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UV-assisted nickel-induced crystallization of amorphous silicon
Thin Solid Films 383 Ž2001. 241᎐243
UV-assisted nickel-induced crystallization of amorphous silicon Ali KhakifiroozU , Saber Haji, S. Shamsoddin Mohajerzadeh Electrical and Computer Engineering Department, Uni¨ ersity of Tehran, North Kargar A¨ e., Tehran 14399, Iran
Abstract
˚ A novel UV-assisted metal-induced crystallization method is reported. A 1000-A-thick amorphous silicon film is deposited onto ˚ a 150-m-thick glass substrate at a temperature of 400⬚C using electron beam evaporation. A 50-A-thick Ni layer is then e-beam deposited to act as the crystallization seed. UV exposure is exploited as an external source of energy to lower the re-crystallization temperature during post-treatment at 400⬚C. Samples exposed to UV during this post-treatment show crystalline structure as confirmed with XRD and SEM analyses, while samples annealed without UV exposure remain amorphous. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal-induced crystallization; UV-exposure; Annealing; Polysilicon; Amorphous silicon
1. Introduction Metal-induced crystallization is a promising technique for low-temperature fabrication of polysilicon thin-film transistors needed for large area applications. Lateral growth of polycrystalline silicon mediated by the formation of nickel silicide, has been successfully employed to fabricate high-performance TFTs at 500⬚C w1x. Carrier mobilities as high as 120 cm2rVs have been achieved using MILC on oxidized silicon wafers w1x. Also, the fabrication of thin film transistors on glass has been reported at a temperature of 450⬚C w2x. However, such temperatures are still needed to be reduced for ordinary glasses. Efforts have also been made to lower the crystallization temperature by incorporating metals other than Ni. Metals such as Pd w3x, Au w4x, Ag w5x and Al w6x can be used for this purpose. In addition, double-metal induced lateral crystallization has been reported in which Ni and Pd pads are placed adjacent w7x. The effect of an electric field on the crystallization of a-Si has also been studied indicating an improved
U
Corresponding author. Tel.: q98-21-8011235; fax: q98-218011235. E-mail address: [email protected] ŽA. Khakifirooz..
rate of lateral growth w8x. Most recently, a lateral growth rate of approximately 500 mrh at 400⬚C has been observed with an electric field of 100 Vrcm and in the presence of an electric current w9x. In this paper we report for the first time, a novel UV-assisted metal-induced crystallization of amorphous silicon on ordinary glass at a record temperature of 400⬚C. The post-treatment of the samples is achieved through exposure to UV light while the substrate is heated to a temperature of 400⬚C using a hot plate. The UV photons seem to impart energy to the lattice, enhancing the possibility of crystallization at reduced substrate temperatures. 2. Experimental One hundred and fifty micrometers thick glass substrates are cleaned using RCA噛1 solution followed by rinsing in DI water and drying with N2 . After reaching ˚ a base pressure of 10y6 torr, a 1000-A-thick amorphous silicon film is e-beam deposited at a substrate temperature of 400⬚C. For this evaporation step 99.999% pure silicon charge is used. The sample is then allowed to cool down without breaking the vac˚ is uum and a layer of nickel with a thickness of 50 A deposited by e-beam evaporation. After unloading the
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 1 3 - 8
A. Khakifirooz et al. r Thin Solid Films 383 (2001) 241᎐243
242
samples from the vacuum chamber, they are annealed on a hot plate while exposed to UV radiation of a 400-W UV-lamp. Fig. 1 presents a schematic diagram of the post-treatment experimental setup. The UV lamp has a distance between 10 and 20 cm away from the top of the hot plate. The effect of UV exposure on the crystallinity of the samples has been studied using XRD and SEM techniques. 3. Results and discussion The morphology of the samples have been studied using SEM. Fig. 2 shows SEM micrographs of the samples annealed at different conditions. Fig. 2a shows the result for the sample annealed at 400⬚C without exposure to UV light. As seen in this figure, no crystallization is observed for this sample. Fig. 2b,c corresponds to the samples prepared under the same conditions but different UV exposures during annealing. The micrograph in Fig. 2b belongs to the sample annealed in the presence of UV light with a distance of 10 cm between the source and the hot-plate. The duration of this post-treatment has been 12 h for all samples. The features on the sample surface are believed to be crystal grains. Comparing this micrograph with the one in Fig. 2a, indicates the usefulness of the UV exposure in obtaining polysilicon films on glass substrates. Fig. 2c displays the result of a sample exposed to UV radiation during annealing while the UV light source was 20 cm away from the hot plate. Although the duration of the annealing in these samples is the same, the density and size of the grains are less significant. The intensity of the UV light was measured at a wavelength of 365 nm and the values found to be 10 and 2.5 mWrcm2 corresponding to a distance of 10 and 20 cm, respectively. The crystallinity of the annealed silicon films has been further investigated by means of X-ray diffrac-
Fig. 2. SEM micrograph of the samples annealed with different UV exposures: Ža. without UV, Žb. 10 mWrcm2 UV, and Žc. 2.5 mWrcm2 UV. All samples were annealed at 400⬚C for 12 h.
