High relative humidity range sensor based on polymer-coated STW resonant device

A polymer-coated surface transverse acoustic wave (STW) resonator is used in high sensitivity relative humidity (RH) sensors ranging from 60 to 100%. The STW device is designed to resonate at 316 MHz and is realized on AT-cut quartz substrate to minimize thermal dependencies. Two kinds of polymeric films, HMDSO and a-C:H, produced by plasma enhanced chemical vapor deposition at room temperature, are tested. The resonant frequency shift of the coated device due to the sorbed water vapor mass is measured at controlled relative humidity and temperature. The most stable and sensitive a-C:H coating is obtained at 50% CH4 of feeding CH4Ar mixture. Average sensitivities for HMDSO and a-C:H coated device are found to be about 15 and 7.5 ppm/% RH, respectively. The response time to a step of 90% RH for both coatings is less than 30 s.