Morphology and microstructure of thermal plasma sprayed silicon splats and coatings

Development of thick silicon films by thermal plasma spray (PS) is of interest for meso-scale electronics applications. The electrical properties of these coatings are impacted by the defected microstructure common to plasma sprayed materials. In particular, blocking effects of splat boundaries, porosity, and grain size variations arising from rapidly solidified splat structure can significantly affect electrical transport and result in anisotropic resistivity in the films [B.D. Kharas, S. Sampath, and R.J. Gambino, (2005). "Anisotropic resistivity in plasma-sprayed silicon thick films," Journal of Applied Physics, 97 (9) 094906.]. In this study, we explore the microstructure and morphology of thick (≈ 65 μm) plasma sprayed silicon films and their individual splat building blocks. The observed microstructure is shown to be unchanged with high temperature furnace annealing and phosphorus diffusion doping. Assessment of single splat shape and flattening ratio by SEM and Zygo white light interference microscopy, is shown to correlate to film porosity. Cross-sectional TEM examinations of thick films and individual splats reveal a 100 nm thick chill zone which is made up of fine equi-axed grains and a thin oxide layer at the splat boundary interface. A numerical simulation of the rapid solidification of an idealized silicon splat on an oxidized silicon surface, is used to estimate the thickness of the chill zone and the grain size of the overlying columnar grains.