Surface acoustic wave humidity sensor based on the changes in the viscoelastic properties of a polymer film

In the present work, a 50 MHz SAW device has been successfully applied as a humidity sensor. A YZ-cut lithium niobate substrate was used. The polymer (polyXIO) was deposited either over the 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 range cannot 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 relative humidity (0-100%). Moreover, the change in attenuation was found to be less for the metallized path. The viscoelastic mechanism appears to dominate for almost the whole range of relative humidity. 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 other gases and seems to be highly selective to humidity. This absence of cross effects makes the device a good sensor candidate for humidity measurements in general and to eliminate humidity effects in electronic nose systems.