Homogeneity and Thermal Stability of Sputtered Al_0.7Sc_0.3N Thin Films

This work presents a study on the homogeneity and thermal stability of Al_0.7Sc_0.3N films sputtered from Al-Sc segmented targets. The films are sputtered on Si substrates to assess their structural properties and on SiO₂/Mo-based stacked acoustic mirrors to derive their piezoelectric activity from the frequency response of acoustic resonators. Post-deposition annealing at temperatures up to 700 °C in a vacuum are carried out to test the stability of the Al_0.7Sc_0.3N films and their suitability to operate at high temperatures. Despite the relatively constant radial composition of the films revealed from RBS measurements, a severe inhomogeneity in the piezoelectric activity is observed across the wafer, with significantly poorer activity in the central zone. RBS combined with NRA analysis shows that the zones of lower piezoelectric activity are likely to show higher surface oxygen adsorption, which is attributed to higher ion bombardment during the deposition process, leading to films with poorer crystalline structures. AFM analysis reveals that the worsening of the material properties in the central area is also accompanied by an increased roughness. XRD analysis also supports this hypothesis, even suggesting the possibility of a ScN non-piezoelectric phase coexisting with the AlScN piezoelectric phase. Thermal treatments do not result in great improvements in terms of piezoelectric activity and crystalline structure.