Attenuated total reflection Fourier transform infrared spectroscopy and spectroscopic ellipsometry were employed for chemical and structural analysis of fluorinated SiO2 deposited in SiF4 and O-2 plasmas. Isotopic substitution of O-16(2) with O-18(2) in the feed gas mixture was used to determine whether F is bonded to Si or O atoms in the film. Isotopic substitution experiments revealed that infrared active SiO-F stretching mode in SiO2 matrix appears at 1372 cm(-1) but only when the SiF4-to-O-2 ratio of the feed gas mixture is reduced below 0.1. The SiOF species in fluorinated SiO2 result from insertion of O into SiF bonds, which have been already incorporated into the film. Deposition experiments using O-16(2) and O-18(2) also helped identify silicon mono-and di-fluorides (O3SiF and O2SiF2), which were most abundant in fluorinated SiO2. The stretching mode of O3Si-F appears at 950 cm(-1), whereas the antisymmetric and symmetric stretching modes of O2Si-F-2 appear at 990 cm(-1) and 920 cm(-1), respectively. The concentration of silicon fluorides increase monotonically with increasing SiF4-to-O-2 ratio. The increase in silicon fluoride concentration causes the Si-O-Si phonon peaks to blueshift and narrow by relaxing the Si-O-Si bond angle and by decreasing the film density. Narrowing of the SiO2 phonon at 1080 cm(-1) is due to SiOSi bond angle relaxation and not due to a more homogenous and ordered fluorinated SiO2 structure than unfluorinated SiO2 as suggested in previous reports. This bond angle relaxation and density reduction are accompanied by a decrease in the refractive index. Although the low refractive index signals low dielectric constant, the chemical susceptibility of SiF to O-2 and H2O limits the maximum allowable silicon fluoride concentration in SiOF films to approximately 11 at.%.