Fabrication of high-aspect-ratio microscale mold inserts by parallel μEDM

The micro-electrical-discharge-machining (μEDM) technique is an alternative to LiGA (Lithographie, Galvanoformung, Abformung) for fabricating metal-based high-aspect-ratio microscale structures (HARMS). Traditional μEDM is a serial subtractive machining technique in which cuts are performed sequentially, and is therefore difficult to use for generating complex micropatterns on surfaces. In this paper, we report results of a hybrid microfabrication approach, combining micropattern definition with LiGA fabricated Ni HARMS with parallel micropattern generation with μEDM. Multiple micropatterns with some geometrical complexity were successfully generated in parallel on elemental Ta and 304 stainless steel. The hybrid LiGA/parallel-μEDM strategy offers a credible alternative to conventional LiGA regarding fabrication of meso- and micro- scale mold inserts, and enables them to be fabricated out of high-temperature metals/alloys not achievable with electrodeposition.     

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.     

Molding of Pb and Zn with microscale mold inserts

Molding of Pb and Zn metal plates with LiGA (Lithographie, Galvanoformung, Abformung) fabricated Ni microscale mold inserts was carried out at 300 °C. Both as fabricated Ni inserts and Ni inserts conformally coated with Ti-containing hydrocarbon (Ti-C:H) coatings were used. The molding performance was characterized both in terms of feature generation on the metal plates and insert condition after molding. The present results demonstrate that, in cases where significant metal/insert chemical interactions exist, surface engineering of the mold insert is necessary to obtain satisfactory performance, and that conformal deposition of suitably engineered ceramic coatings onto Ni microscale mold inserts is an effective means for achieving high temperature micromolding of reactive metals. WJM gratefully acknowledges partial project support from NSF through grant DMI-0124441, Louisiana Board of Regents through contracts LEQSF(2001–04)-RD-A-07 and LEQSF(2000–03)-RD-B-03, and NASA SBIR contract NAS5–01179 through a subcontract with Mezzo Systems Inc.     

Fabrication of LIGA mold inserts

 The present paper describes the fabrication sequence of a LIGA mold insert by electroforming after the patterning steps of the overall process. These tools are applied for large scale fabrication of microcomponents made by molding and embossing processes. The application of an intermediate layer system leads to optimized process performance and to a better surface quality of the mold insert. The plating processes are described and the materials properties, e.g. hardness, are used for the characterization of the recrystallization behavior of the electroformed nickel which yields the high temperature application limit of the tool. Received: 25 August 1997 /Accepted: 22 September 1997     

Helical surface creation by wire electrical discharge machining for micro tools

In mechanical micromachining, micro tooling is one of the key factors affecting the finished geometrical accuracy and surface quality. To overcome the serious tool wear caused by relatively longer micromachining time, micro tools are usually made of ultra-hard materials such as polycrystalline diamond (PCD) or cubic boron nitride (CBN). Wire Electrical discharge machining (WEDM) is a good choice for efficient fabrications of micro tools made of ultra-hard materials. Considering the traces of wire motions form ruled surfaces, in this paper, typical custom micro milling tools with helical surfaces are generated by ruled surfaces. The simulation shows that the selection of guide lines and generating lines for ruled surface is the key point relating to the final geometrical accuracy and machining efficiency in custom micro tool fabrications by WEDM. Based on the mathematical models built in this paper, overcut can be avoided in the process planning stage for complicated helical surfaces. Furthermore, wire locations can be created conveniently by the introduced mathematical models for post processing in dedicated CAM systems.     

