline silicon. Mater Sci Engng Rev Reports 1996;R16(2):43±96. [3] Colgan EG, Gambino JP, Cunningham B. Nickel silicide thermal stability on polycrystalline and single crystalline silicon. Mater Chem Phys 1996;46:209±14. [4] Xiao ZG, Honeycutt JW, Rozgonyi GA. Interface reaction and atomic transport during CoSi2 ®lm formation. Mat Res Soc Symp 1991;202:259. [5] Chen WM, Banerjee SK, Lee JC. Degradation mechanisms and improvement of thermal stability of CoSi2 / polycrystalline Si layers. Appl Phys Lett 1994;64(12):1505±507. [6] Erokhin YN, Hong F, Pramanick S, Rozgonyi GA, Patnaik BK, White CW. Spatially con®ned nickel disilicide formation at 4008c on ion implantation preamorphized silicon. Appl Phys Lett 1993;63:3173. [7] Lee SW, Ihn TH, Joo SK. Fabrication of high-mobility p-channel poly-Si thin ®lm transistor by self-aligned metal-induced lateral crystallization. IEEE Electron Device Lett 1996;17(8):407±09.
Microelectronics Reliability 38 (1998) 1495±1498
Thermal stability of cobalt and nickel silicides M.C. Poon a, *, C.H. Ho a, F. Deng b, S.S. Lau b, H. Wong c a
Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong b Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093-0407, U.S.A. c Department of Electronic Engineering, City University of Hong Kong, Kowloontong, Hong Kong Received 9 October 1997; in revised form 16 March 1998
Abstract Thermal stability of cobalt and nickel silicides on crystalline Si (c-Si) and amorphous Si (a-Si) has been investigated. We have found that CoSi2 is thermally stable on a-Si and c-Si substrates up to 9508C for 30 min. NiSi is stable and shows low resistivity on c-Si at around 7008C for 30 min, but is unstable on a-Si substrate even after annealing at 4008C. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Metal silicides have been widely used in microelectronic applications, particularly as contact material to reduce the series resistance of source, drain and gate regions in MOSFETs. Among all silicides, cobalt-disilicide (CoSi2) and nickel-monosilicide (NiSi) have been demonstrated to be two of the most promising silicide materials for future ULSI, thin ®lm transistor (TFT) and novel devices [1±5]. They have the advantages of having the lowest resistivities (015 mO.cm), good thermal stability (up to 700±9008C), low formation temperature (0400±6008C), and little or no resistivity degradation on narrow lines/gates. Moreover, for CoSi2, it has less lateral gate-source/drain silicide overgrowth, good resistance to HF and plasma etching, and does not react with oxide below 9008C. For NiSi, it has the advantages of less Si consumption (important for device applications on ultra-thin Si layers of 0500 AÊ or less), no reaction with N2, and a simple single-step annealing. This paper aims to provide a ®rst study to explore and compare the thermal stability and process windows of NiSi and CoSi2 in amorphous
* To whom correspondence should be addressed.
Si (a-Si) and single-crystalline Si (c-Si) substrates after 30 min long annealing.
2. Experimental The a-Si layers of 1900 AÊ thick were CVD-deposited at 4608C onto thermally grown SiO2 of 1500 AÊ thick on h100i Si wafers (20 O.cm, p-type, B-doped). About 300 AÊ of Ni ®lm and 210 AÊ of Co ®lm were deposited separately by electron beam evaporation onto the surface of the a-Si and c-Si samples. The evaporation rate was about 5 AÊ/s in a vacuum greater than 5 10 ÿ 7 Torr (base pressure was 05 10 ÿ 8 Torr). The ®lm thickness was measured by a crystal thickness monitor which was calibrated by Rutherford backscattering spectrometry (RBS). After evaporation, the thermal stability test of NiSi on a-Si and c-Si substrates was carried out by furnace annealing of 400±8008C (in steps of 508C) for 30 min in a nitrogen and forming gas ambient. CoSi2 on a-Si and c-Si substrates were annealed at 500±10008C (in steps of 508C) for 30 min. RBS measurements with 2.3 MeV He++ ions with a scattering angle of 1708 were used for the compositional investigations. The sheet resistance was measured with the Van der Pauw con®guration.
