Simulation and Experimental Investigation of a SAW Sensor Containing a Microcavity Array in the Delay Path

Summary

An integrated experimental and theoretical study was carried out to modify the delay path of a SAW device to include an array of cavities having square cross-sectional areas (microcavities). Our results show significant reduction in insertion loss and improved sensitivity to viscous loading. Trends dependent on the device geometry were elucidated through finite element modeling. It is found that devices having microcavities optimized for a particular substrate and wave mode improve sensor characteristics. Using results from the simulations as a basis, we fabricated devices on ST-X Quartz that support shear-horizontally polarized waves.   A standard SAW delay-line and a SAW device containing microcavities were compared by measuring insertion loss and determining the sensitivity to changing glycerol concentrations.  Our experimental results agree well with simulations and introduce delay path modification in the form of microcavities as an effective way to improve SAW sensing performance.

Motivation

Surface Acoustic Wave (SAW) devices can be used as direct, label free biosensors that monitor the interaction between a receptor and its target in real time through changes in the properties of the traveling wave (i.e., frequency, phase, and amplitude)1 . An ideal biosensor will be selective to the species of interest, robust, have high sensitivity and consume a small amount of power. It is common when dealing with SAW sensors to have high insertion losses. Several approaches have been utilized which includes the incorporation of reflective gratings, grooves and corrugated surfaces, and the application of a waveguide material to remedy this problem. Decreasing the amount of power consumed will limit the amount of heat generated that can adversely affect protein activity and increasing sensitivity will allow the detection of antigens at clinically relevant concentrations.  

Results

In this study, we compare finite-element simulations and experiments on SAW devices whose delay path has been modified to include an array of microcavities. Two different substrates commonly used in biosensors were studied, ST-Quartz and 36° YX -LiTaO3. From simulations, we find that microcavities optimized for a particular substrate and wave mode scatter the incident wave causing constructive interference while acting as a waveguide limiting its penetration into the bulk. This results in a decrease in insertion loss as well as a shift in the center frequency, attenuation of higher frequency modes, and increased out of band rejection. The insertion loss of fabricated devices was measured using an Agilent 8753 ES network analyzer. A decrease in insertion loss and a positive shift in center frequency shown experimentally are in agreement with simulated results.