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Materials for LiGA and LiGA-based microsystems
Nuclear Instruments and Methods in Physics Research B 199 (2003) 332–341 www.elsevier.com/locate/nimb
Materials for LiGA and LiGA-based microsystems J. Hormes *, J. G€ ottert, K. Lian, Y. Desta, L. Jian Center for Advanced Microstructures and Devices (CAMD), Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806, USA
Abstract The LiGA technique that has been developed for the inexpensive mass fabrication of microdevices consists of three basic processes: X-ray lithography, electroforming and moulding. In each of these steps the properties of the used materials and the process parameters are strongly correlated to each other. Thus, optimizing processes requires detailed knowledge of the materials properties especially as the LiGA technique offers an extremely broad variety of materials (polymers, metals, alloys, ceramics) for the fabrication of three-dimensional microstructures. When it comes to specific applications of LiGA devices, it is often desirable to tailor the properties of materials and surfaces e.g. in respect to mechanical properties, optical properties, thermo- and electrochemical stability and biocompatibility. Again, a detailed knowledge of the properties of the various materials is crucial to optimize these tasks, keeping in mind that these properties on the microscale can differ from those of bulk materials. 2002 Elsevier Science B.V. All rights reserved. PACS: 07.10.Cm; 07.85.Qe; 81.05.Lg; 81.15.Pq; 85.40.Hp Keywords: LiGA; X-ray lithography; X-ray masks; X-ray-resists; Electroplating; Synchrotron radiation
1. Introduction The LiGA technique has been developed about 20 years ago at the Research Center (FZK) in Karlsruhe (Germany) for the inexpensive mass production of Microsystems [1]. LiGA is the German acronym for the three basic process steps as shown in Fig. 1: Lithographie ( ¼ X-ray lithography), Galvanik ( ¼ electroplating) and Abformtechnik ( ¼ replication techniques such as injection molding and/or hot embossing).
*
Corresponding author. Tel.: +1-225-388-4665; fax: +1-225388-6954. E-mail address: [email protected] (J. Hormes).
As compared to other microfabrication techniques (e.g. silicon-based microfabrication, micromachining, laser technologies), LiGA has distinct advantages: • structures with extreme high aspect ratios (ratio between the height of a structure and the minimum lateral dimension) can be fabricated; • there are no limits concerning the two-dimensional shape of a device; • a broad range of materials (polymers, metals, ceramics) can be used; • with advanced scanner systems ‘‘real’’ threedimensional structures can be fabricated; • partly movable, more complex structures can be fabricated using so-called sacrificial layer techniques.
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 5 7 1 - 9
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Fig. 1. Principle process steps for the fabrication of microstructures by the LiGA technique.
The field has developed mainly empirically, application-by-application, eventually yielding satisfactory, and in many cases even remarkable results, for example the ‘‘LiGA spectrometer’’ that is marketed by MicroParts GmbH (a German microfabrication company) for various applications. LiGA beam lines are in place now in more or less all synchrotron radiation facilities worldwide. In spite of this still growing interest, the LiGA technique is yet not economically successful. This is in contrast for example to silicon-based microfabrication techniques. One can speculate about the reasons for this, but without any doubt one limitation of the LiGA technique is a lack of basic knowledge of: • the properties and the performance of the materials used for LiGA; • the relationship between material properties and processing parameters; this is most probably also the reason for some of the still remaining and much complained process – instabilities; • the techniques for modifying and tailoring the properties of ‘‘LiGA materials’’ according to requirements from specific applications.
This is again in contrast to silicon technologies: silicon is the by far best characterized element of the periodic table! This difference could well be one of the reasons for the ‘‘success’’ of silicon-based microdevices. In this paper we will discuss in the first part the materials science aspects connected to the basic processes. This includes materials for X-ray masks, X-ray resists, materials for electroplating and the materials for ‘‘mass – production’’ using injection molding and/or hot embossing techniques. For specific applications and especially when ‘‘microsystems’’ (i.e. microdevices with ‘‘intelligence’’, movable parts, power supply, etc.) are considered the requirements for the materials are increasing. For biomedical applications biocompatibility is required (and for implants also a lifetime as long as possible) and for chemical and biochemical applications in fluidic systems it is often necessary to tailor the properties of the walls of the channels to optimize a specific functionality. Furthermore, harsh environment applications require extreme thermo- and electrochemical stability. These aspects will be discussed in the second part of this paper.
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2.1. X-ray masks
membranes are made from glass, diamond, silicon and silicon nitride. At CAMD, we have carried out some experiments with kapton membranes, however, the focus of this research effort is to fabricate X-ray masks that meet all requirements of deep and ultra-deep X-ray lithography using off the shelf graphite sheets (DFP-2-3, a grade of graphite supplied by POCO Specialty Materials of Decatur, Texas) as X-ray transparent substrates and thick photo resist, typically SU-8, UV lithography in combination with Au electro-plating to form the absorber pattern [4]. This mask architecture meets the requirements of deep and ultra deep X-ray lithography with Au absorber thickness ranging from a few micrometers up to 50 lm and smallest lateral dimensions of approx. 8 lm. The masks are mounted either onto a 400 stainless steel ring or a 500 NIST ring. These standard formats ensure compatibility to equipment at other X-ray lithography facilities. An example of an X-ray mask mounted to a NIST ring that also provides alignment marks is shown in Fig. 2. The reference marks are patterned on a glass plate together with the absorber pattern. In this case the patterned field is approx. 50 mm in diameter while masks without alignment
Deep X-ray lithography uses X-rays to transfer mask patterns into a suitable resist. The ‘‘ideal’’ Xray mask consists of a mask blank that is transparent to X-rays and a patterned absorber that is opaque. Thus, the unshielded area receives a dose that is high enough so that the resist dissolves in the developer and the dose in the masked area is so low that the shielded resist is not attacked by the developer. Additional requirements for mask materials are mechanical stability, compatibility with standard processes and equipment, and low costs. The various requirements of X-ray masks and the materials that can be used for these masks have been discussed in a paper by Malek et al. [3]. ‘‘Standard’’ X-ray masks for LiGA based on e-beam writing using Ti- or Be-foils as substrates, have always been a ‘‘bottleneck’’ of the process regarding costs and time. Thus, it is crucial to develop new and ‘‘better’’ masks technologies based on ‘‘new’’ materials: better here means: higher transmission for X-rays, good mechanical stability and ‘‘safer’’ handling. Possible alternative
Fig. 2. Graphite X-ray mask mounted to a NIST ring. Reference marks on the glass substrate enable aligned X-ray exposure.
