Monika Leester-Schädel

Surface-Passive Pressure Sensor by Femtosecond Laser Glass Structuring for Flip-Chip-in-Foil Integration

To allow for smaller sizes, smoothness and robustness of exposed surface, and for integration in flexible sensor arrays, an innovative piezoresistive pressure sensor design has been developed. In contrast to known concepts, the sensing elements and the conducting tracks are positioned within the pressure reference chamber and, thus, protected against environmental influences such as water or particles. Sensing elements are electrically accessible from the backside by vias, thus enabling a fully flat surface totally free of electrical elements as desired for flow experiments. The sensor comprises a thin silicon sensing membrane and a body made from glass holding the reference chamber and the vias. The structuring of the sensor body is performed by femtosecond laser ablation. Steep ablation edges are realized, leading to small sensor dimensions. The sensing membrane is fabricated using potassium hydroxide (KOH) wet etching. The glass body and the silicon membrane can be connected with different techniques; hitherto, adhesive bonding by an epoxy resin layer was successfully tested. A sensitivity of 10 mV/V/bar and stable operation up to 7 bar absolute pressure could already be demonstrated. The new concept simplifies micromanufacture and allows for flip-chip-assembly in foil-based flexible systems that can be used in liquids and harsh environments.

Progress in Efficient Active High-Lift

This paper presents some of the progress in research on efficient high-lift systems for future civil aircraft achieved by the Coordinated Research Centre CRC 880 sponsored by the German Research Foundation. Several new approaches to increasing the lift are applied as part of the design of a reference aircraft with short take-off and landing capability: The numerically predicted positive effect of Coanda jet blowing at the trailing edge flap is validated in water tunnel experiments. Robust miniature pressure and hot-fi�lm sensors are developed for the closed-loop control of a piezo-actuated blowing lip. A flexible leading-edge device utilizes composite materials, for which new structural designs are developed. Additionally, a potential de-icing system, as well as a lightning-strike protection are presented. A high power-density electrically driven compressor with a broad operating range is designed to provide the blowing air ow. Different propulsion systems for the reference aircraft are evaluated. An ultra-high bypass ratio engine is considered to be most promising, and thus a preliminary fan stage design process is established. The rotor dynamic influences of the engine on the aircraft structure are investigated through a hybrid approach using a multibody model and modal reduction.

Design of a high-lift experiment in water including active flow control

This paper describes the structural design of an active flow-control experiment. The aim of the experiment is to investigate the increase in efficiency of an internally blown Coanda flap using unsteady blowing. The system uses tailor-made microelectromechanical (MEMS) pressure sensors to determine the state of the oncoming flow and an actuated lip to regulate the mass flow and velocity of a stream near a wall over the internally blown flap. Sensors and actuators are integrated into a highly loaded system that is extremely compact. The sensors are connected to a bus system that feeds the data into a real-time control system. The piezoelectric actuators using the d 33 effect at a comparable low voltage of 120 V are integrated into a lip that controls the blowout slot height. The system is designed for closed-loop control that efficiently avoids flow separation on the Coanda flap. The setup is designed for water-tunnel experiments in order to reduce the free-stream velocity and the systems control frequency by a factor of 10 compared with that in air. This paper outlines the function and verification of the systems main components and their development.

SMA micro actuators for active shape control, handling technologies, and medical applications

The shape memory effect is about to become more and more important as an innovative actuation principle in micro system technologies. Shape memory alloys (SMA) are able to return into a pre-memorized shape when heated. Due to this regeneration force and actuation is produced. This publication reports on the design and the functionality of SMA micro actuators and their applications for active shape control, handling technologies and medical engineering. Thin NiTi foils have been chosen because of their well defined properties and high strength. In order to integrate them into micro systems, different manufacturing methods have been applied and improved at the Institute for Microtechnology (IMT). Laser cutting and wet chemical etching for example are used to fabricate actuator elements for several applications. Different methods for electrical and mechanical connections of the actuators are employed, for example soldering by the use of an additional gold layer. A batch fabrication process of SMA actuators is realized by embedding NiTi-elements into SU-8 structures. Three different micro actuator concepts are presented: A multi-actuator system for deformation of elastic surfaces, which is driven by numerous identical single actuators connected in parallel and in series, a micro gripper for handling and assembling of complex hybrid micro systems and a micro actuator system in medical tools for percutaneous resection of aortic valves.

