Conventional spectroscopic ellipsometry (SE) and synchrotron radiation spectroscopic ellipsometry (SRE) measurements were used to study SiN thin films grown with different techniques. The SiN films were chosen to have low oxygen and hydrogen content (<5%). Analysis of the dielectric function (&egr;) spectra measured by SE and SRE with the tetrahedron model and in conjunction with Rutherford backscattering spectroscopy (RBS) results shows that the dielectric function can be indicative of the stoichiometry of the materials. In addition, we apply a theoretical model to estimate major optical parameters characterizing the materials, such as the fundamental and Penn gaps, the refractive index and the thickness. Furthermore, the film thickness estimated by SE coincides within 10% with that calculated by other techniques, while the stoichiometry is found almost identical with the one estimated from RBS and systematically smaller than the one calculated by Auger sputter profiling.