Q-factor enhancement of piezoelectric MEMS resonators in liquids with active feedback

For a precise determination of the density and viscosity of liquids with MEMS resonators, a high Q-factor is of utmost importance. In such environments, in-plane resonant modes offer a lower damping compared to out-of-plane modes, thus allowing to quantify the increase in Q-factor with and without electrical feedback directly. To stimulate an in-plane oscillation, a sputter deposited aluminum nitride layer on a silicon cantilever is driven by a Lock-In amplifier which is adjusted by the feedback signal originating from the piezoelectric thin film. The exposure to selected liquids, ranging from ethanol to N35 with a dynamic viscosity from 1.2 mPa s up to 73.2 mPa s, features at room temperature intrinsic Q-factors of about 40 and 8, respectively. With a carefully designed amplification circuit, the Q-factors are increased by a factor of 1.4 up to 9.3, which represents Q-factors of 374-12, demonstrating the high potential especially at low viscous liquids.