Aluminum nitride (AlN) thin films were deposited on Si(100) substrate using radio frequency (r.f.) magnetron sputtering. AN/Si interfaces were analyzed by selected-area electron diffraction, cross-sectional high-resolution transmission electron microscopy and Auger electron spectroscopy. The effects of increasing deposition times on the resultant AlN phase and composition, especially in the interface regions, were discussed. AlN film, deposited at r.f, power of 200 W and substrate temperature of 250 degreesC for 1 min, consists of two distinct layers of different phases. At the film/substrate interface area a 3.3-nm thick amorphous AIN with a small amount of oxygen was formed. A polycrystalline AIN phase oriented preferentially to the c-axis was observed in the rest of the film. As deposition time was increased to 2 min, the thickness of the amorphous AIN layer was decreased to 1.5 nn, indicating that the amorphous phase was crystallized during the increased deposition time. Lower substrate temperature of 150 degreesC also certified the trend of the decrease in the amorphous phase with increased deposition time. A heating effect on the substrate by bombardment of energetic neutralized and ions in the deposition ambient and the diffusion of the interface oxygen to the growing film would promote the surface activation for the crystallization.