Phosphosilicate glass, commonly referred to by the acronym PSG, is a silicate glass commonly used in semiconductor device fabrication for intermetal layers, i.e., insulating layers deposited between succeedingly higher metal or conducting layers. (http://www.statemaster....)
Following emitter layer formation, any PSG (phospho-silicate glass) formed from the high-temperature emitter diffusion step is then removed at step 16.