Efficacy of plasma arc treatment for the reduction of boron in the refining of solar-grade silicon

Removal of boron is the major difficulty with refining metallurgical grade silicon into higher purity solar-grade silicon. A plasma arc refining process was developed which efficiently reduces the boron concentration in silicon to below 0.01 ppmW, enabling a low-cost process for manufacturing polysilicon comparable in quality to that made through the Siemens process. The low background boron concentration achieved allows deliberate tailoring of both n-type and p-type ingot resistivity. Silicon refining experiments were performed in a large general-purpose plasma arc melting system. A reaction vessel with independent heating and stirring capabilities was installed in the vacuum chamber, and a single argon plasma torch was used to drive the boron reduction reaction. Reactive gases such as hydrogen and oxygen were introduced to enhance boron removal. A simple half-life model, characterized by the time required to reduce the boron concentration to half its initial value, was particularly useful in comparing a wide range of silicon feedstock purity, different boron reduction processes and the effect of various process parameters on boron reduction. Implications of this model for equipment and process design will be discussed. The effects of several important process variables on the boron reduction half-life were quantitatively determined by experiment. Variables included melt temperature, gas phase chemistry and melt size. The boron reduction half-life was found to increase linearly with silicon melt temperature above the melting point, most likely due to increased silicon vapor pressure, preventing the reactive gas from reaching the molten silicon surface. Although some boron reduction was achieved by adding hydrogen to the chamber atmosphere, the presence of oxygen was found to be crucial in obtaining a short half-life. In general, boron reduction rates were found to be lower for larger melt and crucible sizes, although this is strongly affected by equipment design.