Rapid Prototyping and Rapid Tooling Techniques for the Manufacturing …libvolume6.xyz/.../rapidtooling/rapidtoolingtutorial2.pdf · 2014-06-02 · 4 Rapid Prototyping and Rapid Tooling

4Rapid Prototyping and Rapid ToolingTechniques for the Manufacturingof Silicon, Polymer, Metaland Ceramic Microdevices

T. Hanemann1,2, W. Bauer1, R. Knitter1, and P. Woias2

1Forschungszentrum Karlsruhe, Institut f. Materialforschung III, Postfach 3640, D-76021Karlsruhe, Germany2Albert-Ludwigs-Universitat Freiburg, Institut f. Mikrosystemtechnik, Georges-Koehler-Allee102, D-79110 Freiburg, Germany

1. INTRODUCTION

Rapid prototyping techniques allow the rapid and flexible generation of single designmodels as well as fabrication tools for the replication of small scale series, at present mainlyin the macroworld. A current internet search using one of the established search enginesreveals the following typical results:

• 445000 links for “rapid prototyping”• 55000 links for “rapid” and “prototyping” and “micro”• 5100 links for “micro” and “stereolithography”• 70 links for “microstereolithography”• 50 links for “rapid” and “nano” and “prototyping”• 0 links for “nanostereolithography”.

Approximately half a million of hits for “rapid prototyping” can be found, a significantreduction by a factor of 10 in the number of hits occurs if “rapid prototyping” is combined

188 T. HANEMANN ET AL.

with “micro”, a factor of 10000 in case of “nano”. A similar trend can be observed using“stereolithography” as the fundamental rapid prototyping technology in combination with“micro”, the expression “nanostereolithography” is unknown, despite the fact that severalapproaches for the realization of nanosized structures using stereolithographic methods areunder investigation.

In the macroscopic world a large number of different rapid prototyping (RP) techniqueshave been established for a rapid product development with respect to a significant reductionof the time-to-market-factor, covering the time from the first idea until product launching[249]. During product development certain factors strongly affect the developing time andthe resulting costs:

• Upcoming new fabrication technologies• Material properties• Environmental aspects• Reduced product life time• Maximum acceptable product price• Product design• Market trends• National and international governmental laws and regulations• Product liability aspects.

In this book chapter the authors want to give an overview of the established rapid prototypingtechniques in the macroworld, the basic technological features arising using the top-downapproach in realizing micro and nano rapid prototyping processes and the combinationof established technologies with the application of new physical effects like two photonabsorption a.o.

2. RAPID PROTOTYPING ESTABLISHED IN THE MACROWORLD

2.1. General Considerations

All rapid prototyping techniques have been developed for the generation of 3 dimen-sional (3D) product models. Fifteen years ago mainly design or proportional models wererealized; along with the further improvement of the different techniques more and morefunctional prototypes close to the final product could be produced. In recent years a pro-nounced method diversification has resulted in a large variety of different rapid prototypingtechniques allowing for the generation of prototypes made of polymers, metals and ceramics.At present rapid prototyping can be treated as a generic term for a huge collection of differ-ent methods, which enable the fast realization of a solid, 3D-model starting from computeraided construction or design data (CAD) using generative fabrication processes [118, 239].

The following technology-based definitions have been established:

• Rapid Prototyping (RP): model generation using generative techniques• Rapid Tooling (RT): mold making applying generative and replication techniques• Rapid Manufacturing (RM): small scale fabrication with RP or RT methods

RAPID PROTOTYPING AND RAPID TOOLING TECHNIQUES 189

In general, RP is used in product development, RT in mold making for the realization ofreplication tools and RM for the small scale and in future for mass fabrication [93]. Allgenerative RP methods have in common that the 3D-structure is produced layer by layer bythe deposition of suitable materials.

