Bernd Wagner

Microfabricated electrohydrodynamic (EHD) pumps for liquids of higher conductivity

A traveling-wave-driven electrohydrodynamic micropump without moving parts is discussed. The fundamental operating principles, such as high-frequency traveling waves, a self-stabilizing temperature gradient, and increased wave number, are outlined. The main advantages of the realized pump are its ability to move conductive liquids such as water and weak electrolyte solutions, the lack of any movable parts, and integration. A microfabricated structure demonstrating the pump operation is outlined, and quantitative results are described. Typical parameters characterizing the advantages and limitations of the pumping principle are discussed. Perspectives for optimization of the realized micropump can be seen in further miniaturization and increased number of electrodes. Possible applications are biological, medical, and chemical devices that can deliver accurately metered quantities of fluids in the nl/min and mu l/min range. >

Electrostatically driven micromirrors for a miniaturized confocal laser scanning microscope

A compact two-mirror microscanner has been fabricated to build the central part of a miniaturized confocal laser scanning microscope. This microscope shall be mounted at the tip of an endoscope to provide high resolution imaging for medical diagnostics. In order to achieve a resolution of 500 X 500 image elements large scan angles and also large mirror dimensions have to be realized within a spatially strong limited housing. While bulk silicon technology on the one hand enables fabrication of micromirrors with nearly ideal elastical behavior, those actuators on the other hand often are too fragile for a lot of applications. This paper describes the design, fabrication and assembling of electrostatically driven torsional micromirrors that meet the requirements of fast two-dimensional scanning with high angular precision over large scan angles, compact design and also high shock resistance. This is achieved with the combination of bulk silicon technology with metal surface micromachining. Besides medical diagnostics these microscanners can be used in a wider range of applications such as displays, two-dimensional barcode scanning, multiplexing of fiber optics, etc.

Micro-optic fabrication using one-level gray-tone lithography

This paper reports on a methodology to fabricate arbitrarily shaped structures using technologies common to standard IC manufacturing processes. Particular emphasis is put on the design and use of halftone transmission masks for the lithography step required in the fabrication process of mechanical, optical or electronic components. The algorithms to transfer an initial height profile into a design representation in the common data format GDSII are discussed. This set of data could be used directly by a commercial mask shop. The great data amount of a reticle layout is reduced significantly by a linear working data compaction algorithm. The nonlinear influences of the different process steps on the transfer function are regarded. The specific parameters for the mask making and the resist process are determined. Several components like shaped gratings or lenses are realized in resist up to 10 micrometers thick. In the field of transfering the pattern into a substrate material like silicon, a dry etching process is evaluated.

Wafer-level vacuum packaged resonant micro-scanning mirrors for compact laser projection displays

Scanning laser projection using resonant actuated MEMS scanning mirrors is expected to overcome the current limitation of small display size of mobile devices like cell phones, digital cameras and PDAs. Recent progress in the development of compact modulated RGB laser sources enables to set up very small laser projection systems that become attractive not only for consumer products but also for automotive applications like head-up and dash-board displays. Within the last years continuous progress was made in increasing MEMS scanner performance. However, only little is reported on how mass-produceability of these devices and stable functionality even under harsh environmental conditions can be guaranteed. Automotive application requires stable MEMS scanner operation over a wide temperature range from -40° to +85°Celsius. Therefore, hermetic packaging of electrostatically actuated MEMS scanning mirrors becomes essential to protect the sensitive device against particle contamination and condensing moisture. This paper reports on design, fabrication and test of a resonant actuated two-dimensional micro scanning mirror that is hermetically sealed on wafer level. With resonant frequencies of 30kHz and 1kHz, an achievable Theta-D-product of 13mm.deg and low dynamic deformation <20nm RMS it targets Lissajous projection with SVGA-resolution. Inevitable reflexes at the vacuum package surface can be seperated from the projection field by permanent inclination of the micromirror.

Design of asynchronous dielectric micromotors

Abstract This paper compares asynchronously and synchronously driven dielectric micromotors. The following advantages of an asynchronous motor are discussed. (i) Flexibility of the micromotor geometry. (ii) Simple design which allows for operation of rotors in electrolyte solutions (e.g. water) that may be pumped in this way. (iii) Opportunity to use negative dielectrophoretic forces to stabilise the rotors. To predict some properties of dielectric motors we have modelled their rotors geometrically as ellipsoids. This model is consistent with the experimental results and shows that motor operation is a complex function of geometry, driving frequency and material properties. Although the general motor characteristics (torques) of asynchronously and synchronously driven motors are quite similar both motor types are sufficiently different in detail. Therefore, it is worth to design motors using special properties of the asynchronous principle.

Particle micromanipulator consisting of two orthogonal channels with travelling-wave electrode structures.

