Microstructural evolution of thermally treated low-dielectric constant SiOC:H films prepared by PECVD

Low-dielectric constant amorphous- SiOC:H films were prepared by using plasma-enhanced chemical vapor deposition technique and then treated in vacuum at temperatures ranging between 400 and 900°C for 30 min. The evolution of the film microstructure was investigated by means of vibrational spectroscopy, i.e., by Raman scattering and Fourier transform infrared (FTIR) absorption. FTIR absorption spectra consist of several vibrational bands: namely, Si-O-Si rocking at 450 cm-1, Si-O-Si asymmetric stretching at 1034 cm-1, symmetric deformation of - CH3 group in Si- CH3 configuration at 1270 cm-1, C-H stretching of - CHx groups in the region between 2750 and 3050 cm-1, and -OH related vibrational bands in the range between 3150 and 3700 cm-1. On annealing the samples up to 500°C, it is observed that both spectral shape and intensity exhibit minor changes with increasing temperature, indicating that this material exhibits a good thermal stability. In samples undergoing thermal treatments at temperatures higher than 500°C a progressive hydrogen release occurs which is evidenced by the intensity decrease of the C- Hx related bands. A deeper inspection of the evolution of these spectral features evidenced that thermal treatment of samples induces a preferential release of hydrogen than of - CHx groups, which results in the transformation of - CH3 groups into - CH2 groups. Raman spectroscopy carried out on the same samples evidences the presence of carbon nanoclusters. In fact, in the films treated at temperatures higher than 500°C both D and G bands, typical of sp2 -hybridized carbon, are observed, due to the formation of C-C bonds within the film following the hydrogen release. The intensity of these D and G bands becomes more pronounced in samples annealed at higher temperatures, thus indicating a progressive precipitation of carbon within the amorphous matrix of films. In conclusion, the H-release process is accompanied by a structural rearrangement with the formation of an amorphous silicon oxide matrix, and free-carbon inclusions embedded in it.