High aspect ratio multi-level mold inserts fabricated by mechanical micro machining and deep etch X-ray lithography

Complex microstructures can be fabricated in large quantities by thermoplastic molding processes. The shape of the microstructures is determined mainly by the mold insert. Until now, multi-level mold inserts have been fabricated either by deep etch X-ray lithography and electroforming, Harmening et al. (1992), or by milling of a brass substrate, Schaller et al. (1995). In both cases there are limitations on structuring either by the fabrication effort or by the sizes of the smallest available milling heads. To avoid these limitations on structuring, a new process for manufacturing multi-level mold inserts has been developed at Forschungszentrum Karlsruhe. Milling, drilling, deep etch X-ray lithography and electroforming have been combined to manufacture a mold insert which is characterized by high aspect ratios with small lateral dimensions and various level heights. Samples with two levels and an aspect ratio of 15 have been manufactured. Much higher aspect ratios seem to be achievable. This paper covers the fabrication process, first tests, and experimental results of manufacturing a multi-level mold insert for molding three-dimensional components of a microvalve system. The development of this technology has been supported by the European Community as part of the Esprit project IMICS.     

High aspect ratio multi-level mold inserts fabricated by mechanical micro machining and deep etch X-ray lithography

 Complex microstructures can be fabricated in large quantities by thermoplastic molding processes. The shape of the microstructures is determined mainly by the mold insert. Until now, multi-level mold inserts have been fabricated either by deep etch X-ray lithography and electroforming, Harmening et al. (1992), or by milling of a brass substrate, Schaller et al. (1995). In both cases there are limitations on structuring either by the fabrication effort or by the sizes of the smallest available milling heads. To avoid these limitations on structuring, a new process for manufacturing multi-level mold inserts has been developed at Forschungszentrum Karlsruhe. Milling, drilling, deep etch X-ray lithography and electroforming have been combined to manufacture a mold insert which is characterized by high aspect ratios with small lateral dimensions and various level heights. Samples with two levels and an aspect ratio of 15 have been manufactured. Much higher aspect ratios seem to be achievable. This paper covers the fabrication process, first tests, and experimental results of manufacturing a multi-level mold insert for molding three-dimensional components of a microvalve system. Received: 30 October 1995 / Accepted: 17 January 1996     

Fabrication of a ZnO piezoelectric micro cantilever with a high-aspect-ratio nano tip

This paper reports on a ZnO piezoelectric micro cantilever with a high-aspect-ratio (HAR) nano tip, which is proposed for a ferroelectric material based nano storage system. The system uses the interaction between the nano tip and a storage medium, and the HAR nano tip is needed to suppress undesirable effects caused by the small gap between the cantilever and the storage medium. The fabrication process for the cantilever with the HAR nano tip consists of three parts: the HAR nano tip formation, the cantilever fabrication, and the bonding/releasing process. The HAR nano tip is formed by the Si deep reactive ion etching for a long shaft and the anisotropic wet etching for a nano tip end. The cantilever is made up of 1 m-thick LPCVD poly-Si layer and 0.2 m-thick Si nitride layer, and has 0.5 m-thick ZnO actuation layer. A final releasing process is followed by an anodic bonding process. The fabricated HAR nano tip has 6 m side length, over 18 m height, and less than 15 nm tip radius, which is built on the 85 m-wide, 300 m-long, and 1.2 m-thick cantilever. The experimental results show a linear behavior with respect to input voltage of 1 to 5 V and the first resonance frequency at 17.9 kHz.     

Fabrication of high-aspect-ratio ceramic microstructures byinjection molding with the altered lost mold technique

A fabrication process for complex ceramic microstructures was proposed that combines a lost mold technique and ceramic injection molding. Two key points in this process were studied. First, the solubility of several engineering plastics in various organic solvents was tested to find appropriate combinations of mold material and solvent for dissolving molds. Secondly, the binder extraction rate and the strength of a green body during debinding were investigated. Experimental results indicate that acrylonitrile-butadiene styrene and acetone are the best combinations selected for this lost mold technique. We also propose that using gasoline as the debinding solvent and performing the debinding at room temperature will give a good time-saving effect and avoid toppling the microstructure if paraffin wax, stearic acid, and polyethylene were selected to compound the binder system. This process has been successfully applied to fabricate several ceramic microstructures, such as an integrated punch     

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.     

