Influence of liquid properties on the performance of S0-mode Lamb wave sensors: A theoretical analysis

Sensors based on thin film electroacoustic (TEA) devices have emerged as a promising alternative to quartz crystal microbalance and surface acoustic wave devices, in view of sensibility, miniaturization and easy integration. TEA devices include quasi-shear film bulk acoustic resonators (QS-FBAR) and S0 mode Lamb wave resonators (S0-LWR) based on AlN films. Despite the work done on the application of TEA devices as in-liquid biological and chemical sensors, a theoretical framework for S0-LWRs properly describing their sensing mechanisms is still needed. Here we validate a finite element analysis model on QS-FBARs and study the sensing mechanisms of S0-LWRs in liquid media. We show that S0-LWRs can sense changes in the dielectric permittivity of the liquid and demonstrate different sensitivities to viscosity and density. A complementary assessment of the S0-LWRs losses, dependent in a non-specific manner on the square root of the density viscosity product, provides the ability to discriminate density from viscosity changes on the entire device surface. Finally, with an S0-LWR optimization study we show that resolution improves with the decrease of the membrane thickness; however, a trade-off between sensitivity, quality factor and membrane fragility has to be considered.