Fig. 1. Schematic diagram of experimental setup.
tion. Fig. 3 collects the spectra of the samples discussed in the previous figure. The spectrum Ža. in this figure
A. Khakifirooz et al. r Thin Solid Films 383 (2001) 241᎐243
243
at low temperatures, one can use such films for the fabrication of thin film transistors on glass. We have also observed lateral crystallization in the presence of UV-exposure with a rate of approximately 1 mrh. This recent observation is being investigated and will be presented elsewhere. Acknowledgements The assistance of Dr A. Miri and Mr M. Mozafari is cordially acknowledged. This work has been carried out in Thin Films Laboratory of the University of Tehran. References
Fig. 3. XRD spectra of the samples annealed with different UV exposure: Ža. 10 mWrcm2 and Žb. 2.5 mWrcm2 .
corresponds to the sample annealed in the presence of UV exposure with the UV light source located 10 cm away from the hot plate while the spectrum Žb. belongs to the sample whose annealing has been performed with the UV source 20 cm away. As can be seen from this figure, both samples show a polycrystalline structure. In curve Žb. two significant peaks are observed corresponding to ²111: and ²110: crystal orientations, while curve Ža. shows one major peak at an orientation of ²110: and the other orientation is less observable. The results provided in this figure, confirm the crystallinity of the samples as observed from the previous figure. Although the difference in the XRD results for these two samples is not well understood, we speculate that the sample prepared with lower UV intensity is more similar to those crystallized without UV exposure and at higher temperatures Žsee w10x.. The hump in the spectra is believed to be due to amorphous nature of the glass substrate. The spectrum of the sample without UV exposure indicates its amorphous nature and it is not provided here. 4. Conclusion In conclusion, we have successfully fabricated polysilicon films on ordinary glass substrate at a record temperature of 400⬚C using UV exposure during re-crystallization. The samples prepared using this UV-assisted re-growth, show dominant orientations of ²110: and ²111:. We believe that the exposure of the samples to a UV light source impart energy to the silicon film, which in turn can compensate the reduction in the substrate temperature. Since crystallization can occur
w1x S.W. Lee, S.K. Joo, IEEE Electron. Device Lett. 17 Ž1996. 160. w2x T.-K. Kim, B.-I. Lee, K.-H. Kim, J.-W. Shin, P.-S. Ahn, W.-C. Jeong, S.-K. Joo, Proc. 4th Asian Symp. Information Display, Ž1997., 61. w3x S.-W. Lee, Y.-C Jeon, S.-K. Joo, Appl. Phys. Lett. 66 Ž1995. 1671. w4x S.-Y. Yoon, J.-Y. Oh, C.-O. Kim, J. Jang, Solid-State Comm. 106 Ž1998. 325. w5x B. Bian, J. Yie, B. Li, Z. Wu, J. Appl. Phys. 73 Ž1993. 7402. w6x A.E. Robertson, L.G. Hultman, H.T.G. Hentzell, S.E. Hornstrom, G. Shaofang, P.A. Psaras, J. Vac. Sci. Tech. A5 Ž1987. 1447. w7x B.-I. Lee, W.-C. Jeong, K.-H. Kim, P.-S. Ahn, J.-W. Shin, S.-K. Joo, J. Korean Inst. Telematics Electron. 34D Ž1997. 33. w8x S.-H. Park, S.-I. Jun, K.-S. Song, C.-K. Kim, D.-K. Choi, Jpn. J. Appl. Phys. 38 Ž1999. L108. w9x A. Khakifirooz, S.S. Mohajerzadeh, S. Haji, presented at 47th Int. AVS Symp., Ž2000.. w10x S.Y. Yoon, K.H. Kim, C.O. Kim, J.Y. Oh, J. Jang, J. Appl. Phys. 82 Ž1997. 