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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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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.