Low-temperature Growth of Poly-Si and SiGe Thin Films by Reactive Thermal CVD and Fabrication of High Mobility TFTs over
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Low-temperature Growth of Poly-Si and SiGe Thin Films by Reactive Thermal CVD and Fabrication of High Mobility TFTs over 50 cm2 /Vs Jun-ichi Hanna and Kousaku Shimizu Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan ABSTRACT We have established a new thermal CVD technique, Reactive Thermal CVD, for polycrystalline silicon (poly-Si) and silicon germanium (poly-SiGe) thin films aiming at thin film transistors (TFTs) applications, in which a low substrate temperature of 450oC enables us to use glass substrates. This technique achieved high crystallinity at very early stage of the film growth, resulting no amorphous incubation layer on the substrate surface. We fabricated bottom and top gate n-and p-channel TFTs with these of 200 nm thick films on SiO2 /Si wafers and glass substrates, respectively: the high field effect mobilities as high as 55 cm2 /Vs and 25 cm2 /Vs were achieved in the bottom-gate and top-gate TFTs, respectively. Here, we discuss the technical requirements in the low-temperature CVD technique for the large-area poly-Si thin films and how they can be achieved in the reactive thermal CVD. INTRODUCTION In these 10 years the amorphous silicon TFTs have been well established as an active matrix for liquid crystal display (LCD) devices for computer outputs, and more recently for TV monitors. Due to the increasing demand for large-area and high definition LCDs, the needs for the switching elements faster than amorphous silicon TFTs, whose switching speed is limited by its mobility of 0.1~1 cm2 /Vs, are coming out. In addition, the monolysic fabrication of TFT array and the system circuits including scanning and signal drivers is launched in small-size quality displays for improving reliability and yields, and for the cost reduction. Furthermore, new needs for high mobility TFTs has emerged to realized the active matrix for organic light emitting displays , which request high current density for operation. Thus, there is an increasing demand for poly-Si TFTs whose mobility exceeds 100 cm2 /Vs. Nowadays, the poly-Si thin films on the glass substrate for TFT applications are fabricated in two techniques, that is, excimer laser annealing and solid phase crystallization of a-Si thin films. As for the laser annealing technique, there remains a very serious problem of the cost-expensive production due to high installation and maintenance costs of the excimer laser systems, in addition to a low yield due to inhomogeneous crystallinity of the polycrystalline films caused by a laser scanning process. On the other hand, a high process temperature over 600o C and a long process time over tens hours are a big problem in the solid phase crystallization of amorphous silicon films. With the aid of metal catalysts such as Ni the crystallization process is improved very much, but the residual catalyst is harmful to off-current characteristics and a long-term reliability of the TFTs. Furthermore, these on-going techniques need 3 steps to fabrica
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