From the given restriction in length it is obvious that this paper cannot give a ‘‘complete’’ survey of all the various materials aspects and the diverse developments in the many groups involved in the development of the LiGA technique. The techniques, for example, that have been developed for the proper characterization of the mechanical, optical, magnetic and other properties of ‘‘LiGA materials’’ i.e. materials with small dimensions on the microscale will not be discussed. However, we will try to address at least some important aspects related to LiGA and materials science. We will show mainly examples from our own research groups and from the activities at Louisiana State University (LSU). However, we will also provide the interested reader with additional references for further information. A recent review about materials for LiGA technology providing also a lot of starting references has been published by Ehrfeld et al. [2].
2. Basic materials
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marks allow a maximum patterned area of approx. 2:600 2:600 . This approach allows mask fabrication with high yield and in less than one week as well as provides maximum flexibility with respect to the absorber heights required for different X-ray source. Although graphite substrates are attractive from a fabrication point of view they lack the outstanding performance of other materials with respect to side wall quality. Some preliminary studies indicate that scattering from the graphite substrates result in an increased side wall roughness of typically 200 nm Ra value. This limits the use of these masks to mechanical and fluidic applications at this point. Furthermore, the fabrication process using optical lithography is limiting smallest dimensions to approx. 8 lm. One way of further reducing this number is the use of intermediate masks made from Si membranes and a soft X-ray mask copying process to increase the height on a working mask. Future research at CAMD will concentrate to establish this intermediate mask process using e-beam lithography. Another major focus is modifying the graphite substrate to make better side wall quality possible. 2.2. Resists for X-ray lithography Resist materials for the primary lithography process are of crucial importance for the LiGA technology. There are a variety of criteria that determine the suitability of X-ray resists such as sensitivity, resolution, contrast, process stability, optical properties (variation of the diffraction index, etc.) and adhesion on various other materials. These requirements are so exacting that only very few polymers have been proven to offer an acceptable compromise with respect to all process requirements. Poly(methyl-methacrylate) PMMA is the standard resist used so far in the LiGA process. It has good adhesion to standard titanium oxide surfaces, can be selectively developed with the so-called GG-developer, is stable throughout the process, and offers submicron resolution. However, this polymer/developer system suffers from its very low sensitivity so that long exposure times (corresponding to high exposure costs) are required. To modify one (or more) of the pro-
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perties of a resist in a systematic way, detailed knowledge of the material properties and their changes during the exposure and/or developing process are necessary. This requires detailed investigations using in many cases a variety of different techniques as we have demonstrated by investigating the radiation chemical processes taking place in PMMA the ‘‘standard resist’’ for LiGA applications [5]. From the large number of possible polymer/ developer systems that have been screened over the past 15 years, the most favorable alternative to PMMA seems to be the combination of poly(lactides) as resist and an alcoholic solution of NaOH in water as developer. In a quantitative study of the radiation chemical behavior of poly(lactides) using size exclusion chromatography, in situ mass spectrometry and FT-IR spectroscopy together with titrimetric measurements, and in situ ESR spectroscopy, it was shown that the poly(lactide) system is at least a factor 4 more sensitive than the PMMA/GG system [6–8]. Furthermore, poly(lactides) are biocompatible and also biodegradable, properties that are crucial for new applications of microcomponents in the medical field. Temporary implants, surgical microclips and drug reservoirs for the continuous intracorporal delivery of drugs over a longer period of time – just to name some applications – could be made from poly(lactides). There are a few other polymeric materials that have been used more or less successfully as X-ray resists for the standard LiGA process. Manohara et al. [9] have demonstrated that poly(vinylidene fluoride) PVDF – a polymer with piezoelectric, pyroelectric and ferroelectric properties – can be used for a direct pattern transfer by photo etching. Poly(hexadiene-1,3-sulfone) and some of its derivatives have also been used as a positive X-ray resist [10] and some of the radiation chemistry of this resist has been investigated by X-ray absorption spectroscopy at the S–K-edge and results have been compared with those for poly(butane-1sulfone) a commercially used e-beam resist [11]. Recently also alternative techniques for MEMS fabrication using synchrotron radiation have been developed. In the so-called ‘‘TIEGA’’ process the X-ray lithography step is replaced by direct writing into/photo-etching of Teflon (PTFE). Etching
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rates up to 100 lm/min at temperatures of about 200 C have been achieved. Using a scanning stage with high degree of freedom there are more or less no structural limitations for this process [15]. One way to improve the sensitivity of a resist is to amplify the initial photochemical process step, i.e. applying the principle of chemical amplification. This requires the replacement of the positive tone PMMA with a negative tone resists. There have been several attempts to develop such an Xray resists [12] but only recently these efforts have been really successful. This new negative tone resists – SU-8 – has been developed originally by IBM as an UV-resist. Unlike other negative tone resists, SU-8 has only two components: a photoinitiator and a resin. It was modified by Cremers et al. [13] before being used successfully as an Xray resist. Although PMMA is still the ‘‘standard’’ X-ray resist, recent research in SU-8 proves the great potential of this material. Millimeter tall high aspect ratio microstructures have been successfully patterned at CAMDÕs bending magnet beamlines (see Fig. 3). Compared to PMMA SU-8 is approx. 200 (!!) times more sensitive reducing the exposure times of 500 lm thick resists to a few minutes. Also, side wall roughness and structural accuracy as a function of resist height are superior to optical lithography and comparable with structures made from PMMA. The negative tone of the SU-8 resist
Fig. 4. Three-level SU-8 structure with step heights of 300, 600 and 900 lm fabricated by X-ray lithography.
is also advantageous in fabricating more complex, multi-level, 3D-like microstructures. Fig. 4 illustrates a three-level SU-8 structure fabricated by multiple X-ray lithography. For some special applications also ‘‘exotic’’ resist materials have been used. Photoetchable glasses, for example, are very promising materials for applications in corrosive and high temperature environment. Fabrication technologies and some applications for FOTURAN – a photoetchable glass produced by the Schott Company – are described in a paper by Dietrich et al. [14]. 2.3. Electroforming
Fig. 3. 1500 lm tall SU-8 gears.