5865. A. Khakifirooz received his B.Sc. and M.Sc. degrees in electrical engineering, both from University of Tehran, Tehran, Iran, in 1997 and 1999, respectively. His master thesis was on the study of silicide and germanide materials for sensor applications. In 1997, he joined Thin Films Laboratory at the University of Tehran, where he contributed to setting up the lab. His current works include low-temperature fabrication of polysilicon TFTs, semiconductor sensors, and displays. Mr Khakifirooz is a member of IEEE and Materials Research Society. S. Haji received his B.Sc. degree from Shahid Beheshti University, Tehran, Iran, in 1997. Currently, he is an M.Sc. student at the University of Tehran, working on mobility improvement of polycrystalline semiconductors. S.S. Mohajerzadeh was born in 1964 in Iran. He received his B.Sc. degree from Sharif University of Technology, and M.A.Sc., and Ph.D. degree from the University of Waterloo, Ontario, Canada, all in electrical engineering. His doctoral thesis was on the area of SiGe heterostructure devices. In 1996, he joined the Electrical and Computer Engineering Department of the University of Tehran as an assistant professor. His research interests include material characterization, thin-film transistors, semiconductor sensors, and displays.
UV-assisted nickel-induced crystallization of amorphous silicon Ali KhakifiroozU , Saber Haji, S. Shamsoddin Mohajerzadeh Electrical and Computer Engineering Department, Uni¨ ersity of Tehran, North Kargar A¨ e., Tehran 14399, Iran
Abstract
˚ A novel UV-assisted metal-induced crystallization method is reported. A 1000-A-thick amorphous silicon film is deposited onto ˚ a 150-m-thick glass substrate at a temperature of 400⬚C using electron beam evaporation. A 50-A-thick Ni layer is then e-beam deposited to act as the crystallization seed. UV exposure is exploited as an external source of energy to lower the re-crystallization temperature during post-treatment at 400⬚C. Samples exposed to UV during this post-treatment show crystalline structure as confirmed with XRD and SEM analyses, while samples annealed without UV exposure remain amorphous. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal-induced crystallization; UV-exposure; Annealing; Polysilicon; Amorphous silicon
1. Introduction Metal-induced crystallization is a promising technique for low-temperature fabrication of polysilicon thin-film transistors needed for large area applications. Lateral growth of polycrystalline silicon mediated by the formation of nickel silicide, has been successfully employed to fabricate high-performance TFTs at 500⬚C w1x. Carrier mobilities as high as 120 cm2rVs have been achieved using MILC on oxidized silicon wafers w1x. Also, the fabrication of thin film transistors on glass has been reported at a temperature of 450⬚C w2x. However, such temperatures are still needed to be reduced for ordinary glasses. Efforts have also been made to lower the crystallization temperature by incorporating metals other than Ni. Metals such as Pd w3x, Au w4x, Ag w5x and Al w6x can be used for this purpose. In addition, double-metal induced lateral crystallization has been reported in which Ni and Pd pads are placed adjacent w7x. The effect of an electric field on the crystallization of a-Si has also been studied indicating an improved
U
Corresponding author. Tel.: q98-21-8011235; fax: q98-218011235. E-mail address: [email protected] ŽA. Khakifirooz..
