Attenuated total reflection Fourier transform infrared spectroscopy and spectroscopic ellipsometry were employed for chemical and structural analysis of fluorinated SiO2 deposited in SiF4 and O2 plasmas. Isotopic substitution of 16O2 with 18O2 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-O2 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 16O2 and 18O2 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–F2 appear at 990 cm-1 and 920 cm-1, respectively. The concentration of silicon fluorides increase monotonically with increasing SiF4-to-O2 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 O2 and H2O limits the maximum allowable silicon fluoride concentration in SiOF films to approximately 11 at. %.

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