Excimer Laser Induced Crystallization of Thin Amorphous Si Films on SiO 2 : Implications of Crystallized Microstructures
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EXCIMER LASER INDUCED CRYSTALLIZATION OF THIN AMORPHOUS
Si FILMS ON Si0 2 : Implications of Crystallized Microstructures for Phase Transformation Mechanisms
H. J. Kim*, James S. Im*, and Michael 0. Thompson* Columbia University, HKSM, Metallurgy & Materials Science, New York, NY ** Cornell University, Dept. of Materials Science & Engineering, Ithaca, NY *
ABSTRACT
Using planar view transmission electron microscope (TEM) and transient reflectance (TR) analyses, we have investigated the excimer laser crystallization of amorphous silicon (a-Si) films on Si0 2 . Emphasis was placed on characterizing the microstructures of the single-shot irradiated materials, as a function of the energy density of the laser pulse and the temperature of the substrate. The dependence of the grain size and melt duration as a function of energy density revealed two major crystallization regimes. In the low energy density regime, the average grain size first increases gradually with increases in the laser energy density. In the high energy density regime, on the other hand, a very fine grained microstructure, which is relatively insensitive to variations in the laser energy density, is obtained. In addition, we have discovered that at the transition between these two regimes an extremely small experimental window exists, within which an exceedingly large grain-sized polycrystalline film is obtained. We suggest a liquid phase growth model for this phenomenon, which is based on the regrowth of crystals from the residual solid islands at the oxide interface. INTRODUCTION
The increasing availability of excimer lasers, which are capable of imparting high energy laser pulse beam over a large area, continues to fuel investigations on laser-induced material modifications. One topic, which has recently been receiving increasing attention, involves the excimer laser crystallization of thin a-Si films on transparent substrates [1-41. Technologically, the driving force is the possibility of fabricating high quality thin film transistor devices on excimer laser crystallized Si films for flat panel display applications. Here, the microstructure of the starting material is an important factor in determining the final device performance; in general, the larger the grain size the better the performance. To date, solid phase crystallization (SPO) of a-Si films has been investigated extensively, specifically as a way of producing poly-Si films for TFT devices. Excimer laser crystallization stands as an alternative technique, which can provide several advantages such as selected area processing and rapid crystallization. For these reasons, investigations have been carried out on various aspects of excimer laser induced crystallization; the results suggest that several complex phase transformation scenarios may be involved. In order to obtain a clear, comprehensive picture of these phenomena, we have embarked on a research program that involves extensive microstructural characterizations in addition to in situ analyses. We describe in this paper the depend
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