Temperature stability of Al0.7Sc0.3N sputtered thin films

Al0.7Sc0.3N films are deposited on Si substrates and SiO2/Mo-based acoustic mirrors in an Endeavor-MX cluster tool equipped with a dual-cathode S-gun magnetron and ring-shaped sputtering targets containing alternated Al and Sc segments. The samples are subjected to one-hour thermal treatments in vacuum at 600°C to test the stability of the crystalline structure and piezoelectric activity. Despite the homogeneous radial composition of the films, significant inhomogeneity in their piezoelectric activity is observed across the wafer. RBS-NRA analysis reveals that the samples of low piezoelectric activity are more prone to surface oxidation, which we attribute to excessive ion bombardment during growth probably leading to structurally imperfect films with significant amount of dangling bonds. XRD analysis confirms the defective crystalline structure of the samples located at the center of the wafer, suggesting the presence of non-piezoelectric phases of ScN, accounting for the low piezoelectric activity of the films that does not improve upon thermal treatments.