Surface acoustic wave humidity sensor based on polycrystalline AlN thin film coated with sol–gel derived nanocrystalline zinc oxide film

In this work, the nanocrystalline ZnO/polycrystalline (poly) aluminum nitride (AlN)/Si-layered structure was fabricated for humidity sensor applications based on surface acoustic wave (SAW). The ZnO film was used as a sensitive material layer. The ZnO and AlN films were deposited by a sol–gel process and a pulse reactive magnetron sputtering, respectively. These experimental results showed that the obtained SAW velocity on an AlN film was about 5136 m/s and 5032 m/s, corresponding to the uncoated and coated ZnO structures, respectively. For sensitive ZnO layers coated on an AlN/Si substrate, the films have a hexagonal wurtzite structure after the thin films annealed at 500 °C and 600 °C. The surface of the film exhibits sponginess and a nanometer particle size (below 50 nm). The largest shift in the frequency response was at approximately 160 kHz, the point in the relative humidity change from 10% to 90%, for the structure annealed at 500 °C. The effect of the ZnO layer thickness and the change in the environmental temperature on the frequency response of the SAW humidity sensor was also investigated, along with the affects from changes in the relative humidity.