A microfabricated shear sensor array on a chip with pressure gradient calibration

A micromachined floating element array sensor was designed, fabricated, and characterized. The sensor chip is 1 cm2 and includes 16 separate sensor groups in a 4 by 4 array with a pitch of approximately 2 mm. The device was fabricated using four layers of surface micromachining including copper and nickel electroplating. A capacitance to digital converter IC was used to measure the differential capacitance change resulting from flow forces. The achieved resolution is limited by white noise with a level of 0.24 Pa/√Hz, and linearity is demonstrated to >13 Pa. Experimental characterization in three different duct height laminar flow cells allowed independent determination of the sensitivity to shear stress and pressure gradient. The sensor chip with half the elements acting in parallel has a sensitivity of 77.0 aF/Pa to shear and −15.8 aF/(Pa/mm) to pressure gradient. Pressure gradient sensitivity is found to be an important contributor to overall output, and must be accounted for when calibrating floating element shear stress sensors if accurate measurements are to be achieved. This work is the first demonstration of a shear sensor array on a chip with independent pressure gradient sensitivity calibration.