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LOW TEMPERATURE FABRICATION OF POLY-Si TFTs USING IN-SITU CHEMICALLY CLEANING METHOD NAOKI KONO, TATSURO NAGAHARA, KENJI FUJIMOTO, YUSAKU KASHIWAGI AND HISASHI KAKINOKI Tonen Corporation, Corporate R&D Laboratory, 1-3-1 Nishi-tsurugaoka, Ohi-machi, Iruma-gun, Saitama 354, Japan ABSTRACT Thin film transistors were fabricated using polycrystalline silicon (poly-Si) film which were directly deposited on Coming 7059 glass substrates by plasma chemical vapor deposition method at very low temperature of 450"C. No annealing procedure was carried out in the fabrication process. The dependences of the crystallinity and the electrical properties on the poly-Si film thickness were investigated for three kinds of films deposited under different conditions. These dependences on the film thickness were found to be strongly influenced by the deposition condition, especially the reaction gas pressure. By choosing optimum poly-Si deposition condition carefully, high performance TFTs have been fabricated by this novel method. INTRODUCTION Recently, thin film transistors (TFTs) have been used in many fields such as liquid crystal display (LCD), image sensor and LSI application. Particularly, in the field of active matrix LCD, polycrystalline silicon (poly-Si) TFTs have been vigorously investigated. Because of its superior electrical characteristics, it can be used not only for pixel switching device but also for drive circuit device. As a result, both pixel TFTs and drive circuit TFTs can be fabricated on the same substrate; it may benefit in increasing the reliability and reducing the fabrication cost.[1]. However, since ordinary glass substrates cannot endure high-temperature processing, it is necessary to develop low-temperature poly-Si growth methods. Several techniques have been proposed to prepare poly-Si film at low temperatures, such as low-pressure chemical vapor deposition (LPCVD) [2][3], solid phase crystallization [4][5], laser recrystallization [6][7] and plasma chemical vapor deposition [8][9][10]. In LPCVD, expensive substrates such as quartz glass were required to put up with the process temperature (above 600'C). Solid phase crystallization method required long time for thermal crystallization process, thus increased the fabrication cost. Laser recrystallization had difficulties in achieving uniformity, controllability and productivity of poly-Si films. Poly-Si films prepared by usual plasma CVD method could not be applied to LCD application due to the low electrical properties. We have developed a novel method for preparing high quality poly-Si films on glass substrate at very low temperature (450'C) by plasma chemical vapor deposition method[ 1I]. The method utilizes "in-situ chemically cleaning effect" which enables the crystalline silicon growth at low temperature by chemically cleaning reaction of fluorine radicals, which occurs simultaneously with the film growth. Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society

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In this paper, we report the fabrication of TFTs using these poly-Si fil