Microelectronics Journal 35 (2004) 131–143 www.elsevier.com/locate/mejo
Applications of LIGA technology to precision manufacturing of high-aspect-ratio micro-components and -systems: a review Chantal Khan Maleka,*, Volker Saileb a
Laboratoire de Physique et Me´trologie des Oscillateurs (LPMO)-CNRS 32 Av. de l’Observatoire, Besanc¸on 25044, France b Institut fu¨r Mikrostrukturtechnik (IMT), Universita¨t Karlsruhe and Forschungszentrum Karlsruhe GmbH, and ANKA, Angstro¨mquelle Karlsruhe GmbH, P.O. Box 3640, D-76021 Karlsruhe, Germany Received 9 December 2002; revised 22 September 2003; accepted 3 October 2003
Abstract The by far leading technology for manufacturing MEMS devices is Si-micromachining with its various derivatives. However, many applications of microsystems have requirements on materials basis, geometry, aspect ratio, dimensions, shape, accuracy of microstructures, and number of parts that cannot be fulfilled easily by mainstream silicon-based micromachining technologies. LIGA, an alternative microfabrication process combining deep X-ray lithography, plating-through-mask and molding, enables the highly precise manufacture of high-aspect-ratio microstructures with large structural height ranging from hundreds to thousands of micrometers thick. These tall microstructures can be produced in a variety of materials with well-defined geometry and dimensions, very straight and smooth sidewalls, and tight tolerances. LIGA technology is also well suited for mass fabrication of parts, particularly in polymer. Many microsystems benefit from unique characteristics and advantages of the LIGA process in terms of product performance. The LIGA technology is briefly reviewed. The strengths of the manufacturing method and its main fields of application are emphasized with examples taken from various groups worldwide, especially in micromechanics and microoptics. q 2003 Elsevier Ltd. All rights reserved. Keywords: LIGA; 3D-micromachining; High-aspect-ratio; Micromechanics; Microoptics; Microfluidics
1. Introduction Several microfabrication technologies are available today and are used to fabricate microcomponents and systems. The most successful micromachining technologies have been developed as extensions of standard IC and microelectronics planar silicon-based processing. Others are based on advanced precision engineering and laser structuring. However, individual technologies including Si-micromachining or laser structuring are far from being sufficient to fulfill the needs of the variety of problems posed by: † The great variety of functions of most devices to be made, † The specificity of surroundings in which they will operate, * Corresponding author. E-mail addresses: [email protected]
(C. Khan Malek), [email protected]
(V. Saile). 0026-2692/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2003.10.003
† The optimum cost/performance ratio for the targeted application. Interest in a number of non-Si based machining methods stems from major deficiencies of IC-based machining techniques: † The need for using application-specific materials to optimize the functions and performance of various devices, † The need to reduce cost by choosing low-cost materials, † The difficulty to construct truly 3D objects with planarbased processing continues to be a challenge. Precision and ultra-precision mechanical, electro-discharge, LIGA-based, and laser-based, micromachining techniques, to mention the most current ones, are such alternative techniques, each with their specific application domains and relative merits. LIGA-based processing, a sequence of microfabrication steps combining a step of deep X-ray lithography  [(DXRL), also called by some authors
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deep etch X-ray lithography], and subsequent additive processing of plating-through-mask and molding , has moved from emerging microfabrication technology to wellestablished non-silicon alternative microfabrication technology for MEMS. The LIGA technology provides unique advantages over other manufacturing methods in the fabrication of microstructures. LIGA-based technologies are used and further developed in a number of R&D institutes around the world. Spin-off companies and commercial companies have also evolved around largescale synchrotron facilities. Commercial application of the LIGA process is occurring. This short review will not go back to the physical and technological fundamentals of the technique [3 – 5]; it is rather intended to recall the essential steps of the process sequence and focus on a number of selected examples from recent work performed in various LIGA groups around the world and show the usefulness and advantages of this technology. The LIGA technology has been developed over a rather long time span of two decades. During that time other highaspect-ratio technologies such as UV photolithography in thick resist like SU8, often referred to as ‘UV –LIGA’ and Deep Reactive Ion Etching (DRIE) of silicon have evolved as well and challenge LIGA successfully in some specific application areas. For planning LIGA role in future manufacturing, a review of potential applications may serve as a basis. The fabrication of LIGA-parts concerning in particular the lithographic aspect, materials base expansion through replication technologies such as electroplating and moulding and some associated challenges, as well as some materials issues was reviewed in a former article by one of the authors .1 The basic LIGA process and some aspects of the process are recalled here to illustrate its strengths and discuss challenges, not in terms of materials properties  but in terms of applications. The purpose of this article is thus providing input on the discussion of the LIGA potential by summarizing proposals and ideas for LIGA applications found in literature.
