The effects of natural convection and conduction in a zone-melting-recrystallization chamber
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Zone-melting recrystallization (ZMR) with a graphite strip heater is used to improve the material quality of thin film structures for microelectronic applications. The process takes place in a sealed chamber filled with an inert gas such as argon or helium. The effect of natural convection and conduction at the interface between the gas and film structure was studied both numerically and experimentally. Numerical simulations of the temperature profile in the film structure, and the flow pattern and temperature field in the gas were developed. Experimental observations in a scaled setup using a liquid medium verified the flow patterns calculated from the numerical model of the gas flow in the chamber. Results indicated that the gas is stagnant in the region below the strip heater; consequently, conduction from the strip heater to the wafer is prevalent. Outside the stagnant region, natural convection cools the film structure. These two effects combine to create a steeper thermal gradient across the entire wafer which can increase the thermal stresses in the film. The magnitude of this thermal gradient depends strongly on the thermal diffusivity of the gas. The configuration of the strip heater may significantly affect the amount of heat conduction in the stagnant region.
I. INTRODUCTION
Graphite strip heater
The zone-melting-recrystallization (ZMR) technique has been used over the past decade to melt and recrystallize thin polycrystalline-silicon films on wafers for microelectronic applications (see Fig. 1). The wafer is placed above a lower susceptor heater (~1000 °C) that raises the temperature of the film from an ambient temperature of 30 °C to a temperature closer to its melting point (1412 °C). A graphite strip heater (~2 X 2 mm) operating at a higher temperature (~2300 °C) moves parallel to the wafer surface at a distance of ~ 2 mm and establishes a narrow, moving molten zone in the film. The material downstream of the molten zone recrystallizes in the form of a single crystal. This process takes place in a sealed water-cooled chamber. In order to avoid oxidation of the heating elements and contamination of the film, the chamber is filled with an inert gas such as helium or argon. The steep temperature gradients in the processing chamber induce strong convective gas currents. These currents could be responsible for particle/contaminant transport from the walls of the chamber and the heater to the surface of the film. However, since the primary mode of heat transfer during the process is radiation, heat transfer effects induced by the convective currents of the gas were considered negligible in previous analyses of the process. Yet experiments have shown differences in ^Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 8, No. 3, Mar 1993 http://journals.cambridge.org
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Numerically simulated plane Wafer
Lower susceptor heater FIG. 1. Schematic of the zone-melting-recrystallization process.
the quality of the material when the chamber is filled with different
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