Microfabricated thermal conductivity detector for the micro-ChemLab™

This work describes the design, computational prototyping, fabrication, and characterization of a microfabricated thermal conductivity detector (μTCD) to analyze the effluent from a micro-gas chromatograph column (μGC) and to complement the detection efficacy of a surface acoustic wave detector in the micro-ChemLab™ system. To maximize the detection sensitivity, we designed a four-filament Wheatstone bridge circuit where the resistors are suspended by a thin silicon nitride membrane in pyramidal or trapezoidal shaped flow cells. The geometry optimization was carried out by simulation of the heat transfer in the devices, utilizing a boundary element algorithm. Within microfabrication constraints, we determined and fabricated nine sensitivity-optimized geometries of the μTCD. The nine optimal geometries were tested with two different flow patterns. We demonstrated that the perpendicular flow, where the gas directly impinged upon the membrane, yielded a sensitivity that is three times greater than the parallel flow, where the gas passed over the membrane. The functionality of the μTCD was validated with the theoretical prediction and showed a consistent linear response to effluent concentrations, with a detection sensitivity of 1 ppm, utilizing less than 1 W of power.