Development of multi-functional sensors in thick-film and thin-film technology

The paper describes the results of our studies on material processing, design, fabrication and characterization of two kinds of multi-functional solid state sensor in thick-film and thin-film technology. The first sensing device is based on Cr3+ doped perovskite ferroelectric metal oxide Ba1-xSrxTiO3 (x = 0-1) and is fabricated using thick-film technology. Thick films of porous Ba1-xSrxTiO3 exhibit a change in dielectric constant with respect to temperature variations, while their electrical conductivity is a function of humidity. Hence, temperature and humidity can be simultaneously detected by measuring the film capacitance and the film resistance respectively, without incurring crosstalk. The response times of the thick-film sensor to humidity and temperature are 30 seconds and 60 seconds respectively. The second sensor is based on thermally evaporated n-type semiconducting tungsten trioxide (WO3) thin films with a thickness of about 150 nm. It utilizes water physisorption on grain surfaces of WO3 at ambient temperatures as well as O3 chemisorption at elevated temperatures higher than 200 °C. Humidity and O3 can therefore be detected by operating the sensor at different temperature levels. The response times of the thin-film sensor to humidity and O3 are 10 seconds and 30 seconds respectively. In addition, the thin-film Pt heater at the backside of the sensor substrate serves also as a thermometer measuring the temperature of the sensor substrate. Cross-sensitivities among the three measurands, temperature, humidity and gas, are negligible.