Fabrication of high-aspect-ratio polymer nanochannels using a novel Si nanoimprint mold and solvent-assisted sealing

We present a low cost nanofabrication method to fabricate high-aspect-ratio (HAR) polymer nanochannels using a novel silicon nanoimprint mold fabrication technique and a solvent-assisted sealing method. These nanofluidic channels are being developed for single biomolecule detection. The silicon nanoimprint mold fabrication process is based on the combination of anisotropic etching of silicon by potassium hydroxide (KOH) solution and the local oxidation of silicon (LOCOS) process. The resulting high-aspect-ratio silicon mold has smooth sidewalls owing to the anisotropic KOH etching process along the silicon crystalline geometry as well as the LOCOS process. The nanostructures in the nanoimprint molds that form the nanochannels can be easily controlled by the initial micropattern sizes defined using conventional UV lithography and the oxidation time, making this technique a practical solution for low cost and high-throughput HAR silicon nanoimprint mold fabrication. Nanoimprint molds having aspect ratios of more than 1:5.5 (width: 200 nm, height: 1.1 μm, length: 1 cm) were successfully fabricated. Nanoimprinting technique was used to create poly(methyl methacrylate) (PMMA) nanotrenches out of this nanoimprint mold. A novel solvent-assisted sealing technique was developed in order to seal the HAR PMMA nanotrenches. This technique enables the generation of nanochannels with various nanoscale dimensions without the need for complicated and expensive nanolithography tools.     

Fabrication of diamond micro tools for ultra precision machining

State of the art diamond tools for metal-cutting manufacturing are handcrafted by polishing and grinding of natural diamonds. Tools fabricated by these means are serially made unique copies with a limited variety of shapes. Furthermore, manual fabrication leads to deviation from ideal geometry. We present a novel technique for parallel fabrication of diamond micro tools with high contour accuracy by using lithographic methods followed by a modified ASE-process.     

Fabrication of high-aspect-ratio hydrogel microstructures

Microfabrication of high-aspect-ratio polymeric microstructures via deep X-ray lithography traditionally involves either crosslinking or scissioning a polymer film spun-cast on a substrate. A post-exposure development procedure is usually employed to remove the unwanted polymer, leaving behind lithographically patterned structures. Instead, we use a novel synthesis technique wherein polymerization of a mixture of monomers in solvent is initiated, through a mask, with hard X-rays. The resulting polymer precipitates out of the solvent, thus limiting the spatial propagation of the reaction only to the exposed regions. Such a technique offers a unique way for the patterned synthesis of polymers from a variety of monomer-solvent systems. Here, we present the first results on the synthesis of high-aspect-ratio microstructures of a thermoreversible hydrogel, poly (N-isopropylacrylamide), and an ionic hydrogel, poly (methacrylic acid). These stand-alone, implantable microstructures are envisioned to be potentially useful in such diverse areas as biosensors, microactuators, controlled release applications, and cell and tissue engineering.This paper was first presented at the High Aspect Ratio Microstructures (HARMST) 2003 conference in Monterey California, June 2003.We would like to thank Dr. Francesco De Carlo (APS) for his discussions on beamline simulations. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences (BES), Office of Science, under contract number W-31-109-ENG-38.     

Fabrication of microfluidic structures in quartz via micro machining technologies

Microfluidic channels have been created for quartz material using micromechanical manufacturing technologies such as micro laser machining, micro ultrasonic machining, and ultra-precision machining. Ultra-precision machining has been used to manufacture cross-junction channels 14 µm wide and 28 µm deep with a three-dimensional triangle cross-section. Micro laser machining has been used to manufacture U-shaped and -shaped microfluidic channels. Deep holes and microfluidic channels with a high slenderness ratio (width/depth) can be obtained by using micro ultrasonic machining technology. These three machining techniques are compared with respect to surface profiles and machining quality.

     

Multi-attribute decision making for green electrical discharge machining

This paper aims to develop a combination of Taguchi and fuzzy TOPSIS methods to solve multi-response parameter optimization problems in green manufacturing. Electrical Discharge Machining (EDM), a commonly used non-traditional manufacturing process was considered in this study. A decision making model for the selection of process parameters in order to achieve green EDM was developed. An experimental investigation was carried out based on Taguchi L9 orthogonal array to analyze the sensitivity of green manufacturing attributes to the variations in process parameters such as peak current, pulse duration, dielectric level and flushing pressure. Weighing factors for the output responses were determined using triangular fuzzy numbers and the most desirable factor level combinations were selected based on TOPSIS technique. The model developed in this study can be used as a systematic framework for parameter optimization in environmentally conscious manufacturing processes.     

