Multiple Pulse Irradiation Effects in Excimer Laser-Induced Crystallization of Amorphous Si Films
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SINGLE PULSE IRRADIATION OF LPCVD THIN Si FILMS ON SiQ2 I. PARTIAL MELTING REGIME (Low Energy Density Regime) • Unmelted Si composed of continuous layer (i.e., Melt-depth < Film thickness) * Explosive crystallization, * Vertical regrowth and competitive occlusion of grains * Fine-gralned and small-grained poly-Si (Grain radius < Film thickness) II. NEAR-COMPLETE MELTING REGIME (Super Lateral Growth Regime) * Unmelted Si composed of discrete islands (i.e., Melt-depth - Film thickness) * Significant lateral growth can proceed before the impingement * Up to - jm-sized grains observed EI.
COMPLETE MELTING REGIME (High Energy Density Regime) * No unmelted Si remains * Deep supercooling followed by nucleation and growth of solids * For low substrate temperatures, fine-grained and small-grained poly-Si observed (Grain radius < a few hundred A) * Amorphization observed for thinner films
Figure 1. Various melt-mediated transformation mechanisms Involved in single pulse-induced crystallization of LPCVD a-Si films on SiO 2 [1, 2]. 665 Mat. Res. Soc. Symp. Proc. Vol. 321. c1994 Materials Research Society
In general, the excimer laser-induced crystallization of Sipulse-based films can be divided into single and multiple pulse-based methods. In the single method, much of the film (i.e., except the overlapping areas) is exposed to the beam only once, and in the multiple pulse-based method, the film is exposed to the beam numerous times. Since control and uniformity of the resulting microstructure over the entirefilm is critical for both methods, it is technologically imperative that one properly understands the effects that are associated with multiple-pulse irradiation. To this end, it has recently become apparent that multiple-pulse irradiation of poly-Si films can lead to substantial enlargement of the grains 131. The observed excimer laser-induced grain enlargement (ELGE) phenomenon has the potential to be scientifically noteworthy, as the responsible mechanism may represent a yet uncharacterized process for the microstr-uctural evolution, since it is likely to be quite distinct from well known grain growth processes 141. Here, we report on double- and multiple-irradiation experiments, which were designed In order to better characterize the ELGE process such that the results willl help to unveil the precise transformation scenario through which grain enlargement occurs. Based on the resulting observations, it is suggested that the ELGE phenomenon corresponds to the near-complete melting and solidification process, which involves complete melting of the film occurring locally at and near the grain boundaries. EXPERIMENTAL METHOD Irradiation experiments were conducted with non-hydrogenated LPCVD a-Si films (1000A-thick), which were deposited on oxidized Si substrates [with a 1000OAthick thermal oxide layer). The samples were irradiated in atmosphere with 30nanosecond XeCI exclmer laser pulses (308 nm). We have conducted two types -- double- and multiple- pulse irradiationexperiments. Double-pulse irradiation experiments con
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