2. LIGA process and strengths 2.1. Basic process The basic LIGA process is described in Fig. 1. In the first step of the LIGA process, an X-ray sensitive polymer (resist) layer up to several millimeters thick, typically polymethylmethacrylate (PMMA) is coated onto a conductive substrate. A pattern from a mask is therefore transferred into the thick resist layer via a 1:1 shadow proximity printing scheme using hard X-rays from a synchrotron radiation source. After exposure, selective dissolution of the chemically modified 1
Many references are not repeated in this review.
Fig. 1. (a) Illustration of the basic LIGA process steps. The six panels stand for lithography, polymer components after development, electroplating and overplating the polymer template, a metal mold, and replication. (b) Basic LIGA process sequence.
irradiated parts of the resist layer in a chemical developer results in a polymeric relief replica of the mask pattern. Then, depending on the material and number of parts selected for the final product, different fabrication routes can be chosen, which may include further steps of microreplication through electroforming and/or a variety of molding techniques (injection molding, embossing, casting, compression molding, etc.). The polymeric microstructure can be used: † Simply as-is; † As a lost mold for the formation of ceramic microparts; † As an electroplating template to generate metallic microparts. The microstructures are often further defined by precision lapping to control thickness. † As an electroplating template to produce a metallic master mold, which can then be used multiple times to mold cost-effective replicates in other materials, primarily polymers. When producing large numbers of electroplated components, the molded polymer parts are used as lost molds for a second plating process. The unique processing feature that enables the manufacture of thick microstructures characterized by very steep walls and very tight tolerances is the creation of highly
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precise resist template by deep X-ray lithography using X-ray photons from a synchrotron radiation source. Features characterizing this process are listed below: † As a result of their high energy, these X-rays are capable of deeply penetrating thick (e.g. hundreds of micrometers or even millimeters) layers of polymeric resist, allowing uniform deposition of energy in the depth of the resist and the formation of tall microstructures in one exposure step. † The short wavelengths of X-ray photons provide high resolution for patterning due to low diffraction effects. † The very small vertical angular divergence of the X-ray beam achieves high accuracy in pattern transfer from the mask. Due to their excellent collimation the X-rays penetrate thick resists with extremely low horizontal run-out (less than 0.1 mm/100 mm thickness), thereby producing the substantially vertical walls for which LIGA structures are well known. † The almost parallel (well collimated) light of X-ray beams produced by synchrotron radiation sources also allows printing with large depth-of-field. A large working gap between mask and substrate can then be used in non-traditional pattern transfer as for the manufacture of slanted structures or for pattern formation on substrates presenting a large topography. † The vertical sidewalls are optically smooth with typical local roughness of the order of 10 nm and longer-range waviness such as slope errors or steps determined solely by the accuracy of mask writing. Some aspects in utilizing synchrotron radiation for LIGA are: † A widely perceived drawback in using LIGA-like processes is that synchrotron radiation is the optimum exposure source and thus requires access to large accelerators. However, deep X-ray lithography is one of the demanded services, which has led to improved storage ring access to the point where it may be acquired as a foundry service and can be provided as part of a distributed manufacturing process, as for example ion implantation. It is worth noting in this context that the cost of lithography at synchrotron radiation sources is not a limiting factor in the overall cost of a mold. Entirely different is the case of ‘direct LIGA’  when each batch of parts is produced with X-rays. Then access to a synchrotron facility, intensity of the source, cost per hour of X-rays, exposure field size, and others become crucial factors for the economics of LIGA fabrication. † Specialized equipment for the LIGA process is currently commercially available (from companies such as, e.g. Jena-Optics and Technotrans), besides the custom equipment that various groups have developed. Companies and institutes worldwide provide services for mask making (e.g. ANKA GmbH), X-ray exposure (e.g. ANKA GmbH
