Secondary ion mass spectroscopy (SIMS) and transmission electron microscopy have been used to characterize microstructure and boron penetration in BF2+ implanted amorphous silicon p(+) gated metal oxide semiconductor (PMOS) capacitors, in which oxygen is inadvertently introduced in the gate area during the low pressure chemical vapor deposition (LPCVD) process. Oxygen impurity is found to be incorporated in the LPCVD deposited amorphous silicon film by SIMS. During the break between the two successive depositions for the stacked amorphous silicon (SAS) gate, an ultrathin oxide layer is formed at the interface between the two LPCVD amorphous silicon layers. The two thinner layers lead to smaller grains in the SAS structure. The interfacial oxide layer is a diffusion barrier for oxygen, which causes a higher oxygen concentration and, in turn, retards the crystallization in the SAS structure. Both the interfacial oxide layer and the gate oxide can dissolve a significant amount of fluorine after annealihg. Reduction of free fluorine near or in the gate oxide due to fluorine dissolution in the interfacial oxide layer plays a crucial role for suppressing the boron penetration in the SAS capacitor.