Thermal evaluation of zone-melting recrystallization of thin-film structures over a wide range of melting points
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Zone-melting recrystallization (ZMR) is a lateral epitaxy technique used to recrystallize polycrystalline thin films on substrates. Large-area multilayer structures of thin films processed with ZMR are usable in microelectronics applications. During the processing, slight variations in thermal gradients can lead to different crystalline qualities. Thus, processing uniformity over the wafer is strongly affected by the sensitivity of both the melt width and the solid/liquid interface to changes in the thermal environment. Processing control must either be set initially in a stable operating range or adjusted dynamically to variations in processing. Numerical simulations of the ZMR process were conducted to evaluate the sensitivity of the process over a wide range of temperatures and materials. Results indicate that material with melting points below 900 °C are very sensitive to temperature disturbances. This is due to the increased influence of conductive heating and decreased influence of radiative heating. The increased reflectivity during phase change curbs the amount of absorbed radiation. As the absorbed radiation becomes less influential, the sensitivity of the slush width decreases. Conductive effects should be considered when processing materials with melting points at or below 900 °C.
I. INTRODUCTION Recrystallizing polycrystalline thin films in heterostructures can be accomplished through zone-melting recrystallization (ZMR). In particular, ZMR processing has been applied successfully to silicon-on-insulator (SOI) technology over the past decade.1 Devices made from SOI material can operate at high voltages and high temperatures, and are radiation hardened. Also, microelectromechanical devices such as sensors or actuators have been developed using SOI material processed by ZMR.1 Some of the newer ZMR technologies include the processing of multiple films undergoing recrystallization,2 nonplanar film structures,3'4 large areas films,5-6 and electronic materials other than silicon.7 The heat transfer and crystallization dynamics vary for these different technologies. Thus, to evaluate them, a fundamental understanding of the heat transfer mechanisms in the process is needed. ZMR is a lateral liquid-phase epitaxy process used to recrystallize amorphous or polycrystalline films into a single crystal. Typically, a lower susceptor heats the film structure uniformly to a temperature near its melting point. A line heater situated above the wafer heats a narrow region, creating a molten zone in the thin film (Fig. 1). As the line heater is scanned over the film
"'Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 10, No. 4, Apr 1995
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the material in the wake of the moving molten zone recrystallizes in the form of a single crystal. The narrow heat zone can be generated by several means: scanned laser beam, electron beam, incandescent lamp, or radiant graphite strip. A graphite strip with a square cross section is simulated in this invest
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