Available online at www.sciencedirect.com Sensors and Actuators B 130 (2008) 599–608 TiO 2 thin films from titanium butoxide: Synthesis, Pt addition, structural stability, microelectronic processing and gas-sensing properties Mauro Epifani a,∗ , Andreas Helwig b , Jordi Arbiol c,d , Ra¨ ul D´ ıaz c , Luca Francioso a , Pietro Siciliano a , Gerhard Mueller b , Joan R. Morante c a Istituto per la Microelettronica e i Microsistemi, IMM-CNR, Via Monteroni, 73100 Lecce, Italy b Department of Sensors, Electronics & Systems Integration, EADS Innovation Works, 81663 M ¨ unchen, Germany c EME/CeRMAE/IN 2 UB, Departament d’Electr ` onica, Universitat de Barcelona, C. Mart´ ı i Franqu` es 1, 08028 Barcelona, CAT, Spain d TEM-MAT, Serveis Cientificot` ecnics, Universitat de Barcelona, 08028 Barcelona, CAT, Spain Received 31 July 2007; received in revised form 10 October 2007; accepted 11 October 2007 Available online 22 October 2007 Abstract TiO 2 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 TiO 2 sols. The thin films were investigated by X-ray diffraction, evidencing that Pt promoted the structural transformation of the starting anatase phase of TiO 2 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 TiO 2 contained both anatase and rutile phases and interspersed Pt nanocrystals (2–3 nm). This result allowed attributing the structural transformation in TiO 2 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 TiO 2 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. H 2 gas-sensing tests evidenced that the anatase TiO 2 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 H 2 concentration of about 1000 ppm. © 2007 Elsevier B.V. All rights reserved. Keywords: Chemoresistive sensors; TiO 2 sensors; Solution deposition; Microelectronic processing; Microstructure 1. Introduction Solution-based procedures for the deposition of thin films for sensing applications present several advantages over tradi- tional procedures like sputtering and evaporation, ranging from the low cost of the precursors and the apparatus, the simplic- ity and rapidity of the deposition, and the possibility of easily ∗ Corresponding author. E-mail address: mauro.epifani@le.imm.cnr.it (M. Epifani). controlling the layer composition. A remarkable breakthrough is obtained in case the prepared films can be fully integrated into electronic devices. Such integration will concentrate the sens- ing layer and any complementary functions in a small space and enable complex but portable instruments with small power con- sumption to be built. The compatibility of the solution deposition process with the silicon/silicon dioxide substrate, however, may pose additional problems relating to the solution chemistry and the surface chemistry of the substrate. Thus, each solution- based approach must be carefully investigated with regards to the film uniformity and adhesion to the substrate. Aiming at a 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.10.016