A critical review of thin-film semiconducting materials that are used as gas sensors is presented in this paper. These materials can be approximately divided into two main groups: the first includes sensors that detect oxygen with variations of bulk or surface conductance; the second comprises all the materials that detect oxidizing and reducing gases in air at constant oxygen partial are described. The long-term stability and the selectivity to a particular species are the main properties of a reliable gas sensor; these properties are also discussed in detail in relation to the second group of sensors. The properties of tin oxide, the most investigated material for gas sensing, are reviewed; some new ternary semiconducting compounds are also presented, together with their growth techniques. An almost novel method for growing thin films, the RGTO (rheotaxial growth and thermal oxidation) technique will be presented, since it is capable of preparing mixed oxide thin fimms with high surface area and nanosized crystallites. It is well known that the main possibility for sensor development lies in device arrays; therefore, the utilization of different materials for gas detection and the measurement techniques of signals supplied by sensor arrays will be outlined.

Recent developments in semiconducting thin-film gas sensors

Active elements for humidity sensors based upon MgAl2O4 thin films or sintered pellets were investigated. Thin films were deposited on Si/SiO2 substrates by radiofrequency (rf) sputtering. Sintered MgAl2O4 pellets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dense and homogeneous, made up of clustered particles of about 20–30 nm, while the pellets showed a wide pore-size distribution. X-ray photoelectron spectroscopy (XPS) demonstrated that the thin films have a stoichiometry close to that of MgAl2O4. Sintered MgAl2O4 is crystalline, while it is disordered in thin-film form. The presence of two different components of the Al 2p peaks was correlated with the structural difference between pellets and thin films. The relationship between good film–substrate adhesive properties and the chemical composition at the interface was studied. The electrical properties of the sensing elements were studied at 40°C in environments at different relative humidity (RH) values between 2% and 95%, using ac impedance spectroscopy. MgAl2O4 thin films showed interesting characteristics in terms of their use in humidity-measurement devices. Resistance versus RH sensitivity values showed variations as high as 4 orders of magnitude in the RH range tested for thin films, and 5 orders of magnitude for pellets. The differences in the electrical behavior of MgAl2O4 pellets and thin films were correlated with their different microstructures.

Microstructure and Electrical Properties of MgAl2O4 Thin Films for Humidity Sensing

Stannic oxide sensors were developed to monitor CO concentrations (10–200 ppm). Cross sensitivities of these sensors against humidity (20–80% r.h.) and methane (400–2000 ppm) can be suppressed by evaluating “mode of operation” data. For this purpose, a sequence of switched operation temperatures introduces kinetic parameters. The latter are evaluated by model-based, or data-based algorithms. For this approach to achieve high selectivity, micromachined sensor dies are particular suitable as they show low thermal inertia.

Pulsed mode of operation and artificial neural network evaluation for improving the CO selectivity of SnO2 gas sensors

Workstation, apparatuses and methods for the high-throughput synthesis, screening and/or characterization of combinatorial libraries. The invention relates to an array, which permits various high-throughput methods for synthesis, screening and/or characterization in the same array, without requiring sample transfer from the array. In a preferred embodiment, the synthesis, screening, and/or characterization steps are carried out in a highly parallel fashion, where more than one compound is synthesized, screened, and/or characterized at the same time. The invention may be practiced at the microscale. The array may comprise thermal channels, for regulating the temperature of the wells in the array. The wells of the array may comprise a membrane, which is used in various screening and characterization methods. The invention also relates to a covered array, comprising the array and an array cover, as well as an apparatus comprising the array, which comprises the array, an array cover and a stage. The array, array cover, and the stage may be modified as required for a variety of synthesis and/or analysis techniques. The array is easily interchangeable between different analytical instruments, and in an embodiment, the invention relates to an automated workstation, where the array is transferred between different synthesis, screening, and characterization stations. The invention also relates to a variety of methods for synthesis, screening, and characterization, which are adapted for combinatorial chemistry. Any of the embodiments of the invention may be used either alone or taken in various combinations.

Viscosity and mass sensor for the high-throughput synthesis, screening and characterization of combinatorial libraries

Thin (10–400 nm) polycrystalline SnO2 film gas sensors are currently being fabricated and tested for several applications. Electrical conductivity, σ, and Hall-effect measurements at high temperatures on chemically and structurally characterized films are particularly investigated in the present study. The correlation with the gas tests shows that the best sensitivity for H2 is obtained in a thermal-activation regime of the carrier density, favoured by using films with small crystallites and low donor concentration. A sensitivity S = 200 is obtained at 100 ppm H2.

A. Grisel

Papers

Fabrication of Integrated Thin Film Semiconductor Gas Sensors