Sensing acoustic properties of materials using piezoelectric lateral-mode resonators

In this paper, a novel method for interrogating the acoustic properties of viscous liquids is introduced using lateral-mode thin-film piezoelectric-on-substrate (TPoS) resonators. The bulk acoustic waves which propagate into the substrate through the suspension tethers interact with a sample liquid that fills trenches that are etched through the substrate around the resonator at a proper distance (i.e. reflectors). Depending on the acoustic impedance of the liquid, the acoustic reflection is altered which consequently effects the resonator quality factor. The unique advantage of this technique is that the resonator is not in direct contact with the liquid and therefore, very viscous liquids can be analyzed which would otherwise severely dampen the resonance and render the measurement impractical. To investigate the proposed sensing technique, we have shown that the addition of photoresist into the reflectors of a TPoS resonator can alter the Q of the device based on the photoresist's cured state. The physical change impacts the acoustic velocity of the material which consequently changes the efficiency of the built-in reflectors by allowing more or less acoustic energy to return to the resonator. This method can be expanded to analyze other fluids, notably bio-fluids whose physical properties are helpful in determining health related issues.