Structure changes in LPCVD silicon oxynitride films induced by annealing in wet oxygen

The variation of the Si-O bonds population in the oxynitride films deposited on c-Si has been found by IR spectroscopy after post thermal annealing at 1050°C in wet O₂. The Si-O bonding in SiON film has been related to a sequences of two steps, indicated by the time dependency of the integral absorption band of the asymmetric stretching vibration mode at 1075cm^-1. We have found a fast increase of Si-O population after about 45min of heating. The time dependency shows an almost linear dependence versus t^1/2. The break of the slope is produced only after a preliminary low rate increase process in Si-O bonding roughly speaking being 21 times smaller than in the second process. The annealing behavior of both, the real and the imaginary part of the refractive index n*(λ) = n(λ) + ik(λ) has been extracted using UV-VIS spectra simulation. The result indicates a systematic decrease of n and k with the heating time at 1050°C in wet O₂, suggesting the increasing of oxygen content in SiON films.