and CAMD), development, electroplating (e.g. TVJ— Dover Industrial Chrome, Inc., Dynamics Research Corporation (DRC), etc.), post-processing, molding (e.g. microParts GmbH, Mezzo Systems Inc.,), etc. † Exposure cost is being dramatically reduced by the use of a new very sensitive X-ray resists such as SU8 (which has been developed as a UV resist for photolithography but proved very sensitive to X-rays as well ). † Little effort has been devoted up to now to optimize the cost of exposure and access for LIGA applications on synchrotron facilities that have originally been developed for totally other applications. Therefore, the margin for improvement in that field is very large . † Once the master mold is prepared, the replication steps of microelectroplating and micromolding can be carried out away from the synchrotron source. The development and utilization of new application-specific materials has become one of the key challenges for the commercial production of microcomponents and systems. This applies, in particular, to the LIGA process and is offered through various replication techniques. Electroforming and various types of molding techniques extend the materials basis. They also allow for low-cost mass production. The best strengths of the LIGA technology lie in several key functional areas: † Large to very large structural heights, typically from hundreds to thousands of micrometers thick that can be formed in one single step, † Smallest lateral dimension of a few micrometers with structural details in the sub-micrometer range, † Access to a large base of functional materials: electroplated metals and alloys, molded polymers , ceramics, composites, multilayered materials, gradedmaterials, nanomaterials, etc. The development and utilization of application-specific materials have become one of the key challenges for the commercial production of microcomponents and microsystems. This applies, in particular, to the LIGA process and is offered through the various replication techniques that extend the materials basis. † Formation of complex shapes, * Free lateral shape, * Mastering of the third dimension, not only by achieving deep structures with high aspect ratios, but also more complex structures involving multilevel and oblique shape structures (see paragraph 3), † Structural accuracy of features, * Good dimensional control over the entire structure height, * Very precise shape definition of parts, both laterally in terms of dimensional control and in term of straightness and planarity of sidewalls,
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Low surface roughness of side-walls (rms roughness of 20 nm or better), which is an essential factor for microoptical components , Typical feature sizes of several micrometers with structural height of several 100 mm to several 1000 mm and sub-micrometer details are routinely available . Extreme parallelism/verticality of sidewalls with slopes of the order of 1 mrad.
† Mixing of scales (small features on large parts), which is important for several applications that call for a macroor meso-scale main part with microscale features, e.g. patterning of optical waveguides. † Optical transparency of plastics, mainly PMMA-based, within a wide range of wavelengths (typically between 200 and 900 nm) and low autofluorescence, which makes these materials well suited for optical applications. † Large process latitude. In particular, X-ray lithography functions with large depth of field, allowing for pattern formation on non-planar surfaces . † Combination with CMOS type processes possible as well as with other microfabrication techniques, e.g. high precision mechanical engineering, silicon surface and bulk micromachining, sacrificial layers, membrane technique, etc. † Collective manufacturing by parallel processing of parts (batch fabrication) to reduce cost of fabrication. † Possibility of high-volume low-cost production. The original formulation of the LIGA process included molding, by injection molding or hot embossing, as the technology for mass fabrication. † Finally, fab-lines for LIGA components have been established complying with rigorous quality management systems .
