Wireless and Passive SAW-Based Humidity Sensor Employing SiO₂ Thin Film

A high-sensitive wireless and passive surface acoustic wave (SAW) humidity sensor operating at 433 MHz was designed and developed. SiO2 thin film with large specific surface was employed as the humidity-sensitive interface and deposited along the sensing area of the SAW device patterned by a reflective delay line by using plasma-enhanced chemical vapor deposition (PECVD) to build the sensing chip. Coupling of modes (COMs) model was utilized to simulate the SAW device prior to device fabrication to achieve low time-domain insertion and high signal-to-noise ratio (SNR). The transceiver based on frequency-stepped continuous-wave (FSCW) and planar dipole antennas was developed to construct successfully the wireless measurement system. In wireless humidity sensing experiments, high humidity sensitivity of 11.8°/% RH in the measurement range of 45%–90% RH was achieved at the wireless distance of 0.5 m. The corresponding response and recovery times were evaluated as 60 and 90 s, respectively. Also, excellent repeatability and stability in humidity sensing were obtained.