Within the LiGA technology electroforming is a crucial technology and used in different steps including the fabrication of ‘‘replication tools’’ (mold inserts) and for the direct fabrication of metallic microstructures. Besides Cu and Au, Ni is still the major material used for metallic MEMS systems and especially for mold inserts. The microstructure and thus also the mechanical properties of electrodeposited Ni are closely related with the type of bath and with the process parameters such as current density, bath temperature and pH value. For electrodeposited nickel from a
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Fig. 5. Vickers hardness as a function of the load for Ni electroplated using four different sets of parameters.
sulfate bath a correlation between current densities grain size distribution and mechanical properties was observed [16,17]. Fig. 5 shows as an example for those measurements the Vickers hardness as a function of the load for Ni electroplated using four different sets of parameters. The understanding of these correlations is crucial for tailoring the properties of Ni- LiGA parts. Electrodeposition of metal alloys is important for applications that require very hard microstructures such as mold inserts. Besides Ni–Co also Ni–W [18], Co–W and Ni–Fe have been tested. Though these alloys have favorable properties, the electrodeposition into deep recesses with tight control of the deposit composition and microstructure is still a challenging task. Understanding of the kinetic and transport limitations of the electrochemical reaction rates are to be merged with the dynamic growth behavior so that composition gradients and grain size can be controlled and systematically altered to change the materialÕs property. NiW alloys have been successfully deposited into 500 lm recesses with a microhardness that is nearly three times the conventional electroplated Ni commonly used in the LiGA process [19]. The deposition of the NiW system and others, including NiCu and NiFe, have relied on the use of pulse plating, particularly to avoid the accumulation of undesirable products at the recess bottom.
The lifetime and the performance of MEMS devices are strongly effected by friction and wear. Nickelphosphorous alloys for example with up to 15% phosphorous offer significantly better wear properties than pure Ni. Thus, also these alloys have been used for the electroforming of metallic LiGA parts [20]. Excellent properties concerning hardness and resistance against wear and friction have also been obtained by using compound materials made from metals like Ni filled with microor nanoparticles from materials such as diamond, cBN or Al2 O3 . 2.4. Materials for replication: ceramics, metals and polymers One of the major advantages of the LiGA technique is the ability to produce mold inserts that can be used in either injection molding or hot embossing processes. In the hot embossing process the microstructured mold insert is pressed under vacuum into a semi-finished thermoplastic polymer at a temperature above its glass temperature TG . After cooling the material below TG , the machine is opened and the now patterned plastic foil that remains attached to the embossing tool, is demolded from the mold insert. A broad range of polymers has been used for the hot embossing process, for example: PMMA, POM, PC, PVDF
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and PSU. Areas with diameters up to 400 can be structured by hot embossing and a side wall roughness down to about 50 nm can be obtained making this technology suitable for microoptical applications [21]. Hot embossing is a rather slow process with a cycle time between 10 min and 25 min and thus mainly suitable for prototype production and small series. Nevertheless, when material uniformity is crucial like in microoptical applications, hot embossing is the process of choice. Among the different alternatives for the ‘‘inexpensive’’ mass fabrication of polymeric microparts, injection molding is the more attractive process. Because of the high aspect ratio of LiGA structures, a complete filling of the fine structures requires a low viscosity of the resin and thus also very high tool temperatures. Again a broad variety of polymers have been used for reaction molding, for example PMMA, PSU, PC, POM, HDPE, PA and PEEK. Several parameters are critical for successfully injection molding of polymers for successful molding, for example temperature of the mold during injection and the evacuation of the mold prior to injection. Also the properties of the polymers (TG , adhesion in the mold, crystallinity) are critical for the process. Under favorable conditions and when process parameters are carefully optimized (injection speed, tool temperature, air pressure in the mold cavity) cycle time of about 2–5 min can be achieved [22]. In many cases plastic is not the material of choice for the final microproduct. In these cases the micropowder injection molding process using ceramic or metal powders with grain sizes in the micrometer or even sub-micrometer range can be applied. The incorporation of ceramics into microstructures allows for example much higher flexibility in tailoring properties for a device such as magnetism, piezoelectricity, photochromism and high temperature inertness. For this process the powders are dispersed in suitable organic binder systems and then injected into the LiGA mold. After molding, the molded part is put into a furnace and the binder system is removed in a sintering cycle at high temperature [23]. Applications for low pressure injection molding of aluminum oxide are ceramic microreactors [24]. However, all
these processes do not yet have ‘‘industrial performance’’ and the quality of the molded structures is inconsistent. Thus, a much better understanding of materials, e.g. shrinkage on a microscale, and process parameters for molding and sintering is necessary together with additional materials development, simulation techniques and process control. Before injection molding of ceramic materials had been developed also casting techniques had been applied. Solutions of a pre-ceramic polymer, poly(vinylsilazane), is cast into polymeric LiGA structures. Subsequent pyrolysis leads to complementary bodies of the original structure with very high smoothness of the pyrolyzed ceramics being in an amorphous phase [25]. Casting experiments have also been carried out with cements [26].
3. Materials for special applications Microelectromechanical systems have a vast range of applications for example in precision engineering, information and communication technology, in automotive engineering, in assembly and interconnection technology, and also in biotechnology and medicine. Due to increased demands placed on the devices in terms of expanded functionality, sample processing integration, and interfaces for sensors to the outside world, and to ‘‘intelligence’’, i.e. to microchips and the silicon world, materials with very special properties are necessary. To fulfill those requirements sometimes new materials with tailored bulk properties are needed and in many case it is also necessary to modify the surface properties of a microdevice. In the following paragraphs we will discuss some of these techniques that allow to make microfabrication really successful. 3.1. Modifying polymer-surfaces for biomedical applications Microsystems have recently entered also the forefront of analytical chemistry. In general, these systems are microelectrophoretic devices that are used in separation and detection of various analytical and biological samples. Most of the early
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MEMS systems for analytical applications were made from glass or quartz because these materials have well defined surfaces, excellent optical and good electroosmotic properties. Polymeric materials are now extensively used, because polymer-based MEMS can be fabricated by different methods (injection molding, imprinting, hot embossing) all of which are conducive to mass production once the molding tool has been fabricated. Whereas glass surfaces can be chemically modified quite easily using organosilanes, the development of such techniques for polymers is still in its infancy. In a recent paper, Henry et al. [27] demonstrated a rather versatile technique for modifying PMMA surfaces. By an aminolysis reaction they could produce an amine-terminated PMMA surface. This modification already changed, for example, the flow direction of a microelectrophoretic PMMA device. The amine-modified surface can in a second step be reacted with a variety of different compounds to functionalize PMMA for enzyme immobilization or to change wetability. Further modifications for example by sulfonation and nitration seem to be feasible. Henry and McCarley [28] have used the amine terminated PMMA surfaces also as substrates for the electroless deposition of Au nanoparticle films, the adsorptive deposition of Au colloids, the laterally patterned formation of Au-nanoparticle films, and finally the use of the patterned films to form electroless deposited Ag films with micrometer features. The selective deposition method utilizes a photosensitive amine-protecting organic group. This new technique allows the simple fabrication of microelectrodes within microchannels so that electrochemical detection can be used as an additional detection scheme. The aminated PMMA (and also PC) surfaces can be further modified by attaching to the amineterminated surfaces carboxylic acid groups. Then, sensing probes (antibodies, molecular beacon oligonucleotides, lectins) can be attached to lithographically patterned regions of the carboxylic acid groups (vide infra). In addition, biocompatible layers (such as ethylene glycols) can be formed on the carboxylic acid layers so that undesired materials do not accumulate on the surface of the device.
