Preparation, morphology, and gas-sensing behavior of Cr2-xTixO3+z thin films on standard silicon wafers

The effect of humidity on chromium titanium oxide (Cr_2-xTi_xO_3+z, CTO), on both baseline resistance and sensitivity, is small compared to SnO₂. This has been the key to 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 and, hence, a low cost gas sensor production is possible. CTO thin-film sensors on silicon substrates use 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-μm SiO₂ insulating layer. The polycrystalline p-type CTO is deposited onto the electrodes by oxidizing reactive sputtering or evaporation of Cr/Ti-sandwich structures. 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.