Low temperature deposited titanium boride thin films and their application to surface engineering of microscale mold inserts

Titanium boride thin films were deposited at low temperatures by balanced magnetron sputtering and inductively coupled plasma (ICP) assisted balanced magnetron sputtering. The chemical composition, surface morphology, structure, and mechanical properties of titanium boride thin films were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, and instrumented nanoindentation. As compared to titanium boride films deposited by balanced magnetron sputtering, the increase in plasma density surrounding the substrate surface during film growth afforded by the ICP assist causes significant film densification and mechanical property improvement. The morphology of titanium boride thin films deposited onto microscale non-flat Ta substrates and their effectiveness as barrier coatings for microscale compression molding of Al was characterized by focused ion beam sectioning and SEM. The present results show the potential of low-temperature deposited, conformal, titanium boride thin films for engineering surfaces of microscale mold inserts for microscale pattern replication in reactive metals by compression molding.     

Microstructuring of non-conductive silicon carbide by electrical discharge machining

The electrical discharge machining process is an established process for machining materials regardless of their mechanical properties. Thus this process is especially attractive for materials which are hard to machine with conventional machining methods. The only requirement a material has to fulfil is having a certain electrical conductivity. Ceramic materials, (e.g. zirconia, silicon nitride or silicon carbide) exhibit excellent mechanical properties but are mostly electrically non-conductive. This can be compensated by an applied, electrically conductive assisting electrode. With this modification, the electrical discharge machining of non-conductive ceramic material is enabled. In this study the micro electrical discharge machining of non-conductive sintered silicon carbide is investigated. The drilling process shows instabilities due to the excessive generation of carbon products. A stabilisation of the process up to the maximum depth of 420 μm is realized by two approaches: adapting process parameters and adapting the tool electrode geometry. An analysis of the amount of infeed used in a milling process shows that an infeed of 15 μm has the best material removal rate to tool wear rate ratio. A maximum material removal rate of 3.58 × 10^?3 mm³/min is achieved. Detached microstructures with an aspect ratio of 30 are machined. A conducted surface analysis indicates that the present removal mechanism is thermally induced spalling. Furthermore no heat affected zone is present in the machined near-surface area.     

Fabrication of submicron high-aspect-ratio GaAs actuators

Submicron, single-crystal gallium arsenide (SC-GaAs) actuators have been designed, fabricated, and operated. The fabrication process, called SCREAM-II (single crystal reactive etching and metallization II), uses chemically assisted ion beam etching (CAIBE) and reactive ion etching (RIE) to produce suspended and movable SC-GaAs structures with up to a 25:1 aspect ratio of vertical depth (10 μm) to lateral width (400 nm). Integrated actuators with predominantly vertical sidewall (PVS) aluminum electrodes are used to move the structures. Silicon nitride is used as an etch mask, structural stiffener, and electrical insulator. An x-y stage with integrated actuators produces controllable x-y displacements of ±1.8 μm when a voltage of 54.5 V is applied to either or both of the x and y actuators. The x-y stage resonates for an applied sinewave of 20 V (peak to peak) with f=10.5 kHz and a DC offset of 10 V. The structural vibration amplitude is 0.6 μm     

SOI wafer mold with high-aspect-ratio microstructures for hot embossing process

This paper reports using a Silicon oil insulator (SOI) wafer as a mold insert for the hot embossing process on high-aspect-ratio microstructures to overcome two drawbacks of Inductive Coupled Etching (ICP) process, the area dependent etching and the micrograss. A thin sacrificial wall to eliminate the undercut in the big open area during ICP etching is also described. A good result of final embossed structure on PMMA with aspect ratio of 12 : 1, uniform thickness, and smooth surface is presented.This work is partially supported by grants NSF/LEQSF (2001-04)-RII-02, DARPA DAAD19-02-1-0338, and NASA (2002)-Stennis-22.