Flexible and stretchable systems-in-foil allow easy attachment to bodies with non-planar shapes and therefore offer a very attractive approach to new forms of sensing dynamic 3D shape changes. They may find applications in structural health monitoring or in the medical fields. This paper describes the design and fabrication of a novel 3D flexible curvature sensor-array on a plastic foil substrate which could be used for respiration monitoring of newborns. Each sensor element consists of four strain gages in a Wheatstone bridge configuration. To suppress sensor response on foil stretching and to increase sensitivity to foil bending the strain gages are located on opposite foil surfaces. Two resistors of the Wheatstone bridge are placed on the top and the two others, which are orthogonal to the top side resistors, on the bottom of the foil substrate. Thereby, an output signal can be achieved, which is at least 100% higher when compared with a one-sided sensor design. To characterize the sensor, bending experiments have been performed of both the double-sided and one-sided sensor designs. As the carrier foil material, we used a SU-8 photoresist additionally encapsulated with Polyimide from both sides to protect the sensing elements. The resistors are made of gold and are fabricated by a sputter process with subsequent photolithography. The advantage of our process sequence is that the complete double sided sensor with a thickness below 50 μm can be fabricated without the need to flip over the substrate in between. A key challenge in the fabrication process is the interconnection between the top and the bottom resistors. The interconnect vias are made in a photo definable interlayer and can withstand the bending experiments without disruption.
Using more and more controlled systems in future aircraft the need of flexible sensors to be applied on curved aircraft structures increases. An appropriate substrate material for such flexible sensors is polyimide, which is available both as ready-made foil and as liquid polyimide to be spun-on. Latest results in producing and processing of polyimide layers with a thickness of down to 1 μm including designs for thin foil sensors are presented respectively. The successful processing of liquid polyimide is outlined first including the spin-on procedure, soft bake and curing for polymerization. Parameters for spin-on volume and rotation speed on glass substrates along with a comparison with ordinary polyimide foil are presented. High-precision structuring of the polyimide layer is performed either by etching (wet-etching as well as dry etching in a barrel etcher) or ablative removal using a femtosecond laser. In combination with a layer of silicon nitride as an inorganic diffusion barrier a reliable protection for water tunnel experiments can be realized. The fabrication of a protection layer and test results in water with protected sensors are presented. The design of a hot-film anemometric sensor array made on spin-on polyimide is demonstrated. With a thickness of down to 7 μm the sensors can be applied on the surface of wind tunnel models and water tunnel models without impacting the flow substantially. Additionally both the concept and recent results of a silicon sensor integrated in a polyimide foil substrate that can measure pressure as a complementary measurand for aeronautics are illustrated.