Thermal flow sensor with vacuum-sealed membrane fabricated by surface micromachining

We have proposed a surface-micromachined microflow sensor to solve the problems of poor thermal isolation and low sensitivity presented in typical thermal flow sensors. The membrane of insulating films on a vacuum cavity formed in a silicon substrate supports a n' poly-Si heating resistor and two bidoped poly-Si thermopiles. We have investigated heattransfer characteristics of the sensor microstructure with the 5-μm-depth cavity of 100×100 μm 2 and nitrogen and water as the working fluid using a two-dimensional numerical analysis in terms of the heater power, inlet flow velocity and pressure inside the cavity. The microstructure with the vacuum cavity shows improved thermal isolation compared to the air cavity, suppressing heat loss from the heater to the substrate. With the vacuum cavity having an optimized geometry, it is possible to enhance the performance of the thermal microflow sensor.