Cell manipulation system based on a silicon micro force sensor with self-calibration from backside

In this work a system for cell manipulation is presented. A two axis stage was arranged on an inverted microscope to place cells in focus. Cell manipulation can thereby be observed while the system automatically runs force controlled measurements. A high precision linear motor moves a silicon force sensor, which has been equipped with a stimulation tool e.g. a micro capillary for cell injection. The sensor is made of silicon and consists of a membrane with a boss structure, which enables measurements as low as 120 µN. For the first tests on the sensor, an injection capillary is mounted on the topside, while the backside is fixed to a printed circuit board (PCB) which has an integrated connection for the air pressure. An air pressure was applied under the membrane because the glass capillaries with outer diameters of at least 1 µm do not allow calibration via other force sensors. This setup allows a unique self-calibration of the mounted sensor system, before the measurements of the occurred forces on the cells during the penetration of the capillary.

Liquid polyimide as a substrate for aeronautical sensor systems

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.

MEMS pressure sensors embedded into fiber composite airfoils

The paper describes the integration of pressure sensors into fiber composite in order to obtain flow sensing airfoils to be used in future aircrafts. First, the sensor design and working principle is described, followed by an embedding procedure for damage-free integration. Here the sensors are faced to stresses by vacuum and curing during the embedding process into fiber-reinforced plastic. The mechanical characteristics and the influence of external mechanical stresses on the integrated sensor are further investigated. Finally, a sensor design unsusceptible to external mechanical stresses parallel to the surface of the airfoil is proposed and verified by tensile stress tests.

Flexible hot-film anemometer arrays for flow measurements on curved structures

In this paper, a set of flexible aeroMEMS sensor arrays for flow measurements in boundary layers is presented. The sensor principle of these anemometers is based on convective heat transfer from a hot-film into the fluid. All sensors consist of a nickel sensing element and copper tracks. The functional layers are attached either on a ready-made polyimide foil or on a spin-on polyimide layer. These variants are necessary to meet the varying requirements of measurements in different environments. Spin-on technology enables the use of very thin PI layers, being ideal for measurements in transient flows. It is a unique characteristic of the presented arrays that their total thickness can be scaled from 5 to 52 μm. This is essential, because the maximum sensor thickness has to be adapted to the various thicknesses of the boundary layers in different flow experiments. With these sensors we meet the special requirements of a wide range of fluid mechanics. For less critical flow conditions with much thicker boundary layers, thicker sensors might be sufficient and cheaper, so that ready-made foils are perfect for these applications. Since the presented sensors are flexible, they can be attached on curved aerodynamic structures without any geometric mismatches. The entire development, starting from theoretical investigations is described. Further, the micro-fabrication is explained, including all typical processes e.g. photolithography, sputtering and wet-etching. The wet-etching of the sensing element is described precisely, because the resulting final dimensions are critical for the functional characteristics.

Robust pressure sensor for measurements in boundary layers of liquid fluids with medium total pressures

In this work, the latest results of the design, fabrication and characterization of a new MEMS piezoresistive pressure sensor are presented. It is made of silicon using a boron diffusion process to create piezoresistors. Significant changes in the layout as well as in the micro-fabrication process have been made, e.g. anodic bonding of a Pyrex cover on the backside. These lead to a very precise pressure sensor, which is tailor made for high dynamic measurements in fluids with a total pressure up to 4 bar. This new piezoresistive pressure sensor has been developed in order to meet the special requirements of measurements in fluid mechanics, particularly with regard to the non-intrusive nature of the sensor. The sensor development, starting with the simulation of mechanical stresses within the diaphragm is described. These calculations have lead to an optimized placement of the piezoresistors in order to achieve a maximum sensitivity. The result of this work is a sensor which has well known properties. Important parameters including sensitivity, resonance frequency and maximum load are described precisely. These are necessary to enable new measurements in the boundary layer of fluids. The experiments and the initial results, e.g. its linearity and its dynamic capability are demonstrated in several figures.

Silicon grating microfabrication for long-range displacement sensor

Silicon gratings are fabricated using micromachining techniques. The gratings are used with fiber optic probes to measure highresolution and long-range linear displacements. Different parameters of the fabrication process such as the etching solution, the concentration of the etchant, and the temperature are optimized to achieve a mirror-like surface quality of the grating steps. For each parameter set, the resulting roughness and flatness are analyzed and discussed. Finally, linear displacement measurements are performed with the optimized grating as a component of a long-range fiber optic sensor. A resolution better than 34 nm and a measurement range up to 8.7 mm are obtained.