Development and fabrication of magnetic thin films

For designing and fabricating electromagnetic microactuators as pursued within the collaborate research center “Design and Fabrication of Active Microsystems” (Sonderforschungsbereich 516), soft and hard magnetic materials are required to create and guide magnetic flux. The investigations on the development of suitable materials and their deposition technologies are presented in this paper. In the area of soft magnetic materials, the application of Cobalt–Iron (CoFe) as an alternative to Nickel–Iron (NiFe, Permalloy) was investigated. The benefit of CoFe over NiFe is its greater saturation flux density. The technology utilized for the deposition was electroplating. In the area of hard magnetic materials, gas flow sputtering was applied for depositing Samarium–Cobalt (SmCo). This technology enables the deposition of pure SmCo layers at high deposition rates and without high vacuum. Furthermore, the dependence of the magnetic properties of the SmCo on the film composition was examined.     

High aspect ratio micromolds for the electroplating of micro electro discharge machining tools

A new approach using micro systems technology for the fabrication of micro electro discharge machining tool electrodes using a combination of positive and negative photoresist fabricated with near UV-lithography as high aspect ratio micromolds was developed. Micromolds with heights varying from 200 to 650 μm and an aspect ratio of up to 26:1 were fabricated using SU-8?, a negative photoresist, as a micromold for the electrodeposition of the micro tool electrodes. Besides serving as a micromold, SU-8? is also used as an insulator to protect the sidewalls of the tool electrodes from corrosions. The fabrication of the SU-8? micromold was optimized to avoid cracks and delaminations. In the experiments, electrodes made of Cu, WCu, and CoFe were deposited into these High Aspect Ratio Micro Structure Technology (HARMST) micromolds. Furthermore, the process technique for electroplating in deep micromolds is discussed in this paper.     

Numerical study of enhancing the mixing effect in microchannels via transverse electroosmotic flow by placing electrodes on top and bottom of the channel

This paper reports the enhancement of the mixing effect via the transverse electroosmotic flow by using a 3D microelectrode system, which is structured by aligning two layers of electrodes face-to-face placed on both the top and bottom sides of the channel. The fluid was stretched and folded due to the transverse electroosmotic flow generated by applying an electric field on the electrodes. In this paper, two type of electrode designs (a parallel type electrode design and squarewave type electrode design) were chosen and six design patterns with different combinations of these two types of electrode designs were investigated by using the numerical method. The mixing effect at different design patterns was investigated via comparing the flow structure, mixing mechanism, Poincaré map, and the index of mixing performance. An optimum pattern was obtained when squarewave type electrodes were placed on both top and bottom of the channel. A minimum mixing length of 1.6 mm is required for the optimum pattern when the flow velocity is 1.5 mm/s, and the amplitude of the applied electric potential is 1.2 Volts. The effects of the geometric size and flow rate for the optimum pattern are discussed.     

Hard and soft magnetic materials for electromagnetic microactuators

An important aspect of the development of electromagnetic microactuators is the search for suitable materials as well as the development of the respective deposition and patterning processes. Within the Collaborative Research Center 516 “Design and Fabrication of Active Microsystems”, it is the task of the subproject B1 “fabrication of magnetic thin films for electromagnetic microactuators” to perform these investigations. The materials of interest can be divided into two groups: hard magnetic materials and soft magnetic materials. Materials with optimized properties and fabrication processes have been developed within both groups. An example is Samarium–Cobalt (SmCo), which can either be deposited using magnetron sputtering as Sm₂Co₁₇ with a very high energy product or in the SmCo₅ phase using gas flow sputtering with very high deposition rates. In the area of soft magnetic materials, investigations on Nickel-Iron (NiFe) especially NiFe81/19 were followed by the evaluation of NiFe45/55, which features a higher saturation flux density B _s and relative permeability μ _r. Furthermore, current investigations focus on Cobalt-Iron (CoFe) and its further increased saturation flux density B _s and relative permeability μ _r. Current tasks include the stabilization of the fabrication processes to achieve good material properties (i.e. electroplating of CoFe) or a shortening (e.g. by using heated substrates during deposition) by using process alternative not used so far. Another topic is the integration into fabrication processes, i.e. the investigation of process stability and compatibility.     

