Humidity sensing properties of plasma polymer coated surface transverse wave resonators

Results from an experimental study on the humidity sorptive properties of plasma polymer coated surface transverse wave (STW) resonators for possible applications in humidity sensors are presented. We used 767 MHz STW resonators coveted with hexamethyldisiloxane (HMDSO) polymer films, obtained in a glow discharge process. The STW resonator frequency shift, versus the sorbed water vapour mass was measured at controlled relative humidity (RH) and temperature. An average sensitivity of 1.4 ppm/% change in RH was obtained which was found to be 4 to 10 times higher compared to 14 MHz bulk crystal resonators coated with the same polymer. The STW devices demonstrated a hysteresis of 5% between increasing and decreasing RH. It was found that the process of sorption and desorption is reversible, however, 30 to 60 minutes of sorption time are necessary to completely eliminate hysteresis