2.2. Towards building 3D systems with LIGA Lithography is often considered as a planar process step. Deep X-ray lithography extends to a third out-of-plane dimension through forming microstructures with high vertical or oblique sidewalls. Typically LIGA structures allow for the free choice of the lateral 2D pattern that is projected into the third dimension to form prismatic or cylindrical geometries. This technique has generally been used to produce structures with straight walls. However, for all major LIGA process steps, variations have been developed that increase the fabrication flexibility. Geometrical variations in that third vertical dimension are possible and can be obtained in different ways by modifying or combining process steps, in particular for producing shapes with increased dimensionality. The 3D structural construction with LIGA technique can be primarily classified in two categories:
† One based on the sequential planar formation of individual levels to form 3D structures . Fabricating such 3D structures by multiple level, aligned X-ray exposures requires additional processes such as * An accurate replanarization process such as precision lapping after each electroplating step enabling subsequent resist application, * A mask-alignment procedure between each conventional DXRL cycle with special alignment gear and alignment marks on the mask and the substrate. † The other with spatial variation of the absorbed dose. Off-axis X-ray exposures and exposures where mask and/or substrate are moved in non-standard schemes during irradiation are capable of forming complex threedimensional structures. They require: * Tilting and/or rotation of the mask and/or substrate system during exposure [17 – 20] or oscillation of the mask ; the use of specialized exposure fixtures or sophisticated stages permit angular positioning and rotation of the substrate and/or mask during X-ray exposure. The capability of nearly arbitrary angled resist patterning allows for the formation of multiple angled cylindrical surfaces and a wide range of geometries. * Software packages modeling the dose deposited inside the resist with complex exposure scheme. The technique can also be extended to the manufacture of 3D, more complex systems by: † Stacking and assembling various levels of microfabricated structures or by bonding several superimposed structure levels * Multiple materials (e.g. non-silicon materials, e.g. for magnetic actuation) and multiple levels; free-moving parts, * The release of parts through the use of sacrificial layers, * Requires small part handling. † Forming stepped structures, thus shaping the third dimension, for example by combining DXRL with other process steps or other techniques, e.g. by machining the substrate into a 3D geometry before applying the resist layer. The fabrication of micromechanical sensors and actuators often requires movable parts, which can be achieved by combining the LIGA process with a sacrificial layer technique. Many microsystems make use of a combination of various microfabrication techniques. Micromechanical and microoptical structures can be combined with microelectronic devices to form intelligent microsystems. Several methods have been used to combine various functions and materials.
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Many devices successfully demonstrated interfacing of bulk or surface silicon micromachining and LIGA through assembled and co-processed integration in advanced, integrated systems. However, in most cases a monolithic approach where the LIGA components such as sensors and actuators are co-processed on the same chip with the silicon-based processing circuitry is difficult due to low compatibility of processes, though some examples exist such as the fabrication of LIGA microstructures on CMOS wafers using deep X-ray lithography  or molding techniques [23,24]. Alternative hybrid concepts for integrating components and circuitry made from the most appropriate materials made on separate substrates offer the possibility to build up miniaturized systems with optimum performances and reduced cost. For example an accelerator sensor has been integrated with ASICs . To date, the majority LIGA –MEMS has been accomplished by hybrid integration of individual LIGA components with electronics. In practice, only components with the highest precision requirements are fabricated by LIGA, whereas the other parts are produced by other methods. Approaches to microassembly of high aspect-ratio metal mechanisms benefit from conventional and silicon micromachining technology. Bonding approaches such as diffusion bonding or press-fit assembly proved powerful approaches to achieving multilevel mechanisms. Complex three-dimensional millimeter-sized structures can also be constructed using a ‘peg-board’ approach, which offers increased flexibility for batch assembly of LIGA fabricated devices . A flip-chip assembly technique by means of electroplating at a low processing temperature has also been applied to the integration of high-aspect-ratio microstructures with substrates that have pre-fabricated microelectronics massively and in parallel. As such, it provides a powerful way to achieve the integration of meso- and microscopic electromechanical systems . In many cases, hybrid integration and in particular hybrid assembly imposes good tolerances and alignment accuracy. The evolution of microfabrication technologies from laboratory-scale prototyping to a viable manufacturing process requires the ability to set dimensional tolerances and clearance fits at the design stage. To do this requires evaluation, characterization and understanding of the magnitude and origin of dimensional variation in microfabricated components  as well as development of a consistent set of specifications for clearance and tolerance in microfabricated and micro assembled parts [29,30]. Specific tools and methods for automated and parallel assembly of LIGA-fabricated systems are being developed [31 – 43].
3. Applications of LIGA Applications for deep microstructures exist in many sectors of R&D activity and as industrial products,
worldwide [44 – 50]. The presence of thick, deep and highly precise microstructures with high aspect ratios is a requirement for a number of micromechanical, optical or packaging applications, as well as in other fields.