Fabrication of monolithic multilevel high-aspect-ratio ferromagnetic devices

This paper describes a process to fabricate monolithic multilevel high-aspect-ratio microstructures (HARMs) for ferromagnetic devices built on silicon wafers using aligned X-ray lithography in conjunction with electrodeposition. Two X-ray masks were fabricated, each consisting of gold (Au) absorber structures on a transparent polyimide membrane. One mask was used to print a polymethyl methacrylate (PMMA) resist layer. Then, a second PMMA layer was applied to the same wafer, and the second mask was used to pattern it. Transparent alignment windows in the second mask, combined with a piezoelectrically controlled X-ray aligner, allowed for high alignment accuracy between the two print patterns over large areas (>4 inch in diameter). Au circuits were electroplated into first PMMA layer from a sulfite-based electrolyte, and nickel-iron (NiFe) ferromagnetic HARMs were formed in second PMMA resist from a sulfate-based bath. The deposition resulted in well-defined NiFe structures with aspect-ratios up to 67:1 as well as smooth sidewalls and top surfaces. Chemical composition measurements with energy X-ray dispersive spectroscopy (EDS) and wavelength X-ray dispersive spectroscopy (WDS) showed that Fe content increased during the electrodeposition process. To electrically isolate the NiFe posts and Au circuits, both wet chemical etching and sputter etching were explored to remove the exposed seed layer, and the latter approach completely removed the seed layers without damaging the electroplated features.     

High aspect ratio gold nanopillars on microelectrodes for neural interfaces

Microelectrode arrays (MEA) have become an established tool in applied and fundamental research. Low impedance at the interface between tissue and conducting electrodes is of utmost importance for the electrical recording or stimulation of electrophysiological active cells such as cardiac myocytes and neurons. A common way to improve this interface is to increase the electrochemically active surface area of the electrode. In this paper the fabrication of microelectrodes covered with very high aspect ratio (AR > 100) gold nanopillars is presented and electrode biocompatibility is investigated using cell culture experiments. The nanopillar electrodes show decreased impedance over the entire scanned frequency range of 1 Hz–100 kHz and an impedance improvement of up to 89.5 at 1 kHz depending on nanopillar height. Neurons adhere well to the substrate and electrodes and signals with amplitudes up to ten times higher than with conventional gold electrodes were recorded in cell culture experiments.     

Batch mode micro-electro-discharge machining

This paper describes a micro-electro-discharge machining (micro-EDM) technique that uses electrode arrays to achieve high parallelism and throughput in the machining. It explores constraints in the fabrication and usage of high aspect ratio LIGA-fabricated electrode arrays, as well as the limits imposed by the pulse discharge circuits on machining rates. An array of 400 Cu electrodes with 20 μm diameter was used to machine perforations in 50-μm-thick stainless steel. To increase the spatial and temporal multiplicity of discharge pulses, arrays of electrodes with lithographically fabricated interconnect and block-wise independent pulse control resistance-capacitance (RC) circuits are used, resulting in >100× improvement in throughput compared to single electrodes. However, it was found to compromise surface smoothness. A modified pulse generation scheme that exploits the parasitic capacitance of the interconnect offers similarly high machining rates and is more amenable to integration. Stainless steel workpieces of 100 μm thickness were machined by 100 μm×100 μm square cross-section electrodes using in 85 s using an 80-V power supply. Surface smoothness was unaffected by electrode multiplicity. Using electrode arrays with four circuits, batch production of 36 WC-Co gears with 300 μm outside diameter and 70 μm thickness in 15 min is demonstrated     

High-rate through-mold electrodeposition of thick (>200 μm)NiFe MEMS components with uniform composition

