Borophosphosilicate glassy (BPSG) films, used as a premetal dielectric in metal-oxide-semiconductor field-effect transistor devices, can be a serious constraint in the device technology due to the nucleation and growth of boron-phosphate phase (BPO4), especially in BPSG films grown with a high boron-to-phosphorous ratio. It has been commonly believed that BPO4 forms during the film flow/annealing process. This investigation focused on the mechanism of possible BPO4 nucleation in the films, deposited by means of plasma-enhanced chemical vapor deposition (PECVD) before high-temperature annealing and its evolution during heating. Our study indicates that the dopant-related precipitates in the as-deposited films are a key factor for the ultimate manifestation of BPO4 during hi.-h-temperature annealing. B2O3 and P2O3/P2O5 compounds, in conjunction with H2O, are essential for BPO4 materialization in SiO2 matrix. The impacts of each of the above reactants on the film instability are discussed. The optimization of the PECVD BPSG film structure to improve the film reliability is proposed. (c) 2008 The Electrochemical Society.