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Asic gas sensors based on ratiometric principles

By Jaleed Ejaz Khawaja


The wide-scale usage of VOCs in industrial processes requires monitoring the\ud concentrations of these vapours to keep a safe operating environment. Most combustible\ud hydrocarbons can be ignited as a gas-air mixture in the range of 0.5% to 15% by\ud volume. This has led to the development of several portable air quality monitoring\ud instruments. However, the high costs and lack of durability of these instruments has\ud remained an issue to be addressed. This PhD thesis reports on the development and\ud characterization of a novel low cost smart gas sensor technology adaptable for use in a\ud portable instrument. The smart gas sensor devices have been developed to target four\ud different VOCs in air.\ud The smart gas sensor device combines a smart ASIC (SRL 194 designed at\ud SRL, Warwick University) fabricated in standard 0.7 μm CMOS technology and two\ud alkyl-dithiol based self-assembled gold nanoparticle chemoresistive sensors (fabricated\ud at Sony Deutschland GmbH) in a ratiometric array to offer a robust system which can\ud address the common mode variations found in polymer based gas sensor systems. The\ud ratiometric ASIC sensor array architecture allows for the reduction of the baseline\ud value’s dependence on environmental variations and the elimination of baseline drift\ud due to long term application of DC voltage.\ud Three ratiometric array arrangements - mono-type uni-variate with only one\ud chemosensor per device, mono-type bi-variate with two chemosensors of the same film\ud material per device and duo-type with a polar and a non-polar chemosensor per device\ud and their variations were characterized in an automated FIA test station against\ud exposure to methanol, ethanol, propan-1-ol, and toluene at 30°C and 0-5% rh. It was\ud determined that the devices’ response output to VOC analytes was entirely dependent\ud on the variation of the resistance ratio of the chemoresistive sensors in the ratiometric\ud sensor array. The effects of variations of the temperature and rh on the smart sensor\ud output were calibrated. The mono-type devices gave a high magnitude response to the\ud vapours whereas the duo-type arrangement offered a high degree of discrimination\ud between the test analytes with little post-processing steps.\ud Three different alkyl-dithiol chemoresistive sensor films on gold electrodes\ud were successfully used as the VOC vapour sensitive elements in each arrangement. The\ud effects of using a silicone sealant gel as a partitioning layer were characterized and it\ud was observed that at vapour concentrations less than 3000 ppm the silicone\ud encapsulated chemosensor devices reported a larger response to the VOC analytes as\ud compared to those without the silicone. The test devices reported promising response\ud repeatability and reproducibility with excellent return to baseline properties, a negligible\ud hysteresis and an error margin of under 10%. Ideal operating temperature was\ud determined to be 40°C at which rh variations were found to be minimal. The test\ud devices were found to be robust with little variation in the quality of the device output\ud over the course of 18 months.\ud The novel research demonstrated that it is possible to get high level of\ud diversification between analytes from a low cost and robust gas sensor system for\ud monitoring VOCs. The work carried out here has opened the opportunity to develop\ud highly integrated programmable hand-held gas sensor and e-nose systems for\ud environmental monitoring use in health and safety applications

Topics: TP
Year: 2009
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