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

Abstract

Key to realizing practical resonators for liquid-phase sensing applications is efficient electromechanical transduction and reasonable Q in liquid, which determine the motional resistance (Rm). Both lower Rm and high liquid phase Q are important for realizing a more stable close-loop oscillator to allow a lower detection limit. But Rm 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 Rm 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 Rm 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 Rm of only 2.68 kQ in water without feedthrough cancellation. Scaling down the device to 300 μm by 200 μm, we demonstrate an Rm of just 5.5 kQ and Q of 245 in water. The proposed EPR topology boasts the lowest Rm among resonators immersed in liquid after normalizing over the device area.