An electrodeposition process for achieving good uniformity, growth rate, and yield in NiFe microgears is described. Microgears are electrodeposited from a mixed nickel sulfanate/iron chloride electrolyte through a 230-μm-thick poly methylmethacrylate mold patterned using synchrotron X-ray radiation. Despite the use of a plating cell with nearly ideal wafer-scale electrolyte mixing characteristics [the uniform injection cell (UIC)], a degree of compositional variation in the microgears can arise. The composition variation is shown to be due primarily to nonuniformities in microscopic electrolyte mixing patterns within the mold. To a lesser extent, nonuniformity in the local current distribution also contributes to feature-scale composition variation. Improved composition uniformity is achieved when the plating bath is formulated to reduce the sensitivity to electrolyte agitation. Electrodeposition of MEMS components from a low-flow sensitivity electrolyte using the UIC results in NiFe growth rates greater than 60 μm/h, yields in excess of 90% and good compositional uniformity. Analysis of mechanical properties illustrates that NiFe parts made using this technique compare favorably to typical electrodeposited MEMS components made from nickel and copper     

Sidewall morphology of electroformed LIGA parts-implications for friction, adhesion, and wear control

LIGA fabricated parts are finding application in a wide variety of micro-mechanical systems. For these systems to operate reliably, friction between contacting sidewall surfaces must be understood and controlled. The roughness of the as-plated sidewall is an important determinate of friction forces at such contacts. LIGA sidewalls were characterized in order to provide a basis for predicting the friction, adhesion, and wear behavior of LIGA micromachines. A variety of unexpected sidewall morphologies were observed during this investigation. Three morphologies were identified: a fine scale roughness, a linear through thickness feature, and a group of larger high aspect ratio features. Each morphology has been associated with a specific aspect of the LIGA manufacturing process. Potential friction, adhesion, and wear management strategies suggested by these features have been discussed. In addition, the asperity behavior in a LIGA sidewall contact has been predicted based on the finest roughness observed.     

Fabrication of compliant high aspect ratio silicon microelectrode arrays using micro-wire electrical discharge machining

This paper reports on the fabrication of high aspect ratio silicon microelectrode arrays by micro-wire electrical discharge machining (μ-WEDM). Arrays with 144 electrodes on a 400 μm pitch were machined on 6 and 10 mm thick p-type silicon wafers to a length of 5 and 9 mm, respectively. Machining parameters such as voltage and capacitance were varied for different wire types to maximize the machining rate and to obtain uniform electrodes. Finite element analysis was performed to investigate electrode shapes with reduced lateral rigidity. These compliant geometries were machined using μ-WEDM followed by a two step chemical etching process to remove the recast layer and to reduce the cross sections of the electrodes.     

A novel spark erosion technique for the fabrication of high aspect ratio micro-grooves

Micro groove is an important geometrical feature of components used in microsystem technology (MST). Straight micro grooves are the predominant features in microsystem components such as micro heat exchangers and diffraction gratings. Micro Electrical Discharge Machining (micro EDM) is a complementary microfabrication technique adopted from the conventional EDM machining process for the purpose of micro machining. Using micro EDM it is possible to machine all electrically conductive materials irrespective of their hardness. High aspect ratio microgroove machining for length as high as 20 mm is a formidable task for the conventional micro EDM. In the present work, a novel spark erosion technique has been described wherein a graphite foil has been used instead of the traditional pin shaped tool electrode, for the purpose of making straight grooves. In a single setup microgroove of 20 mm length and an aspect ratio of about 2.3 has been achieved on hardened tool steel by this technique. This process is further refined by using the gravitational effect for the effective debris removal, which has improved the aspect ratio to about 8.Accepted: September 2003     