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3.2. Hardening of microstructures The detailed understanding of tribological behaviour on the microscale is of crucial importance in the design of movable LiGA parts such as turbines or motors. Early tribological studies of LiGA microstructures with a special designed microtribometer that could be used inside of a scanning electron microscope have been carried out by Bieger and Wallrabe [29]. In a systematic study of the friction coefficient of LiGA processed nickel rotor samples on an alumina substrate, Mathieson et al. [30] observed a correlation between the mass of the rotor samples and the measured coefficient of friction. They observed that for very small rotors and light loads the friction coefficients were very high. These results show the importance of engineering the surfaces of microdevices. Suitable methods for modifying the surfaces of quasi-3D-LiGA MEMS devices are just beginning to be explored. Two techniques – inductively coupled plasma assisted sputter deposition and chemical vapor deposition (CVD) – have been used at LSU for surface modification of LiGA structures. A wide variety of ceramic and metal coatings have already been deposited, including mixed metal nitride systems. It could be shown that a uniform deposition could be achieved even with high aspect ratio structures. The CVD technique is very versatile so that metals, oxides, nitrides, carbides, or other ceramic materials, as well as polymers and semiconductors can be deposited. Given sufficient understanding of the surface reaction mechanism for a specific coating/substrate system, the CVD technique is well-suited for producing thin layers of wellcontrolled thickness, down to the atomic scale. In a study by Cao et al. [31] LiGA parts made of nickel have been coated with amorphous hydrocarbon (a-C:H) and metal containing amorphous hydrocarbon films (Me–C:H). These films possess moderately high hardness, chemical inertness, low coefficients of friction and low wear rate. Thus, the conformal deposition of these nanostructured ceramic films offers possibilities to improve in a rather defined way the tribological characteristics of movable LiGA parts. This technique offers also – as had been demonstrated – the
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opportunity to fabricate freestanding, high-aspect ratio Ti–C:H microtubes. 3.3. Integration of microelectronic devices and LiGA structures Integration of electronic circuitry to LiGAbased sensors and actuators is a basic requirement for microsystems applications. This integration of the materials from LiGA world with the silicon world is rather troublesome and a final solution is not at the horizon. There are two basic approaches for integration of circuits to microsensors and microactuators. In the monolithic approach, the control, sensing and/or signal processing circuits are located on the same chip together with the LiGA microstructures. This approach is desirable when the measured signals are very small and must be amplified or processed immediately. An example would be a two-dimensional neural probe array where measured signals are in the range of a few mV or lower and must be amplified and multiplexed immediately to increase signal/noise ratio. The second is a hybrid or modular approach where LiGA microstructures and their controlling circuitry are fabricated on separate substrates and mounted on a common carrier. This approach is desirable for convenience and cost reasons for applications such as gas sensors. A technique for monolithic integration has been developed for example by M€ uller [32] for the fabrication of movable microstructures on processed silicon wafers using an aligned hot embossing approach in combination with sacrificial layer and electroplating techniques. The process ensures a reliable electrical contacting between the sensor and the CMOS evaluation board. Another monolithic technique has been developed by Stadler et al. [33] for integration of LiGA structures with standard CMOS process, whereby the circuits are fabricated first on the chip and the LiGA structure later. High aspect ratio structures along with sacrificial layer etching require extremely long etching times during which the fabricated integrated circuit must be protected. The standard processes available in the integrated circuit technology fail completely and alternative processes must be developed to carry out this integration. It has been
observed that materials problems specifically the control of stress and interfacial adhesion forces in deposited thin film layers are of paramount importance in development of this integration process [34,35]. As an alternative to monolithic integration, a hybrid scheme for integrating LiGA devices and ASICs has been developed recently by Strohrmann et al. [36] for connecting an accelerator sensor with a CMOS device. However, also here additional research is required to optimize materials and processes. Recently, a novel device structure has been developed to directly integrate LiGA-based highaspect ratio microstructure motion to electronic circuits on the same chip [37]. Fabrication techniques need to be developed to successfully utilize this new device for a variety of Microsystem applications. 4. Conclusions One of the most important characteristics of the LiGA technique as compared to other microfabrication techniques is the extremely broad variety of materials that can be used for the fabrication of microstructures. However, this advantage is also a challenge as materials can only be used successfully when their properties and the changes of these properties during the various process steps on a microscale are understood. There has been considerable progress in this area over the past years, however there are still a lot of challenges remaining. To solve these problems, and also in order to broaden the range of materials and to evaluate the techniques to tailor desired properties for specific MEMS applications will be a precondition for the economic success of the LiGA technique. Acknowledgements We would like to thank all our colleagues at CAMD and LSU especially Profs. P. Ajmera, K. Kelly, R.L. McCarley, M.C. Murphy, L. Podlaha and S.A. Soper for their continuous support and for providing us with partly unpublished material from their ongoing research. This work is partly
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supported by DARPA under contract no. N 66001-98-1-8926.
