Thick-film-based metal-oxide sensors

An early example of an integrated gas sensor array is a thick-film implementation developed by Hitachi [49], with six discrete metal oxide sensor areas on an alumina substrate (figure 14.5). The chip is heated to 4(K) °C with a platinum heater printed on the bottom of the substrate. More recently, other Sn02-based arrays have been reported. A low-power, 1.5 mm foursensor array employing Sn02 has been demonstrated for detection of toxic... 

Gas sensors — (c) Metal oxide gas sensors — Figure 7. Gas sensor based on SnC>2 thick-film... 

Although the above-described examples have all been based on relatively simple nanocrystalline metal oxides, additional phases might have been introduced. The use of more complex metal oxides has also been investigated, with nanocrystalline thick films of both barium titanate [96] and cobalt titanate [97] having been considered as possible sensor materials. When the response of such barium titanate films doped with 10% CuO and 10% CdO was studied with respect to CO, LPG, H2S, and H2 [96], sensor selectivity was improved for LPG over the other gases at 250 °C. However, the addition of 0.3 wt% Pd resulted in an even greater selectivity to LPG at a lower temperature, of 225 °C. 

It then addresses the micro-hotplates concept that has led to the development of different types of micromachined gas sensor devices. The different reahzations of micromachined semiconductor gas sensors are presented thin- and thick-film metal-oxide, field effect, and those using complementary metal-oxide semiconductors (CMOSs) and silicon-on-insulator (SOI) technologies. Finally, recent developments based on gas sensitive nanostructures, polymers, printing and foil-based technologies are highlighted. 

In this chapter, we will describe and discuss the fabrication and characterization of metal oxide thick- and thin-film sensors (which are based on the conductivity modulation principle) and SAW devices (which are based on frequency modulation). The sensing properties of thick or thin semiconductor films with various components will be discussed, focusing on sensitivity and stability. In addition, SAW-type CWA sensors will be introduced for different pairs of inteidigitated transducer (IDT) fingers, various wave lengths, and different polymer coatings on the input and output IDTs of the device. 

Some examples of thick-film impedimetric sensors can also be found in the bibliography. In Haeusler and Meyer (1996) interdigitated gold electrodes were used and BaTiOj-CuO was obtained from pulverized powders of both materials. The sensing device was printed onto an alumina substrate provided with a heater on its reverse. They found the response strongly dependent on the firing parameters and on the metal oxide additives added to the base material, such as CaCOj and La203. 

Xu C, Xu G, Liu Y, Zhao X, Guanghou WG (2002) Preparation and characterization of SnO nanorods by thermal decomposition of SnC O precursor. Seri Mater 46 789-794 Yang HX, Qian JF, Chen ZX, Ai XP, Cao YL (2007) Multilayered nanocrystaUine SnO hollow microspheres S3mthe-sized by chemically induced self-assembly in the hydrothermal environment. J Phys Chem 111 14067-14071 Yuan L, Hyodo T, Shimizu Y, Egashira M (2011) Preparation of mesoporous and/or macroporous SnO -based powders and their gas-sensing properties as thick film sensors. Sensors 11 (2) 1261-1276 Yue W, Zhou W (2008) Crystalline mesoporous metal oxide. Progr Nat Sci 18 1329-1338... 

Costello BPD, Ewen RJ et al (2003) Thick film organic vapour sensors based on binary mixtures of metal oxides. Sens Actuators B-Chem 92(1-2) 159-166... 

---