Abstract In the paper experimental and numerical results for a simple particle micromanipulator fabricated in silicon technology are presented. It consists of three orthogonal liquid filled channels above meander-shaped planar electrode strips. By applying appropriate alternating, rotating or travelling electric fields in the chamber dielectric forces acting on particles suspended in the liquid are induced allowing trapping, movement and separation of them. The efficient manipulation of small particles with typical dimensions of several micrometers using electrogradient techniques (EGT) requires electrode structures of the same size. Due to the complex electrode geometry a numerical procedure is used for calculation of particle trajectories and optimising the design. In the micro range small fabrication defects are likely to cause large changes of the properties of the manipulator. Therefore, a test procedure based on electrode processes in aqueous media and the pH-dependent fluorescence intensity of a marker solution which enabled us to visualise the working states, surface coatings and fabrication defects of microstructured electrodes via a microscope is introduced.

Surface micromachined piezoresistive pressure sensors with step-type bent and flat membrane structures

Surface micromachined pressure sensors with step-type bent and nearly flat membranes are compared. The influence of the membrane structure on the sensor characteristics is investigated. The sensors contain four polysilicon piezoresistors arranged at the underside of a polysilicon membrane. For the step-type bent version of the sensor polysilicon is used as sacrificial layer and aqueous TMAH solution for underetching. Devices with a nearly flat structure are fabricated by applying a combined TMAH and HF sacrificial layer etching technique of a polysilicon/oxide sandwich. Compared to step-type bent, nearly flat structures provide decisive advantages for fabrication and show improved performance. Pressure response, noise behavior as well as the thermal drift of both types of sensors is presented.

Fabrication of microrelief surfaces using a one-step lithography process

To fabricate arbitrarily shaped microrelief surfaces you should be able to design and control beside the X- and Y- direction also the Z-direction. With common micromachining technologies this could not be obtained. The surface micro machining technology, with sacrificial layer etching as a key process, is a planar technology, which offers no degree of freedom to design the surface in Z-direction. The bulk micro machining technology, with anisotropic KOH etching of silicon as a key process, offers only restricted possibilities for 3D design. To overcome this limitation binary optical elements have been fabricated using multi-mask processes or multidose e-beam or laser writing. For refractive optical lenses resist melting is a good compromise. This paper reports on a new methodology to fabricate the above discussed arbitrarily shaped structures using a one step lithography process. This technique is called one-level gray-tone lithography, which is common to standard IC manufacturing processes, supplemented by some processes like spincoating and developing of thick resist layer, electroplating of thick metal layers and dry etching. This group of processes has been collected into a new technology category with free design capabilities in Z- direction up to 20 micrometer: relief micro machining. Particular emphasis is put on the design of the halftone transmission masks. The algorithms to transfer an initial height profile into a design representation in the common data format GDSII are discussed. The great data amount of a reticle layout is reduced significantly by a first order data compaction. The specific parameters for the mask making and the resist process are determined. Several components like shaped gratings or lenses are shown in resist up to 15 micrometers thick. In the field of transferring the pattern into a substrate material like silicon or glass, a dry etching process is evaluated.

A Novel Fabrication Technology for Waferlevel Vacuum Packaged Microscanning Mirrors

This paper presents a novel fabrication technology for wafer level hermetic encapsulated MEMS micro scanning mirrors. This wafer level package enables vacuum operation of the electrostatically driven resonant scanners reducing the required driving voltage and enhancing the mechanical deflection angle even at resonance frequencies of more than 100 kHz. Two nearly independently structured 30 microns thick epipoly silicon layers offer a high design variability due to the possibility to create active silicon structures with two different thicknesses. A buried isolated interconnection layer between the two thick epipoly silicon layers enables the fabrication of lateral feedthroughs which are vital to set up a hermetic package. A structured glass wafer with up to 900 microns deep cavities seals the device wafer at the front side via anodic bonding. An AuSi-eutectic bonded silicon wafer completes the wafer scale vacuum packaging process.

Progress in gray-tone lithography and replication techniques for different materials

The fabrication of 3D-microstructures with well-defined curved surface contours is of great importance for various mechanical, optical and electronic devices and subsystems. Complex geometrical structures or topographies are necessary to obtain a certain mechanical stability, a specific surface property or a predetermined electrostatic field configuration. Obviously in the micro-optic domain, there is a great demand to produce sophisticated surface topographies for refractive or diffractive optical elements, e.g. Fresnel lenses. This paper reports on progress in graytone lithography using subresolution pixeled chromium glass masks and introduces some replication techniques for different materials. In continuation of our work on graytone lithography, reported elsewhere, detailed view on reproducibility, fidelity and process latitude will be presented. Based on this results infrared diffractive optical elements have been fabricated in silicon using an 1:1 dry etching process, where the surface roughness of the shaped areas after etching has an 1 (sigma) value of 18 nm. For low cost application in the visible wavelength region a replication technique in polycarbonat by injection modeling is described. First results are shown.