(93j) Surface Acoustic Wave Sensors with Filled Microcavities and Waveguide

Surface acoustic wave (SAW) devices have been used for sensing for many years. The surface acoustic wave generated by the interdigital transducers (IDTs) of the device can be influenced by the mass loading on the device surface. When the molecules are bound on the surface, the velocity of the surface acoustic wave is changed, leading a shift in the phase or the frequency of the output signal. In this way, SAW devices can be used as the biosensors or chemical sensors.

To realize the potential of SAW devices in portable sensing applications, it is important to reduce the power consumption and improve sensitivity. In this work, we designed several SAW sensors based on ST 90^o-X quartz. 3D finite element models (FEM) were constructed to model SAW devices with different structures, including bare device, device with a silicon dioxide (SiO₂) waveguide, device with microcavities in the delay path and device which incorporate both waveguide layer and microcavities. Then, the insertion loss (IL) and mass sensitivity of these SAW sensors were simulated to systematically evaluate the advantages of using waveguide and microcavities. We also compared the performances of two different materials (Tantalum and SiO₂) filled in the microcavities.

Our simulations demonstrated that the filled microcavities and waveguide layer can increase the displacements of the particles on the surface, and can also concentrate the acoustic energy to the surface of the device. As the acoustic wave propagates more closely to the device surface, less energy of the wave leaks to the interior of the device, which leads to a dramatic improvement in insertion loss. The SAW device becomes more sensitive to the mass loading on the surface because of the larger particle displacements and the concentration of energy. Therefore, the mass loading sensitivity of the SAW sensor is significantly increased by the filled microcavities and waveguide. The optimal thickness of the waveguide layer and the depth of the microcavities were obtained for the various devices and were found to be strongly dependent on the nature of the microcavity filling material.

Our FEM simulation results illustrated that a sensor device comprised of SiO₂ waveguide layer with Tantalum filled microcavities has the lowest insertion loss whereas a device configuration consisting of a SiO₂ waveguide layer with SiO₂ filled microcavities displays the highest sensitivity, compares well with our experimental observation. The SAW devices were fabricated on ST 90^o-X quartz substrate. This study allows for construction of sensitive SAW biosensors, which work is underway.