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[18] T. Yamasaki, R. Tomohira, Y. Ogino, P. Schlossmacher, K. Ehrlich, Plating & Surface Finishing 87 (2000) 148. [19] E.J. Podlaha, P.B. Hubbard, L. Namburi, A. Panda, AIChE Annual Meeting, Los Angeles, CA, 12–17 November 2000. [20] T. Bieger, Microsyst. Technol. 3 (1997) 155. [21] M. Heckele, W. Bacher, K.D. M€ uller, Microsyst. Technol. 4 (1998) 122. [22] M.S. Despa, K.W. Kelly, J.R. Collier, Microsyst. Technol. 6 (1999) 60. [23] V. Piotter, W. Bauer, T. Benzler, A. Emde, Microsyst. Technol. 7 (2001) 99. [24] R. Knitter, D. G€ ohring, P. Risthaus, J. Haußelt, Microsyst. Technol. 7 (2001). [25] H. Freimuth, V. Hessel, H. Kolle, M. Lacher, W. Ehrfeld, T. Vaahs, M. Bruck, J. Am. Ceram. Soc. 79 (1996) 1457. [26] S.L. Colston, P. Barnes, H. Freimuth, M. Lacher, W. Ehrfeld, J. Mater. Sci. Lett. 15 (1996) 1660. [27] A.C. Henry, T.J. Tutt, M. Galloway, Y.Y. Davidson, C.S. McWhorther, S.A. Soper, R.L. McCarley, Anal. Chem. 72 (2000) 5331. [28] A.C. Henry, R.L. McCarley, J. Phys. Chem. B 105 (2001) 8755. [29] T. Bieger, U. Wallrabe, Microsyst. Technol. 2 (1996) 63. [30] D. Mathieson, U. Beerschwinger, S.J. Yang, R.L. Reuben, M. Taghizadeh, S. Eckert, U. Wallrabe, Wear 192 (1996) 199. [31] D.M. Cao, T. Wang, B. Feng, W.J. Meng, K.W. Kelly, Thin Solid Films 398 (2001) 553. [32] K.-D. M€ uller, Forschungszentrum Karlsruhe, Scientific Report, FZKA 6254, 1999. [33] S. Stadler, P.K. Ajmera, Sensors Materials 14 (2002) 151. [34] I. Stadler, P.K. Ajmera, Mater. Lett. 35 (1998) 18. [35] S. Stadler, Integration of a post-fabricated accelerometer on a chip with standard CMOS circuitry using deep X-ray lithography, MS Thesis, Louisiana State University, 1997. [36] M. Strohrmann, F. Eberle, O. Fromhein, W. Keller, O. Kr€ omer, T. K€ uhner, K. Lindemann, J. Mohr, J. Schultz, Microsystem Technologies 94, Berlin, 1994, p. 969. [37] Xiaodong Wang, P.K. Ajmera, Laterally movable gate field effect transistors for microsensors and microactuators, US Patent #6,204,544, granted 20 March 2001.
Materials for LiGA and LiGA-based microsystems J. Hormes *, J. G€ ottert, K. Lian, Y. Desta, L. Jian Center for Advanced Microstructures and Devices (CAMD), Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806, USA
Abstract The LiGA technique that has been developed for the inexpensive mass fabrication of microdevices consists of three basic processes: X-ray lithography, electroforming and moulding. In each of these steps the properties of the used materials and the process parameters are strongly correlated to each other. Thus, optimizing processes requires detailed knowledge of the materials properties especially as the LiGA technique offers an extremely broad variety of materials (polymers, metals, alloys, ceramics) for the fabrication of three-dimensional microstructures. When it comes to specific applications of LiGA devices, it is often desirable to tailor the properties of materials and surfaces e.g. in respect to mechanical properties, optical properties, thermo- and electrochemical stability and biocompatibility. Again, a detailed knowledge of the properties of the various materials is crucial to optimize these tasks, keeping in mind that these properties on the microscale can differ from those of bulk materials. 2002 Elsevier Science B.V. All rights reserved. PACS: 07.10.Cm; 07.85.Qe; 81.05.Lg; 81.15.Pq; 85.40.Hp Keywords: LiGA; X-ray lithography; X-ray masks; X-ray-resists; Electroplating; Synchrotron radiation
1. Introduction The LiGA technique has been developed about 20 years ago at the Research Center (FZK) in Karlsruhe (Germany) for the inexpensive mass production of Microsystems [1]. LiGA is the German acronym for the three basic process steps as shown in Fig. 1: Lithographie ( ¼ X-ray lithography), Galvanik ( ¼ electroplating) and Abformtechnik ( ¼ replication techniques such as injection molding and/or hot embossing).
*
Corresponding author. Tel.: +1-225-388-4665; fax: +1-225388-6954. E-mail address: [email protected] (J. Hormes).
As compared to other microfabrication techniques (e.g. silicon-based microfabrication, micromachining, laser technologies), LiGA has distinct advantages: • structures with extreme high aspect ratios (ratio between the height of a structure and the minimum lateral dimension) can be fabricated; • there are no limits concerning the two-dimensional shape of a device; • a broad range of materials (polymers, metals, ceramics) can be used; • with advanced scanner systems ‘‘real’’ threedimensional structures can be fabricated; • partly movable, more complex structures can be fabricated using so-called sacrificial layer techniques.
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 5 7 1 - 9
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Fig. 1. Principle process steps for the fabrication of microstructures by the LiGA technique.
The field has developed mainly empirically, application-by-application, eventually yielding satisfactory, and in many cases even remarkable results, for example the ‘‘LiGA spectrometer’’ that is marketed by MicroParts GmbH (a German microfabrication company) for various applications. LiGA beam lines are in place now in more or less all synchrotron radiation facilities worldwide. In spite of this still growing interest, the LiGA technique is yet not economically successful. This is in contrast for example to silicon-based microfabrication techniques. One can speculate about the reasons for this, but without any doubt one limitation of the LiGA technique is a lack of basic knowledge of: • the properties and the performance of the materials used for LiGA; • the relationship between material properties and processing parameters; this is most probably also the reason for some of the still remaining and much complained process – instabilities; • the techniques for modifying and tailoring the properties of ‘‘LiGA materials’’ according to requirements from specific applications.
This is again in contrast to silicon technologies: silicon is the by far best characterized element of the periodic table! This difference could well be one of the reasons for the ‘‘success’’ of silicon-based microdevices. In this paper we will discuss in the first part the materials science aspects connected to the basic processes. This includes materials for X-ray masks, X-ray resists, materials for electroplating and the materials for ‘‘mass – production’’ using injection molding and/or hot embossing techniques. For specific applications and especially when ‘‘microsystems’’ (i.e. microdevices with ‘‘intelligence’’, movable parts, power supply, etc.) are considered the requirements for the materials are increasing. For biomedical applications biocompatibility is required (and for implants also a lifetime as long as possible) and for chemical and biochemical applications in fluidic systems it is often necessary to tailor the properties of the walls of the channels to optimize a specific functionality. Furthermore, harsh environment applications require extreme thermo- and electrochemical stability. These aspects will be discussed in the second part of this paper.
