Investigations on plasma-polymer-coated SAW and STW resonators for chemical gas-sensing applications

Results from gas probing with various analyte vapors on high-Q low-loss surface transverse wave (STW) and surface acoustic wave (SAW) resonators coated with thin plasma-polymer films of hexamethyldisiloxane (HMDSO), styrene, and allyl alcohol at different polymerization conditions are presented in this paper. At the same acoustic wavelength of 7.22 μm and identical film thicknesses, HMDSO-coated STW devices feature substantially higher relative sensitivities to all analytes compared to their SAW counterparts. When operated in a microwave oscillator loop, plasma-poly-styrene and allyl-alcohol-coated STW devices generate strong sensor signals, even at low analyte concentrations, retaining an oscillator short-term stability in the 1×10^-9/s to 1×10^-8/s range. A 250 kHz sensor signal with 7×10^-9/s stability was obtained from a styrene coated 700 MHz STW resonator oscillator at a 1400 parts per million concentration of xylene vapor, which results in a measurement resolution of less than 40 parts per billion for xylene in the ambient air. It is shown that, with respect to sensitivity and stability over long probing periods, plasma-polymer films may become a serious competitor to the more or less unstable soft polymer coatings currently used in SAW-based gas sensors for applications in wireless systems for environmental control and protection