Characterisation of LPCVD silicon oxynitride films by optical spectroscopy

The Si-H and Si-N vibration modes contributions to IR absorption bands in the wave number range of 600-960 cm^-1 has been investigated for the as-deposited LPCVD SiON layers prepared at different N₂O/NH₃ ratios. The increasing of N/O atomic ratio was related to the specific changes of the wide IR absorption band profile. 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 sequence 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 40 min of heating. The time dependency shows an almost a 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 s peaking being 20 times smaller than in the second process. The high rate of Si-O bond formation at long annealing times has been found to be related to the removal of the low frequency wing of the Si-H stretching absorption band at about 2100 cm^-1. This feature is also connected to the already reported Si-H frequency shift in local tetraedra as a result to Si-O bonding. The annealing behaviour 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 film. We have proposed a preliminary physical model for the Si-O bond formation in SiON at 1050C in wet O₂, based on two-step process. The first step assumes that at an early stage a local oxygen re-bonding to the tetraedra (like: Si[Si₂,A,H], where A can be N, OH, (NH )₂ or H) changes the local chemical structure by breaking Si-Si bonds, especially close to the gas/solid interface. The partial reconstruction of SiON layer in a hot and wet O₂ atmosphere facilitates the incorporation of the oxidative species based on a diffusion limited process through a Si-O-Si network, embedded in the SiON film.