The structural and acoustic properties of sputtered aluminum nitride on silicon

The structural and acoustic properties of reactively sputtered aluminum nitride (AlN) on silicon have been characterized. The AlN films were grown on (100) silicon wafers, having a 200 nm thick layer of PECVD silicon nitride, at substrate temperatures in the 300°C to 500°C range, with a low background sputter pressure, and a high deposition rate. X-ray, atomic force microscopy and transmission electron microscopy measurements defined the structural properties. Surface acoustic wave (SAW) propagation measurements in the frequency range from 50 MHz to 1.5 GHz defined the acoustic properties. A strong correlation was found between the structural properties and the measured SAW velocity and propagation loss characteristics. One micrometer thick films with high intensity (002) plane x-ray diffraction intensities, closely packed uniform grains, approximately 30 nm in diameter, and a surface roughness less than 10 nm, had SAW velocity dispersion characteristics, coupling factors and propagation losses close to those of epitaxial AlN. Electron diffraction characteristics taken over several grains perpendicular to the columnar structure resembled a single crystal diffraction pattern. Deviation of the films structural properties from these epitaxial-like conditions was observed when more than one-half percent of oxygen was incorporated in the films