Porous SiOCH thin films for gravimetric gas sensors

The increasing need for on-site and real-time analyses of complex gas mixtures drives the development of several sensing approaches. In case of gravimetric sensors, a chemical sensitive layer is often required to improve the target molecule detection. Numerous materials are studied in the literature but only few are really compatible with the manufacturing of silicon-based sensing devices. This paper is focused on the gas detection ability of porous organosilicates thin films deposited by plasma enhanced chemical vapor deposition that are already used as low-k dielectric in microprocessors. A large panel of porous SiOCH were deposited on quartz crystal microbalances and were studied in the case of toluene and pentane detection. By using three different synthesis approaches, variations in open porosity, mean pore size and chemical composition were investigated. A large affinity to toluene and pentane is observed with partition coefficients higher than 20,000 in the case of toluene. These values are far above those reported for usually known polymers. The kinetic responses are in the range of a few minutes which is fast compared to the literature. Ellipsometry porosimetry (EP) characterizations, performed to obtain adsorption isotherms, highlight the role of porosity and more precisely of microporosity. Materials with smaller pores present higher affinity to pentane and toluene than materials with larger pores. Besides, EP appears to be an efficient technique to classify chemical sensitive layers. Finally, these porous SiOCH that present a good chemical stability and a hydrophobicity are promising sensitive layers for the manufacturing of NEMS-based gravimetric sensors integrated on silicon.