Fabrication of high-aspect-ratio microstructures on planar andnonplanar surfaces using a modified LIGA process

Large surface areas (tens of square centimeters to square meters) covered with high-aspect-ratio microstructures (HARMs) have potential applications in a wide range of fields including heat transfer, adaptive aerodynamics, acoustics, catalysts, seal and bearing design, and composite materials. HARMs are typically hundreds of micrometers in height, with widths ranging from a few micrometers to tens of micrometers, and they can be manufactured from a variety of materials such as metals, polymers, and ceramics. Three of the barriers to extensive use of large HARM-covered surfaces are cost, nonplanarity of typical surfaces, and adhesion of the microstructures to the surface. A starting point for inexpensive reproduction of large arrays of HARMs is the plastic molding step of the LIGA micromanufacturing process. In order to address the latter two problems, the standard LIGA process was modified/extended. Free-standing polymer sheets, perforated with a pattern of high-aspect-ratio throughholes, were clamped to conductive substrates. The sheets provide a template for electrodeposition of nickel microstructures onto the target surface. This process makes it economically feasible to electroform metal microstructures directly onto large planar and nonplanar metal surfaces (cylinders)     

Micro injection molding for mass production using LIGA mold inserts

Micro molding is one of key technologies for mass production of polymer micro parts and structures with high aspect ratios. The authors developed a commercially available micro injection molding technology for high aspect ratio microstructures (HARMs) with LIGA-made mold inserts and pressurized CO₂ gasses. The test inserts made of nickel with the smallest surface details of 5 μm with structural height of 15 μm were fabricated by using LIGA technology. High surface quality in terms of low surface roughness of the mold inserts allowed using for injection molding. Compared to standard inserts no draft, which is required to provide a proper demolding, was formed in the inserts. To meet higher economic efficiency and cost reduction, a fully electrical injection molding machine of higher accuracy has been applied with dissolving CO₂ gasses into molten resin. The gasses acts as plasticizer and improves the flowability of the resin. Simultaneously, pressurizing the cavity with the gasses allows high replication to be obtained. Micro injection molding, using polycarbonate as polymer resins, with the aspect ratio of two was achieved in the area of 28 × 55 mm² at the cycle time of 40 s with CO₂ gasses, in contrast to the case of the aspect ratio of 0.1 without the gasses.     

Direct, high throughput LIGA for commercial applications: a progress report

Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 μm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases. New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase. Received: 7 July 1999/Accepted: 30 July 1999     

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     

Multi-functional valve components fabricated by combination of LIGA processes and high precision mechanical engineering

 The advantage of thermoplastic molding as a convenient method for fabricating large quantities of microstructures is restricted by the possibilities and the necessary efforts for structuring the required molding tools. In order to increase the complexity of mold inserts without significantly increasing the fabrication expenditure a new process combining LIGA techniques and precision mechanics had been suggested by Research Center Karlsruhe. Recent work on the optimization of this process made it possible to manufacture multi-leveled mold inserts with which different three-dimensional microcomponents have successfully been molded. The two- and three-level structures feature among other details integrated alignment aids which worked very well during the assembly of the valve system the components were designed for. This paper deals with the process optimization, the manufacturing of the mold inserts, the fabrication of the three-dimensional microstructures by hot embossing in PMMA as well as in polymers with high thermal resistance and the application in a microvalve system. Received: 25 August 1997/Accepted: 22 September 1997     

Rapid replication of polymeric and metallic high aspect ratio microstructures using PDMS and LIGA technology

This paper present a method of rapid replication of polymeric high aspect ratio microstructures (HARMs) and a method of rapid reproduction of metallic micromold inserts for HARMs using polydimethylsiloxane (PDMS) casting and standard LIGA processes. A high aspect ratio (HAR) metallic micromold insert, featuring a variety of test microstructures made of electroplated nickel with 15:1 height-to-width ratio for 300 μm microstructures, was fabricated by the standard LIGA process using deep X-ray lithography (DXRL). A 10:1 mixture of pre-polymer PDMS and a curing agent were cast onto the HAR metallic micromold insert, cured and peeled off to create reverse images of the HAR metallic micromold insert in PDMS. In addition to the replication of polymeric HARMs, replicated PDMS HARMS were coated with a metallic sacrificial layer and electroplated in nickel to reproduce another metallic micromold insert. This method can be used to rapidly and massively reproduce HAR metallic micromold inserts in low cost mass production manner without further using DXRL.     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

Various replication techniques for manufacturing three-dimensional metal microstructures

