Fabrication and Testing of a MEMS Low-Flow Meter

A compact MEMS flowmeter was fabricated and tested for small gas flow rates from 0.5 to 10 sccm. The flowmeter was fabricated by integrating a silicon chip consisting of three surface-micromachined sensor clusters with a 60μm tall, 1.25 mm wide, and 10 mm long flow channel. Each sensor cluster in the chip consists of a pressure sensor, a surface shear stress sensor, and a temperature sensor. As gas flows through the channel, the volumetric flow rate is determined by measuring either the pressure drop in the flow direction or the local surface shear stress. The temperature sensors in the clusters mainly function as a reference for temperature compensating of the thermally based shear stress sensors. The flowmeter was calibrated in an open-loop pipe-flow facility using nitrogen as the gas medium. Based on the test results, it is clear that the pressure sensors, while capable of handling high flow rates, do not have the necessary precision level for small flow measurements. By comparison, the shear stress sensors, after proper temperature compensation, provide consistent and repeatable measurements for small flow rates. Additionally, the voltage output of the shear stress sensors also follows the 1/3 power trend, previously predicted for macro flow situations, in the small flow range tested.