Structural properties of fluorinated SiO2 thin films

Fluorinated SiO2 (SiOF) films, prepared by plasma enhanced chemical vapour deposition from SiH4, N2O and CF4 precursors, have been analyzed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to extract chemical and structural information. Notwithstanding XPS reveals that fluorine concentrations are quite low (less than 4 at.%), the analysis of the Si-O-Si vibration modes in the IR spectra indicates that CF4 addition involves a deeper modification of the film structure, than the simple formation of Si-F bonds. In particular, by increasing the F concentration in the oxides, the stretching frequency of the Si-O-Si bonds increases, while the bending frequency decreases. On the basis of the central force model, both observations are consistent with the occurrence of a Si-O-Si bond angle relaxation phenomenon, the importance of which increases with the fluorine concentration in the films.