Characteristics of Excimer-Laser-Crystallized Polysilicon Films by Line Beam Scanning Method
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YOUNG MIN JHON, DONG HWAN KIM, HONG CHU, CHANG WOO LEE, and SANG SAM CHOI Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea ABSTRACT Characteristics of polysilicon films, crystallized by excimer laser annealing, have been investigated. The entire amorphous silicon film of 50 mm X 50 mm has been crystallized by scanning a line shape excimer laser beam, which basically reduces the nonuniformity in the beam overlap region of the 2-dimensional scanning method. The laser beam had a Gaussian profile in the scanning direction, which ensured good crystallization by the reversible transitions between the crystalline and amorphous states and was expected to give step annealing effect. The laser energy density and substrate temperature were varied. INTRODUCTION The interest in polycrystalline silicon thin-film transistors (poly-Si TFT's) are increasing, since they have large mobilities suitable for high-speed devices as well as for pixel TFT's in high-resolution active-matrix-addressed liquid crystal displays (AMLCD's), which can be used for high-definition televisions (HDTV's) [1,2]. However, the conventional method, low-pressure chemical vapor deposition (LPCVD), of fabricating poly-Si requires high-quality glass as a substrate because of its high processing temperature (600-1000°C) and also requires a time-consuming long processing time [3]. Excimer laser annealing has been shown to be a novel method to crystallize amorphous silicon on a low-cost glass substrate [3-6). Moreover, selective crystallization is possible by using localized irradiation of the laser beam, so that the driver circuits and the low-mobility pixel TFT's can be fabricated on a single substrate [7,81. Nonuniformity is found in the overlap region of the laser beams when a large area is scanned by a rectangular-shaped beam [9], and thus the small-pitch scanning method [10,11] has been introduced to improve the uniformity in one dimension, but the nonuniformity in the other dimension remains inevitable. This nonuniformity can be basically reduced by using a line shape laser beam which covers the entire range of one dimension of the substrate leaving only the other dimension to be scanned [12]. In the present work, we investigate the characteristics of poly-Si films crystallized by scanning a line shape excimer laser beam with a Gaussian profile in the scanning direction. The varying spatial energy distribution of the Gaussian profile ensures good crystallization since, even if laser amorphization occurs, the laser-amorphized silicon can be recrystallized by the rear part of the laser beam due to the reversible transition between crystalline and amorphous states [13,141. Moreover the front part of the laser beam which has a spatially increasing energy distribution is expected to give a step annealing effect which increases the uniformity [11]. EXPERIMENTAL Amorphous silicon films were prepared on a substrate 50 mm X 50 mm large. 300 nm SiNx buffer layers were deposited onto Coming 7059 glass substrates followed by 50
