Characterisation of LPCVD silicon oxynitride films by optical spectroscopy

The Si-H and Si-N vibration modes contributions to IR absorption bands in t he wave number range of 600-960 cm(-1) has been investigated for the as-dep osited LPCVD SiON layers prepared at different N2O/NH3 ratios. The increasi ng of N/O atomic ratio was related to the specific changes of the Wide IR a bsorption band profile. The variation of the Si-O bonds population in the o xynitride films deposited on c-Si has been found by IR spectroscopy after p ost thermal annealing at 1050 degreesC in Wet O-2. The Si-O bonding in SiON film has been related to a sequence of two steps, indicated by the time de pendency of the integral absorption band of the asymmetric stretching vibra tion mode at 1075cm(-1). We have found a fast increase of Si-O population a fter 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 p reliminary low rate increase process in Si-O bonding, roughly speaking bein g 20 times smaller than in the second process. The high rate of Si-O bond f ormation at long annealing times has been found to be related to the remova l 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 f requency shift in local tetraedra as a result to Si-O bonding. The annealin g behaviour of both, the real and the imaginary part of the refractive inde x n*(lambda)=n(lambda)+ik(lambda) has been extracted using UV-VIS spectra s imulation. The result indicates a systematic decrease of n and k with the h eating time at 1050 degreesC in wet O-2, suggesting the increasing of oxyge n content in SiON film, We have proposed a preliminary physical model for t he Si-O bond formation in SiON at 1050C in wet O-2, based on two-step proce ss. The first step assumes that at an early stage a local oxygen re-bonding to the tetraedra (like: Si[Si-2,A,H], where A can be N, OH, (NH)(2), or H) changes the local chemical structure by breaking Si-Si bonds, especially c lose to the gas/solid interface. The partial reconstruction of SiON layer i n a hot and Wet O-2 atmosphere facilitates the incorporation of the oxidati ve species based on a diffusion limited process through a Si-O-Si network, embedded in the SiON film.