rate of lateral growth w8x. Most recently, a lateral growth rate of approximately 500 mrh at 400⬚C has been observed with an electric field of 100 Vrcm and in the presence of an electric current w9x. In this paper we report for the first time, a novel UV-assisted metal-induced crystallization of amorphous silicon on ordinary glass at a record temperature of 400⬚C. The post-treatment of the samples is achieved through exposure to UV light while the substrate is heated to a temperature of 400⬚C using a hot plate. The UV photons seem to impart energy to the lattice, enhancing the possibility of crystallization at reduced substrate temperatures. 2. Experimental One hundred and fifty micrometers thick glass substrates are cleaned using RCA噛1 solution followed by rinsing in DI water and drying with N2 . After reaching ˚ a base pressure of 10y6 torr, a 1000-A-thick amorphous silicon film is e-beam deposited at a substrate temperature of 400⬚C. For this evaporation step 99.999% pure silicon charge is used. The sample is then allowed to cool down without breaking the vac˚ is uum and a layer of nickel with a thickness of 50 A deposited by e-beam evaporation. After unloading the
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 1 3 - 8
A. Khakifirooz et al. r Thin Solid Films 383 (2001) 241᎐243
242
samples from the vacuum chamber, they are annealed on a hot plate while exposed to UV radiation of a 400-W UV-lamp. Fig. 1 presents a schematic diagram of the post-treatment experimental setup. The UV lamp has a distance between 10 and 20 cm away from the top of the hot plate. The effect of UV exposure on the crystallinity of the samples has been studied using XRD and SEM techniques. 3. Results and discussion The morphology of the samples have been studied using SEM. Fig. 2 shows SEM micrographs of the samples annealed at different conditions. Fig. 2a shows the result for the sample annealed at 400⬚C without exposure to UV light. As seen in this figure, no crystallization is observed for this sample. Fig. 2b,c corresponds to the samples prepared under the same conditions but different UV exposures during annealing. The micrograph in Fig. 2b belongs to the sample annealed in the presence of UV light with a distance of 10 cm between the source and the hot-plate. The duration of this post-treatment has been 12 h for all samples. The features on the sample surface are believed to be crystal grains. Comparing this micrograph with the one in Fig. 2a, indicates the usefulness of the UV exposure in obtaining polysilicon films on glass substrates. Fig. 2c displays the result of a sample exposed to UV radiation during annealing while the UV light source was 20 cm away from the hot plate. Although the duration of the annealing in these samples is the same, the density and size of the grains are less significant. The intensity of the UV light was measured at a wavelength of 365 nm and the values found to be 10 and 2.5 mWrcm2 corresponding to a distance of 10 and 20 cm, respectively. The crystallinity of the annealed silicon films has been further investigated by means of X-ray diffrac-
Fig. 2. SEM micrograph of the samples annealed with different UV exposures: Ža. without UV, Žb. 10 mWrcm2 UV, and Žc. 2.5 mWrcm2 UV. All samples were annealed at 400⬚C for 12 h.
Fig. 1. Schematic diagram of experimental setup.
tion. Fig. 3 collects the spectra of the samples discussed in the previous figure. The spectrum Ža. in this figure
A. Khakifirooz et al. r Thin Solid Films 383 (2001) 241᎐243
243
at low temperatures, one can use such films for the fabrication of thin film transistors on glass. We have also observed lateral crystallization in the presence of UV-exposure with a rate of approximately 1 mrh. This recent observation is being investigated and will be presented elsewhere. Acknowledgements The assistance of Dr A. Miri and Mr M. Mozafari is cordially acknowledged. This work has been carried out in Thin Films Laboratory of the University of Tehran. References
Fig. 3. XRD spectra of the samples annealed with different UV exposure: Ža. 10 mWrcm2 and Žb. 2.5 mWrcm2 .