3.1. MEMS, high-precision parts and tooling One primary application area for the LIGA technique is the batch fabrication of precision actuators and mechanisms [51,52]. † The goal is producing mechanically sturdy and stable, yet highly precise structures for microactuators for a variety of applications. † Large structural heights are also advantageous to achieve microdevices, which are capable of generating sufficient force and/or torque to be functional. Most difficulties for producing powerful microactuators arise from the use of essentially planar technologies and severe materials restrictions associated with them. † The material basis is enlarged in particular towards metallic and magnetic materials. The possibility of accurately shaping magnetic materials with a large volume through electroplating or casting and sintering allows large magnetic forces. Typical examples are magnet rings, motor parts . † High accuracy and tight tolerance for a variety of moving elements such as shafts, contacting parts, bearings  and gears. Miniaturized gear systems  are a typical example of micromechanical systems where LIGA technology demonstrates its superiority by producing the various gears with sufficient height and excellent tolerances. The precision of LIGA allows for the fabrication of gears with minimized backlash and friction. The free design in two dimensions allows for the creation of structures with specific optimized geometries [56,57]. The steep and precise profile in conjunction with the smoothness of side-walls are very well suited to good fitting and assembly of various components such as rotor and shaft. † Many components with those improved performances have a high potential in application fields such as microsurgery, robotics and assembly, automotive and aerospace industry, military safing and arming devices for munitions [58,59] (force, precision and reliability): * Micromotors [60 – 67]-rotors and -drivers with appreciable torques , microturbines  generating power for a variety of applications, various actuators [53,70 – 73] and sensors based on electrostatic or electromagnetic-based principle [74,75], e. g. mechanical microconnectors using magnetic force , high precision micromechanical actuators for electrical switches and relays, optical switches, fluid control valves and pumps, etc.
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Various sensors and actuators where surface area dependent structures benefit from the height/mass/volume for better performances such as improved sensitivity/resolution, for actuators systems, lower driving voltage, etc. Examples are found in microgyroscopes [77,78], and capacitive acceleration sensors  for automotive and aerospace industry, trajectory sensing devices, mass spectrometers , switches and relays , magnetic sensor , magnetoelectric devices , electrostatic actuators, e.g. for magnetic head tracking system of hard disk drives , new type of transistors , various types of transducers [86 – 88], and detectors [89 – 96]. More robust microprobes [97,98], grippers [99 – 102], and manipulators for interfacing the micro/nanoworld with the macro-world or for precision handling and manipulating mechanical or biological micro/nano-objects; Flexural stiffness of microstructures constraining the motion to a plane, e.g. springs and coils; the height provides greater structural strength and rigidity.
† Nozzles for a variety of applications. The attractive feature is here the high precision combined with the smooth surface state as well as application-specific tailored geometries and materials. Examples are: * Spinnerets for textile fiber  of better quality and lifetime, * Nozzles for ink-jet printing type applications, * Nozzles for fluid injection systems in various applications: automotive, aerospace, etc. † Passive high-added-value highly precise parts for integrated tooling and mold manufacturing. The precision of fabrication, the accuracy of assembly process, and the resistance to wear are here attractive features. The examples include microtools for various techniques : * Mold inserts/dies for a number of replication technologies [105,106,50]. The capability to have straight and smooth sidewalls facilitates the demolding process of the molded microparts. In many cases, complex molds are built up in a modular way by combining levels made by LIGA with those made by precision engineering techniques, * Electrode arrays for microEDM , * IC leadframe punches ; * 3D carriers for microdevices have also been produced by LIGA . † LIGA is also a powerful tool for manufacturing positioning structures with the highest of precision. Structural heights are sufficiently thick to form passive, robust fixturing guides for other components such as fibers, lenses, etc. (see next paragraph) and sufficiently
precise to form edge reference structures in a number of applications. 3.2. MOEMS, optics and communication Another application, for which LIGA components provide superior performance, is in the field of microoptical components and systems [110,111]. The technical viability of the LIGA-approach for fabricating microoptics has been demonstrated for a variety of cases such as: † Miniature optically-based sensors such as microspectrometers for different wavelength ranges [112 –114], interferometers for various applications ,distance sensor , polarization sensor , microchopper , wavefront sensor , etc. * The use of microoptical systems in optical telecom and datacom . Applications in this field require a variety of optical microcomponents, both active and passive, many of which can be economically produced through LIGA. In particular, through LIGA, polymers offer a broad range of materials that can also be tailored to specific requirements, providing flexibility and cost advantage over other materials. Additionnally, the LIGA-based replication processes enable high-volume production of parts and systems of low-cost and high performance. They include: – Batch fabrication of optical components  such as microlenses [122,123], gratings , mirrors , and filters. The sidewalls of LIGA components are inherently optical flats with surface roughness (of 30 nm or under) low enough to make those structures suitable for optical applications as well as compatible with industry specifications on elimination of scattering. Some possible applications of multiple oblique X-ray exposure techniques are the fabrication of optical devices such as microprisms , cat-eye reflectors, two and 3D photonic band gap microstructures [127 – 129]…2 – The high aspect ratio capabilities of LIGA combined with its excellent spatial resolution and patterning accuracy over large field sizes are utilized for manufacturing a variety of passive optical components based on light-guiding elements, optical benches, base plates and high precision mechanical holders and fixtures, as well active components. They include: – Complex planar optical waveguiding structures [130,131] are achieved through stacking a combination of transparent optical polymer layers of adequate refractive index on top of each other. This layer stack can be structured 2
A special paragraph further down in the text deals on X-ray components, including crossed lenses for X-ray focusing produced by multiple oblique X-ray exposures (Ref. ).
