Computational and experimental development of ultra-low power and sensitive micro-electro-thermal gas sensor
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In this research a state-of-the-art micro-thermal conductivity detector is developed based on MEMS technology. Its efficient design include a miniaturized 100×2 µm bridge from doped polysilicon, suspended 10 µm away from the single crystalline silicon substrate through a thermally grown silicon dioxide sacrificial layer. The microbridge is covered by 200 nm silicon nitride layer to provide more life time. Analytical models were developed that describe the relationship between the sensor response and ambient gas material properties. To obtain local temperature distribution and accurate predictions of the sensor response, a computational three dimensional simulation based on real geometry and minimal simplifications was prepared. It was able to handle steady-state and transient state, include multiple physics such as flow, heat transfer, electrical current and thermal stresses. Two new methods of measurement for micro TCD were developed; a time resolved method based on transient response of the detector to a step current pulse was introduced that correlates time constant of the response to the concentration of gas mixture. The other method is based on AC excitation of the micro detector; the amplitude and phase of the third harmonic of the resulting output signal is related to gas composition. Finally, the developed micro-sensor was packaged and tested in a GC system and was compared against conventional and complex FID for the detection of a mixture of VOCs. Moreover compact electronics and telemetry modules were developed that allow for highly portable applications including microGC utilization in the field.