0026-2714/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 8 ) 0 0 0 4 5 - 6
1496
M.C. Poon et al. / Microelectronics Reliability 38 (1998) 1495±1498
Fig. 1. Sheet resistance vs annealing temperature for cobalt silicide on single crystalline and amorphous silicon substrates.
3. Results and discussion Fig. 1 shows the sheet resistance of cobalt silicide on a-Si and c-Si substrates after annealing at 450±10008C for 30 min. After the 4508C annealing, the sheet resistances of a-Si and c-Si are around 140 and 114 O/sq, respectively. For a-Si, the resistance decreases to 9.0 and 3.0 O/sq after 500 and 5508C annealing, and remains almost constant with the temperature after 550±9508C annealing. After 10008C annealing, the resistance increases to about 8.2 O/sq. For c-Si, the resistance decreases to about 54, 3.0, 2.6 and 2.2 O/sq after 500, 550, 600 and 6508C annealing, respectively, and remains almost constant with temperature for 650±9508C annealing. After 10008C annealing, the resistance increases slightly to 2.6 O/sq. Fig. 2 compares the RBS spectra for cobalt silicide on a-Si and c-Si substrates after 5508C and 30 min annealing. For samples with a-Si layers [Fig. 2(a)], from the RBS computer ®t (depth uncertainty 05%), the layer structure is found to be CoSi2±Si±SiO2 and the thicknesses are around 750±1050±1500 AÊ. For c-Si samples [Fig. 2(b)], the silicide is still a mixture of CoSi and CoSi2. CoSi has not been fully transformed into CoSi2. Nevertheless, the entire silicide layer becomes CoSi2 after annealing at 6008C. Both samples on a-Si and c-Si then retain similar RBS spectra and the CoSi2 structure remains after annealing at 600±9508C. The annealing results show that CoSi2 is thermally stable on both c-Si and a-Si even after 9508C for 30 min; these results are in agreement with the reported experimental resistivity (015 mO.cm) and thermal stability of CoSi2 on c-Si [1, 2]. It is known that for Co ®lm on c-Si, Co reacts with Si at approximately 200±3008C to form Co2Si. At around 5008C,
CoSi forms by consuming more Si. CoSi is stable on c-Si up to around 5508C and CoSi2, the low resistivity phase, forms at around 6008C. At around 10008C, the resistance increases due to oxidation of the CoSi2 layer and agglomeration. The formation of the CoSi2 layer for a-Si at 5508C is in agreement with the reported result by Xiao et al. [4], where CoSi2 can be formed at lower temperature (by about 508C) on a-Si compared with c-Si, due to the enhanced nucleation of CoSi2 on a-Si [5]. Fig. 3 compares the sheet resistance of nickel silicide on a-Si and c-Si substrates after annealing at 250±8008C for 30 min. After the 2508C annealing, the resistance of a-Si and c-Si are around 12.5 and 8.6
Fig. 2. RBS spectra for cobalt silicide on (a) amorphous silicon (o curve) and (b) crystalline silicon (- curve) after 5508C and 30 min annealing.
M.C. Poon et al. / Microelectronics Reliability 38 (1998) 1495±1498
1497
Fig. 3. Sheet resistance vs annealing temperature for nickel silicide on single crystalline and amorphous silicon substrates.
