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 °C. The film thickness after heat-treatment at 500 °C 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 °C 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 °C 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 °C and allowing for the detection of a minimum H2 concentration of about 1000 ppm.