AlN shear mode solidly mounted resonator with temperature compensation for in-liquid sensing

This paper proposes several solutions to adapt AlN-based acoustic resonators to sensors for in-liquid applications. The main requirements of these devices are addressed: the growth of AlN films with properly tilted grains for the excitation of shear modes, the isolation of the electrical connections from the microfluidic system that leads the liquid to the sensor active area, and the stability of the frequency response with temperature. We present guidelines for designing acoustic reflectors that allow reducing the TCF of the overall device without degrading the response of the resonators, as well as an optimized layout of the electrical extensions required to separate the electrical contacts from the sensing area. We fabricate shear mode resonators based on AlN tilted films with electrical contacts extended up to 1.5 mm, exhibiting a reduced TCF of -17 ppm/°C, electromechanical coupling factors of 4% and quality factors of 150 when operating in water.