MEMS milliactuator for hard-disk-drive tracking servo

This paper describes the design, fabrication, and operational characteristics of a MEMS milliactuator designed for servo tracking in a hard-disk drive (HDD). The actuator is designed to increase the bandwidth of an HDD tracking servo and pack more recording tracks on a disk. An Invar (low thermal expansion metal) electrode position process was developed to meet the thermal stability requirement. The electroplated Invar's thermal coefficient of expansion is as low as 6.3×10^-6/K, which is almost half of that of pure nickel. For the plating mold pattern definition, a high-aspect-ratio polymer etching technique was developed. A high-aspect-ratio structure line-and-gap definition is required to achieve both a high directional stiffness ratio and electrode efficiency for the actuator. The etching technique described can etch through a thick (<40 μm) polymer layer with an aspect ratio of 16:1 at an etch rate of <2 μm/min. Low-cost/high-volume manufacturing is achievable by this batch fabrication technique. A milliactuator was fabricated and assembled with a suspension and a slider weighted at around 2 mg. The slider was successfully driven by the milliactuator while the slider was flying on a spinning disk. The operational characteristics (frequency response) of the in-flight milliactuator were measured, and the results indicate that the actuator is suitable for high-bandwidth HDD servo-tracking applications     

Synergistic effects of micro/nano modifications on electrodes for microfluidic electrochemical ELISA

Microfluidic electrochemical sensing has been considered to be highly efficient. However, we showed, by using numerical simulations in this study, that a planar electrode formed on the bottom of a microchannel is exposed to only a small fraction of analytes in amperometric detection. We also showed that three-dimensional (3D) micropillar electrodes significantly improve the detection current. The practical performance was evaluated using 3D micropillar electrodes fabricated by photolithography. The output current increased as the diameters of the micropillars decreased, as predicted by the simulations. It is noteworthy that the current enhancements obtained with the 3D electrodes were larger than those expected from an increase in the surface area. Further increase in current was achieved by electrical deposition of nanoporous gold-black onto the surface of the 3D electrode: when a 3D electrode with micropillars 30 μm in diameter was used, the output current was approximately 20 times that obtained with a 2D electrode without modification. The applicability of the micropillar electrodes was demonstrated in electrochemical enzyme-linked immunosorbent assay (ELISA) of bone metabolic marker proteins. Although an increase in the surface area of the electrode leads to more noise in general, there is no significant difference in the signal-to-noise ratio between the modified 3D electrode and the 2D electrode without modification in the ELISA experiments. This nanoporous micropillar electrode could potentially be a useful component for the development of on-site diagnosis systems.     

Fabrication of high-aspect-ratio electrode array by combining UV-LIGA with micro electro-discharge machining

In this paper, the combination of UV-LIGA with the Micro electro-discharge machining (Micro-EDM) process was investigated to fabricate high-aspect-ratio electrode array, and an easy and rapid process for fabricating ultra-thick SU-8 microstructures up to millimeter depth was described. First, the modified UV-LIGA process was used to fabricate the copper hole array, and then the hole array electrode was employed as a tool in the Micro-EDM process to fabricate the multiple-tipped electrodes. Electrode array of various shapes have been fabricated by this technique. The aspect ratio is up to 17.65.     

Tool electrode geometry and process parameters influence on different feature geometry and surface quality in electrical discharge machining of AISI H13 steel

Electro discharge machining process (EDM) is frequently used when machining of high complex and accurate features is required. Indeed, it is specially recommended for hard materials and micro-machined features. However, due to the process nature, there is still incomprehension on process parameters influence at the final quality features, ending up by lower productivity and quality ratios. On the other hand, fashioning and re-shaping of required electrodes for each feature are time consuming phases and the number of stored electrodes is very high. Therefore, in order to increase the global EDM process productivity, quality and flexibility, standardized simple electrode shapes, capable to machine different features, must be found. This study presents the influence of the main EDM process parameters and different tool geometries on basic process performance measures. A set of designed experiments with varying parameters such as pulsed current, open voltage, pulse time and pulse pause time are carried out in H13 steel using different geometries of copper electrodes. In addition, material removal rate , surface roughness and different dimensional and geometrical micro-accuracies are analyzed through statistical methods. Results help to select appropriate EDM process parameters to machine parts depending on product requirements.     

