Design, Fabrication, and Characterization of Surface Skimming Bulk Wave (SSBW) Resonator-Based Humidity Sensor

In this study, we present a high-frequency Surface Skimming Bulk Wave (SSBW) resonator-based humidity sensor which is fabricated on a SAW-grade ST-X quartz substrate utilizing metallic aluminum interdigital transducers (IDTs). The uncoated resonator exhibited a resonance frequency of 355.71 MHz, which exhibited a slight down shift 355.699 MHz due to polyaniline (PANI) thin film deposition. Leveraging PANI's notable hygroscopic properties, it is perceived that the sensor’s resonance frequency increased to 355.853 MHz at 17% relative low humidity (RH) and subsequently decreased to 355.758 MHz at relative high 78% RH. This response underscores the sensor's pronounced sensitivity to water vapor adsorption. The quality (Q) factor of SSBW resonator was ~925, while the electromechanical coupling coefficient was 0.11% after coating of PANI layer. To analyze the equivalent electrical behavior of the SSBW resonator, a modified Butterworth-Van Dyke (mBVD) circuit model was created using Advanced Design System (ADS) software. This model highlights a significant change in static arm capacitance in response to increased water vapor content between the electrodes. The sensor demonstrated exceptional linearity, with a fitting parameter (R2) of 0.9982, and a remarkable sensitivity of 1.48 kHz/%RH, stable behavior, fast response and recovery speed affirming its potential for precise humidity sensing applications. This kind of room temperature operating sensor are highly useful for RH sensing applications.