Investigation on anchor and base design for aluminum nitride flexural mode resonators

This paper investigates the impact of base and anchor on the quality factor (Q) of piezoelectric microelectromechanical system (MEMS) tuning fork resonators vibrating in in-plane flexural mode, and proposes a new strategy to improve Q significantly. Finite element method simulation and measured results reveal that base and anchor geometries have a significant impact on the thermoelastic damping (TED) which dominates the overall resonator quality factor. For the first time, we accurately extract Q value related to TED of piezoelectric tuning fork resonators by cryogenic temperature experiment, which is consistent with FEM simulation results. According to the analysis, a wider anchor or a longer base should promote overall Q by restraining the thermal conduction induced by TED. On the other hand, increase of width or length of anchor will force the out-of-plane flexural modes closer to the target mode, reducing the overall Q by multimode effect. With a wide anchor and a pillar structure under the base to suppress multimode effect, the resonator Q could be improved by 65% to more than 11 000 and impedance at series resonant frequency (f (s)) could be reduced by 78% to 6.5 k & omega;. To our knowledge, the proposed resonator exhibits the highest fx Q and lowest motional impedance among the reported in-plane mode piezoelectric MEMS tuning fork resonators.