High sensitivity humidity sensors using flexible surface acoustic wave devices made on nanocrystalline ZnO/polyimide substrates

Flexible ZnO/polyimide surface acoustic wave (SAW) based humidity sensors were fabricated and their performances were investigated. ZnO films deposited on polyimide substrates are (0002) oriented columnar nanocrystals with grain sizes of 50–60 nm. SAW devices with different wavelengths showed high transmission performance, and the sensors exhibited a good repeatability in response to the cyclic change in humidity from ∼5% relative humidity (RH) to ∼87% RH. The responses of the sensors to RH change increase with the increase in humidity. The sensitivity increases with the decrease in wavelength owing to the improved characteristics of the sensors with shorter wavelengths. A high sensitivity of 34.7 kHz/10% RH has been obtained from a SAW sensor with no surface treatment, demonstrating that the flexible SAW humidity sensors are very promising for applications in flexible sensors and microsystems.