In this work, we investigated the deposition of the AlN thin films on silicon (100) substrates by mid-frequency pulsed magnetron sputtering of a metal Al target in an Ar-N2 gas mixture at room temperature. The films were characterized by various means for the composition, the crystal structure, the surface morphology, and the hardness and Young’s modulus. AFM surface RMS (root mean square) roughness analysis revealed that the surface morphology has relation with the nitrogen flow rate in the Ar–N2 gas mixture. The highest surface smoothness was observed at the nitrogen flow rate of 30-50%. The phenomenon was interpreted by the action of the vapor-solid interface on the film growth, as well as the nonequilibrium processes occurred in the film growth.