O/sq, respectively. For a-Si, the resistance decreases with the temperature and is around 5.7, 3.5 and 3.5 O/sq after 300, 350 and 4008C annealing, respectively, and increases to 6.7, 6.3, 13.2 and 16 O/sq after 500, 600, 700 and 8008C annealing, respectively. For c-Si, the sheet resistance decreases to 4.9, 3.1, 2.4 after 300, 350, 4008C annealing, respectively, and remains almost constant with temperature at around 2.4 O/sq after 450±6508C annealing. The resistance then increases gradually with temperature and is around 3.7 O/sq after 700±8008C annealing. Fig. 4 compares the RBS spectra for nickel silicide on a-Si and c-Si substrates after 5008C annealing. For c-Si [Fig. 4(a)], using computer ®t, the silicide is NiSi and the thickness is around 700 AÊ. The spectrum is similar to the spectra after annealing at 400±7508C. For a-Si [Fig. 4(b)], however, the layer is found to be a mixture of NiSi,
NiSi2 and poly-Si instead of a distinct NiSi/Si structure. The experimental results show that NiSi is thermally stable on c-Si up to 7508C for 30 min. No report has been found on the thermal stability of NiSi on c-Si after prolonged annealing. It is known that for Ni ®lm on c-Si, Ni reacts with Si at about 2508C to form Ni2Si. At around 3508C, NiSi forms by consuming more Si. NiSi is stable on c-Si up to around 7008C, and NiSi2, the high resistivity phase, forms at around 7508C [2, 3]. However, NiSi is unstable on a-Si even after a 4008C/30 min annealing. Formation of NiSi2 at low temperature has been reported by Erokhin and co-workers in their study of Ni in ion implanted amorphized Si [6], but no report has been found on Ni in CVD a-Si and its thermal stability after long time annealing. Recently, NiSi has been applied using metal induced lateral crystallization technology to obtain high mobility a-Si TFT after 4508C/30 h annealing [7]. The instability of NiSi in a-Si might suggest possible diusion of Ni in the TFT channel, and might aect the performance of TFT and the use of NiSi in a-Si applications.
4. Conclusion
Fig. 4. RBS spectra for nickel silicide on (a) crystalline silicon (- curve) and (b) amorphous silicon (o curve) after 5008C and 30 min annealing.
In summary, we have found that CoSi2 is stable on a-Si and c-Si substrates, and shows low resistivity after 9508C and 30 min annealing. NiSi is stable and shows low resistivity on c-Si after around 7008C and 30 min annealing, but is unstable on a-Si substrate even after 4008C annealing. No distinct NiSi/Si structure is formed and both NiSi and NiSi2 are formed and mixed with the Si substrate. Results might suggest that CoSi2 is a better silicide material for a-Si device applications.
1498
M.C. Poon et al. / Microelectronics Reliability 38 (1998) 1495±1498
Acknowledgements This work was sponsored by the research grant (grant number HKUST 564/94E) of the Research Grant Council of Hong Kong. The work performed at UCSD was sponsored by the US National Science Foundation. The authors would like to thank J. K. O. Sin of the Hong Kong University of Science and Technology, and D. J. Qiao of the University of California of San Diego, for their technical help and valuable discussions.
References [1] Philips JR, Revesz P, Olowolafe JO, Mayer JW. Thermal stability of CoSi2 on single crystal and polycrystalline silicon. In: Materials Research Society Proceedings, vol. 182. 1990:57±63.
[2] Colgan EG, Gambino JP, Gambino P. Formation and stability of silicides on polycrystalline silicon. Mater Sci Engng Rev Reports 1996;R16(2):43±96. [3] Colgan EG, Gambino JP, Cunningham B. Nickel silicide thermal stability on polycrystalline and single crystalline silicon. Mater Chem Phys 1996;46:209±14. [4] Xiao ZG, Honeycutt JW, Rozgonyi GA. Interface reaction and atomic transport during CoSi2 ®lm formation. Mat Res Soc Symp 1991;202:259. [5] Chen WM, Banerjee SK, Lee JC. Degradation mechanisms and improvement of thermal stability of CoSi2 / polycrystalline Si layers. Appl Phys Lett 1994;64(12):1505±507. [6] Erokhin YN, Hong F, Pramanick S, Rozgonyi GA, Patnaik BK, White CW. Spatially con®ned nickel disilicide formation at 4008c on ion implantation preamorphized silicon. Appl Phys Lett 1993;63:3173. [7] Lee SW, Ihn TH, Joo SK. Fabrication of high-mobility p-channel poly-Si thin ®lm transistor by self-aligned metal-induced lateral crystallization. IEEE Electron Device Lett 1996;17(8):407±09.