Selective Vacuum Evaporation by the Control of the Chemistry of Gas Phase in Vacuum Refining of Si.

The evaporation of P from liquid Si under vacuum and reduced pressures of H, He, and Ar was studied to evaluate the feasibility of effective P removal with insignificant Si loss. It was found that the introduction of Ar and He inert gases at low pressures reduces the rate of P removal, and their pressure decrease will increase the process rate. Moreover, the kinetics of P removal was higher in He than in Ar, with simultaneous lower Si loss. Under reduced pressures of H gas, however, the P removal rate was higher than that under vacuum conditions with the lowest Si loss. Quantum chemistry and dynamics simulations were applied, and the results indicated that P can maintain its momentum for longer distances in H once it is evaporated from the melt surface and then can travel far away from the surface, while Si atoms lose their momentum in closer distances, yielding less net Si flux to the gas phase. Moreover, this distance is significantly increased with decreasing pressure for H, He, and Ar gases; however, it is the largest for H and the lowest for Ar for a given pressure, while the temperature effect is insignificant. The rate of P evaporation was accelerated by applying an additional vacuum tube close to the melt surface for taking out the hot gas particles before they lose their temperature and velocity. It was shown that this technique contributes to the rate of process by preventing condensing gas stream back to the melt surface.