Manufacturing of flexible micro hot-film probes for aeronautical purposes

• We present a new application for flexible hot-film sensors manufactured on 25 μm thick polyimide foil.
• Design rules and geometric uncertainties are discussed briefly.
• The micro-fabrication of the hybrid layering including a Nickel sensing element and Copper wiring is explained.
• Single sensor elements are presented as well as a sensor array consisting of 32 sensors.
• The results of tests to determine the sensitivity are described.

In this work we present the design, fabrication and characterization of flexible AeroMEMS hot-film probes. The hot-films are fabricated on polyimide foil (PI). The foil is bonded onto a glass or silicon substrate in order to get a rigid structure for using standard silicon bulk-micromachining processes. After micro fabrication is finished the PI foil can be easily removed from the substrate. The foil itself is flexible enough to be placed on complex curved contours typically used in aerodynamic tests. This study discusses the failures introduced by geometry effects which occur due to isotropic wet etching of the metal layers. After these investigations, one hot-film design was chosen to create a sensor array for special measurements of wall shear stress in fluid mechanics. The array consists of 32 hot-film sensors custom made for an aerodynamic test. The properties of the sensing elements are optimized for the measurement set-up, providing a simple implementation. The sensing elements were finally made of Nickel, while the wiring was electroplated with 2 μm of Copper. Using the advantages of both materials is unique. It is assumed that due to the extremely flat design, the sensor including its Cu wiring has no influence on the flow. The foil, including the thin wiring is bendable enough to lead the contacts into the wing, making the electrical connection very convenient.

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