Solidification Interface Instabilities During Zone Melting Recrystallization Processing of Multilayer thin Film Structur
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SOLIDIFICATION INTERFACE INSTABILITIES DURING ZONE MELTING RECRYSTALLIZATION PROCESSING OF MULTILAYER THIN FILM STRUCTURES Sharon M. Yoon, Christopher K. Hess, and Ioannis N. Miaoulis* Thermal Analysis of Materials Processing Laboratory, Mechanical Engineering Department, Tufts University, Medford, MA 02155
ABSTRACT This paper describes a stability analysis of the solidification interface during graphite-strip zone-melting-recrystallization of Silicon-On-Insulator thin film structures. The study focused on instabilities induced by i) variations in the optical properties due to thickness perturbations in the structure and ii) changes in optical properties during phase change. Reflective and emissive interference effects between multilayers play a significant role in the temperature distributions during processing. The presence of a step perturbation imbedded within the film structure affects local heat absorption and resulting temperature profiles. Such disturbances that trigger instabilities at the solid-liquid interface were investigated numerically. Processing speeds which cause interface instability due to optical property variation during phase change were identified.
INTRODUCTION Zone-melting-recrystallization (ZMR) processing of silicon-on-insulator structures (SOI) is a common method of recrystallizing silicon films from a polycrystalline form to a single-crystal for use in microelectronics fabrication. As a strip heater scans the surface of the SOI structure, a moving narrow molten region is formed (Fig. 1). The film recrystallizes into a single crystal following the motion of the strip heater. Careful control of this process and monitoring of the temperature gradients within the structure is crucial for obtaining high quality crystals. However, due to the small size of the structures involved, trial and error methods or numerical techniques must be used to determine the correct parameters for optimal results. The quality of the crystal is determined by the temperature profiles within the silicon film and the morphology of the solidification interface during recrystallization. The morphology of the interface is significant because defect trails and subboundaries are generated at the cusps of cellular 1 morphologies or at the interior corners of faceted interfaces. When an interface becomes 1 2 unstable, a nonplanar morphology results which is a precursor to subboundary formation. This paper describes a stability analysis of the solidification interface during graphite-strip zonemelting-recrystallization of SOI thin film structures. Two phenomena which may trigger instabilities were analyzed: optical property variation due to film thickness perturbation and optical property variation due to phase change. Since the thickness of the stacked films is Grphite on the order of magnitude of the wavelength of strp the incident thermal radiation, considerable interference can result within the multilayers. Depending on the thickness and number of Wafer layers, the reflectivity and emission from the Liquid structure
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