Silicon nitride thin films were prepared on polished silicon wafers by reactive radio frequency sputtering. The optical constants and the thicknesses of the deposited layers were determined by spectroscopic ellipsometry in the wavelength range from 250 nm to 1690 nm. The dielectric function of the silicon nitride thin films was modeled by two different ways: on the one hand the Tauc-Lorentz dispersion relation was applied; on the other hand the combination of the Cauchy dispersion relation and the Urbach absorption tail was used. The evaluations of the measured ellipsometric spectra based on the two different modeling yielded similar results concerning the thickness of the surface roughness layer, as well as the thickness and the refractive index of the silicon nitride layers.