Gettering impurities from crystalline silicon by phosphorus diffusion using a porous silicon layer

The possible benefits of phosphorus-based gettering applied to crystalline silicon wafers have been evaluated. The gettering process is achieved by forming porous silicon (PS) layers on both sides of the Si wafers. The PS layers were formed by the stain-etching technique, and phosphorus diffusion using liquid POCl3-based source was done on both sides of the Si wafer. The realized phosphorus/PS/Si/PS/phosphorus structure undergoes a heat treatment in an infrared furnace under an O2/N2 controlled atmosphere. This heat treatment allows phosphorus to diffuse throughout the PS layer and to getter eventual metal impurities towards the phosphorus doped PS layer. The gettering effect was evaluated using four probe points, Hall effect measurements and the light beam induced current (LBIC) technique. These techniques enable to measure the density and the mobility of the majority carrier and the minority carrier diffusion length (Ld) of the Si substrate. We noticed that the best gettering is achieved at 900 °C for 90 min of heat treatment. After gettering impurities, we found an apparent enhancement of the mobility and the minority carrier diffusion length as compared to the reference substrate.