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 paper 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 μm by 400 μm EPR, which delivers an R_m of only 2.68 kΩ in water without feedthrough cancellation. Scaling down the device to 300 μm by 200 μm, we demonstrate an R_m of just 5.5 kΩ 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.