Semiconducting metal oxides, such as for example SnO_2, ZnO, TiO_2, and WO_3, have been intensively studied as preferred class of materials for the functionalization of solid-state conductimetric gas sensors [1,2]. It has been well established that, at high temperature, the charge-carrier concentration near the metal oxide surface exposed to atmospheric air is sensitive to the presence of reactive compounds. The model developed by Windischmann et al. [3] explained that atmospheric oxygen, chemisorbed as O_2 - or O-, acts as an electron acceptor state, lying within the band gap of the metal oxide but located at the surface of the material. Reactions at the surface change the fractional surface coverage of this acceptor state, thus inducing a change in conductivity. Thanks to this phenomenon, the electron depleted surface is highly gas sensitive. Reducing gases like CO or H_2 react with the chemisorbed oxygen at the surfaced removing it, decreasing the depletion region and hence increasing the conductivity; oxidizing gases like NO_2 act in the opposite way, increasing the depletion region and hence decreasing the conductivity. Due to the superficial nature of the gas sensing interaction, nanostructuring the sensing layer, providing nanoscale porosity (i.e. high specific surface), would result in an increased sensitivity or in the possibility of extreme miniaturization. Several deposition techniques for the production of the oxide sensing layer have been investigated during the last years, such as paste/slurry depositions (screen-printing, drop deposition, spray deposition), chemical vapor depositions and physical vapor depositions (sputtering, evaporation). Most of the commercially available conductimetric gas sensors are realized with screen-printing technique on small and thin ceramic substrates [5]. The major advantage of this approach is the possibility to deposit thick-films of metal oxides on batches of alumina platforms, making it very attractive for a high-throughput fabrication. One of the major operative drawbacks of screen-printed ceramic gas sensors is the power consumption for high temperature operation, that is in the order of hundreds of mW, excluding battery-driven operations. The evolution of microfabrication technology and silicon micromachining has led to the development of a class of new devices that, besides the reliability of the well-established production process, the low cost, the possibility of extreme miniaturization and integration of different functionalities on the same platform, offers the possibility to reduce the power needed for high temperature operations. This is achieved by thermal isolation of the active area from the rest of the platform, by using special type of microstructures generally named micro-hotplates [4]. These devices can include elements for measuring and controlling temperature, and measuring the electrical resistance of the active sensing layer, as well as transducers for physical measurements, such as temperature or gas flow (hybrid platform). Furthermore they can add on-chip circuitry for the amplification and elaboration of the electrical signal. Therefore the integration of nanostructured sensing layers on micro-hotplates can be considered the starting point for the fabrication of a novel class of miniaturized devices with superior performances and new integrated functionalities. This result can be obtained by developing nanostructured materials production and deposition methods fully compatible with MEMS microfabrication technologies. Batch deposition with very high lateral resolution, structural damage and cross-contamination of platform during deposition, control on sensing layer thickness, structure, porosity and reproducibility are the main issues to be addressed. In this paper we present a method based on supersonic cluster beam deposition (SCBD) for the deposition of nanostructured metal oxides on microfabricated platforms, which can represent a solid and reliable t
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