Phononic crystal based liquid sensor governed by localized defect resonances

Phononic crystal sensors are promising for liquid sensor applications. We have already shown that the frequency of narrow transmission bands depends on properties of liquids confined within a 2D phononic crystal. In comparison to almost all liquid sensor platforms where sensors respond to effects close to the sensor surface, e.g. mass load due to absorption of molecules in a recognition layer, the objective of the liquid cavity resonators is the determination of volumetric (bulk) properties of the liquid. The quality factor is the most crucial parameter of sensor performance. Therefore, classical microacoustic resonance sensors must avoid radiation of acoustic energy into the liquid. The phononic crystal sensor concept tailors acoustic wave propagation in a way to excite a specific mode within the band gap of the phononic crystal. We apply surface acoustic wave (SAW) devices as reliable platform for the realization of phononic crystal sensor. It performs both excitation of a selected liquid cavity resonance and its detection. The liquid cavity microchannel is realized within an overlayer of the SAW device. The liquid in the microchannel becomes a part of vibrating overlayer and determines its acoustic properties. The sensor development contains three parts: development of the SAW platform including etching of periodic elements, design of the overlayer containing the microchannels, and optimization of acoustic coupling between the two elements. We present simulation results of the overlayer with the acoustic field penetrating the liquid. We further report on technology to realize phononic crystal structures with well-defined shape and depth of etched structures in SAW substrates which prove the correctness and feasibility of our approach.