SAW characteristics of sputtered aluminum nitride on silicon and gallium arsenide

A study was undertaken to evaluate the SAW properties of sputtered aluminum nitride on silicon and gallium arsenide substrates with different interface layers and different AlN film thickness. Thick (2.9 μm) AlN films were deposited on Si. The experimental results suggested that the SAW properties of the thick films were not improved as expected from theoretical consideration. The deviation from the theoretical prediction resulted from the degradation of the preferred orientation of the grains as the films grew thicker. Thinner films (1.0 μm) were then applied to Si, silicon-on-insulator (SOI), and GaAs wafers for studying the effect of substrate materials. Also multilayered metal and dielectric (MLM/D) structures were deposited on Si and GaAs substrates to evaluate the effect of these layers on the AlN. The experimental results showed that AlN SAW transducers fabricated on silicon substrates with a thin dielectric buffer layer provided the best performances. The buried oxide in the SOI substrate increased the velocity dispersion. The addition of MLM/D layers to the Si substrate also increased the velocity dispersion. The addition of MLM/D layers to GaAs slightly lowered the phase velocity but significantly increased the propagation loss. The AlN transducers fabricated on GaAs substrates without MLM/D layers gave the lowest propagation losses. The primary effect on the transducer losses is related to the film growth characteristics as evidenced by the x-ray data, surface morphology and grain orientation of the reactivity sputtered AlN films. The SAW velocity dispersion is controlled by the relation between the SAW velocities of films, layer and substrate materials