Comparative studies on polymer coated SAW and STW resonators for chemical gas sensor applications

This paper presents and compares experimental data from performance tests on polymer coated 433 MHz surface acoustic wave (SAW) and 1 GHz surface transverse wave (STW) based two-port resonators for chemical gas sensor applications. The acoustic devices were coated with gas sensitive polymer films of different thickness' and viscoelastic properties as parylene C, poly-(2-hydroxyethylmethacrylate) (PHEMA) and poly-(n-butyl-methacrylate) (PBMA). Then they were gas probed using perchloroethylene and water. The SAW versus STW sensor sensitivities, insertion loss, loaded Q and distortion of the frequency and phase responses during gas probing were evaluated and compared. It was found that STW devices, when coated with thin sensitive polymer layers, retain low loss, high Q and low noise and feature substantially higher relative gas sensitivities compared to their SAW counterparts. Coated SAW sensors can stand substantially thicker soft polymer films than STW ones but at the expense of highly degraded electrical and noise performance and moderate increase in relative sensitivity.