An Integrated Mass Flow Sensor with On-Chip CMOS Interface Circuitry

This paper reports a monolithic mass flow sensor capable of measuring gas flow velocity, direction, type, temperature, and pressure. The transducers are supported on micromachined dielectric windows 0.5 mm x 0.5 mm in size. Using on-chip interface circuitry to deliver high-level buffered signals, true mass flow can be computed. The flow velocity sensor operates over the 1 cm/s to 5 m/s range, giving a flow sensitivity that agrees well with simulated performance. Self-test circuitry on the chip allows the thermal time constant of the window to be measured at any time, resolving the buildup of surface films well below the level required to degrade device resolution. A thermopile-based transducer allows flow direction to be resolved within 5°, while a CMOS bandgap temperature sensor has been implemented with a sensitivity of 4 mV/°C. This last device uses the substrate n-p-n transistors that occur naturally in CMOS to obtain the bipolar transistors required for this transducer with no additional process steps. Finally, a polysilicon-bridge pressure sensor delivers a pressure sensitivity of 12 μV/V.mmHg, resolving 1 mmHg over a 750 mmHg dynamic range. The on-chip circuitry accurately sets and maintains the operating window temperatures, implements self-testing, and provides a standardized interface to higher level system control, while requiring an increase in die area of 30% and reducing the number of required package pins from 24 to 10. The flowmeter chip requires 13 masks and measures 3.5 mm x 5 mm in 3-am features.