Sputter Process Optimization for Al0.7Sc0.3N Piezoelectric Films

Al_0.7Sc_0.3N films were reactively sputtered from AlSc segmented targets by ac powered dual-cathode S-gun magnetron. 0.5-2.0 μm thick films with homogeneous Sc concentration within 30 +/- 0.5 at. % across 200-mm wafers were grown at ambient temperature directly on Si (100) and on highly (110) textured Mo electrodes. Sputter process optimization, focused on improving film crystalline quality and reducing surface roughness, was performed using XRD, SEM, EDS, TEM, and AFM methods. In contrast to AlN films, which crystallinity typically improves with increasing film thickness due to growth of more thorough grains, Al_0.7Sc_0.3N texture degrades in thicker films. Technological solutions for smooth and highly c-axis oriented Al_0.7Sc_0.3N films were defined and tested. Introduction of AlN seed layer and deposition at low gas pressure remarkably suppressed growth of abnormal grains and improved crystallinity of the films. Highly c-axis oriented films with RC FWHM of 1.6° and surface roughness RMS of 2.3 nm were grown on Si as well as on Mo electrodes.