Fully-differential Piezoelectric Button-like Mode Disk Resonator for Liquid Phase Sensing.

We present a unique lateral shear resonance mode excited in a micro-electro-mechanical (MEM) disk resonator. We refer to this proposed mode as the Button-like (BL) mode. The BL mode has a characteristic lateral strain profile (based on the sum of orthogonal strain components in the plane of fabrication) resembles a shirt button. Hence our choice of name for this mode. The strain profile of the BL mode is highly suited for piezoelectric transduction. Like the more widely reported Wine Glass (WG) or elliptical mode, the BL mode offers feedthrough cancellation through fully-differential transduction. But compared to the WG mode, the BL mode possesses higher coupling coefficient (keff2) and higher quality (Q) factor for the same disk radius. These advantages make the BL mode highly-attractive for realizing electrically-addressed MEM resonators for liquid phase sensing. This paper examines various design aspects pertaining to the BL mode: tether geometry, characterization setup, size of disk, and even the effect of the gap around the disk on Q factor. The highest Q factor measured in water is 410 based on a disk with a radius of 150 μm. The lowest motional resistance in water is 1.36 kΩ based on a disk with a radius of 400 μm.