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2.1. X-ray masks
membranes are made from glass, diamond, silicon and silicon nitride. At CAMD, we have carried out some experiments with kapton membranes, however, the focus of this research effort is to fabricate X-ray masks that meet all requirements of deep and ultra-deep X-ray lithography using off the shelf graphite sheets (DFP-2-3, a grade of graphite supplied by POCO Specialty Materials of Decatur, Texas) as X-ray transparent substrates and thick photo resist, typically SU-8, UV lithography in combination with Au electro-plating to form the absorber pattern [4]. This mask architecture meets the requirements of deep and ultra deep X-ray lithography with Au absorber thickness ranging from a few micrometers up to 50 lm and smallest lateral dimensions of approx. 8 lm. The masks are mounted either onto a 400 stainless steel ring or a 500 NIST ring. These standard formats ensure compatibility to equipment at other X-ray lithography facilities. An example of an X-ray mask mounted to a NIST ring that also provides alignment marks is shown in Fig. 2. The reference marks are patterned on a glass plate together with the absorber pattern. In this case the patterned field is approx. 50 mm in diameter while masks without alignment
Deep X-ray lithography uses X-rays to transfer mask patterns into a suitable resist. The ‘‘ideal’’ Xray mask consists of a mask blank that is transparent to X-rays and a patterned absorber that is opaque. Thus, the unshielded area receives a dose that is high enough so that the resist dissolves in the developer and the dose in the masked area is so low that the shielded resist is not attacked by the developer. Additional requirements for mask materials are mechanical stability, compatibility with standard processes and equipment, and low costs. The various requirements of X-ray masks and the materials that can be used for these masks have been discussed in a paper by Malek et al. [3]. ‘‘Standard’’ X-ray masks for LiGA based on e-beam writing using Ti- or Be-foils as substrates, have always been a ‘‘bottleneck’’ of the process regarding costs and time. Thus, it is crucial to develop new and ‘‘better’’ masks technologies based on ‘‘new’’ materials: better here means: higher transmission for X-rays, good mechanical stability and ‘‘safer’’ handling. Possible alternative
Fig. 2. Graphite X-ray mask mounted to a NIST ring. Reference marks on the glass substrate enable aligned X-ray exposure.
From the given restriction in length it is obvious that this paper cannot give a ‘‘complete’’ survey of all the various materials aspects and the diverse developments in the many groups involved in the development of the LiGA technique. The techniques, for example, that have been developed for the proper characterization of the mechanical, optical, magnetic and other properties of ‘‘LiGA materials’’ i.e. materials with small dimensions on the microscale will not be discussed. However, we will try to address at least some important aspects related to LiGA and materials science. We will show mainly examples from our own research groups and from the activities at Louisiana State University (LSU). However, we will also provide the interested reader with additional references for further information. A recent review about materials for LiGA technology providing also a lot of starting references has been published by Ehrfeld et al. [2].
2. Basic materials
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marks allow a maximum patterned area of approx. 2:600 2:600 . This approach allows mask fabrication with high yield and in less than one week as well as provides maximum flexibility with respect to the absorber heights required for different X-ray source. Although graphite substrates are attractive from a fabrication point of view they lack the outstanding performance of other materials with respect to side wall quality. Some preliminary studies indicate that scattering from the graphite substrates result in an increased side wall roughness of typically 200 nm Ra value. This limits the use of these masks to mechanical and fluidic applications at this point. Furthermore, the fabrication process using optical lithography is limiting smallest dimensions to approx. 8 lm. One way of further reducing this number is the use of intermediate masks made from Si membranes and a soft X-ray mask copying process to increase the height on a working mask. Future research at CAMD will concentrate to establish this intermediate mask process using e-beam lithography. Another major focus is modifying the graphite substrate to make better side wall quality possible. 2.2. Resists for X-ray lithography Resist materials for the primary lithography process are of crucial importance for the LiGA technology. There are a variety of criteria that determine the suitability of X-ray resists such as sensitivity, resolution, contrast, process stability, optical properties (variation of the diffraction index, etc.) and adhesion on various other materials. These requirements are so exacting that only very few polymers have been proven to offer an acceptable compromise with respect to all process requirements. Poly(methyl-methacrylate) PMMA is the standard resist used so far in the LiGA process. It has good adhesion to standard titanium oxide surfaces, can be selectively developed with the so-called GG-developer, is stable throughout the process, and offers submicron resolution. However, this polymer/developer system suffers from its very low sensitivity so that long exposure times (corresponding to high exposure costs) are required. To modify one (or more) of the pro-
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perties of a resist in a systematic way, detailed knowledge of the material properties and their changes during the exposure and/or developing process are necessary. This requires detailed investigations using in many cases a variety of different techniques as we have demonstrated by investigating the radiation chemical processes taking place in PMMA the ‘‘standard resist’’ for LiGA applications [5]. From the large number of possible polymer/ developer systems that have been screened over the past 15 years, the most favorable alternative to PMMA seems to be the combination of poly(lactides) as resist and an alcoholic solution of NaOH in water as developer. In a quantitative study of the radiation chemical behavior of poly(lactides) using size exclusion chromatography, in situ mass spectrometry and FT-IR spectroscopy together with titrimetric measurements, and in situ ESR spectroscopy, it was shown that the poly(lactide) system is at least a factor 4 more sensitive than the PMMA/GG system [6–8]. Furthermore, poly(lactides) are biocompatible and also biodegradable, properties that are crucial for new applications of microcomponents in the medical field. Temporary implants, surgical microclips and drug reservoirs for the continuous intracorporal delivery of drugs over a longer period of time – just to name some applications – could be made from poly(lactides). There are a few other polymeric materials that have been used more or less successfully as X-ray resists for the standard LiGA process. Manohara et al. [9] have demonstrated that poly(vinylidene fluoride) PVDF – a polymer with piezoelectric, pyroelectric and ferroelectric properties – can be used for a direct pattern transfer by photo etching. Poly(hexadiene-1,3-sulfone) and some of its derivatives have also been used as a positive X-ray resist [10] and some of the radiation chemistry of this resist has been investigated by X-ray absorption spectroscopy at the S–K-edge and results have been compared with those for poly(butane-1sulfone) a commercially used e-beam resist [11]. Recently also alternative techniques for MEMS fabrication using synchrotron radiation have been developed. In the so-called ‘‘TIEGA’’ process the X-ray lithography step is replaced by direct writing into/photo-etching of Teflon (PTFE). Etching
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rates up to 100 lm/min at temperatures of about 200 C have been achieved. Using a scanning stage with high degree of freedom there are more or less no structural limitations for this process [15]. One way to improve the sensitivity of a resist is to amplify the initial photochemical process step, i.e. applying the principle of chemical amplification. This requires the replacement of the positive tone PMMA with a negative tone resists. There have been several attempts to develop such an Xray resists [12] but only recently these efforts have been really successful. This new negative tone resists – SU-8 – has been developed originally by IBM as an UV-resist. Unlike other negative tone resists, SU-8 has only two components: a photoinitiator and a resin. It was modified by Cremers et al. [13] before being used successfully as an Xray resist. Although PMMA is still the ‘‘standard’’ X-ray resist, recent research in SU-8 proves the great potential of this material. Millimeter tall high aspect ratio microstructures have been successfully patterned at CAMDÕs bending magnet beamlines (see Fig. 3). Compared to PMMA SU-8 is approx. 200 (!!) times more sensitive reducing the exposure times of 500 lm thick resists to a few minutes. Also, side wall roughness and structural accuracy as a function of resist height are superior to optical lithography and comparable with structures made from PMMA. The negative tone of the SU-8 resist
Fig. 4. Three-level SU-8 structure with step heights of 300, 600 and 900 lm fabricated by X-ray lithography.
is also advantageous in fabricating more complex, multi-level, 3D-like microstructures. Fig. 4 illustrates a three-level SU-8 structure fabricated by multiple X-ray lithography. For some special applications also ‘‘exotic’’ resist materials have been used. Photoetchable glasses, for example, are very promising materials for applications in corrosive and high temperature environment. Fabrication technologies and some applications for FOTURAN – a photoetchable glass produced by the Schott Company – are described in a paper by Dietrich et al. [14]. 2.3. Electroforming
Fig. 3. 1500 lm tall SU-8 gears.
