Modelling and Characterization of an Electro-Thermal MEMS Device for Gas Property Determination

The increasing requirements for monitoring of gas composition e.g. in fuel gas control have led to a strong demand for cost-effective gas sensing solutions. This work presents the modelling and characterization of a miniaturized thermal gas sensor. In the proposed measurement technique, a polysilicon heater placed on a thin membrane is excited with a periodic heating voltage and the resulting temperature response is evaluated in the frequency domain using thermopiles and a lock-in amplifier. The thermo-physical properties of the surrounding gas alters the heat loss of the heating element to the environment leading to a gas-specific thermal characteristic. A thermal lumped element model interprets amplitude and phase of the thermopile signal yielding the estimation of thermal conductivity and volumetric heat capacity. The simultaneous and selective measurement of these two independent thermal properties offers the possibility to improve the performance of existing thermal gas sensors. We were able to distinguish gases over a wide range of properties with high reproducibility based on our thermal model.