For preparation of niobium oxide films radio frequency sputtering was used with Nb2O5 crystalline powder. The sputtering gas was an argon-oxygen mixture (Ar/O2=4/1). During the sputtering the substrate was water-cooled, and chamber pressure was kept at 0.3 Pa. The deposited film thickness was adjusted to about 50-350 nm by selecting an appropriate sputtering time. The precise knowledge of the optical properties and the thickness are important in the design of optical film systems. The thickness and the complex dielectric functions of the as-prepared and annealed niobium oxide films were determined by spectroscopic ellipsometry using the Cauchy dispersion model and the Tauc-Lorentz oscillator model. The insertion of a roughness layer between the ambient and the niobium oxide film in the optical model resulted in better agreement between the experimental and the generated spectroellipsometric data. The composition of the layers was studied using Rutherford Backscattering Spectrometry with 2 MeV 4He+ ion beam. The Nb/O ratio was found in the close vicinity of the stoichiometric one. The films contained small amount of argon. Upon annealing (500 oC, 1 hour), the extinction coefficient decreased by about 20% in the ultraviolet spectral range. The optical properties of the sputtered films were compared to those obtained by plasma enhanced chemical vapor deposition .
 J.-P. Masse, H. Szymanowski, O. Zabieda, A. Amassian, J. E. Klemberg-Sapieha, L. Martinu, Thin Solid Films 515 (2006) 1674.