Ulrike Wallrabe

Mechanical properties of silicones for MEMS

This paper focuses on the mechanical properties of polydimethylsiloxane (PDMS) relevant for microelectromechanical system (MEMS) applications. In view of the limited amount of published data, we analyzed the two products most commonly used in MEMS, namely RTV 615 from Bayer Silicones and Sylgard 184 from Dow Corning. With regard to mechanical properties, we focused on the dependence of the elastic modulus on the thinner concentration, temperature and strain rate. In addition, creep and thermal aging were analyzed. We conclude that the isotropic and constant elastic modulus has strong dependence on the hardening conditions. At high hardening temperatures and long hardening time, RTV 615 displays an elastic modulus of 1.91 MPa and Sylgard 184 of 2.60 MPa in a range up to 40% strain.

A fully MEMS-compatible process for 3D high aspect ratio micro coils obtained with an automatic wire bonder

We report the fabrication of 3D micro coils made with an automatic wire bonder. Using standard MEMS processes such as spin coating and UV lithography on silicon and Pyrex® wafers results in high aspect ratio SU-8 posts with diameters down to 100 µm that serve as mechanical stabilization yokes for the coils. The wire bonder is employed to wind 25 µm insulated gold wire around the posts in an arbitrary (e.g. solenoidal) path, yielding arrays of micro coils. Each micro coil is bonded directly on-chip, so that loose wire ends are avoided and, compared to other winding methods, coil re-soldering is unnecessary. The manufacturing time for a single coil is about 200 ms, and although the process is serial, it is batch fabrication compatible due to the high throughput of the machine. Despite the speed of manufacture we obtain high manufacturing precision and reliability. The micro air-core solenoids show an RF quality factor of over 50 when tested at 400 MHz. We present a flexible coil making method where the number of windings is only limited by the post height. The coil diameter is restricted by limits defined by lithography and the mechanical strength of the posts. Based on this technique we present coils ranging from 100 µm diameter and 1 winding up to 1000 µm diameter and 20 windings.

Unconventional applications of wire bonding create opportunities for microsystem integration

Automatic wire bonding is a highly mature, cost-efficient and broadly available back-endprocess, intended to create electrical interconnections in semiconductor chip packaging. Modern production wi ...

Optical characterization of adaptive fluidic silicone-membrane lenses

We present an extended optical characterization of an adaptive microfluidic silicone-membrane lens at a wavelength of 633 nm, respectively 660 nm. Two different membrane variations; one with a homogeneous membrane thickness, and one with a shaped cross section, have been realized. This paper includes the theoretical predictions of the optical performance via FEM simulation and ray tracing, and a subsequent orientation dependent experimental analysis of the lens quality which is measured with an MTF setup and a Mach-Zehnder interferometer. The influence of the fabrication process on the optical performance is also characterized by the membrane deformation in the non-deflected state. The lens with the homogeneous membrane of 5 mm in diameter and an aperture of 2.5 mm indicates an almost orientation independent image quality of 117 linepairs/mm at a contrast of 50%. The shaped membrane lenses show a minimum wave front error of WFE(RMS) = 24 nm, and the lenses with a planar membrane of WFE(RMS) = 31 nm at an aperture of 2.125 mm.

Movable Microstructures Manufactured by the LIGA Process as Basic Elements for Microsystems

By the LIGA process microstructures with structural heights of several hundred micrometers and dimensions down to one micrometer can be produced in submicron accuracy. Using a special sacrifical layer technique it is possible to fabricate movable as well as fixed microstructures in one process step. These microstructures are used as microsensors or microactuators. As first examples capacitive acceleration sensors as well as microgears with a shaft bearing used as microturbines have been produced by this extended LIGA technique.

Review on Electrodynamic Energy Harvesters—A Classification Approach

Beginning with a short historical sketch, electrodynamic energy harvesters with focus on vibration generators and volumes below 1dm3 are reviewed. The current challenges to generate up to several milliwatts of power from practically relevant flows and vibrations are addressed, and the variety of available solutions is sketched. Sixty-seven different harvester concepts from more than 130 publications are classified with respect to excitation, additional boundary conditions, design and fabrication. A chronological list of the harvester concepts with corresponding references provides an impression about the developments. Besides resonant harvester concepts, the review includes broadband approaches and mechanisms to harvest from flow. Finally, a short overview of harvesters in applications and first market ready concepts is given.

Design, Simulation, and Fabrication of a Quadstable Monolithic Mechanism With X- and Y-Directional Bistable Curved Beams

This paper demonstrates a novel quadstable monolithic mechanism (QsMM), which provides four stable equilibrium positions within its planar operation range. The QsMM has been realized from the use of both X- and Y-directional bistable structures, which utilize curved snapping beams. Two pairs of curved beams were attached to an inner frame in both X and Y directions to present an independent bistable behavior in the directions. It was found out that the design of the inner frame is crucial for the quadstability and dynamic responses of the mechanism. A millimeter-scale brass mechanism was actually fabricated by ultraprecision milling to test the quadstability and the force-displacement behavior. The prototype clearly demonstrates four distinct stable positions in its millimeter-scale operation range. The design concept, finite element simulation, fabrication, and experimental measurement of the proposed multistable mechanism have been presented. The mechanical multistability of the proposed QsMM can be utilized for multiple switching and optical networking applications, yielding low power consumption operations.

Microactuators fabricated by the LIGA process

Using the LIGA process combined with a sacrificial layer technique, microstructures with great structural heights and lateral dimensions in the micrometer range have been fabricated. These structures are used as microactuators. By using electrostatic linear actuators large displacements are achieved. A large torque is realized by a micromotor with a toothed rotor, friction being diminished by a rolling movement. Bimaterial elements based on PMMA and metal are designed to achieve a movement parallel to the substrate, to fabricate, for example, a gripper. Electromagnetic activation has been used to measure the fatigue behavior of LIGA micromechanical elements.

Axial scanning in confocal microscopy employing adaptive lenses (CAL)

In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ~2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results.

Theoretical and experimental results of an electrostatic micro motor with large gear ratio fabricated by the LIGA process

It is known that the extension of the LIGA process by a sacrificial layer technique allows the fabrication of movable microstructures with great structural height without the need to assemble individual components. With this technique an electrostatic stepping micro motor has been fabricated and tested. The design of the motor has been optimized with regard to large torque by finite-element-method calculations. The radii of the rotor and the axle have been adjusted to the step width of the motor in order to realize a rolling movement of the rotor on the axle, which minimizes the friction.<<ETX>>