Origin of Inhomogeneity in Pulsed Excimer-Laser Crystallized Silicon Films Studied by Computer Simulation
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Origin of Inhomogeneity in Pulsed Excimer-Laser Crystallized Silicon Films Studied by Computer Simulation
Toshio Kudo, Daiji Ichishima and Cheng-Guo Jin* Research & Development Center, Sumitomo Heavy Industries Ltd., 63-30 Yuuhigaoka, Hiratsuka, Kanagawa 254-0806, JAPAN [email protected] *ACT Center, TIC Corporation, 2-9-30 Kitasaiwai, Nishiku, Yokohama, Kanagawa 220-0004, JAPAN ABSTRACT In order to understand the occurrence and development of inhomogeneous poly-Si texture in the polycrystallization process of poly-SiTFT fabrication, we have simulated 1) the influence of pulse energy (PE) and optical axis (OA) fluctuations on the texture of poly-Si films and 2) the interference between both fluctuations, under the single-pulsed XeCl laser irradiation at the overlapping ratio 98%. The frequency of fatal irregular pulses in the PE fluctuation is much more than that in the OA fluctuation (pointing stability). Once the poly-Si texture suffers a fatal irregular pulse over the complete melting (CM) point, the inhomogeneous texture caused is never recovered by the following overlapping irradiation and no columnar structure is formed. However, when no irregular pulses exceed the CM point, the partial inhomogeneous texture caused in the poly-Si films is recovered by the thinning out of small Si grains in the texture and a columnar structure is formed. The key to the issue of fatal irregular pulses should be not only the reduction of PE and OA fluctuations but also the optimum selection of pulse energy not over the CM point. INTRODUCTION An excimer laser annealing (ELA) process is still the bottleneck in the fabrication of low temperature polycrystalline silicon thin film transistors (p-SiTFTs). The highly overlapped irradiation of a line-shaped laser is inevitable to the fabrication of high performance p-SiTFTs[1]. At present the result of high overlapping makes the polycrystallization throughput slower. The reason the line-shaped laser is irradiated at the high overlapping ratio (OR) is that the inhomogeneous texture of p-Si films, which makes an appearance in the polycrystallization process, can be recovered by the repetition of the irradiation. The inhomogeneous p-Si texture, the multiformity in Si grain sizes comes from fluctuations of pulse energy (PE) [2,3], of opitical axis (OA), and of spatial and temporal profiles of the pulsed-laser [4,5]. In order to understand the occurrence and development of inhomogeneous textures in the p-Si films, it is indispensable to observe visually the change in texture for each irradiation during a step scanning of the line beam. Experimentally, however, the visual observation is hard, because the duration of the laser irradiation is so short. For this purpose our simulator was developed as a tool of observing visually the Si grain growth process [2]. In this paper we report on knowledge obtained from simulations of the fluctuations in PE and OA and of the interference between them, and then discuss how we reduce the occurrence of inhomogeneous texture in the p-Si films.
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