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

Arman Hoseinpur,Stefan Andersson,Jafar Safarian,Kai Tang

doi:10.1021/acs.langmuir.1c00876

Arman Hoseinpur, Stefan Andersson + Show 2 more

Open Access

https://doi.org/10.1021/acs.langmuir.1c00876

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Abstract

The evaporation of P from liquid Si under vacuum and reduced pressures of H2, 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 H2 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 H2 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 H2, He, and Ar gases; however, it is the largest for H2 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.

Highlights

Vacuum evaporation of metals and alloys has been researched for more than a century and has found a lot of applications in industries such as refining metals to remove volatile species,[1−6] evaporation of metals from melt pools in physical vapor deposition (PVD) applications,[7] and so forth
This reveals that H2 compared to Ar has a specific effect on the vacuum evaporation rate of P from liquid
The following remarks are highlighted as the outcomes of this research: 1. The gas-phase mass transport is a rate-controlling step for P removal from Si melt under reduced pressures, and it is affected by the type of adjacent gases of Ar, He, and H2

Summary

Introduction

Vacuum evaporation of metals and alloys has been researched for more than a century and has found a lot of applications in industries such as refining metals to remove volatile species,[1−6] evaporation of metals from melt pools in physical vapor deposition (PVD) applications,[7] and so forth. Vacuum evaporation of liquid metals was first studied and theorized by Hertz in 188218 and Knudsen in 1915,19 and they showed that when a matter evaporates into the vacuum, the atoms emitted from the surface travel in cometary trajectories in a thin layer with a thickness of few mean-free paths (λ), without having interactions and moving freely between the collisions. This layer is called the “Knudsen layer,” and the gas particles have a full-range Maxwellian velocity distribution which relaxes to a continuum flow.

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