Metallic Impurities in Mono and Multi-crystalline Silicon and Their Gettering by Phosphorus Diffusion

Abstract

The concentrations of 42 metal impurities in mono-crystalline microelectronic grade (EG) and mono-crystalline solar cell grade (SOG) and cast multi-crystalline SOG silicon have been measured by ICP-MS. In mono-crystalline EG material, all 42 elements were found to be below the detection limit. In mono and multi SOG material we could measure Al, Ti, Cr, Mn, Fe, Ni, Co, Cu, Ga, Mo and Sn. No significant difference was observed between the metal content of the top, middle and bottom of cast multi-crystalline Si. In contrast, a significant difference was observed in top, middle and bottom of solar grade mono-crystalline Si. The outer part of cast multi-crystalline silicon contains higher metal impurities than the center part, likely caused by impurities from crucibles. In cast wafers, phosphorus diffusion gettering (PDG) at 900 C leads to complete gettering of Cu and Ni. Fe can be partially removed, effectively by ramp-down annealing to 700 C. In this case we found that the Fe segregation coefficient increases by a factor of 36 for lightly doped Si. In some cast material PDG could reduce metal impurities significantly without a strong improvement of lifetime. Depth profiling of metal impurities in PDG areas shows no Cr because it evaporates during PDG annealing. Also Fe is found to evaporate to some extent.