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to perform optical functions both horizontally and vertically, for example with mirrors, lenses, 3D gratings, Hollow waveguides and waveguide cavities have also been made for visible light, longer wavelength in the IR, and for the terahertz range . – Various high precision mechanical positioning structures such as mounts, supports and connectors for accurate and robust aligning, mounting, coupling and interconnecting [133 – 141], assembling, and packaging of components [142,143], devices, and systems. – Splices, directional couplers, holders for optical fibers or optical stops can be produced by deep X-ray lithography, where features such as grooves or pins allow for reproducible positioning of ball lenses, detectors, light sources, fibers, etc. without time consuming manual alignment. Waveguides and fiber positioning grooves (including V-grooves with arbitrary slope, which is an advantage compared to what can be produced with silicon-based grooves) can also be achieved using aligned and stepped exposures and/or embossing elements. The extremely high precision achieved garantees the exact position of the optical components, minimizing optical losses. – For hybrid optical systems and free space microoptical set-ups, an optical bench shows advantages upon actively aligned counterparts [144,145]. The reduction of the number of degrees of freedom in alignment leads to distinct savings in cost and time. In addition the use of LIGA allows extending the materials base for microoptical benches from polymers to ceramics for better thermal management and lower thermal expansion. Micrometer to sub-micrometer positioning accuracy of the microoptical elements is guaranteed by the LIGA technique. LIGA is also a scalable approach well suited for monolithic integration in one substrate of optical and mechanical functionalities, allowing the reduction of the degrees of freedom for critical alignment. For example, passive fiberto-chip coupling with fiber alignment to optical chips based on mechanical alignment can be realized by structures for guiding pins. The accuracy of patterning such systems, even in multiple levels with accuracies in the range of several micrometers, is ideally suited for assembly and packaging of hybrid optical systems at micron or sub-micron precision without the need for active alignment.
