Polycrystalline aluminum nitride (AlN) thin films with wurtzite structure were deposited on silicon substrate by an unbalanced magnetron sputtering system equipped with a pulse dc power supply. Microstructure and chemistry of the AIN-coated substrates under different deposition time were characterized by x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), energy-dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS). XRD results show that the thin films exhibit enhanced (002) preferred orientation. It was obtained from FE-SEM and TEM results that the AIN films have a columnar structure, and that the size of the columns increases with the distance from the substrate and the deposition time. Furthermore, AFM analysis indicates that the surface roughness of the coatings increases with the deposition time. In addition, EDS and EELS analyses give the chemical composition of the coating and the Al-N bonding state present in the coating. (c) 2065 American Vacuum Society.