Highly Sensitive Surface Acoustic Wave (SAW) Humidity Sensors Based on Sol–gel SiO2 Films: Investigations on the Sensing Property and Mechanism

The sensing property and mechanism of surface acoustic wave (SAW) humidity sensors based on sol–gel SiO2 films were investigated. The SiO2 films were fabricated with a sol–gel method. The SEM, AFM, and porosity investigations showed that the SiO2 films had a porous structure with a porosity of 53.4%. Through a comparative study between SAW type and resistive type humidity sensors based on semi-conducting oxide (ZnO, Co3O4, CuO, TiO2) and SiO2 films, it was found that not only the change of film conductivity (electric loading effect) but also the change of mass loading on the films (mass loading effect) contributed to the response of SAW humidity sensors when the RH (relative humidity) value changed. Moreover, the mass loading effect made the major contribution to the responses of SAW humidity sensors based on SiO2 films due to the films’ porous structure. Consequently, the sensors based on SiO2 films showed much higher responses, and the responses increased with the increase of the thickness of SiO2 films. The sensor with the thickest SiO2 film (∼230nm) showed a response of −520kHz when the RH value increased from 30% to 93%.