A sub-micron metallic electrothermal gripper

We report the design, fabrication, and characterization of a multiple bent beam, sub-micron metallic electrothermal gripper. A bottom electroplating mold for electrodes was patterned using electron beam lithography in an SU-8, followed by nickel electroplating. A top electroplating mold for a sub-micron metallic gripper with high aspect ratio bent beams (thickness of 1 μm, width of 350 nm) was prepared using electron beam lithography in a polymethyl methacrylate (PMMA), followed by nickel electroplating and dry release of the top and bottom molds. The sub-micron gripper was characterized using a nanomanipulator system installed in a dual column scanning electron microscopy/focused ion beam system. The ability of the jaw to close up to 1.39 μm displacement with high precision and reliability has been reproducibly observed at an applied current of 28 mA, corresponding to the maximum power consumption of 11.2 mW. Finite element modeling displacement results performed using ANSYS for effective bent beam widths of 370 nm showed a good agreement with the measured displacement results. The sub-micron gripper demonstrated herein will enable the reproducible manipulations with nano-scale resolution displacement and could provide an effective means of interface between nano-scale objects and the micro/macro scale robotic systems.     

Fabrication of High-Aspect-Ratio Electrode Arrays for Three-Dimensional Microbatteries

Silicon-micromachining techniques have been combined with conventional material-synthesis methods to develop microelectrodes for 3-D microbatteries. The resulting electrodes feature an organized array of high-aspect-ratio microscale posts fabricated on the current collector to increase their surface area and volume for a given footprint area of the device. The diameter of the posts ranges from a few micrometers to a few hundred micrometers, with aspect ratios as high as 50. The fabrication approach is based on micromolding of the electrode materials and subsequent etching of the mold to release the electrode structures. Deep reactive-ion-etching or photo-assisted anodic etching has been used to form an array of deep holes in the silicon mold. Electroplating or colloidal-processing method has been used to fill the mold with battery-electrode materials. Measurements on electrochemical half-cells indicated that the 3-D electrode arrays, which are composed of vanadium oxide nanorolls or carbon, exhibited much greater energy densities (per-footprint area) than that of the traditional 2-D electrode geometries. The use of electroplating enabled us to fabricate 3-D interdigitated arrays of nickel and zinc; and battery operation was demonstrated. [2006-0293].     

An electrostatic microactuator using LIGA process for a magnetic head tracking system of hard disk drives

 We demonstrate an electrostatic micro actuator which is fabricated by LIGA process. The actuator is designed for a magnetic head tracking system of hard disk drives (HDDs). The actuator is essential to achieve very high track density of HDDs. We realize the aspect ratio of 125 by the LIGA process using a Si-Au mask. We propose to use PMMA molds both as the mechanical structure and as the insulator between electrodes. We believe there are great opportunity for the LIGA process in making micro actuators of HDDs. Received: 25 August 1997 / Accepted: 24 October 1997     

Electrorotation chip consisting of three-dimensional interdigitated array electrodes

We have developed an electrorotation (ER) chip that has a sandwich structure in which interdigitated array (IDA) electrodes are arranged face-to-face. These IDA electrodes on the top and bottom of the chip were orthogonally arranged to form over 2000 square regions having rotating electric fields between the IDA electrodes. Since rotating electric fields can be generated by arranging the electrical connections to produce a π/2 phase difference between adjacent electrodes, a large number of measurement areas for ER were incorporated within a single ER chip. The ER properties of glass microrods at the individual measurement areas were investigated using this ER chip. The present ER chip was found to be a useful tool for performing high-throughput assays to analyze the dielectric properties of microparticles.     

128电极电阻抗断层成像数据采集系统设计

以实现旋转电极法电阻抗断层成像数据采集自动化为目的,设计开发了一种基于NIOS Ⅱ处理器、拥有128个电极的旋转电极法电阻抗断层成像数据采集系统.进行了数据采集实验,在PC机上获得了采集结果,验证了系统的可靠性.