Study of Crystal Growth in Grain-Filters for Location-Controlled Excimer Laser Crystallization
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Study of Crystal Growth in Grain-Filters for Location-Controlled Excimer Laser Crystallization Paul Ch. van der Wilt, B.D. van Dijk, G.J. Bertens, and R. Ishihara Delft Institute of Microelectronics and Submicron Technology (DIMES), P.O. box 5053, 2600 GB Delft, The Netherlands. ABSTRACT Amorphous silicon films were transformed into large-grain polycrystalline silicon films by excimer laser induced rapid melting and solidification. Growth is seeded by residual unmolten silicon left in a grid of holes made in the underlying silicon oxide layer. Growth thus starts with a vertical growth phase, during which occlusion of grains occurs, ideally filtering out only a single grain that subsequently seeds the lateral growth. Yield was increased by sharpening the edges of the hole to improve the filtering mechanism and very uniform polycrystalline layers were obtained. Furthermore, simulations were performed to analyze parameters such as growth velocity and melt depth. It was found that heat from laser light absorbed in the neighborhood of the hole strongly contributes to melt depth and thus to the filtering mechanism. Substrate heating or longer pulse durations can be utilized to ensure the occlusion of grains during vertical growth, to reduce thermal stresses, and to increase grain size to a value sufficient for single crystal thinfilm transistors. INTRODUCTION Previously we presented the grain-filter method to create location-controlled single-crystal regions in a thin silicon film by excimer laser crystallization [1]. Such a method could be applied to make high performance thin-film transistors on low melting temperature substrates such as glass. The single crystal regions were obtained by combining two phenomena common to excimer laser crystallization. One is the dependence of the complete melt threshold on the film thickness and the other is the occlusion of grains observed in vertical regrowth of partially molten films [2]. This was achieved with the structure shown in figure 1(a), which is irradiated at an energy density far above the threshold for complete melt (ECM) of the surrounding film, i.e. sufficient to melt most of the silicon filling of the hole. The residue left is poly-Si, as the melt was preceded by explosive crystallization (XC) [3]. The regrowth then starts with a vertical growth phase in which occlusion of grains will occur, hence the name grain-filter. Ideally, only a single grain survives, which subsequently seeds the lateral growth of a single crystal region. It was argued that when the melt-depth to hole-diameter ratio (η) is increased, this would lead to a high yield of single crystal regions. At that time, however, only an estimation of the melt depth was given based on a one-dimensional experiment in which both the ECM and the agglomeration threshold (EAGG) were experimentally determined as a function of film thickness. Agglomeration occurs as a result of the poor wetting properties of molten silicon on silicon dioxide and when either peak temperature and/or melt duration exceeds a certain value
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