L. Arés

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.

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 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.

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.

Association of the Lung Immune Prognostic Index With Immune Checkpoint Inhibitor Outcomes in Patients With Advanced Non–Small Cell Lung Cancer

Importance Derived neutrophils/(leukocytes minus neutrophils) ratio (dNLR) and lactate dehydrogenase (LDH) level have been correlated with immune checkpoint inhibitor (ICI) outcomes in patients with melanoma. Objective To determine whether pretreatment dNLR and LDH are associated with resistance to ICIs in patients with advanced non–small cell lung cancer (NSCLC). Design, Setting, and Participants Multicenter retrospective study with a test (n = 161) and a validation set (n = 305) treated with programmed death 1/programmed death ligand 1 (PD-1/PD-L1) inhibitors in 8 European centers, and a control cohort (n = 162) treated with chemotherapy only. Complete blood cell counts, LDH, and albumin levels were measured before ICI treatment. A lung immune prognostic index (LIPI) based on dNLR greater than 3 and LDH greater than upper limit of normal (ULN) was developed, characterizing 3 groups (good, 0 factors; intermediate, 1 factor; poor, 2 factors). Main Outcomes and Measures The primary end point was overall survival (OS). Secondary end points were progression-free survival (PFS) and disease control rate (DCR). Results In the pooled ICI cohort (N = 466), 301 patients (65%) were male, 422 (90%) were current or former smokers, and 401 (87%) had performance status of 1 or less; median age at diagnosis was 62 (range, 29-86) years; 270 (58%) had adenocarcinoma and 159 (34%) had squamous histologic subtype. Among 129 patients with PD-L1 data, 96 (74%) had PD-L1 of at least 1% by immunohistochemical analysis, and 33 (26%) had negative results. In the test cohort, median PFS and OS were 3 (95% CI, 2-4) and 10 (95% CI, 8-13) months, respectively. A dNLR greater than 3 and LDH greater than ULN were independently associated with OS (hazard ratio [HR] 2.22; 95% CI, 1.23-4.01 and HR, 2.51; 95% CI, 1.32-4.76, respectively). Median OS for poor, intermediate, and good LIPI was 3 months (95% CI, 1 month to not reached [NR]), 10 months (95% CI, 8 months to NR), and 34 months (95% CI, 17 months to NR), respectively, and median PFS was 2.0 (95% CI, 1.7-4.0), 3.7 (95% CI, 3.0-4.8), and 6.3 (95% CI, 5.0-8.0) months (both P < .001). Disease control rate was also correlated with dNLR greater than 3 and LDH greater than ULN. Results were reproducible in the ICI validation cohort for OS, PFS, and DCR, but were nonsignificant in the chemotherapy cohort. Conclusions and Relevance Pretreatment LIPI, combining dNLR greater than 3 and LDH greater than ULN, was correlated with worse outcomes for ICI, but not for chemotherapy, suggesting that LIPI can serve as a potentially useful tool when selecting ICI treatment, raising the hypothesis that the LIPI might be useful for identifying patients unlikely to benefit from treatment with an ICI.

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.