Borophosphosilicate glass (BPSG) has been widely used as a premetal dielectric to achieve excellent gap fill and planarization due to its reflow capability. In this paper, we present some characterizations of subatmospheric chemically vapor deposited BPSG, aiming at developing a mechanistic understanding of this process. By comparing the effect of controlling variables on the film properties of BPSG and undoped silicon oxide, we conclude that this deposition process is controlled by the gas-phase diffusion of the reaction intermediates on the Si substrate. Therefore, chamber pressure and susceptor spacing are the two major process variables other than the reactant flows. The helium carrier gas process shows better film properties in terms of high deposition rate, smooth film, and good reflow capability.