Approximately 2.2 mu m thick aluminum nitride (AlN) thin films were deposited on silicon and glass substrates using a reactive direct current (DC) unbalanced magnetron sputtering system. Two asymmetric bipolar-pulsed DC generators were used to sputter the aluminum targets in Ar+ N-2 plasma. The AlN coatings were deposited at different substrate bias voltages, substrate temperatures and nitrogen flow rates. The coatings were characterized using X-ray diffraction (XRD), micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy, ellipsometry and nanoindentation techniques. Peaks corresponding to (100), (002), (10 1) and (102) planes of hexagonal AlN were observed in the XRD data, indicating polycrystalline nature of the AlN coatings. The Raman data showed broad peaks corresponding to E-2 and A, (transverse optic) modes of hexagonal AlN. The N 1s and the Al 2p(3/2) XPS spectra confirmed the formation of AlN phase. The AlN coating deposited under the optimized process conditions exhibited an average nanoindentation hardness of 12 GPa and an elastic modulus of 225 GPa. The refractive index of the AlN coating was found to be in the range 1.94-2.11 in the wavelength region of 300-1200 nm and the extinction coefficient was almost zero throughout the wavelength range. (C) 2007 Elsevier B.V All rights reserved.