Reactive magnetron sputtering of piezoelectric Cr-doped AlN thin films

Doped aluminum nitride (AlN) films are considered as prospective materials for various elecroacoustic devices based on potentially stronger piezoelectric response and thermal stability of frequency. In this work, 0.1 - 3 μm thick Cr-doped AlN films were reactively sputtered on Si, Mo, and Pt substrates using an ac powered dual-target S-gun magnetron with Al-Cr alloy targets. The films exhibited single c-axis texture corresponding to hcp-AlN. Two specific features of the AlCrN films were revealed: an anomalous surface roughness that dramatically increased in thicker films and a strong tendency to high compressive stress. The film microstructure is characterized by large-dimension crystalline grains embedded within a matrix of nano-size grains. In contrast to AlN, stress control via reduced ion bombardment and increased Ar gas pressure during deposition led to inferior crystal orientation and higher roughness of the AlCrN films. A multi-step deposition process reduced stress and roughness and preserved strong texture. Preliminary assessments of AlCrN piezoelectric activity using SMR BAW and CMR devices demonstrated lower values of quality factor and electromechanical coupling coefficient, while TCF was higher compared with pure AlN films. Results may be due to both the material properties and the film structural features discovered.