Solid Phase Crystallization of LPCVD Amorphous Si Films by Nucleation Interface Control
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ABSTRACT A novel method for the fabrication of poly-Si films with a large grain size is reported using solid phase crystallization (SPC) of LPCVD amorphous Si films by nucleation interface control. The reference films used in this study were 1000 A -thick a-Si films deposited at 500'C at a total pressure of 0.35 Torr using Si 2H6/He. Since the deposition condition changes the incubation time, i.e. nucleation rate, and since nucleation occurs dominantly at a-Si/SiO2 interface, we devised the following deposition techniques for the first time in order to obtain the larger gain size. A very thin a-Si layer (- 50 A) with the deposition conditions having long incubation time is grown first and then the reference films (- 950 A) are grown successively. Various composite films with different combinations were tested. The crystallization kinetics of composite films was observed to be determined by the deposition conditions of a thin a-Si layer at the a-Si/Si0 2 interface. Nucleation interface was also observed to be modified by interrupted gas supply resulting in the enhancement of the grain size. INTRODUCTION Polycrystalline silicon films have received a great deal of attention since they can be applied to various applications such as a gate electrode for MOSFET, the emitter in bipolar transistors and interconnects. Recently polycrystalline silicon thin film transistors (poly-Si TFT's) have attracted considerable interest for the fabrication of active-matrix-liquid-crystal-display (AMLCD). In these applications the large grain size of poly-Si films is desirable since the presence of grain boundaries lead to a degradation in device performance. Directly deposited LPCVD (low pressure chemical vapor deposition) poly-Si films exhibit a very fine grain size with a columnar microstructure [1]. Enhancement of the grain size is generally achieved by solid phase crystallization (SPC) of LPCVD amorphous silicon (a-Si) films. Solid phase crystallization of a-Si films proceeds by nucleation and growth. Deposition conditions such as deposition temperature and deposition pressure can affect the initial state of asdeposited a-Si films[2]. The degree of structural disorder and the purity of as-deposited a-Si films can in turn influence nucleation and growth kinetics during solid state transformation [3]. To obtain poly-Si films with the larger gain size the nucleation rate should be retarded and the growth rate should be enhanced during phase transformation [4,5]. Since the temperature at which solid phase crystallization is usually conducted is at around 600'C the phase transformation occurs at a temperature far from the equilibrium point between amorphous and crystal phases [6]. Since crystallization is carried out at a relatively low temperature regime the phase transformation is limited by kinetic control. In other words both the nucleation rate and the growth rate decrease as the crystallization temperature decreases. The activation energy for nucleation in the case of a-Si, however, is greater than that for growth. As a result the
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