Optical Effects Induced by the Multilayer Nature of SOI Films During Transient Thermal Processing with a Radiant Line He
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OPTICAL EFFECTS INDUCED BY THE MULTILAYER NATURE OF SOl FILMS DURING TRANSIENT THERMAL PROCESSING WITH A RADIANT LINE HEAT SOURCE.
Peter Y. Wonga. loannis N. Miaoulisa* , and P. Zavrackyb aThermal Analysis of Materials Processing Laboratory. Mechanical Engineering Department. Tufts University. Medford, MA 02115 b Kopin Corp., Taunton MA 02780 ABSTRACT Radiation heat transfer has been found to have the greatest impact on the quality of the thin recrystallized silicon film during zone-melting recrystallization (ZMR) processing. This study focused on the radiation effects during ZMR with an infrared radiant line heat source such as a graphite strip heater. The multilayer nature of the capped silicon-oninsulator (SOl) structure induces complex optical effects which affect the temperature distribution during processing. A two dimensional numerical model of the ZMR process has been developed using a finite difference scheme. The effect of the radiant line heat source's emission into the wafer has been modeled with a matrix method using Fresnel coefficients. A numerical parametric study was conducted to observe the effects of varying the thickness of the different layers in a capped SOT wafer on the maximum temperature and melt width attained. Results indicate that the variation of either the capping or insulating silicon oxide layer causes significant fluctuations of the reflectivity and temperature profile of the film. Increasing the thickness of the Si layer results in a nearly linear increase in temperature and melt width after complete melting. Layering schemes that are sensitive to small variations in thickness that may result in large changes in reflectivity were identified. INTRODUCTION Zone Melting Recrystallization (ZMRI with a radiant line heat source is currently being used to recrystallize the thin silicon layer of a SOI (silicon-on-insulator) wafer used for advanced memory chips. The SOI structure (Fig. 1) consists of a thick silicon crystal substrate (-400 jim), an electrically insulating silicon oxide film (-1 gm), and a polycrystalline silicon film (-1 gm). Often there is also a silicon oxide capping layer (-1 •rm). The ZMR processing technique involves passing a radiant line heat source over a wafer which is being heated from the underside by a susceptor. The thin polycrystalline silicon film is melted as the heat source passes over it. forming a moving molten zone. As the material cools, it recrystallizes in the form of a single crystal. The crystal grows and spans the entire wafer as additional material refreezes at the moving solid-liquid interface. Because of the low levels of impurities within the wafer, the crystal growth is almost completely heat transfer dominated (1]. In particular, it is the temperature profiles in the Graphite Strip Heater
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Figure 1. Schematic of Silicon-On-Insulator (SOt) Film author to whom correspondance should be addressed Mat. Res. Soc. Symp. Proc. Vol. 201. c 1991 Materials Research Society
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