Detail: A mixture with high concentration of HNO3 and HF & H2SO4 is applied to etching the edge and rear side of wafer. The surrounding N-type silicon is erased to insulation upper and lower surfaces. Alkaline Rinse the wafer dipped into KOH solution to remove the porous silicon and dipped into HF solution to remove phosphorus silicon glass (PSG) formed during diffusion process. However, HF react with PSG very fast but almost don't react with Si. So acidic rinse can remove PSG and also can ensure the P-N junction in good condition.