– Optical processing elements to perform functions such as filtering, beam splitting, attenuating, and redirecting optical signals: 3D diffraction gratings; a variety of switches [146 – 149,47,150], which are key components for optical networks: by-pass switch, cross-connect matrix switch for reconfiguration of systems in high-speed optical datacom networks; – Coupling and interconnecting components for optical backplane for computer applications such as 1 £ N splitters and N £ N star couplers . † 3D high-power microwave, millimeter-wave photonics and RF components  and devices , transmission lines, antennas, couplers, filters and resonators , etc. Since the sidewalls of these structures can be made accurate, smooth, and highly vertical, closely spaced, tall microstructures in thick metal able to conduct heat in highly coupled circuit topologies can be fabricated. † X-ray optical components [155 –162] † Various components for accelerators: mm-wave linac , W-band klystron, and accelerator structures . 3.3. Microfluidics and bio-MEMS The development of miniaturized devices for microfluidics holds great promise for high throughput screening and drug discovery as well as therapeutic drug delivery: in particular multifunctional analytical systems in the rapidly growing field of lab-on-a-chip devices or microTotalAnalysis Systems (m-TAS). Besides the medical/biological fields, microfluidics systems also find applications in many other fiels, such as chemical and biotech systems , optical systems, ink jet printers, heat exchange, etc. They consist of a number of components and systems that can be fabricated in part or in total by LIGA and exploit a number of characteristics linked to the high aspect ratio nature of the microfabricated structures. For many applications such as reactions of fluids in crossing channels well-defined interaction volumes provided by LIGA are of crucial importance. Furthermore, plastic microstructures are desirable as low-cost material for microfluidic components for a number of portable analytical and single-use disposal medical or biological applications. LIGA was used in diverse microrofluidic building blocks [166,167] such as microchannels, reaction chambers, integrated heat exchangers , mixing or separation  capabilities, micropumps [169 – 172], microvalves [173 –175], microsieves, filters and membranes [176 – 178], microinjectors  and-extractors, catalyst carriers, microplates , various sensors and actuators [181 –184], as well as various integrated systems such as inhalers , electrophoretic devices [186,187], gas chromatographs , microthrust and heat generator , as well as complete systems such as miniature modular microfluidic
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chips with processing units, monitoring and diagnosing systems . Cells combining electrochemistry and spectroscopic applications have also been produced, for the control of electrochemical microreactions .
industries and will push fabrication of such molds forward. Much effort still has to be devoted to the commercialization of LIGA-based products, in particular, time-to-market, process stability, quality, and cost are key issues for a broad industrial implementation of LIGA.
4. LIGA products 5. Conclusion Though there is active R&D in LIGA in several groups around the world and prototyping with commercial companies is active, commercialization of LIGA products has been rather slow but is developing. Products manufactured by using LIGA and associated processes have emerged worldwide in companies [192 –196]. In addition, LIGA foundry capabilities have been established, providing easy access to synchrotron radiation [197,13]. Several groups are also offering process know-how that can be licensed by commercial entities interested in LIGA production. Applications in the defense sector are also expanding, especially in the US . Exactly what market share will be for LIGA still remains to be determined. A few products with LIGA-parts are presently commercially available, such as: † MicroSpectrometer (see Fig. 2) for visible and IR ranges , † Mechanical gear systems [193,194], † Optical fiber connectors , † Microalignment flexures for photonics platform . This success was made possible in part by a consistent and consequent focus on making the technology reliable for manufacturing purposes . Nevertheless, LIGA has been limited to niche markets, predominantly due to the complex production infrastructure required. Unlike for Si microfabrication, where a vast array of processes, originally developed with billions of US$ for the electronics industry, is available, LIGA processes have been developed at institutes with comparatively modest resources. However, the superior qualities, in particular of LIGA molds, for mass production of polymer parts are increasingly recognized by
LIGA is a flexible technology that offers several advantages over other microfabrication techniques. The fabrication of components, systems or molds using Deep-X-Ray-Lithography remains the most precise batch technique available for the manufacture of microobjects with large structural height and high aspect ratio. Most of the interest in LIGA is associated with the ability to provide these microstructures in a large selection of materials, in particular in metal or polymer, through replication techniques. The usefulness and flexibility of the technique is further extended through advanced processing, sacrificial layers, multilevel and oblique exposures, and combination with other microstructuring processes through multiprocessing or assembly. LIGA technology is particularly well suited for fabricating polymer components where high aspect ratios, smooth surfaces and submicron accuracies are required. The main fields of current applications are in mechanical devices and microoptics, in particular, for sensing, optical telecom and datacom networks. Future high-impact fields may include large-area high-precision patterning and, in general, the fabrication of moulding tools for various applications including polymer fluidics. In the latter example the extremely smooth surfaces are the most attractive feature of LIGA when compared to more traditional fabrication techniques such as milling or microelectro-discharge machining. A second route, producing components directly with Deep-X-Ray-Lithography is currently being explored. Here cost has been prohibitively high due to the long exposure times of the PMMA resist. However, recent progress with new resists, in particular EPON SU-8 with dramatically increased sensitivity as compared to PMMA
Fig. 2. LIGA-Spectrometers (Steag Microparts GmbH) in various products for color determination in dentistry, quality control, and health care.
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has been encouraging. After establishing a robust process with such a resist ‘Direct-LIGA’ could become feasible, at least for small and medium size volume production of specific components.
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