A thin-film sensing element for ozone, humidity and temperature

A compact, robust and multi-functional thin-film sensor for ozone, humidity and temperature has been developed using thin-film technology. The sensor has a sapphire base of 4.3×6×0.25 mm with a pair of interdigitated Pt-film electrodes and a Pt-film heater fabricated on both sides of sapphire using wet chemical etching and lift-off techniques. Sensing films of WO3 have been thermally evaporated onto the interdigital electrodes and subsequently crystallised at elevated temperatures. The new heater configuration is capable of providing a homogeneous distribution of temperature across the entire sensing area. The heater has an electrical resistance of 200 Ω and simultaneously serves as a thermometer. Working at ambient temperatures, the sensor exhibits a high sensitivity to humidity, while working at elevated temperatures, the sensor is very sensitive to ozone. The cross-sensitivity between ozone, humidity and temperature is negligible.