corresponds to the sample annealed in the presence of UV exposure with the UV light source located 10 cm away from the hot plate while the spectrum Žb. belongs to the sample whose annealing has been performed with the UV source 20 cm away. As can be seen from this figure, both samples show a polycrystalline structure. In curve Žb. two significant peaks are observed corresponding to ²111: and ²110: crystal orientations, while curve Ža. shows one major peak at an orientation of ²110: and the other orientation is less observable. The results provided in this figure, confirm the crystallinity of the samples as observed from the previous figure. Although the difference in the XRD results for these two samples is not well understood, we speculate that the sample prepared with lower UV intensity is more similar to those crystallized without UV exposure and at higher temperatures Žsee w10x.. The hump in the spectra is believed to be due to amorphous nature of the glass substrate. The spectrum of the sample without UV exposure indicates its amorphous nature and it is not provided here. 4. Conclusion In conclusion, we have successfully fabricated polysilicon films on ordinary glass substrate at a record temperature of 400⬚C using UV exposure during re-crystallization. The samples prepared using this UV-assisted re-growth, show dominant orientations of ²110: and ²111:. We believe that the exposure of the samples to a UV light source impart energy to the silicon film, which in turn can compensate the reduction in the substrate temperature. Since crystallization can occur
w1x S.W. Lee, S.K. Joo, IEEE Electron. Device Lett. 17 Ž1996. 160. w2x T.-K. Kim, B.-I. Lee, K.-H. Kim, J.-W. Shin, P.-S. Ahn, W.-C. Jeong, S.-K. Joo, Proc. 4th Asian Symp. Information Display, Ž1997., 61. w3x S.-W. Lee, Y.-C Jeon, S.-K. Joo, Appl. Phys. Lett. 66 Ž1995. 1671. w4x S.-Y. Yoon, J.-Y. Oh, C.-O. Kim, J. Jang, Solid-State Comm. 106 Ž1998. 325. w5x B. Bian, J. Yie, B. Li, Z. Wu, J. Appl. Phys. 73 Ž1993. 7402. w6x A.E. Robertson, L.G. Hultman, H.T.G. Hentzell, S.E. Hornstrom, G. Shaofang, P.A. Psaras, J. Vac. Sci. Tech. A5 Ž1987. 1447. w7x B.-I. Lee, W.-C. Jeong, K.-H. Kim, P.-S. Ahn, J.-W. Shin, S.-K. Joo, J. Korean Inst. Telematics Electron. 34D Ž1997. 33. w8x S.-H. Park, S.-I. Jun, K.-S. Song, C.-K. Kim, D.-K. Choi, Jpn. J. Appl. Phys. 38 Ž1999. L108. w9x A. Khakifirooz, S.S. Mohajerzadeh, S. Haji, presented at 47th Int. AVS Symp., Ž2000.. w10x S.Y. Yoon, K.H. Kim, C.O. Kim, J.Y. Oh, J. Jang, J. Appl. Phys. 82 Ž1997. 5865. A. Khakifirooz received his B.Sc. and M.Sc. degrees in electrical engineering, both from University of Tehran, Tehran, Iran, in 1997 and 1999, respectively. His master thesis was on the study of silicide and germanide materials for sensor applications. In 1997, he joined Thin Films Laboratory at the University of Tehran, where he contributed to setting up the lab. His current works include low-temperature fabrication of polysilicon TFTs, semiconductor sensors, and displays. Mr Khakifirooz is a member of IEEE and Materials Research Society. S. Haji received his B.Sc. degree from Shahid Beheshti University, Tehran, Iran, in 1997. Currently, he is an M.Sc. student at the University of Tehran, working on mobility improvement of polycrystalline semiconductors. S.S. Mohajerzadeh was born in 1964 in Iran. He received his B.Sc. degree from Sharif University of Technology, and M.A.Sc., and Ph.D. degree from the University of Waterloo, Ontario, Canada, all in electrical engineering. His doctoral thesis was on the area of SiGe heterostructure devices. In 1996, he joined the Electrical and Computer Engineering Department of the University of Tehran as an assistant professor. His research interests include material characterization, thin-film transistors, semiconductor sensors, and displays.