Artificial Formation of Nucleation Seed in Excimer Laser Recrystallization Of Poly-Si
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ABSTARCT Excimer laser annealing method employing artificial nucleation seed is proposed to increase the grain size of polycrystalline silicon(poly-Si). We utilize Si component incorporated in aluminum(Al)-sputtering source for the nucleation seed. Si clusters which are to be used as nucleation seed are successfully formed on the substrate by deposition and etch-back of Siincorporated Al layer. Irradiation of excimer laser on amorphous silicon(a-Si) film deposited on the substrate prepared by our method results in enlargement of poly-Si grains, compared with conventional laser recrystallization. Poly-Si thin film transistor also shows much improved electrical perfbrmance which directly reflects the quality of poly-Si film recrystallized by our method.
INTRODUCTION Excimer laser recrystallization of amorphous Si (a-Si) is useful method for various poly-Si devices such as thin film transistors (TFTs) because of low density of defects in laser annealed poly-Si film[l,2,3]. The excellent electrical characteristics of poly-Si TFTs are closely related to grain size and defect density. In order to improve the quality of laser annealed poly-Si film, there have been considerable efforts to increase the grain size[4,5,6]. It is well known that melting and solidification occurs during laser irradiation on a-Si film[7]. This is the main reason for low defect density of excimer laser annealed poly-Si, in contrast to solid phase crystallized (SPC) poly-Si which does not undergo phase transformation from liquid to solid during the recrystallization. However, the grain size of laser annealed poly-Si film varies sensitively with laser energy density, showing a variety of device performance[7,8]. In the excimer laser recrystallization, it is also known that there exists some critical energy level which maximizes the grain size, and this energy level is the so-called SLG (super lateral growth) regime[9]. At the SLG regime, a-Si layer is nearly fully melted down to the substrate. Some surviving Si clusters, which are not melted, act as nucleation seed and conduct the lateral growth of grains. This energy level may be considered favorable for the best electrical performance, however does not ensure the reproducibility because the pulse-to-pulse energy of excimer laser fluctuates rather significantly. At higher energy level over the SLG regime, a-Si melts completely to the bottom and the homogeneous nucleation, which leads to abrupt reduction of grain size, becomes dominant. Under such full melt energy condition, further increase in the energy density would hardly affect the grain size so that some energy fluctuation may ensure the nearly identical size of grain. The purpose of this paper is to propose an excimer laser annealing technique to increase the grain size of poly-Si film with the insurance of reproducibility. Our method is carried out at the full melt energy condition which is immune to the laser energy fluctuation. In order to increase the grain size, our method employs the artificially formed nucleation seed. The proposed m
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