In this article we report the structure, deposition rate, density and optical properties of tantalum oxynitride films prepared by reactive direct current magnetron sputtering. Thin films of tantalum oxynitrides were deposited on Si (100), graphite and glass substrates at room temperature from a metallic Ta target, which has been sputtered in an argon-oxygen-nitrogen mixture. These films have been characterized by a variety of techniques including Rutherford backscattering, X-ray photoelectron spectroscopy, X-ray diffraction, X-ray reflectometry, optical spectroscopy and spectroscopic ellipsometry. Addition of nitrogen leads to a beneficial increase of film properties. The sputter rate increases from 0.27 to 0.49 nm/s and the film density increases from 7.15 to 8.65 g/cm(3). This leads to an increase of refractive index of the films. Even for films with a high refractive index of around 2.5, the band gap is found to be above 2.5 eV, i.e., fully transparent films are obtained. Wafer curvature measurements of the samples show that the films possess a high compressive stress. (c) 2005 Elsevier B.V. All rights reserved.