Etude d’un capteur chimique à base d’un transducteur à électrodes interdigitées.

Abstract

The extreme volatility of volatile organic compounds (COVs) molecules gives them the ability to diffuse far from their source of emission, resulting in direct and indirect impacts on ecosystems and human health. The exposure to high concentrations of certain COVs even for short term may cause serious irreversible effects or disease. The detection and the evaluation of the chemical species activity present in the environment require very efficient and often expensive analysis materials. As an alternative method of analysis, is to design a chemical sensor with equivalent characteristics in terms of reliability, simplicity, speed and selectivity at lower cost. Chemical sensors are often simple and compact devices that transform the chemical signal into an electrical signal that can be easily exploited. Capacitive type sensors have generated considerable interest because they have a low fabrication cost, can operate at room temperature, consume a very small amount of energy and can be easily integrated into the associated electronics circuit. In this work, chemical sensors based on IDCs (interdigital capacitance) and organosilicon films were elaborated and tested for the detection of organic volatile compounds. Aluminum interdigital electrodes have been deposited on glass substrates using microelectronics technique with different geometrical configurations. The sensitive films used as dielectric layers were deposited by PECVD technique from the HMDSO monomer with different pressures. The sensitivity of the sensors in terms of capacity change was assessed for different concentrations of methanol, ethanol and acetone ranging from 100 ppm to 400 ppm. The simulation results showed that the IDC structural based sensors can be successfully designed by COMSOL Multiphysics software using the finite element method. The observed results on the variation of the capacity of the structure show that an increase in the width and the number of fingers induces a significant increase in the value of the capacity. On the other hand, the thickness of the sensitive layer must be taken into account because it greatly affects the total capacity of the structure. The simulation work revealed the optimal design of the IDC structure for specific applications such as chemical sensors. Four types of sensors were manufactured with sensitive layers deposited with different pressure of the monomer to assess its impact on the sensitivity and affinity of the sensor. Then, four other capacitive sensors based on interdigitated structures were manufactured with different spacing between electrodes to evaluate the effect of the gap on the sensor’s detection properties. The variation in monomer pressure during plasma discharge produces layers with different detection properties which have been correlated with the results of characterizations by FTIR, contact angle and AFM. The results of COV detections were interpreted by the growth of a nanoporous structure with a large specific surface area associated with a highly hydrophobic surface behavior. The sensor manufactured with a low gap (36 µm) and high monomer pressure (50 Pa) has the best detection characteristics with a sensitivity of about 0.32, 0.24 and 0.20 pF/ppm towards methanol, ethanol and acetone, respectively. All the developed sensors show a good affinity to methanol vapors due to their small molecular sizes and high values of the dielectric constant. The performance of the sensors studied in terms of sensitivity values and detection limit obtained from measurements at different analyte concentrations indicates that these sensors have good response characteristics compared with those reported in the literature and a potential application for the detection of volatile organic molecules.