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

Abstract

A high-sensitive wireless and passive surface acoustic wave (SAW) humidity sensor operating at 433 MHz was designed and developed. SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 reflective delay line by using plasma enhanced chemical vapor deposition (PECVD) to build the sensing chip. Coupling of modes (COM) 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 were developed to construct successfully the wireless measurement system. In wireless humidity sensing experiments, high humidity sensitivity of 11.8 °/% RH in measurement range of 45%-90% RH was achieved at the wireless distance of 0.5 m. The corresponding response and recovery time were evaluated as 60 s and 90 s respectively. Also, excellent repeatability and stability in humidity sensing were obtained.