High frequency thickness shear mode devices for organic vapor sensing

Thickness shear mode (TSM) quartz resonators at a fundamental frequency of 96 MHz were evaluated for their performance in organic vapor sensing applications and results were compared with the performance of 10 and 20 MHz resonators. These devices were produced by chemical milling of AT-cut quartz. Seven test organic vapors were utilized at concentrations ranging from 0.2 to 13.7 vol.% in the vapor phase. In all cases, the rubbery polymer poly(isobutylene) was used as a sensing layer. Detailed results for various sensor parameters such as sensitivity, baseline noise and drift, limit of detection, dynamic range, and repeatability for this 96 MHz device were compared with those for 10 and 20 MHz devices. The test case of benzene/poly(isobutylene) was chosen to make these detailed comparisons, and the general conclusions were found to be similar with other solvents. The 96 MHz device was found to be more sensitive than the lower frequency ones. Device sensitivity was dependent on the benzene concentration and an enhancement factor of 8–27 when compared to the 10 MHz device was seen as the benzene concentration ranged from 0 to nearly 7 vol.% in the vapor phase. Significantly higher enhancements were limited by difficulty in coating thicker sensing layers without damping out the response, for this 96 MHz device. Limit of detection was found to be higher for the 96 MHz device, due to increased baseline noise. Response times decreased with analyte concentration. Sensor response was in reasonable agreement with the perturbation model of Sauerbrey at lower concentrations and deviated at the higher concentrations for the 96 MHz device. It is suggested that higher frequency TSM devices can be very useful as organic vapor sensors in process monitoring applications. TSM devices have advantages of simpler electronics, easier design and fabrication and well-developed models when compared to other acoustic wave devices.