Piezoelectric Elliptical Plate Micromechanical Resonator with Low Motional Resistance for Resonant Sensing in Liquid

Key to realizing practical resonators for liquid-phase sensing applications is efficient electromechanical transduction and reasonable ${Q}$ in liquid, which determine the motional resistance ( ${R}_{m}$ ). Both lower ${R}_{m}$ and high liquid phase ${Q}$ are important for realizing a more stable close-loop oscillator to allow a lower detection limit. But ${R}_{m}$ usually increases when scaling down resonator size, leading to weak output signals in liquid. This article describes a piezoelectrically transduced micromechanical elliptical plate resonator (EPR) targeting liquid-phase sensing applications. The proposed EPR delivers lower ${R}_{m}$ relative to other disk-based modes and has a reasonable ${Q}$ in water. These two features are critical for eventually realizing a closed-loop system to enable real-time frequency tracking for sensing applications. The low ${R}_{m}$ arises from enhanced transduction efficiency associated with the modal lateral strain profile. The EPR’s moderate liquid phase ${Q}$ stems from transducing a stiff lateral bulk mode that increases energy storage. The proposed EPR can be scaled down more efficiently compared to other disk-based modes in the limit of mode shape distortion by anchors when scaling down the resonator below a threshold. Experimental results in water are demonstrated for a $500~\mu \text{m}$ by $400~\mu \text{m}$ EPR, which delivers an ${R}_{m}$ of only 2.68 $\text{k}{\Omega }$ in water without feedthrough cancellation. Scaling down the device to $300~\mu \text{m}$ by $200~\mu \text{m}$ , we demonstrate an ${R}_{m}$ of just 5.5 $\text{k}\Omega $ and ${Q}$ of 245 in water. The proposed EPR topology boasts the lowest ${R}_{m}$ among resonators immersed in liquid after normalizing over the device area.