| In recent years, more and more attention has been focused on the development of smart or MEMS (Micro Electro-Mechanical System) infrared gas sensors featuring high selectivity, high sensitivity, fast response, low cost and mass production. The development of miniaturized infrared gas sensors is now realistic due to the advance of MEMS technology.An infrared photo-acoustic (PA) gas sensor system with MEMS components has been designed. Centered around the system, three research topics have been done: the research and development of the MEMS modulating infrared thermal emitter, the novel longitudinal PA cells for MEMS photo-acoustic gas sensors and the single-chip silicon condenser microphone. Based on the research work, a portable non-dispersive infrared gas sensor using the MEMS infrared modulating emitter has been developed.A MEMS thermal emitter based on heating thin film resistors has been studied for the MEMS infrared PA gas sensor as an infrared (IR) source. The IR source, with an effective emitting area of 2.0×2.0 mm2, is fabricated using heated platinum thin film resistors deposited on a Si3N4/SiO2 membrane. IR radiation power of the emitter is up to 60mW and the energy efficiency reaches 16.6% with the effective blackbody temperature ranged from 300 to 850K. Moreover, it has very good dynamic parameters with a response time of 12.8ms.Based on the study of MEMS modulating IR emitters with a compound film, a novel mono-film MEMS IR emitter is first fabricated using platinum thin film heating resistors deposited on a low stress PECVD SiNx membrane. The response time of the mono-film IR emitter is reduced to less than 10ms.Systematic research has been performed to optimize the pattern design of the electrode, which has the most significant effect on the performance of the emitter. By the comparison among different patterns, we find that a bi-spiral electrode emitter will have the maximum heating efficiency if the ratio between the line width and spacing of lines is in the range of 1.0 to 1.5.The dependence of the performance on the effective emitting area has been investigated. The results show that the emitter with larger effective emitting area does not relevantly show better performance. The emitter with an emitting area of 1.6× 1.6mm2 has higher radiation power and longer life span than the one with emitting area of 2.0×2.0mm2. The former has an energy efficiency up to 10.62% while the latter has an energy efficiency of 6.54% for the same power consumption of 195mW/mm2.A novel technique using the oxygen ions bombardment for surface roughening is proposed to improve the performance of the emitter. The IR emitter with a roughened surface can emit strong IR radiation within the range of 2-15μm. The energy efficiency of emitter is up to 2.86% at 6V supply voltage, which is much better than the emitter with Si3N4 and SiO2 passivation layer.A portable non-dispersive infrared (NDIR) system based on Lambert-Beer law has been developed using the MEMS modulating IR emitter developed. The system mainly consists of the MEMS infrared emitter, a thin Fabry-Perot interference filter and a pyro-electricity IR detection for the analysis of gases. A few gases, including CO, CO2, NH3 and SO2, have been measured with the device. The results of the measurement prove that the system has good sensitivity and stability; the detectionlimit is several tens ppm and the response time is less than 20sec. It has been proven that the emitter can serve as a stable IR source in non-dispersive infrared (NDIR) gas sensor and has potential market prospect.In this paper, the study of a one-dimensional longitudinal acoustic resonator has also been done for the MEMS PA gas sensor system. Based on the transmission line model, a new approach of the one-dimensional longitudinal acoustic resonator simulation model named LC vibrating circuit is proposed. The model shows a good conformability with experimental data. The model can fast calculate the character of the PA cell, such as structural parameters, quality factor and resonance frequency et al, and it can visibly modulate the relation of the PA signal with the gas concentration, temperature and the variation of structural parameters. According to the results of simulation, a PA cell has been fabricated with MEMS techniques, which can be used as a component for the MEMS PA sensor system. This work would provide some useful information for the PA design and serves as the basis for the further research and development work of the MEMS PA system.A novel single-chip silicon condenser microphone has been proposed. The condenser microphone consists of a flexible single-crystal diaphragm using a P+ etching stop technique and a rigid polyimide back-plate with acoustic holes. The sacrificial layer is etched using a dry etching technique. This condenser microphone features simple structure and high sensitivity, and is easy to process. Aimed to gain correct design parameters, the structure features of the sensor are simulated. Analyses of the sensor for static performance, dynamic response and pull-in effect are implemented, using finite-element method (FEM). The optimization principles found are that a high sensitivity microphone must have a relatively narrow air gap, small acoustic holes, and a low initial stress membrane. According to the design of the condenser microphone, fabrication process has been proposed. Some key processes have been developed and a series of important process parameters obtained. These studies lay a solid foundation for the further development of the condenser microphone.
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