Excimer-Laser Growth of Si Large-Grain Arrays
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Excimer-Laser Growth of Si Large-Grain Arrays Masakiyo Matsumura Advanced LCD Technologies Development Center Co., Ltd. 292 Yoshida-cho, Totsuka-ku, Yokohama, 244-0817 Japan ABSTRACT Amorphous silicon (a-Si) Thin Film Transistors (TFT) will continue to play an important role in large-area liquid crystal displays (LCD), however, there is also a strong demand for ultra-high performance TFTs aimed at intelligent small (or medium) size displays. For this reason, “Crystal Si on glass” has become an increasingly attractive solution. This paper reviews recent work performed at ALTEDEC (Advanced LCD Technologies Development Center Co., Ltd, with regard to growing an array of large Si grains at low temperature. The results obtained so far indicate that satisfactory progress has been made towards the achievement of this goal. 1. INTRODUCTION A-Si TFTs will continue to be a leading technology in the foreseeable future, addressing the need for devices requiring high impedance liquid-crystal pixels in panels of more than 15” in diagonal size. However, various and recently devised display applications, such as intelligent and fine displays [1], require TFTs having both a higher switching speed and lower current leakage. Another technological trend, that towards organic electroluminescent (EL) displays, require new TFTs having higher current drivability, since EL diodes are forward-biased and have low impedance. Poly-Si TFTs are unlikely to be satisfactory with respect to mobility, leakage, or on-off transition, and show large fluctuations in their characteristics. Thus, development of single-crystal Si (x-Si) TFTs will be essential for next-generation displays. The current authors have not, however, been able to grow x-Si on large non-heat-tolerant (amorphous) substrates such as those of glass or plastics. An appropriate solution is, therefore, to grow arrays of high quality Si grains, which are as large as the TFT feature sizes, with a desired spatial pitch on a large substrate at low temperature. Si film production, using an ultra-short-time melt-regrowth method, based on high-power pulsed lasers, such as excimer lasers, may provide the best technology to meet this new requirement, because this method causes only slight thermal damage to non-heat-tolerant substrates. Although excimer laser annealing (ELA) has been studied for 20 years, only extremely small grains could be grown until a lateral growth mode [2] was discovered in 1992. Many groups are now studying the lateral growth mode, leading to the development of, for example, the gradient [3], SLS [4] and µ-Cz [5] methods. The SLS method is based on a step-and-repeat approach; a large number of shots are required to crystallize the entire substrate surface, because the growth length per shot is as short as a few micrometers. The µ-Cz method requires ultra-fine substrate etching technology, and is an impractical method at present. The
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gradient method had a serious problem in how to microscopically modulate the laser light intensity on the substrate surface.
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