AlN films were deposited on Si substrates using radio frequency(RF) magnetron sputtering.The orientation,microstructure,morphology and mechanical properties of the films were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),Fourier transform infrared spectroscopy(FTIR) and nanoindention.The results show that the deposited AlN films are transformed from non-crystalline to polycrystalline and the crystal quality is improved with the decrease of the substrate-target distance.Meanwhile,the surface of the films becomes more compact and the size of the grains becomes larger as the substrate-target decreases.For polycrystalline AlN films,a longer substrate-target distance is beneficial for the growth of the(100) and(110) planes,while a shorter distance is conducive to the formation of the(002) plane.The preferential orientation of the AlN thin films is also explained from the viewpoint of the mean free path of sputtered particles and the Al-N band(B1 band and B2 band).Moreover,good adhesion of the AlN piezoelectric film to the substrate,which is critical for the performance of the surface acoustic wave(SAW) devices,can be obtained with shorter target-substrate distance.