Surface acoustic wave humidity sensor based on a thin PolyXIO film

Abstract

In the present work, a polymer-coated surface acoustic wave (SAW) dual delay line (DL) oscillator of 50 MHz fundamental frequency has been successfully applied as a humidity sensor. The polymer (polyXIO) was deposited over the bare substrate or over a metal film on the substrate. A turn-around in the frequency shift and a jump in the attenuation were observed when the polymer was deposited over the substrate directly and the relative humidity (RH) range could not be covered completely. However, when a metal was deposited between the polymer and the substrate, both the turn-around and the jump disappeared and the curve was very smooth for the whole range of RH (0–100%). Moreover, the change in attenuation was found to be less for the polymer on the metallized path. The viscoelastic mechanism appears to dominate for almost the whole range of RH. The sensor was found to be reproducible with a moderate hysteresis effect, of the order of 5%. The sensor appears to have no cross effects from small concentrations of other light molecular weight gases.