Location Control of Laterally Columnar Si Grains by Dual-Beam Excimer-Laser Melting of Si Thin-Film
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Location Control of Laterally Columnar Si Grains by Dual-Beam Excimer-Laser Melting of Si Thin-Film Ryoichi Ishihara Laboratory of Electronic Components, Technology and Materials (ECTM) Delft Institute of Microelectronics and Submicrontechnology (DIMES) Delft University of Technology Feldmannweg 17, POBox 5053, 2600 GB Delft, The Netherlands ABSTRACT The offset of the underlying TiW is introduced in the island of Si, SiO2 and TiW on glass. During the dual-beam excimer-irradiation to the Si and the TiW, the offset in TiW acts as an extra heat source, which melts completely the Si film near the edge, whereas the Si inside is partially melted. The laterally columnar Si grains with a length of 3.2 µm were grown from the inside of the island towards the edge. By changing the shape of the edge, the direction of the solidification of the grain was successfully controlled in such a way that the all grain-boundaries are directed towards the edge and a single grain expands. The grain-boundary-free area as large as 4 µm × 3 µm was obtained at a predetermined position of glass. INTRODUCTION TFTs inside a large Si grain, i.e., crystal-Si (c-Si) TFTs [1] will solve the problems of the excimer-laser crystallized polycrystalline-Si (poly-Si) TFTs [2], such as the high leakage current, the relatively low field-effect mobility and the large deviation of those characteristics [3]. In order to produce the c-Si TFTs on a large glass substrate, it is essential to control the position of the large Si grain at the predetermined position on a glass. The position of surviving crystal seeds, from which crystal grains are grown, can be controlled either by shaping the irradiation beam [4,5], by shadowing the beam with a cap layer [6] or by local structural variation of the underlying material [7,8] which locally modifies the heat extraction rate towards the substrate. The number of the seeds can be controlled to be the unity by making the size of the lowtemperature area small enough. In the last method, a single crystal grain as large as 6.5 µm can successfully be grown at the predetermined position on glass [9]. Another way to have a single crystal area is to select one crystal out of many crystal seeds using temperature gradient so that the grain boundary is swept away [10]. The method, however, has a limitation of the size of single grain since the width of Si film has to be gradually narrowed along the growing direction. In this study, a novel location-control method of the single crystal area is proposed. This method is based on the local heat supply that melts Si completely beside the surviving seeds and selects one single crystal during the growth using the lateral temperature gradient. The method allows one to have a large crystal Si area thanks to (1) slow cooling rate of the melt and (2) a widening Si region rather than narrowing. The method would also allow one to have a preferred crystallographic orientation, which is an important parameter for the c-Si TFTs.
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Figure 1: Schematic cross sectional view of the structure propo
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