Low power micro-calorimetric sensors for analysis of gaseous samples

This work discusses the design, finite element method modeling (FEM), fabrication and characterization of a silicon-based, catalytic micro calorimetric sensor. The sensing area is comprised of two titanium silicide (TiSi2) – polysilicon (poly-Si) resistive temperature sensors symmetrically positioned relative to a poly-Si heater on which an oxidation promoting catalyst is deposited. The resistive structures are located on a suspended, thereby, thermally isolating, low mechanical stress membrane and integrated into a glass flow channel. The micro-calorimetric sensor is applied for measuring propane and hydrogen concentrations in air. An approach to optimization of thermal and fluidic design of the microsensor is presented based on developed models: (i) a 3D thermo-electric analysis of the suspended heater and (ii) a 2D thermo-chemical analysis of the catalytic oxidation of propane in the flow channel. Influence of the design and material of the membrane on the power consumption and temperature distribution across the sensing area are analyzed. A relationship between the thermal design of the sensor, reaction conditions and its operation as a thermal actuator and sensor of reaction heats are discussed. Various thermo-electrical characterizations (electrical, infrared surface imaging and transient thermal response measurements) in the context of microcalorimetric sensing are performed. Microsensors with a 50 μm × 50 μm sensing area consume ca. 12 mW at an operational temperature of 350 °C. Thermal imaging with an infrared camera indicates local heating with a temperature gradient across the active area estimated to be 4 °C μm−1 (at ca. 500 °C). The heating and cooling times are found to be ca. 1 and 8 ms, respectively. Temperature vs. power curves are determined for both stationary and constant flow conditions of various gases. Based on the experimental and modeling results we envision that these microsensors can be successfully used for calorimetric sensing and analysis of gaseous samples.