During the phosphorus predeposition a layer of phosphosilicate glass was grown on the surface of the silicon. This layer will act as a constant source of dopants for subsequent diffusion steps, thus fixing the surface concentration of the phosphorus doped areas to the solid solubility limit.

In order to remove the PSG, a three step etch will be performed:

 

remove PSG:	HF etch
chemical oxidation:	H2SO4 /HNO3
remove chemically oxidized layer:	HF etch