Influence of rear side coating on emitter formation during POCL₃ diffusion process

Abstract

The influence of a SiNx coating of a Si wafer on sheet resistance (RSh) of a neighbouring wafer during POCl3 diffusion process is investigated. Wafers facing the SiNx layer of the neighboring wafer in the next slot of the quartz boat show a lower RSh compared to those facing a bare Si wafer, e.g. a reduction from 61 Ω/sq to 52 Ω/sq and a thicker PSG layer are determined. The active doping profile, measured by ECV, shows a deeper plateau region while the tail region is unchanged. Accordingly, the emitter saturation current density rises from 130 fA cm-2 to 193 fA cm-2. We propose that the thicker PSG layer originates from a lower consumption of the reactive gases (POCl3-N2 and O2) at the SiNx coated surface and thus a higher availability of them at the bare Si surface. On the other hand, we also investigate the influence of the thickness of the SiNx rear coating on the emitter at the non-coated Si surface. Already a very thin layer of 20 nm SiNx causes a significant change in RSh and the emitter profile, while there was no difference observed for the SiNx thickness in the range from 20 nm to 160 nm. Facing a rear coated neighbor wafer during diffusion seems to improve the uniformity of RSh on 6-inch large area wafers.