 In microsystem technology a large range of different 6materials will be available only after the necessary micromanufacturing techniques have been developed or adapted. Existing manufacturing techniques are structuring or shaping techniques producing three-dimensional microstructures out of silicon (silicon etching, silicon surface micromechanics), mostly unfilled plastics (lithographic techniques, injection molding, hot embossing, reaction molding) or a few pure metals or binary alloys (electroforming). The choice of materials for microcomponents is determined by the function and conditions of use of microsystems. Especially the range of metals is still restricted considerably because the only processes available are electroforming and thin-layer techniques. It is for these reasons that we are developing various processes for manufacturing three-dimensional metal microstructures. In addition to direct electroforming of injection molding lost plastic micromolds, these are a new microcasting process and Micro Metal Injection Molding (Micro MIM). Microstructures have already been molded from mold inserts made by micromechanical cutting or by the LIGA technique. The results achieved, and future prospects, are outlined below. Received: 25 August 1997/Accepted: 3 September 1997     

Micro powder metallurgy for the replicative production of metallic microstructures

 Reproductive techniques like injection molding or embossing of feedstock provide microstructures of a wide variety of materials for a reasonable price to micro system technology. In this paper, the dependencies and barriers to produce high aspect ratio structures by micro metal injection molding are described; some results of embossing of metal powder based feedstocks are presented, too. The investigations show different influencing parameters for reaching high aspects ratios. The main factor is the used powder, finer powders allow higher aspect ratios. Moreover, the binder system, the feedstock (mixture of powder and binder) and the quality of the injection mold influence the reproduction process. Received: 10 August 2001/Accepted: 24 September 2001     

Performance and simulation of thermoplastic micro injection molding

 Originally developed for the replication of high aspect ratio LIGA structures, micro injection molding is presently on its way to become an established manufacturing process. Enhanced technological products like micro optical devices are entering the market. New developments like the different kinds of injection molding with several components open up opportunities for increasing economic efficiency as well as for new fields of applications. Software tools for the simulation of the thermal household of the molding tool and/or the moldfilling process itself can provide useful but not wholly sufficient assistance for the optimization of micro injection molding. Received: 10 August 2001/Accepted: 24 September 2001     

Injection molding for microstructures controlling mold-core extrusion and cavity heat-flux

 In this work we constructed an injection press molding system with a mold-core extrusion mechanism and a small sensor assembly for effectively duplicating microstructures to the mold products. The mold-core extrusion mechanism is driven by a piezo element to apply force on important area with microstructures. For example, after injection it increases the cavity pressure from 20 to 34 MPa. Small sensors consist of the pressure, displacement, and heat flux sensor assemblies, arranged around the small cavity. The signals showed us the physical phenomena inside the mold and may be further used as control signal. In order to evaluate this injection press molding system, we formed micro triangular grooves of pitch 1 μm and angle 140°. The mold-core extrusion gave better diffraction intensity by several percents. Received: 16 May 2001/Accepted: 24 July 2001 The authors would like to thank FANUC Ltd. for cutting the triangular grooves on the mold-core. This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

A study of integration of LIGA and M-EDM technology on the microinjection molding of ink-jet printers’ nozzle plates

Common processes to manufacture nozzle plates of ink-jet printer heads are electroplating or laser machining. In order to reduce the production cost and improve the performance of nozzle plates, a new approach, microinjection molding, is introduced to manufacture nozzle plates in this study. The micro mold was made by integration of the LIGA and M-EDM technology to improve the positioning and alignment accuracy. After assembling the micro mold, microinjection-molding technique was applied to produce four nozzle plates in one shot. There are 60 micro through-holes on each plate. The diameters were measured by an optical microscopy and in the range of 101±1 microns. Experiment and simulation result indicated that the most significant factor for molding these thin films with micro through holes is the mold temperature. This work recommends the mold temperature during filling of the vario-thermal mold system is about 10∼15°C higher than the glass transition temperature of the polymer. The manufacture procedures proposed in this study are believed to be more accurate and economical.     

“LIGA2.X” process for mass production of single polymeric LIGA micro parts

The new modification of the LIGA process “LIGA2.X” is a promising process chain to replicate high accuracy single polymeric LIGA micro parts in large scale to feasible costs. The advantages of this new approach compared to the standard LIGA process are the reduction of fabrication costs, the elimination of any rework after replication, industry suitable mold technology for injection molding using a new developed demolding concept, optimized injection molding parameters for every single cavity, shot volumes of the parts below 0.5 mm³ and a new freedom in the arrangement of structure cavities in multicavity molds. Aspects covered in this paper are the introduction of the complete process chain of the new “LIGA2.X” process and the used mold concept for the micro-män 50 micro injection molding machine. Furthermore replicated single polymeric LIGA microparts fabricated using the new process chain are shown. To conclude, potential process improvements and future work will be outlined.     

