J. Getino

A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons

Abstract Preliminary studies to obtain an integrated solid state gas microsensor are presented in this work. FEM thermal simulations have been carried out to optimize the membrane and heater layouts with the aim of increasing temperature uniformity on the sensitive area and decreasing power consumption. Heater materials and full process flow have been defined in order to achieve compatibility with a CMOS technology with minor changes. On the other hand, first tests of sensitivity with pure tin oxide and Pt-doped tin oxide deposited by a sputtering process on alumina substrates, for benzene and toluene, have given good results for low concentrations (100 ppm) at 400 and 300°C respectively.

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.

Environmental applications of gas sensor arrays: combustion atmospheres and contaminated soils

Abstract Gas sensor arrays have been used to analyze volatile organic compounds in contaminated soils and exhaust gases coming from combustion processes. Sputtered thin films of semiconductor metal oxides were used as gas sensors in the sensor arrays. Combustion gases such as NOx, SO2 and benzene were detected in a highly toxic atmosphere formed by N2, O2, H2S, HF, HCl and water vapour. Sensitivities by 100% were obtained for different sensors when exposed to NOx and SO2. Six volatile organic compounds coming from contaminated soils were successfully identified using different pattern recognition methods such as principal component analysis and backpropagation neural networks. In both cases the use of the normalized fractional conductance change as preprocessing algorithm was decisive. Quantitative determinations of the mixtures of the volatile compounds were performed with relative prediction errors ranging from 2 to 50% for the calibration set. Higher errors were found using the validation data set. Backpropagation neural networks with partially connected hidden layer resulted in general more satisfactory than multiple linear regression methods because the response models were not well satisfied for all the sensors. The use of the normalized fractional conductance change as preprocessing algorithm gave the best results with the neural networks.

Integrated sensor array for gas analysis in combustion atmospheres

Abstract A semiconductor-based sensor array has been developed for highly toxic gas analysis in atmospheres with low oxygen content and in the presence of humidity and corrosive gases. The device consists of 16 discrete sensing elements formed by tin oxide thin layers deposited by sputtering. The sensor array was exposed to a gas mixture formed by N 2 , O 2 , CO 2 , H 2 S, HF, HCl and water vapour with a constant flow rate of 500 ml min −1 . Once their electric resistance at different temperatures between 150°C and 350°C was stabilized, the response to polluting gases coming from combustion processes (NO x , SO 2 , C 6 H 6 ) was studied.

Long-term reliability of sensors for detection of nitrogen oxides

Abstract In this work, the reliability and reproducibility of tin oxide as sensor material for the detection of nitrogen oxides as well as its behaviour when operating for long periods are studied. The experiments were carried out with thin films (3000 A) of tin oxide doped with aluminium. The sensors were operating for six months in synthetic air at the constant temperature of 525 K. During this time they were exposed with regularity to nitrogen oxide in order to determine the frequency at which calibration checks should be carried out and the frequency at which sensors may have to be replaced.

Discrimination of grape juice and fermented wine using a tin oxide multisensor

Abstract A multisensor was applied to discriminate between grape juice and fermented Albillo white wine variety. The multisensor consists of 15 sensing elements formed by tin oxide thin layers deposited by r.f. reactive sputtering. Some components were doped with platinum and chromium, aiming to improve their sensitivity and selectivity to the wine varieties tested. Each sensor was characterised by resistance measurements from 200 to 350°C, in order to find the optimum detection temperature. The detection of the volatile compounds present in wine was useful to distinguish between the different wine phases. The response to the fermented wine was always better than the corresponding one to the grape juice. All the sensors had a high sensitivity and the best responses were obtained in doped sensors at 350°C. At this temperature, the different wine phases (grape juice, fermented wine and wine) were detected. The presence of volatile organic compounds in the different phases of wine elaboration is also discussed.

Hall effect measurements to calculate the conduction control in semiconductor films of SnO2

Abstract Hall effect measurement is one of the most powerful methods for obtaining information about transport mechanisms in polycrystalline semiconductor compounds that constitute the basis for understanding the sensing function of semiconductor gas sensors. The presence of grain boundaries represents the essential difference between single-crystal and polycrystalline semiconductors. The boundaries are important because they generally contain fairly high densities of interface states which trap free carriers from the bulk of the grains. In this paper the grain size of the semiconductor (calculated by the XRGA technique) and Hall effect measurements are used in order to obtain conduction-band profiles. Depending on the preparation method (reactive sputtering, electron beam, serigraphy), three types of conduction control can be distinguished. Similar results are obtained from analysis of the material microstructure.

Electrical characterization of a thin film tin oxide sensor array for VOCs detection

Abstract A tin oxide sensor array has been fabricated using the r.f. reactive sputtering technique. Some elements of the array have been doped with Pt, Pd and Al by sputtering. Sensors have been characterized by DC electrical measurements for different volatile organic compounds (VOCs) such as benzene, toluene, chloroform, propanal, methyl–ethyl–ketone (MEK) and octane, in a concentration range from 50 to 500 ppm in dry air at 300°C. The best response has been obtained by Pt and Pd doped sensors for the oxygenated compounds, but for all gases good sensitivities are shown. The impedance spectra were obtained by AC variable-frequency measurements for all the sensors in air, in benzene and in propanal atmospheres at 300°C. The VOCs peak frequencies changed with regard to air peak frequencies for all the sensors. The roles of dopants and doping level are discussed.

Integrated Sensor Array For Gas Analysis In Combustion Atmospheres

A semiconductor-based sensor array has been developed for highly toxic gas analysis in atmospheres with low oxygen contents and in the presence of humidity and corrosive gases. The device consists of 16 discrete sensing elements formed by tin oxide thin layers deposited by sputtering. The sensor array was exposed to a gas mixture formed by N/sub 2/, O/sub 2/, CO/sub 2/ H/sub 2/S, HF, HCI and water vapour with a constant flow rate of 500 ml/min. Once stabilized their electric resistance at different temperatures from 150-350/spl deg/C, the response to polluting gases coming from combustion processes (C/sub 6/H/sub 6/, SO/sub 2/, NO/sub x/) was studied.

Microsystems for in situ measurement of vocs in groundwaters and soils

Publisher Summary This chapter summarizes the substitution of conventional analytical procedures by suitable selective chemical sensors. Devices allow continuous monitoring of concentrations in field, which is significant for environmental analysis. Components of multi-sensors are doped with Pt, Pd, and Al by sputtering. Each sensor is characterized by electrical measurements for various volatile organic compounds (VOCs) in a concentration range from 50 to 500 ppm in synthetic air at 300 °C. The best responses are obtained from Pt and Pd doped sensors mainly for oxygenated compounds. The influence of the oxygen species, ionosorbed on the sensing surface, on the VOCs detection is also discussed. The single component classification and the multi-component analysis of the VOCs are carried out. The principal component analysis (PCA) classification is permitted to distinguish the three families of VOCs. The best results from the multi-component analysis are obtained for the ILS and PCR methods.