TiO2 thin films from titanium butoxide: synthesis, Pt addition, structural stability, microelectronic processing and gas-sensing properties

TiO2 thin films were prepared by spin-coating of a Ti butoxide-derived sol onto oxidized silicon wafers, followed by a heat-treatment at temperatures ranging from 500 to 800 oC. The film thickness after heat-treatment at 500 oC was 50 nm. Pt addition, with a Pt:Ti nominal atomic ratio ranging from 0.01 to 0.1, was achieved by adding solutions of Pt(II) acetylacetonate to the TiO2 sols. The thin films were investigated by X-ray diffraction, evidencing that Pt promoted the structural transformation of the starting anatase phase of TiO2 to rutile, with a more enhanced effect with increasing the Pt concentration and/or the heat-treatment temperature. High-resolution transmission electron microscopy evidenced that, when a Pt:Ti atomic ratio of 0.05 and a heat treatment at 500 oC were used, the TiO2 contained both anatase and rutile phases and interspersed Pt nanocrystals (2-3 nm). This result allowed attributing the structural transformation in TiO2 to the strain created by the Pt nanocrystals--a conclusion which was further corroborated by the observation that Pd-modified films, prepared under similar conditions, were only composed of anatase TiO2 and did not contain any Pd nanocrystals. The films heat-treated at 500 oC were able to withstand a full microelectronic processing sequence, including dry etching for gas sensors sensitive area definition, Ti/Pt contact formation, and heater processing on the backside of the sensor substrates. H2 gas-sensing tests evidenced that the anatase TiO2 phase was much more sensitive than the rutile one. The presence of Pt further enhanced the gas-sensing properties, lowering the optimum sensor operation temperature to about 330 oC and allowing for the detection of a minimum H2 concentration of about 1000 ppm.