BPSG is an oxide primarily used as a field dielectric.
It is deposited in a PECVD reactor using a mixture of SiH4, B2H6, and PH3 with N2O in a temperature and pressure controlled environment. BPSG is used principally because of its lower melting point (viscous flow temperature) compared to other oxides.
BPSG can be deposited over delineated polysilicon and can flow at temperatures low enough to not significantly alter the dopant profiles in the underlying device silicon. This smoothing improves metal-level step coverage. BPSG is not a good passivation material because it is hydroscopic in nature. (출처 : 세미파크)