VHF-band biconvex AlN-on-silicon micromechanical resonators with enhanced quality factor and suppressed spurious modes

This paper reports experimental results demonstrating the use of biconvex-edge designs to enhance the quality factor (Q) in aluminum nitride (AlN)-on-silicon micromechanical resonators. The proposed biconvex design serves to confine the acoustic energy to the center of the resonators, thus reducing out-of-plane bending on the supporting tethers that contribute to acoustic energy leakage, thereby enhancing Q. We here demonstrate that the biconvex design concept can be scaled and applied across a range of operating frequencies from 70 to 141 MHz with the notable effect of boosting Q relative to conventional flat-edge designs. Our measurements of several resonators have shown that the biconvex designs result in an increase in Q by 4-10 times compared to conventional flat-edge designs. In addition, we have also investigated the effect of using different lengths of supporting tethers on Q for both biconvex and flat-edge designs. From the measurement results of devices under test, we have found that the variation in Q as a function of tether length was insignificant compared to the increase in Q going from a flat-edge to biconvex design. As such, the level of enhancement for Q using the biconvex design is much more significant compared to varying the geometry of the support structures. Interestingly, the biconvex shape causes a modal split that gives rise to an additional anti-symmetric mode not found in the flat-edge design. We show experimentally that this spurious anti-symmetric mode can be suppressed by over 54 dB by applying a novel center-loaded electrode design that matches the strain field pattern of the desired symmetric mode. Close agreements between the 3D coupled-domain finite element simulations and the measured results of fabricated devices have been obtained for the resonant frequencies and motional capacitances.