Characterization of thin-film devices for gas sensing

In this report are summarized the results of our investigations of gas sens ors, based on thin films of metal oxides and multilayer structures, carried out by using surface analysis techniques during the last few years, For th e studies of surface chemical composition and its variation in depth, the f ollowing experimental techniques have been employed: XPS, selected-area XPS depth profiling, scanning Auger-microscopy (SAM) and SIMS, The morphology of the sensors was investigated by means of SERI and scanning tunnelling mi croscopy (STM), Most of the devices investigated in our work were based on thin films of polycrystalline tin oxide, doped or surface-activated with di verse metals (Pt, Au, Cu, Ag, etc.). In addition, the thin films of mixed T i-W oxides with very promising gas-sensing characteristics were analysed. A new family of multilayer devices consisting of one or more ultrathin meta l films (Pt, Au, Mo, Ni, etc.) and a top layer of tin oxide was also examin ed. Another type of multilayer structure with interesting gas-sensing param eters was based on an ultrathin film of Pt covered with titanium bis-phthal ocyaninate. The experimental cases of surface analysis and optimization of different ty pes of gas sensors are discussed, revealing the main uncertainties in prepa ration technology and emphasizing the benefits of surface-sensitive techniq ues for this application. Copyright (C) 2000 John Wiley & Sons, Ltd.