Design of chemoresistive silicon sensors for application in gas monitoring
The growing concerns over our exposure to hazardous substances have been addressed by stringent legislation to ensure air quality. A wide variety of applications have therefore arisen which require the reliable detection of hazardous gases. Hence, the motivation behind the research presented in this thesis was the aim of developing a portable gas monitor to detect nitrogen dioxide, carbon monoxide and volatile organic compounds (e.g. benzene, toluene). The need to improve gas sensor technology for suitability to this demanding application has been identified. Thus, the objectives were to develop a number of ultra-low power devices consisting of an array of chemoresistive gas sensors for incorporation into an intelligent sensor system. The operation of these sensors relies on the measurement of a change in resistance of a gas-sensitive material when exposed to specific gases. Silicon technology has been employed in order to obtain reproducible, miniaturised sensors with a low unit cost. Furthermore, chemoresistors employing metal oxide semiconductor (MOS), metal-substituted phthalocyanine (XPc) and conducting polymer (CP) materials have been used because of their sensitivity to the gases of interest. Common problems associated with these materials are poor specificity to a target gas and poor stability. However, the approach to minimising these problems was to design arrays of cross-sensitive chemoresistors for use in a microprocessor-based intelligent sensor system. The microprocessor applies a pattern recognition algorithm to the sensor outputs to extract the required information. This thesis describes the design, fabrication and characterisation of these sensor arrays. MOS and XPc materials have shown an optimum performance at elevated temperatures. Micromachining techniques have therefore been employed to integrate resistance heaters in a micro-hotplate structure, which can allow temperatures of 600°C to be attained in —15 ms with a typical power consumption of —150 mW/sensor. A pulsed mode of operation should provide average power consumptions of less than 1 mW. A low power consumption is critical for a portable batterypowered instrument. The design, modelling and characterisation of the micro-hotplate structures have also been described. The design and development of a novel automated gas sensor test system was also fundamental to this research, in order to accurately characterise sensor responses and to validate theoretical models. The research objectives have been fulfilled in that a number of sensor array devices have been produced, which are suitable for a portable intelligent instrument. The different designs and materials are compatible for integration into a hybrid sensor. The advancements achieved in sensor technology provide a foundation for future research into the production of a portable intelligent sensor system.