Within the LiGA technology electroforming is a crucial technology and used in different steps including the fabrication of ‘‘replication tools’’ (mold inserts) and for the direct fabrication of metallic microstructures. Besides Cu and Au, Ni is still the major material used for metallic MEMS systems and especially for mold inserts. The microstructure and thus also the mechanical properties of electrodeposited Ni are closely related with the type of bath and with the process parameters such as current density, bath temperature and pH value. For electrodeposited nickel from a
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Fig. 5. Vickers hardness as a function of the load for Ni electroplated using four different sets of parameters.
sulfate bath a correlation between current densities grain size distribution and mechanical properties was observed [16,17]. Fig. 5 shows as an example for those measurements the Vickers hardness as a function of the load for Ni electroplated using four different sets of parameters. The understanding of these correlations is crucial for tailoring the properties of Ni- LiGA parts. Electrodeposition of metal alloys is important for applications that require very hard microstructures such as mold inserts. Besides Ni–Co also Ni–W [18], Co–W and Ni–Fe have been tested. Though these alloys have favorable properties, the electrodeposition into deep recesses with tight control of the deposit composition and microstructure is still a challenging task. Understanding of the kinetic and transport limitations of the electrochemical reaction rates are to be merged with the dynamic growth behavior so that composition gradients and grain size can be controlled and systematically altered to change the materialÕs property. NiW alloys have been successfully deposited into 500 lm recesses with a microhardness that is nearly three times the conventional electroplated Ni commonly used in the LiGA process [19]. The deposition of the NiW system and others, including NiCu and NiFe, have relied on the use of pulse plating, particularly to avoid the accumulation of undesirable products at the recess bottom.
The lifetime and the performance of MEMS devices are strongly effected by friction and wear. Nickelphosphorous alloys for example with up to 15% phosphorous offer significantly better wear properties than pure Ni. Thus, also these alloys have been used for the electroforming of metallic LiGA parts [20]. Excellent properties concerning hardness and resistance against wear and friction have also been obtained by using compound materials made from metals like Ni filled with microor nanoparticles from materials such as diamond, cBN or Al2 O3 . 2.4. Materials for replication: ceramics, metals and polymers One of the major advantages of the LiGA technique is the ability to produce mold inserts that can be used in either injection molding or hot embossing processes. In the hot embossing process the microstructured mold insert is pressed under vacuum into a semi-finished thermoplastic polymer at a temperature above its glass temperature TG . After cooling the material below TG , the machine is opened and the now patterned plastic foil that remains attached to the embossing tool, is demolded from the mold insert. A broad range of polymers has been used for the hot embossing process, for example: PMMA, POM, PC, PVDF
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and PSU. Areas with diameters up to 400 can be structured by hot embossing and a side wall roughness down to about 50 nm can be obtained making this technology suitable for microoptical applications [21]. Hot embossing is a rather slow process with a cycle time between 10 min and 25 min and thus mainly suitable for prototype production and small series. Nevertheless, when material uniformity is crucial like in microoptical applications, hot embossing is the process of choice. Among the different alternatives for the ‘‘inexpensive’’ mass fabrication of polymeric microparts, injection molding is the more attractive process. Because of the high aspect ratio of LiGA structures, a complete filling of the fine structures requires a low viscosity of the resin and thus also very high tool temperatures. Again a broad variety of polymers have been used for reaction molding, for example PMMA, PSU, PC, POM, HDPE, PA and PEEK. Several parameters are critical for successfully injection molding of polymers for successful molding, for example temperature of the mold during injection and the evacuation of the mold prior to injection. Also the properties of the polymers (TG , adhesion in the mold, crystallinity) are critical for the process. Under favorable conditions and when process parameters are carefully optimized (injection speed, tool temperature, air pressure in the mold cavity) cycle time of about 2–5 min can be achieved [22]. In many cases plastic is not the material of choice for the final microproduct. In these cases the micropowder injection molding process using ceramic or metal powders with grain sizes in the micrometer or even sub-micrometer range can be applied. The incorporation of ceramics into microstructures allows for example much higher flexibility in tailoring properties for a device such as magnetism, piezoelectricity, photochromism and high temperature inertness. For this process the powders are dispersed in suitable organic binder systems and then injected into the LiGA mold. After molding, the molded part is put into a furnace and the binder system is removed in a sintering cycle at high temperature [23]. Applications for low pressure injection molding of aluminum oxide are ceramic microreactors [24]. However, all
these processes do not yet have ‘‘industrial performance’’ and the quality of the molded structures is inconsistent. Thus, a much better understanding of materials, e.g. shrinkage on a microscale, and process parameters for molding and sintering is necessary together with additional materials development, simulation techniques and process control. Before injection molding of ceramic materials had been developed also casting techniques had been applied. Solutions of a pre-ceramic polymer, poly(vinylsilazane), is cast into polymeric LiGA structures. Subsequent pyrolysis leads to complementary bodies of the original structure with very high smoothness of the pyrolyzed ceramics being in an amorphous phase [25]. Casting experiments have also been carried out with cements [26].
