Single-Shot Excimer-Laser Crystallization of an Ultra-Large Si Thin-Film Disk
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Single-Shot Excimer-Laser Crystallization of an Ultra-Large Si Thin-Film Disk Mitsuru NAKATA *, Chang-Ho OH and Masakiyo MATSUMURA Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan E-mail : [email protected] URL : http://silicon.pe.titech.ac.jp/ Abstract An excimer-laser-induced large-grain growth method has been proposed which utilizes non-uniform heat diffusion along the Si thin-film and also along the underlayer. A single-shot of KrF excimer-laser light pulse with uniform intensity could crystallize a circularly pre-patterned Si thin-film of 20μm in diameter, much larger than TFT feature size in present AM-LCD panels. Introduction Studies on polycrystalline silicon (poly-Si) crystallized by excimer-laser annealing (ELA) have been pursued so as to implement high driving-current and low leakage-current in thin-film transistors (TFTs). To produce single-crystal Si TFTs, following three conditions should be satisfied. The one is to grow grains larger than the TFT feature size [1,2], the second is to control position of these large grains [3], and the third is to keep the excess-process cost low, for example, by a single shot and uniform irradiation using a simple ELA system. ELA-induced lateral growth can potentially satisfy these requirements, since long lateral growth can take place when there is a sufficient retardation of a start time of nucleation along the Si thin-film. There are two dominant ways in elongating the retardation, that is, introduction of a non-uniform light intensity distribution [4,5,6] or introduction of a non-uniform sample structure to initiate non-uniform heat out-diffusion from the uniformly molten Si thin-film. And we have studied here the latter way. Namely, a nucleation controlled ELA (NCELA) method has been proposed where the unmolten amorphous-Si (a-Si) region is localized to a very small volume by lateral heat diffusion in the Si thin-film. The NCELA method has been combined with pre-patterned ELA (P2ELA) method where the molten Si film is heated up non-uniformly by the underlayer. By this IP2ELA (improved P2ELA) method, a Si disk of as large as 20μm in diameter was crystallized by a single shot and uniform irradiation, and a central part of a Si disk of 10μm in diameter was single-crystallized. Proposal of IP2ELA In the conventional ELA method, Si grains start to grow from a lot of nuclei born simultaneously in the unmolten a-Si region. Since collision with neighboring grains interrupts their growth along the lateral direction, grown grains are column-like-shaped with a diameter much smaller than the TFTs feature size. The long lateral grain growth can take place, however, when the nucleation time is delayed along the molten Si layer. Retardation of the nucleation time along the Si layer, a necessary condition for the sufficiently-long lateral growth, may be introduced by several ways. One of them is to generate a temperature gradient after the uniform laser light irradiation by a non-uniform sample struc
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