It was found that helium dilution in reactant gas on plasma-enhanced chemical vapor deposition of fluorine-doped silicon oxide film increases the film deposition rate while decreasing hygroscopicity and dielectric constant. He dilution has also been found to decrease the intensity of the infrared absorption band at 980 and 923 cm(-1), but affected band intensity only negligibly at about 948 cm(-1) Absorption bands at 980 and 923 cm(-1) are due to silicon difluoride sites, while that at 948 cm(-1) is due to silicon monofluoride sites. He presumably plays an active role in chemical vapor deposition, not acting only as a dilution gas, increasing film deposition rate and reducing fluorine at silicon difluoride sites. The selective reduction of fluorine at silicon difluoride sites is also assumed to decrease the film hygroscopicity and dielectric constant.