Metal oxide gas sensors

The lifetime of metal oxide gas sensors in general is several years. Nevertheless poisoning effects can occur when exposed to silicones. An example of commercial Ga203-semiconductor gas sensors is shown in Fig. 5.32. 

During the last years, so-called microhotplates (pHP) have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal-oxide gas sensors [7,9,15]. Microhotplates consist of a thermally isolated stage with a heater structure, a temperature sensor and a set of contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (< 100 mW). Moreover, small time constants on the order of 10 ms enable applying temperature modulation techniques with the aim to improve sensor selectivity and sensitivity. 

Most microhotplate-based chemical sensors have been realized as multi-chip solutions with separate transducer and electronics chips. One example includes a gas sensor based on a thin metal film [16]. Another example is a hybrid sensor system comprising a tin-oxide-coated microhotplate, an alcohol sensor, a humidity sensor and a corresponding ASIC chip (Application Specific Integrated Circuit) [17]. More recent developments include an interface-circuit chip for metal oxide gas sensors and the conccept for an on-chip driving circuitry architecture of a gas sensor array [18,19]. 

The central topic of the book was the integration of microhotplate-based metal-oxide gas sensors with the associated circuitry to arrive at single-chip systems. Innovative microhotplate designs, dedicated post-CMOS micromachining steps, and novel system architectures have been developed to reach this goal. The book includes a multitude of building blocks for an application-specific sensor system design based on a modular approach. 

G. Sberveglieri, W. Hellmich, and G. Muller. Silicon hotplates for metal oxide gas sensor elements . Microsystem Technologies 3 (1997), 183-190. 

I. Simon, N. Barsan, M. Bauer, and U. Weimar. Micromachined metal oxide gas sensors opportunities to improve sensor performance . Sensors and Actuators B73 (2001), 1-26. 

P.F. Ruedi, P. Heim, A. Mortara, E. Franzi, H. Oguey, and X. Arreguit. Interface circuit for metal-oxide gas sensor Digest IEEE Custom Integrated Circuits Conference (2001), 109-112. 

A. Friedberger, R. Kreisl, E. Rose, G. Muller, G. Kuhner, J. Wdllenstein, and H. Bottner. Micromechanical fabrication of robust low-power metal oxide gas sensors . Sensors and Actuators B 93 (2003), 345-349. 

N. Barsan and U. Weimar. Conduction model of metal oxide gas sensors , Journal of Electroceramics 7 (2001), 143-167. 

S. Muller. CMOS-Micro-Hotplate with MOS Transistor Heater for Integrated Metal Oxide Gas Sensors, Diploma thesis, ETH Zurich, Switzerland (2002). 

D. Barrettino, M. Graf, S. Taschini, S. Hafizovic, C. Hagleitner, and A. Hierlemann. CMOS monohthic metal-oxide gas sensor Microsystems , IEEE Sensors Journal 6 (2006), 276-286. 

When working with sensors, one of the most important issues is cross-sensitivity. Due to the sensing principle, this notably affects metal oxide gas sensors, especially in the case of measurements performed in real life conditions. To prove real life feasibility, it is necessary to keep as close as possible to the real life conditions of the application. In the present case, the real life conditions are mainly represented by the use of ambient air as a carrier gas, but also by the chosen experimental set up. 

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

Rothschild, A. and Komem, Y., The effect of grain size on the sensitivity of nanoc-rystaUine metal-oxide gas sensors, J. Appl. Phys. 95 (2004) 6374—6380. 

A number of sensor arrays consisting of an assortment of commercial metal oxide gas sensors have been reported [45 7], For controlled tests, the sensors are mounted in an air-tight chamber fitted with gas inlets and outlets for controlled gas flow. Each sensor s heating element is controlled externally and resistance changes of the gas sensors are monitored by a computer data acquisition system. A significant effort in this area exists at the University of Warwick, Coventry, where for many years, sensor arrays, made from discrete Sn02 sensors or miniature integrated sensors, have been studied for ultimate application to food quality and food process control [47, 48]. 

A. Setkus, Heterogeneous reaction rate based description of the response kinetics in metal oxide gas sensors, Sens. Actuamrs B 87 (2002) 348-359. 

Volatile samples were collected and analysed in the field using the sampling system in conjunction with an array of metal oxide gas sensors. The sensors were calibrated using vapour from an ethanol/water solution and this was used as a reference. A sample of room air was used as a control -background odour. The SPME sampler was introduced to the suspect area, and after sampling the fibre was desorbed into the heated sensor block. When many response patterns are compared it is difficult to visualise... 

Lorenzelli, L., Benvenuto, A., Adami, A. et al. (2005) Development of a gas chromatography silicon-based microsystem in clinical diagnostics. Biosens Bioelectron, 20 (10), 1968-1976. Zampolli, S., Ehni, I., Stiirmann J. et al. (2005) Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column. Sens Actual B, 105 (2), 400-406. Bessoth, F.G., Naji, O.P., Eijkel, J.C.T. and Manz, A. (2002) Towards an on-chip gas chromatograph the development of a gas injector and a dc plasma emission detector. J Anal Atom Spectrom, 17 (8), 794-799. 

In the next section, a variety of solid state environment gases sensors (NO,, CO2, CO, SO2, O2, etc.) are reviewed, and attention is also paid to semiconducting metal oxide type. Also discussed are the extension of the operating temperature to the near-human temperature regimes and better sensing properties derived from the nanostructured semiconducting metal oxide gas sensors. 

Various types of solid-state NO2 sensors have been proposed based on semiconducting metal oxides (including heterocontact materials) [42-50,58,59,234-238], solid electrolytes [1,239,240], metal phthalocyanine [241], and SAW devices [242]. Among these NO2 sensors, the semiconducting metal oxides and solid electrolytes appear to be the best. Specifically, semiconducting metal oxide gas sensors are most attractive because they are compact, sensitive, of low cost, and have low-power consumption. Their basic mechanism is that the NO2 gas is adsorbed on the surface of the material this decreases the free electron density into the space-charge layer and results in a resistance increase [243]. 

Zampolli S., Elmi I., Sturmann J., Nicoletti S., Dori L., and Cardinal G. C., Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column, Sens. Actuators B, 105(2), 400, 2005. 

General discussion about sensing with semiconducting metal oxide gas sensors... 

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