Energy efficient micro-machined thermal flow-rate sensor based on transient operation mode

We report on a micro-machined thermos-resistive flowrate sensor optimization with respect to energy consumption while using a large thermal conductivity substrate, silicon for instance. Two aspects of the device are optimized for this purpose: its geometry and its operating scheme. Large thermal conductivity materials are not recommended for thermal flowrate sensors since they induce large thermal leakage. However, they can be needed for some lab-on-chip devices where the co-integration of several sensors and components is the main constraint. Under these conditions, we show that the flow-rate sensor thermal efficiency can be increased by geometric optimization and its energy consumption significantly reduced by operating the device in transient mode. The original reference device is fabricated on bulk silicon where the heating element is made of platinum (Pt). Then, a geometric optimization is performed to increase the heating resistor thermal insulation by creating a micro-pillar in the middle of a cavity to support the heater. The pillar structure flow-rate sensor exhibits 2.5 °C temperature under no-flow while the bulk silicon-based sensor shows 0.8 °C only under 30 mA current supply. This larger temperature increase under no flow leads to a larger sensitivity of the former. In addition, transient operating mode enables the minimization of the pre-measurement waiting time, hence a reduction of the device energy consumption. While a steady-state measurement requires a heating time of the order of 20 minutes, a transient mode measurement can be performed after a heating time lower than 3 minutes.