Technology- MEMS Thermal Mass Flow Sensors

Flow measurement is critical in many applications, including semiconductor manufacturing processes, chemical processes, medical devices and natural gas metering, etc. 

MEMS thermal mass flow sensors have been explored extensively for their simple structure and implementation. MEMS technology is amenable to creating micro-heaters and thermal sensors with no moving parts, thus simplifying fabrication and operational requirements. Other advantages of thermal mass flow sensors is small size, short response time, low power consumption, higher sensitivity to low flow rates.

The thermal mass flow sensor typically consists of upstream and downstream temperature sensors (thermopiles) and a heater located between the two temperature sensors as shown below.

The flow rate is detected by the MEMS thermal mass flow sensor. The sensor chip, produced in MEMSIC proprietary CMOS compatible technology, is composed of a central heater source (micro heater) and two temperature sensors (thermopiles), which are placed symmetrically upstream and downstream of the micro-heater. If no gas flows over the sensor surface, the symmetric thermopiles measure the same rise in temperature, resulting in the same output voltage of the two thermopiles. If a non-zero gas flows from the inlet to the outlet of the meter, the velocity of a fully-developed laminar air flow unbalances the temperature profile around the heater and heat is transferred from upstream thermopiles to the downstream thermopiles, causing a change in the voltages of the thermopiles. Larger gas flow rates result in larger asymmetry in the temperature profile.

Ideally, sensors are thermally isolated so only heat transfer due to flow can occur. Other heat transfer pathways such as through substrate or electrical leads result in thermal losses that degrade sensor performance and is minimized in the device design.

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