Determining Thermal Properties of Liquids: Membrane-based versus Bridge-based Micromachined Sensors

For the determination of thermal material properties, micromachined sensors are of particular interest. Membrane-based structures facilitate an effective thermal decoupling of the actual sensing region from the sensor substrate, resulting in high sensitivity and a short response time. For some liquids, however, the remaining spurious heat flow in the membrane is still not negligible. These unwanted thermal shunts can be further decreased by utilizing structures, like cantilevers or micro-bridges. To investigate this aspect experimentally, two different sensors were designed, both intended to determine the thermal conductivity and diffusivity of liquids using a heating element and a spatially separated temperature sensor. In the former case, heater and thermistor are located on a closed membrane structure whereas in the latter case the membrane is selectively patterned yielding an individual micro-bridge for each element. The measurement results confirm, that the novel bridge-based sensor allows to determine the thermal properties more accurately and independently from the thermal properties of the membrane.