Design and fabrication of a direction sensitive MEMS shear stress sensor with high spatial and temporal resolution

Shear stress at the fluid-wall interface is one of the most frequently studied parameters in fluid dynamics. It is also a parameter of very small magnitude and calls for high resolution force sensors. Macroscopic sensors compromise dynamic bandwidth for the required high resolution and therefore cannot resolve shear stress data in space and/or time, which is very important for fundamental understanding in non-laminar fluid dynamics. We exploit the linear reduction in stiffness accompanied by cubic reduction in mass by miniaturization to design and fabricate a novel micro-electro-mechanical sensor (MEMS) for direct measurement of shear stress along and across the direction of fluid flow, with 0.01 Pa resolution and 50 kHz bandwidth along the flow. The mechanical component of the sensor is a floating beam element and capacitive comb drives supported by an in-plane torsional spring. A resonant RLC circuit, capable of sub-femtofarad capacitive sensing, is used to sense the displacement in the floating beam under shear. Fabrication of the sensor is demonstrated using silicon-on wafer (SOI) technology. The small overall size of the sensor, wide range of measurement, large bandwidth and high spatial and temporal resolution will make it useful in a wide variety of civil and military applications such as aerospace, automotive, marine and biomedical.