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Design, manufacturing, calibration, and basic characterization of a microelectromechanical systems (MEMS) wall hotwire sensor on a flexible polyimide substrate are presented. A configuration exhibiting bondpads on the top side of the foil, as well as an improved setup featuring a through-foil metallization and bottom side bondpads were established. Both sensor designs make use of a highly sensitive nickel thin-film resistor spanning a reactive ion etched cavity in a polyimide substrate. The polyimide base material enables the sensor to be adapted to curved aerodynamic surfaces, e.g., airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers is avoided. The combination of polyimide's low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low-power consumption (<25 mW). Fluctuations in wall shear stress up to 85 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.166 V/(N/m 2 ) is achieved in a wall shear stress range from 0 to 0.72 N/m 2 . The specifically designed through-foil metallization process allows for electrical contacts to be positioned on the backside of the substrate, thus effectively minimizing aerodynamic disturbances.Index Terms-Microelectromechanical systems (MEMS), polyimide, via, wall hot-wire.

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Micromachined wall hot-wire sensors composed of a highly sensitive, nickel, thin-film resistor spanning an airfilled cavity in a mechanically flexible substrate are presented. Cavity design and sensor materials are optimized to reduce thermal losses, thus enabling measurement of high-frequency fluctuations in fluid flows. Successfully realized sensors featuring wire widths of 2 and 5 m, wire lengths from 400 to 2000 m, and various cavity dimensions were characterized in wind-tunnel experiments. Static sensor calibration, cutoff frequency determination using a sine sweep, and recording of angular characteristics were conducted at wall shear stresses of up to 1 N=m 2 , obtained at 20 m=s freestream velocity in an open wind tunnel on a flat plate with fully developed turbulent flow. An overheat ratio of 1.8 was used for hours without thermal failure of the sensors, and a maximum cutoff frequency in still air of 73 kHz was obtained. The highest average sensitivity of 0:196 V=N=m 2 was recorded for a sensor of 5-m wire width and a length-to-width ratio of 400 in a wall shear stress range from 0 to 1 N=m 2 with a power consumption of less than 30 mW. Nomenclature a W = overheat ratio of the hot wire d C = depth of the cavity, m k = yaw factor l C = length of the cavity, m l W = length of the hot wire, m P = electrical power consumption, W R W = electrical resistance of the hot wire in operation, R 0 = electrical resistance of the hot wire at room temperature, S = sensitivity of the sensor to changes in wall shear stress, V=N=m 2 T W = temperature of the heated wire, K T 0 = ambient fluid temperature, K t F = thickness of the polyimide foil, m t W = thickness of the hot wire, m U = anemometer output voltage with flow in constanttemperature operation, V U 0 = anemometer output voltage without flow in constanttemperature operation, V U 0 = anemometer output voltage with flow normal to the wire length in constant-temperature operation, V v = flow velocity at the hot wire, m=s v eff = flow velocity contributing to cooling of the hot wire, m=s v n = component of flow velocity normal to the hot-wire length, m=s v 1 = freestream velocity, m=s w C = width of the cavity, m w W = width of the hot wire, m = yaw angle, deg = wall shear stress, N=m 2

Design, simulation, manufacturing, calibration, and basic characterization of a MEMS wall hot-wire anemometer is presented. A highly sensitive nickel thin film resistor spanning a reactive ion etched cavity in a polyimide foil is employed. This sensor is the first in literature to feature both a thermally insulating cavity and a flexible base material. The polyimide base material allows adopting of the sensor to aerodynamic surfaces, e.g. airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers, is avoided. The combination of polyimide's low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low power consumption (<30 mW). Fluctuations in wall shear stress up to 80 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.44 V / (N/m²) is achieved in a wall shear stress range from 0 to 0.25 N/m².

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