(758g) Silicon Particles Preparation By Silane Thermal Decomposition: Crystallinity Modification By Sintering Treatment

One of the most important issues for polysilicon production by chemical vapor deposition in fluidized-bed reactor (CVD-FBR) is the fine particle (silicon powders) formation from silane vapor decomposition, which causes major negative impacts on product yield and purity and has been concerned for years. Many studies have been carried out to explore the detailed mechanism of silane pyrolysis. Yuuki, Giunta, Switart and Girshick et al. proposed that silane pyrolysis approximated a polymerization process of silylenes (divalent Si_nH_2n), and the heavy silicon hydrides (Si atom  ≥ 10) were involved in a comprehensive mechanism with the maximum 20 silicon atoms responsible for the powders. While these results give us a clear understanding on silane thermal decomposition, an effective way is still unable to be figured out for the fine particle suppression as a percentage of silicon powder high up to 20-40% appears in practice. Therefore, more detailed studies are imperatively required on crystallinity modification of the silicon particles. This work aims to improve the crystallinity of silicon powders from silane pyrolysis through sintering treatment, with special focuses on sintering temperature, duration time and vacuum condition. 

The silicon powders, prepared by silane thermal decomposition using high purity nitrogen as diluted gas in a horizontal quartz tube reactor at 923K, was applied as sample powders for sintering. The characterization methods mentioned below were used. The crystal structure of the powders was detected by x-ray diffraction (XRD) measurement at the rate of 4°/min in the range of 20 to 60°. The morphology of the silicon powders was observed by scanning electron microscopy (SEM). Zeta potential analyzer (ZPA) was applied for accounting the powder size distribution (PSD). Diffuse reflection Fourier transform infrared spectroscopy (DR-FT-IR) was used to analyze the bond structure of Si-H and Si-O in the powders.

The results showed that the crystallinity of the silicon particles can be improved obviously with the characteristic peaks of crystalline silicon in the sintered powders strongly enhanced and the grain size greatly increased when elevating the sintering temperature and a polydispersed PSD appears when the sintering time prolonged. Besides, vacuum supply can enhance the hydrogen release. As a result, a study on a two-step silane CVD-FBR process with the first pyrolysis reaction and the second sintering treatment is in progress.