Preparation of Metal Oxide Films from Precursor Solutions and Application to Humidity Sensors

INTRODUCTION: SnO2 thin film has usually been used in various sensors due to its low adsorption energy of water vapor and various gases. Ceramic sensors have the properties of being heatproof and corrosion resistant, etc. and so have been able to be used in acidic or alkaline environments. Porous ceramics have been used for their large surface area and have had high sensitivity, but heat cleaning has been necessary due to their large thermal capacity. To eliminate this necessity and maintain reproducibility, we attempted to manufacture the ceramic thin film and to apply it to humidity sensors. SnO2, SnO2– IrO2, and IrO2 /SnO2, in several tenth nano size thicknesses were prepared from precursor solutions of SnO2 and IrO2 by the advanced sol-gel method , which is most convenient. EXPERIMENTAL: We prepared SnO2 and IrO2 precursor solutions in the same way as TiO2 precursor solution 1) except that tin tetra chloride or hydrated iridium chloride were used as a starting material. The single or mixed oxide precursor solutions were all transparent. They were coated onto quartz substrates and heat-treated at various temperatures. The morphologies were observed by FESEM. The analysis of the residual chlorine was carried out by using XRF and GDSA. The sensor performance was measured by resistance change of the sensor mounted in the chamber. RESULTS and DISCUSSION: The SnO2 thin films of 50 nm thickness were heat-treated at various temperatures, and the resistance changes of the elements caused by the change of conductivity of the surface are shown in Fig.1. The higher the heat-treating temperature, the greater the resistance and slope of the curve become. When the heattreating temperature was lower, the sensor performance was not good as shown by the lower resistance and less steep slope. It is considered that the residual chlorines contained in the films used as a starting material are influencing factors for conductivity in the same way as the florin in FTO glass is. To clarify this, we observed the depth profile of the elements (shown in Fig.2). The residual chlorine contained in the film becomes gradually less when the heat-treating temperature is higher, but the results at 500C and 700C do not exhibit such large differences. The film surface morphologies heat-treated at various temperatures are shown in Fig.3. When heattreated at 1000C, about 20 or 30 nm crystalline grains were observed. At lower heat-treating temperatures, it is observed that the films liberated organic solvents and gradually crystallized. The reproducibility of sensors is shown in Fig.4. SnO2 film heat-treated at 1000C showed good reproducibility without heat cleaning. We think nano size grains cause this ‘refresh effect’ and that this is an important factor for sensor performance. In addition, we will present the sensor performances for SnO2 –IrO2, IrO2 /SnO2. 1) El-M.Mohamed, Y.Nakamura, Y.Fujii, M.Kamiya, S.Rengakuji, Electrochemistry,.72, 455 (2004) Fig.1 The relations between humidity and resistance of SnO2 thin films annealed at various temperatures