Study of Hot Embossing Using Nickel and Ni–PTFE LIGA Mold Inserts

Hot embossing is one of the main process techniques for polymer microfabrication, which helps X-ray lithography, electroplating, and molding (LIGA) to achieve low-cost mass production. Most problems in polymer micromolding are caused by demolding, especially for hot embossing of high-aspect-ratio microstructures. The demolding forces are related to the sidewall roughness of the mold insert, the interfacial adhesion, and the thermal shrinkage stress between the mold insert and the polymer. The incorporation of polytetrafluoroethylene (PTFE) particles into a nickel matrix can have the properties such as antiadhesiveness, low friction, good wear, etc. To minimize the demolding forces and to obtain high-quality polymer replicas, a Ni-PTFE composite microelectroforming has been developed, and the hot embossing process using Ni and Ni-PTFE LIGA mold inserts has been well studied in this paper. The morphologies, sidewall roughness, and friction coefficient have been explored in the fabricated Ni-PTFE LIGA mold insert. Finally, the comparison of embossed microstructures with various aspect ratios and the comparison of the embossing lifetimes of mold inserts have been carried out between Ni and Ni-PTFE mold inserts, which show a better performance of the Ni-PTFE mold and its potential applications.     

Microcomposite electroforming for LIGA technology

 Effective methods to improve hardness and thickness uniformity in electroforming for the LIGA process to produce metallic microstructures are described. The result shows the internal stress of Ni-Co alloy deposit can be well tuned between 2 to −2 kg/mm² by adding a stress release agent into the electroplating bath. A secondary cathode is applied to improve the thickness uniformity in electroforming without loss plating rate at the center of the features. Received: 7 July 1999/Accepted: 25 November 1999     

Combination of LIGA with other microstructure technologies

 The combination of the LIGA technique with other microstructure technologies like silicon technology, membrane technologies, and mechanical micromachining gives access to the development and manufacturing of new microsystems. The quasi-monolithic integration of LIGA microstructures on top of a CMOS-Wafer using a molding process will be shown which has the potential to integrate a LIGA component directly with microelectronic circuits. By combining mechanical micromachining, plastic molding, and membrane technologies a micropump has been developed which is one of the key components of the fluid handling module of microanalysis systems together with the well known LIGA microspectrometer. In the case of the micro-spectrometer, a small-scale series production was installed. Received: 30 October / Accepted: 1 December 1995     

Method for polymer hot embossing process development

Molding technologies associated with fabricating macro scale polymer components such as injection molding and hot embossing have been adapted with considerable success for fabrication of polymer microparts. While the basic principles of the process remain the same, the precision with which the processing parameters need to be controlled especially in the case of molding high aspect ratio (HAR) polymer microparts into polymer sheets is much greater than in the case of macro scale parts. It is seen that the bulk effects of the mold insert fixture and molding machine have a dominant influence on the molding parameters and that differences in material parameters such as the glass transition temperature (T _g) of polymer sheets are critical for the success and typically differ from sheet to sheet. This makes it very challenging to establish standard processing parameters for hot embossing of sheet polymers. In the course of this paper, a methodology for developing a hot embossing process for HAR microstructures based on known material properties and considering the cumulative behavior of mold, material, and machine will be presented. Using this method force–temperature–deflection curves were measured with the intent of fine tuning the hot embossing process. Tests were carried out for different materials using a dummy mold insert yielding information that could be directly transferred to the actual mold insert with minimum development time and no risk of damage to the actual microstructures.     

Molding of LIGA microstructures from fluorinated polymers

 For further applications microsystem technology needs a wider range of materials and processes suited to manufacture microstructures of high chemical resistance, medical compatibility and low friction. Therefore, the fundamentals of stepped-microstructure fabrication from fluorinated polymers were developed at Forschungszentrum Karlsruhe in cooperation with the Bürkert GmbH & Co. company. It was the aim to enhance the applicabilities of the LIGA technique. First, the process for the fabrication of LIGA molding tools with two or more levels, and second, the process of vacuum hot embossing of PVDF microstructures were developed. The potentials of both developments are demonstrated by the example of molded stepped micro valve plates of PVDF. These micro valve plates were implemented in miniaturized values and tested successfully. Received: 30 October 1995 / Accepted: 12 January 1996     

Movable microstructures made by a sub-micron LIGA process

 Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied. Received: 30 October 1995 / Accepted: 17 January 1996     

Visualizing analysis for weld line forming in micro injection molding by experimental method

In micro injection molding, the melt flow behavior is important for the final product quality. However, the current process monitoring and measurement technology are not adequate enough to provide a direct analysis access. In the presented study, a glass insert mold designed for performing the direct visual analysis for melt flow phenomena in micro injection molding is introduced. The micro tensile specimen with 0.1 × 0.4 mm² (depth × width) cross section dimension is chosen as the objective part. The correlation between processing parameters (injection pressure, injection speed, mold temperature) and flow behavior was investigated and analyzed. The results show that the injection pressure put an obvious effect on the filling speed through micro cavity. Injection speed can influence the filling time dramatically also. Higher mold temperature brings positive influence with the flowing speed, due to the lower viscosity of polymers in higher mold temperature.