Microcavity assisted acoustic wave channeling can lead to high sensitivity and ultra-low power SAW sensors

Shear horizontal SAW devices are used as biosensors to detect breast cancer markers,¹ E. Coli bacteria,² and in DNA hybridization studies.³ A current trend in these biosensing systems is to move away from clinical laboratories where expensive bulky equipment and highly skilled personnel are needed and move to point-of-care-testing (POCT). Monitoring a physiological signal such as blood glucose levels in a patient with a wireless sensor provides a good example.⁴ A major challenge to the incorporation of wireless sensors for biosensing/medical applications is maintaining high sensitivity while simultaneously lowering power consumption. Inspired by the concept of phononic crystals (PCs), we introduce a SH-SAW channel sensor based on 90 ° ST-X Quartz substrate where microcavities incorporated in the delay path in the form of periodic inclusions confine acoustic wave propagation to a narrow channel. We utilize a three-dimensional (3-D) finite element model (FEM) to compare acoustic wave propagation characteristics, insertion loss (IL) and mass sensitivity of SAW channel sensors. We show acoustic wave channeling translates into increased sensitivity and reduced power loss. To harness the potential of a SAW channel sensor, we have systematically evaluated properties such as size, periodicity and nature of the filling materials. Tungsten filled cavities show the best performance in terms of device sensitivity. The origin of the improvement in power loss and sensitivity are traced to the acoustic impedance mismatch between the substrate and nature of the filling material in the microcavities. Based on our simulation results, SAW devices with tantalum and SiO₂ filled microcavities are being fabricated to validate the simulation results. Preliminary experimental results appear promising and suggest that further optimization of microcavities based design variables is needed to realize SAW devices that operate in the ultra-low power range. Our findings thus offer encouraging prospects for designing low power highly sensitive portable biosensors.