PSG trapping of metal contaminants during belt furnace inline phosphorus diffusion in crystalline Si wafers

Inline wet chemical phosphorus coating and thermal diffusion in a conveyor furnace is a high-throughput alternative to the conventional batch POCl₃ process. There is now significant interest in the adoption of inline diffusion on an industrial scale in order to achieve a continuous flow, low-cost process which interfaces well with other inline processing steps. A concern about inline diffusion in conveyor furnaces has been the potential for conveyor belts to contribute to wafer contamination. The objective of this study is to use Secondary Ion Mass Spectrometry (SIMS) to (i) determine if commonly used nickel alloy conveyor belts are a potential source of wafer metal contamination during the diffusion firing and (ii) to investigate the effectiveness of phosphorus coating and the resulting post-diffusion phosphosilicate glass (PSG) to act as a contaminant isolating barrier, or trapping layer. We report SIMS depth and concentration profiles of Ni, Cr, and Fe, which indicate that the phosphosilicate glass produced during the diffusion process acts as an effective trap of metal contaminants.