Spectroscopic ellipsometry (SE) was used to characterize AlN thin films deposited on SiO2/Si substrates by r.f. magnetron sputtering. Refractive index and thickness of each layer were extracted from the SE data analysis employing a multi-layer model and effective medium approximation. Non-destructive characterization of multi-layer films, with total thickness up to 2.5 mu m, was successfully carried out, despite of the strong correlation between the film parameters such as thickness and surface roughness. The refractive indices of the AlN films were in the range of 1.98-2.15, which are typical values for polycrystalline AlN. The good agreement between the results of SE and those from X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM), regarding crystallinity, rms roughness, and thickness of the films, confirms that spectroscopic ellipsometry is a useful tool to extract the information on the film quality and uniformity.