Structural, mechanical and piezoelectric properties of polycrystalline AlN films sputtered on titanium bottom electrodes

Polycrystalline AlN coatings were deposited on Ti-electrode films by reactive magnetron sputtering. During the deposition, processing parameters such as the reactive gas pressure and time of deposition have been varied. The purpose was to obtain an optimized AlN/Ti system coating with suitable properties for applications such as piezoelectric sensors, which could monitor the wear rate and the remaining coating life of a specific part. The chemical composition, the structure, and the morphology of the multilayered films were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques, respectively. These measurements showed the formation of highly (1 0 1), (1 0 2) and (1 0 3) oriented AlN films with piezoelectric and mechanical properties suitable for the desired purpose. A densification of the AlN coating was also observed, caused by lower nitrogen pressures, which has led to an improvement of the crystallinity along with an increase of hardness. The coating stability at high temperatures was also examined. Consequently, an improvement of the piezoelectric properties of the AlN films was observed, inferred from the enhancement of c-axis (0 0 2) orientation after annealing. Furthermore, the mechanical characteristics (hardness and Young's modulus) were significantly improved after heat treatment. These two parameters decrease rapidly with the increase of the indentation depth, approaching constant values close to those of the substrate after annealing. Thus, thermal annealing promotes not only the rearrangement of Al-N network, but also a surface hardening of the film, caused by a nitriding process of unsaturated Al atoms.