Research On The Influence Of Holding Temperature On The Quality Of Ingot During The Vacuum Directional Solidification Of Polysilicon

Nowadays,the main method for the preparation of solar-grade silicon in metallurgy industry is directional solidification,but preparing solar-grade polysilicon by directional solidification is a n invisible process which needs high vacuum and high temperature,accompanied by heat transfer,thermal stress,phase change,complex phenomena like liquid phase flow,and there is a mutual coupling relationship between these phenomena.Hence this process needs to be deeply studied and understood from the technique and theory.In this paper,#3303 industrial silicon is used for polyc rystalline vacuum directional solidification experiments.This study notices that the Marangoni effect caused by the free surface tension gradient of silicon melt,which has influence on the flow rate to silicon melt,internal stress-temperature field and shape of solid-liquid interface.Based on the resistance furnace for polysilicon directional solidification,a multi-physics coupling model for internal silicon material on temperature-flow-stress field under the 1:1 vertical Bridgman system is designed.The crystal growth orientation,impurity removal efficiency and impurity distribution situation inside the ingot during the vacuum solidification process of polysilicon vacuum were mainly studied through combing experiments and numerical simulations.Research results show that various indexes such as the crystal growth orientation,impurity removal efficiency,impurity distribution of polycrystalline silicon ingot formed reach optimal under a pull-down speed of 10μm/s,holding time of 0.5h and holding temperature of 1730K;when the holding temperature is too high(≥1760K),on the one hand,the solid silicon material in the solidification process will be re-melted,the release of thermal stress inside the silicon ingot will be weakened,meanwhile with the crystal defects increased;on the other hand,cracking occurs inside the solid-liquid interface after accomplishing the final solidification process,which seriously affects the formation of large-sized columnar crystals and the removal of impurities.Un der the same technique conditions,low holding temperature lead s to the weakening of the diffusion of volatile impurities,which seriously decreases the performance of the polycrystalline silicon ingot,the result of directional solidification and impurity removal as well.The best crystal morphology and the solid-liquid interface morphology can be obtained when holding temperature is 1730 K,which indicates that the experimental results and numerical simulation results are consistent with each other.At the same time,the effect of changing the holding temperature of the polycrystalline vacuum vacuum solidification on the internal heat transfer characteristics of the ingot and the quality of the ingot was also studied.The simulation results show that the solid-liquid interface tends to be flat or slightly convex,the flow state is more uniform,and the flow velocity of the melt is increased when appropriately reducing the solidification temperature at the end o f qualitative solidification.Consequently,it is necessary to properly lower the heat retention temperature at the end of the polycrystalline silicon solidification process.