High-sensitivity humidity sensors with ZnO nanorods based two-port surface acoustic wave delay line

This paper describes the fabrication and characterization of zinc oxide (ZnO) nanorods based surface acoustic wave (SAW) humidity sensors. In this work, the ZnO nanorods were synthesized and coated on nanocrystalline ZnO/AlN/Si-layered structures. SAW properties (e.g., resonance frequency) over a relative humidity (RH) ranged from 10% to 90% at 25 °C were investigated. The surface morphologies of these SAW structures (AlN/Si, nanocrystalline ZnO/AlN/Si and ZnO nanorods/nanocrystalline ZnO/AlN/Si) were also evaluated. Experimental results showed that the performance of SAW structures was slightly inferior compared with that of nanocrystalline ZnO/AlN/Si-layered structures. However, the sensor deposited ZnO nanorods showed the largest resonance frequency shift of approximately 750 kHz due to the high surface-to-volume ratio of the ZnO nanorod sensing layer.