3. Materials for special applications Microelectromechanical systems have a vast range of applications for example in precision engineering, information and communication technology, in automotive engineering, in assembly and interconnection technology, and also in biotechnology and medicine. Due to increased demands placed on the devices in terms of expanded functionality, sample processing integration, and interfaces for sensors to the outside world, and to ‘‘intelligence’’, i.e. to microchips and the silicon world, materials with very special properties are necessary. To fulfill those requirements sometimes new materials with tailored bulk properties are needed and in many case it is also necessary to modify the surface properties of a microdevice. In the following paragraphs we will discuss some of these techniques that allow to make microfabrication really successful. 3.1. Modifying polymer-surfaces for biomedical applications Microsystems have recently entered also the forefront of analytical chemistry. In general, these systems are microelectrophoretic devices that are used in separation and detection of various analytical and biological samples. Most of the early
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MEMS systems for analytical applications were made from glass or quartz because these materials have well defined surfaces, excellent optical and good electroosmotic properties. Polymeric materials are now extensively used, because polymer-based MEMS can be fabricated by different methods (injection molding, imprinting, hot embossing) all of which are conducive to mass production once the molding tool has been fabricated. Whereas glass surfaces can be chemically modified quite easily using organosilanes, the development of such techniques for polymers is still in its infancy. In a recent paper, Henry et al. [27] demonstrated a rather versatile technique for modifying PMMA surfaces. By an aminolysis reaction they could produce an amine-terminated PMMA surface. This modification already changed, for example, the flow direction of a microelectrophoretic PMMA device. The amine-modified surface can in a second step be reacted with a variety of different compounds to functionalize PMMA for enzyme immobilization or to change wetability. Further modifications for example by sulfonation and nitration seem to be feasible. Henry and McCarley [28] have used the amine terminated PMMA surfaces also as substrates for the electroless deposition of Au nanoparticle films, the adsorptive deposition of Au colloids, the laterally patterned formation of Au-nanoparticle films, and finally the use of the patterned films to form electroless deposited Ag films with micrometer features. The selective deposition method utilizes a photosensitive amine-protecting organic group. This new technique allows the simple fabrication of microelectrodes within microchannels so that electrochemical detection can be used as an additional detection scheme. The aminated PMMA (and also PC) surfaces can be further modified by attaching to the amineterminated surfaces carboxylic acid groups. Then, sensing probes (antibodies, molecular beacon oligonucleotides, lectins) can be attached to lithographically patterned regions of the carboxylic acid groups (vide infra). In addition, biocompatible layers (such as ethylene glycols) can be formed on the carboxylic acid layers so that undesired materials do not accumulate on the surface of the device.
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3.2. Hardening of microstructures The detailed understanding of tribological behaviour on the microscale is of crucial importance in the design of movable LiGA parts such as turbines or motors. Early tribological studies of LiGA microstructures with a special designed microtribometer that could be used inside of a scanning electron microscope have been carried out by Bieger and Wallrabe [29]. In a systematic study of the friction coefficient of LiGA processed nickel rotor samples on an alumina substrate, Mathieson et al. [30] observed a correlation between the mass of the rotor samples and the measured coefficient of friction. They observed that for very small rotors and light loads the friction coefficients were very high. These results show the importance of engineering the surfaces of microdevices. Suitable methods for modifying the surfaces of quasi-3D-LiGA MEMS devices are just beginning to be explored. Two techniques – inductively coupled plasma assisted sputter deposition and chemical vapor deposition (CVD) – have been used at LSU for surface modification of LiGA structures. A wide variety of ceramic and metal coatings have already been deposited, including mixed metal nitride systems. It could be shown that a uniform deposition could be achieved even with high aspect ratio structures. The CVD technique is very versatile so that metals, oxides, nitrides, carbides, or other ceramic materials, as well as polymers and semiconductors can be deposited. Given sufficient understanding of the surface reaction mechanism for a specific coating/substrate system, the CVD technique is well-suited for producing thin layers of wellcontrolled thickness, down to the atomic scale. In a study by Cao et al. [31] LiGA parts made of nickel have been coated with amorphous hydrocarbon (a-C:H) and metal containing amorphous hydrocarbon films (Me–C:H). These films possess moderately high hardness, chemical inertness, low coefficients of friction and low wear rate. Thus, the conformal deposition of these nanostructured ceramic films offers possibilities to improve in a rather defined way the tribological characteristics of movable LiGA parts. This technique offers also – as had been demonstrated – the
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opportunity to fabricate freestanding, high-aspect ratio Ti–C:H microtubes. 3.3. Integration of microelectronic devices and LiGA structures Integration of electronic circuitry to LiGAbased sensors and actuators is a basic requirement for microsystems applications. This integration of the materials from LiGA world with the silicon world is rather troublesome and a final solution is not at the horizon. There are two basic approaches for integration of circuits to microsensors and microactuators. In the monolithic approach, the control, sensing and/or signal processing circuits are located on the same chip together with the LiGA microstructures. This approach is desirable when the measured signals are very small and must be amplified or processed immediately. An example would be a two-dimensional neural probe array where measured signals are in the range of a few mV or lower and must be amplified and multiplexed immediately to increase signal/noise ratio. The second is a hybrid or modular approach where LiGA microstructures and their controlling circuitry are fabricated on separate substrates and mounted on a common carrier. This approach is desirable for convenience and cost reasons for applications such as gas sensors. A technique for monolithic integration has been developed for example by M€ uller [32] for the fabrication of movable microstructures on processed silicon wafers using an aligned hot embossing approach in combination with sacrificial layer and electroplating techniques. The process ensures a reliable electrical contacting between the sensor and the CMOS evaluation board. Another monolithic technique has been developed by Stadler et al. [33] for integration of LiGA structures with standard CMOS process, whereby the circuits are fabricated first on the chip and the LiGA structure later. High aspect ratio structures along with sacrificial layer etching require extremely long etching times during which the fabricated integrated circuit must be protected. The standard processes available in the integrated circuit technology fail completely and alternative processes must be developed to carry out this integration. It has been
observed that materials problems specifically the control of stress and interfacial adhesion forces in deposited thin film layers are of paramount importance in development of this integration process [34,35]. As an alternative to monolithic integration, a hybrid scheme for integrating LiGA devices and ASICs has been developed recently by Strohrmann et al. [36] for connecting an accelerator sensor with a CMOS device. However, also here additional research is required to optimize materials and processes. Recently, a novel device structure has been developed to directly integrate LiGA-based highaspect ratio microstructure motion to electronic circuits on the same chip [37]. Fabrication techniques need to be developed to successfully utilize this new device for a variety of Microsystem applications. 4. Conclusions One of the most important characteristics of the LiGA technique as compared to other microfabrication techniques is the extremely broad variety of materials that can be used for the fabrication of microstructures. However, this advantage is also a challenge as materials can only be used successfully when their properties and the changes of these properties during the various process steps on a microscale are understood. There has been considerable progress in this area over the past years, however there are still a lot of challenges remaining. To solve these problems, and also in order to broaden the range of materials and to evaluate the techniques to tailor desired properties for specific MEMS applications will be a precondition for the economic success of the LiGA technique. Acknowledgements We would like to thank all our colleagues at CAMD and LSU especially Profs. P. Ajmera, K. Kelly, R.L. McCarley, M.C. Murphy, L. Podlaha and S.A. Soper for their continuous support and for providing us with partly unpublished material from their ongoing research. This work is partly
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supported by DARPA under contract no. N 66001-98-1-8926.
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