The optical characterization of boron-doped silica films grown by plasma-enhanced chemical vapor deposition (PECVD) is reported. The refractive index of the deposited film is found to increase with the B 2 H 6 flow rate, which is in contrast to the theoretical expectation. Infrared absorption studies show that the Si-O main peak for the boron-doped sample shifts to lower wavenumber and broadens, as compared with that for the undoped sample. This suggests that the observed refractive index behavior is due to the contribution of the Si-N bond. To alleviate the undesired nitrogen incorporation, the refractive index of the boron-doped film was studied as a function of deposition power. It is found that the refractive index value decreases with increasing power level. Based on the above results, a boron doping mechanism for PECVD silica films is proposed. The annealing effect on the refractive index of boron-doped silica is also described.