High Efficiency Crystallization of Silicon Thin Films Using Continuous Wave Infrared Laser
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0910-A14-02
High Efficiency Crystallization of Silicon Thin Films Using Continuous Wave Infrared Laser Naoki Sano1, Masato Maki2, Nobuyuki Andoh2, and Toshiyuki Sameshima2 1 Hightec Systems Corporation, 3-19-5, Shin Yokohama, Kohoku-ku, Yokohama, Kanagawa, 222-0033, Japan 2 Tokyo University of Agriculture and Technology, Koganei, Tokyo, 184-8588, Japan
ABSTRACT Laser crystallization of silicon thin films on inexpensive glass substrates with a photoabsorption layer of diamond-like-carbon (DLC) film was investigated. Scanned near-infrared beam emitted from a continuous wave (CW) laser diode at a speed of 25 cm/s with a power density of 22.5 kW/cm2 and a wavelength of 940 nm was effectively absorbed by DLC films, and 50-nm-thick silicon films were well crystallized by the thermal diffusion from DLC heated by the laser irradiation. As a result, crystallization of silicon films was achieved. Raman scattering spectra were measured for the structural analysis of silicon films and the sharp TO phonon peak at wave number of 519.5 cm-1 was observed. It shows that silicon films were effectively crystallized using the near-infrared laser diode. On the other hand, no crystallization was observed when the laser diode was directly irradiated to 50-nm-Si/glass, because silicon has too small optical absorption coefficient at 940 nm.
INTRODUCTION Polycrystalline silicon films have been applied to various kinds of devices such as thin film transistors (TFTs) and solar cells. Laser crystallization processes using the XeCl excimer laser have been widely used for the mass production of low temperature polycrystalline silicon (LTPS) TFTs as the switching devices for the pixels and the peripheral driver circuits of liquid crystal displays (LCDs) [1-4]. However laser in near infrared wavelength region were not used for crystallization of silicon films because of its low absorption efficiency by silicon films. We have already proposed silicon crystallization using the diamond-like-carbon (DLC) films as photo-absorption layer by irradiation of 308-nm-XeCl excimer laser [5-7] and 1064-nmNd:YAG laser [8]. In resent years, it has been much interested in DLC because of its outstanding properties, such as high hardness, thermal conductivity, wear resistance, thermal durability and chemical inertness. Therefore it is widely utilized for technological and industrial applications such as wear resistant and protective hard coats. We paid special attention on its optical properties. It has low refractive indices from 1.3 to 1.9 and high extinction coefficients from 0.8 to 0.9 for wavelength from 250 to 1100 nm. Therefore laser diodes with wavelength of 600 ~ 1000 nm can be used for crystallization of silicon films by the assistance of DLC. Now, commercially available cheap laser diodes with energy conversion efficiency above 40 % can be easily assembled to have power much higher than that of conventional XeCl excimer laser used
for mass production of LCD panels. If high power laser diodes are used for crystallization of silicon, a large reduction
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