I. Sayago

Detection of low NO2 concentrations with low power micromachined tin oxide gas sensors

Abstract Semiconductor gas sensors integrated on silicon substrates with thermally isolated structures are presented and technological processing steps of their fabrication are described. Tin oxide sensitive layers have been deposited by reactive sputtering technique due to the compatibility with IC fabrication. The active area has a size of 500×500 μm2 and is supported by a membrane of silicon nitride. Polysilicon is used as heating material and the power consumption is below 50 mW at the operating temperature of 350°C for every sensor prepared. Good isolation among chip devices was guaranteed from FEM thermal simulations [A. Gotz, I. Gracia, C. Cane, E. Lora-Tamayo, M.C. Horrillo, J. Getino, C. Garcia, J. Gutierrez, A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons, Sensors and Actuators B 44 (1997) 483–487.]. Very low concentrations of NO2 have been detected with such type of device obtaining good sensitivity and short response time for various thin-film thicknesses of tin oxide.

Analysis of VOCs with a tin oxide sensor array

Using a sensor array of 15 thin film tin oxide sensors, both the single-component classification and the multicomponent analysis of volatile organic compounds (VOCs) have been carried out. The classification has been accomplished through the techniques of principal component analysis (PCA) and artificial neural networks (ANNs). The multicomponent analysis has been carried out in two stages: first, linearization of the responses, secondly, multivariate linear regression. Four multivariate (MVA) regression methods have been used: classical least squares (CLS), inverse least squares (ILS), principal component regression (PCR) and partial least squares (PLS). The PCA classification permitted to distinguish three families of VOCs: aliphatic and aromatic, chlorinated and oxygenated compounds. ANNs classification discriminated six VOCs gases with a success rate of 71%. The best results from the multicomponent analysis were obtained for the ILS and PCR methods.

Identification of typical wine aromas by means of an electronic nose

In the field of electronic noses it is not very usual to find many applications in wine detection. Most of them are related to discrimination of wines in order to prevent their illegal adulteration and detection of off-odours but their objective is not the identification of wine aromas. In this paper, an e-nose using headspace as an extraction technique is used for the identification of typical aromatic compounds present in white and red wines. The descriptors of these compounds are: fruity, floral, herbaceous, vegetative, spicy, smoky, and microbiological and they are responsible for the usual aromas in wines; concentrations differ from 2 to 10 times the threshold concentration humans can smell. Some of the measured aromas are: pear, apple, peach, coconut, rose, geranium, cut green grass, mint, vanilla, clove, almond, toast, wood and butter. Principal component analysis (PCA) shows datasets of this group of compounds are clearly separated and radial basis neural networks (RB-NN) show a 98% rate of success in classification.

Use of complex impedance spectroscopy in chemical sensor characterization

Abstract The complex impedance method is a powerful tool in the characterization of gas adsorption in semiconductor structures used as gas sensors. Different tin oxide structures have been used. In tests of sensitivity to dry air, air + H2O, O2, etc, the operating temperature has increased to 175 °C. A variable frequency (10 μHz–32 MHz) a.c. voltage is applied to the sensor structure to perform the complex impedance spectrum measurements. The different parameters are a function of the type of gas in the atmosphere. A model has been proposed based on adsorption on the grain boundary.

Detection of gases with arrays of micromachined tin oxide gas sensors

Abstract A good detection of NO 2 , CO and toluene at low concentrations has been carried out by using a micromachined gas sensor array composed of three devices working at different temperatures. The structure is fabricated using standard microelectronic technologies and tin oxide layers as sensitive material. The total power consumption of the array is in the range of 150 mW and a good uniformity of temperature is achieved, thanks to a silicon plug placed under the active area of each sensor. With this device type, it is possible to discriminate gases in a mixture when each array microsensor is heated at a proper temperature.

Differentiation of red wines using an electronic nose based on surface acoustic wave devices

An electronic nose, utilizing the principle of surface acoustic waves (SAW), was used to differentiate among different wines of the same variety of grapes which come from the same cellar. The electronic nose is based on eight surface acoustic wave sensors, one is a reference sensor and the others are coated by different polymers by spray coating technique. Data analysis was performed by two pattern recognition methods; principal component analysis (PCA) and probabilistic neuronal network (PNN). The results showed that electronic nose was able to identify the tested wines.

Detection of toxic gases by a tin oxide multisensor

A tin oxide multisensor with eight sensor elements has been utilized to detect different gases that are emitted from the chemical industry. The different sensors have been prepared using the RF reactive sputtering technique. Some components of this multisensor were doped with Pt and Cr by sputtering. Each sensor has been characterized by electrical measurements for detection of atmospheric pollution (nitrogen dioxide, carbon monoxide, toluene, and propanal). The detected gases are toxic and their extreme concentrations recommended in air oscillate from 1 ppm to hundreds of ppm. Detections were carried out with a single gas or a gas mixture (two gases) in dry air at 250/spl deg/C. The sensitivity and selectivity to these gases were studied. Backpropagation neural networks and their classification results were discussed.

Results on the reliability of silicon micromachined structures for semiconductor gas sensors

Abstract Thin film semiconductor gas sensors fabricated on thermally isolated silicon substrates have been proposed as good alternative to thick film devices that are on the market as they show low power consumption. However, for their industrial success, it is necessary to assess good yield and high reliability for maintaining the functionality of the device during a long period of time. In this paper, a set of thermo-mechanical tests has been applied to gas sensors based on silicon micromachined structures with dielectric membranes. The aim of the tests is to determine the survivability of the devices under aggressive conditions of use. The tests have been carried out on two specific structures, a single Si 3 N 4 membrane; and the same device that also includes a silicon plug below the sensor active area. Results are compared as a tool for improving the structure in the future.

The influence of the tin-oxide deposition technique on the sensitivity to CO

Abstract The sensor parameters for CO detection on semiconductor films are strongly dependent on the film-preparation techniques. This is observed for semiconductor films prepared by sputtering and screen-printing. The influence of sensor-film porosity on the sensitivity to CO has also been observed. The effect of catalysts on semiconductor sensors combines the catalytic activity of the metals with the surface properties of the semiconductor oxides. It is very important that the catalyst is highly dispersed on the particles of the semiconductor and that the added quantity is adequate. We have studied the effect of catalysts (Pt and Pd on films prepared by reactive sputtering and screen-printing by analysing their respective roles. Depending on whether the catalyst is Pt or Pd, the catalytic mechanism is different. Wiht Pt the effect is purely chemical, whereas with Pd the effect is electronic. Surface analysis technique (GAXRD, XPS, EDX) have been used to compare the two different preparation techniques with regard to thickness, composition and morphology.

The interaction of different oxidizing agents on doped tin oxide

Abstract The interaction of NO2, NOx and O2 and O2 on an SnO2 thin-film surface is studied. Experiments have been carried out for different tin-oxide films, undoped and doped with Pt, In and Al, prepared by reactive sputtering (r.f.) on alumina substrates. These films should allow the development of detectors for nitrogen oxides at concentrations of about a few ppm. Conductance measurements have been done in a constant flow of inert atmosphere (N2) and in synthetic air containing oxidizing agents (NO2 and NOx) to study directly adsorbed NO2 and NO and their reaction with chemisorbed oxygen. The results are discussed in terms of surface reactions.