Study of Low-Frequency Narrow Bandwidth Surface Acoustic Wave Sensor for Liquid Applications

Surface acoustic wave (SAW) devices have been applied as telecommunication filter for decades. Due to its simple interdigitated transducer (IDT) layout and geometry-dependent frequency, the SAW filter operates at the designed frequency and its working bandwidth could be designed to fulfill specific applications. Researchers also use SAW devices for sensing the mass or pressure in air. Furthermore, SAW device can be employed in liquid environments. The main focus of this paper is to present a Love mode device for liquid sensing. The Love mode device included a shear-horizontal surface acoustic wave (SH-SAW) delay-line configuration with a photoresist waveguide, which was deposited on split-electrode IDT and reflectors. The substrate was ST-cut quartz, and the SH-SAW propagated between the waveguide and the piezoelectric substrate. Using the Love mode device, we monitored the frequency shift corresponding to a water drop. We demonstrate that the insertion loss level is not critical for S-parameter transmission signal readout. The signal quality within the resonant narrowband is very important for water sensing. In this study, two types of SH-SAW devices were fabricated and tested; SH-SAW resonator and SH-SAW delay-line. We also demonstrate single and split electrodes electrode configurations to generate acoustic waves. Four different waveguide thickness values were tested to prove the benefit of thick polymer waveguide. This research also offers a standard method to fabricate SAW on ST-quartz for liquid application. In the future, we plan to integrate the Love mode device with a cell-culturing chamber to obtain a biosensor.