This paper presents a detailed study of the influence of deposition conditions on structural and morphological properties of AlN thin films synthesized on c-sapphire substrates by radio frequency (RF) reactive sputtering. After the optimization of deposition parameters such as RF power and substrate temperature, the substrate bias has been identified as a critical variable to improve the structural properties of the AlN layers. The use of negative bias leads to a decrease of the full-width at half-maximum (FWHM) of the rocking curve of the AlN-(0002) x-ray reflection and an increase of the grain size. However, 2 theta/omega x-ray scans of layers grown under negative bias reveal lattice disorder at the AlN/sapphire interface, which is attributed to the highly accelerated positive ions (Al+, N+, N-2(+)) arriving to the substrate at the initial stages of the deposition process. In order to prevent this interface degradation, we propose a two-step deposition method which consists of starting the growth with an unbiased AlN buffer layer, at least 30 nm thick, followed by AlN deposition under negative bias. This procedure results in high-quality AlN layers with FWHM of the rocking curve of the (0002) reflection of 1.63 degrees, grain size of similar to 40 nm and root-mean-square surface roughness of 0.4 nm. (C) 2013 Elsevier B. V. All rights reserved.