L. Zheng, G. Wei, H. Zhang, State University of New York at Stony Brook, Stony Brook, NY; W. Zhang, Edison Welding Institute, Columbus, OH
Thermal plasma spraying (PS) is a versatile deposition technique which has historically found uses for formation of ceramic thermal barrier (TBC) and wear resistant coatings. There have been recent efforts to expand the PS capabilities for deposition of inexpensive electronic and magnetic materials for meso-scale electronics and sensor applications. Plasma sprayed coatings are formed by injecting a powder feedstock through a thermal plasma flame, where the powder is melted, and molten droplets are projected at a desired substrate. The molten droplets spread into disk like pancake shapes called splats and rapidly solidify. Agglomerating splats build a thick film coating with a distinctive lamellar microstructure, characterized by the splat boundary interfaces. Since the molten droplets fly through an air ambient, they readily react to form a surface oxide, which is preserved. Incoming droplets may also adsorb surface gasses, or fail to wet underlying topography and form both lamellar and globular pores. For electronic materials deposited by PS methods the presence of porosity, splat boundaries, and microstructures formed during rapid solidification, contribute additional blocking interfaces not found in traditional polycrystalline silicon systems deposited by CVD, PVD, or casting methods. Therefore understanding and control of the mechanisms responsible for the formation of these defect structures is of interest. Despite the inherent defects previous work has shown that PS silicon thick films show promise as electronic materials. Recently, substrate heated by plasma gun or by external laser bean has been proposed to enhance the mechanical and thermal properties of the coatings. Studies have been found that with sufficient substrate heating, substrate melting may happen. When droplets impact on a thin layer of liquid on the top of the substrate, conditions will be very similar to early crystal growth processes. Epitaxy film growth is possible. It is therefore possible that using substrate melting as tool to make epi-layer growth happen in PS technology. The coatings will have significantly improved quality. It is possible to extend the traditional PS technologies into meso-scale electronics applications. In this study we will exam the possibility of this technology for the first time. To make simulation simple, silicon droplets impact on a Germanium substrate will be studied using plasma gun to heat the substrate. The morphology of individual silicon splats will be examined and related the splat shape distribution to the initial powder feed and resulting thick film microstructure and porosity. A splat formation model that includes undercooling, nucleation, and non-equilibrium solidification is used to model the observed microstructure formation. The optimal conditions, such as standoff distance and substrate melting thickness, will be determined for substrate melting. After numerical work, experiments and characterization of splat morphology will be performed to study film microstructure under epitaxy growth conditions.
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