Effect of thickness on humidity sensing properties of RF magnetron sputtered CaCu3Ti4O12 thin films on alumina substrate

Abstract

Resistive type relative humidity (RH) sensors, which were based on CaCu3 Ti4O12 (CCTO) nanostructure, were deposited on interdigitated silver electrode-patterned alumina substrates at temperatures 300 °C by radio frequency magnetron sputtering. The CCTO thin films thickness effect on morphological, structural, and humidity sensing properties were investigated by X-ray diffraction, field emission scanning electron microscopy-energy dispersive spectroscopy (EDS), and $I$ – $V$ characterization. The grain size increased from 32 to 37 nm with increasing film thickness. Intergranular porosity and microstructure of the CCTO thin film were observed by FESEM. Film thicknesses of 200, 400, and 600 nm were observed from the cross-sectional image of samples. The EDS was used to estimate chemical composition of the films. Hygroscopicity of Ag–CCTO/Al2O3 sensor was studied to observe the relationships between the RH and the resistance recorded in different thicknesses. The measurement of the resistance of the CCTO thin films was done under various RHs (% RH) 30%–90% at room temperature by using a direct current analysis method. The humidity sensing behavior was strongly dependent on % RH and applied voltage. A good humidity response, negligible humidity hysteresis, high stability, and response/recovery time were obtained. Response times were 10, 24, and 30 s while the recovery times were 450, 510, and 630 s for 200-, 400-, and 600-nm CCTO thin films, respectively. The results illustrated that CCTO thin film with 200 nm thickness had the best humidity sensing property. Therefore, CCTO thin film could be described as a promising humidity sensor with wide range of application.