Preparation, Morphology, and Gas-Sensing Behavior of Cr2-xTixO3+z Thin Films on Standard Silicon Wafers

The effect of humidity on chromium titanium oxide (Cr2-xTixO3+z, CTO), on both baseline resistance and sensitivity, is small compared to SnO2. This has been the key to the successful commercial development of thick-film sensors based on CTO, for detection of carbon monoxide and ammonia in synthetic air. Thin-film structures on silicon substrates offer the possibility to use fabricating, bonding and housing equipment, which is common for mass production of microelectronic devices and, hence, a low cost gas sensor production is possible. We present a novel approach to produce CTO thin-film sensors on silicon substrates using conventional photolithography, sputtering and evaporation techniques. A Ta/Pt resistance layer (25/200-nm thick) for heating the device to its operating temperature and interdigital electrodes are evaporated and structured on a silicon substrate which is covered by a 1-mu m SiO2 insulating layer. The polycrystalline p-type CTO is deposited onto the electrodes by oxidizing reactive sputtering or evaporation of Cr/Ti-sandwich structures. A thermal treatment at similar to 900 degrees C in synthetic air was performed in order to alloy the sandwich, to oxidize and to stabilize the gas sensing material. The resulting sensors were characterized by means of energy dispersive X-ray analysis, secondary electron microscopy, and X-ray diffraction pattern. Also, gas responses toward NO2, NH3, CO and CH4, and different humidity, were investigated.