The general requirements for a rapid prototyping process can be summarized as fol-lows:

1. Process acceleration: The rapid prototyping concept has to provide a significantacceleration of the device fabrication, decreasing processing time from month andweeks as in conventional technology down to weeks or days. It should in particularbe capable of producing derivatives of a certain design “on demand” without anytime lag.

2. Functional reproducibility: In the macroworld quite often haptic prototypes espe-cially for the visualization of design studies were generated. More and more a rapidprototype has to be functional to be used for tests under realistic operation condi-tions. At best, the rapid prototyping process has to use exactly the same materialsas intended for the latter mass fabrication.

3. Geometrical reproducibility: As mentioned above, a rapid prototype should ex-actly mimick the functional behavior of the latter device. Therefore, the geometricalreproducibility must be accurate down to the critical scales of the respective design.

4. Process compatibility: The two criteria named above demand a high process com-patibility between rapid prototyping and latter mass fabrication technology.

5. Prototyping costs: The machinery used for micro rapid prototyping should beof lower complexity than standard fabrication technology. Also the service levelshould be lower to reduce the continuous costs of ownership. It would therefore beadvantageous to use only a small number of technological steps that have a highfunctionality.

In general the RP process can be subdivided in two basic physical steps: Firstly, the gen-eration of the mathematical data set containing the complete geometrical information andsecondly, the transformation of the data into a physical model layer by layer using generativefabrication techniques. Of particular interest is the capability of the interface between themodel generation software tool and the applied prototyping machinery. Besides all structurerelated geometric parameters additional informations like supporting or stabilizing struc-tures a.o. have to be translated and adapted into a standardized format. The most commoninterface for 3D-geometries is the STL (Standard Triangulation Language)-interface. Laterthe whole structure is divided into slices. The thickness of the slices depends mainly on thelayer resolution of the used RP machinery, typical layer thickness values in the macroscopicworld are around 0.1 mm.

The mathematical description of the model surface in the STL-standard uses smalltriangles as in finite element methods (FEM). The smaller the triangles the more accu-rate is the reproduction of details on the surface, however, the calculation effort increasessignificantly. Figure 1 shows the geometry development starting from the CAD until thesliced format using a simple hemisphere as model structure. As depicted from figure 1,geometrical steps occur at the corners depending on the RP machinery accuracy limitingthe realization of round shapes and smooth corners in z-direction. For applications in the


a)

c)

b)

d)

FIGURE 1. Contour description using a) CAD, b) STL course, c) STL fine and d) slices.

macro world accessible structural details around 0.1 mm in any direction are sufficient. Asimulation of the rapid prototyping processes using virtual prototyping or the applicationof process planning methods helps to optimise the individual process steps prior to thephysical model fabrication [51, 150].

In the following a short overview of the basic principles of the most important RPtechnologies will be given. Among the huge number of different existing methods onlythese few with relevance to microsystem technology will be described in more details:

• Stereolithography (SLA)• Selective Laser Sintering (SLS)• Inkjet Printing• Extrusion Techniques.

In literature the above mentioned techniques are described with a large number of dif-ferent abbreviations, to retain consistence only SLA for Stereolithography and SLS forSelective Laser Sintering are used. Different process combinations allow a rapid tooling formold making exploiting the capability of master formation via stereolithography or conceptmodellers.

Considering the micro- or nanometer scale the given definition of rapid protyping hasto be expanded further. Firstly, all techniques, which allow in general a rapid fabrication ofa micro- or nanostructured compound as well as a micro- or nanosized part, are named asrapid prototyping regardless whether a generative or subtractive method is used. Secondly,in case of silicon, which is not established as RP material in the macroscopic world, moreor less subtractive techniques like chemical etching or laser ablation processes have beenestablished.

2.2. Stereolithography

Stereolithography was the first RP technique invented in 1984 by 3D Systems [117].The basic principle of stereolithography is the in-layer-solidification of low viscous poly-mer reactive resins, i.e. a solution of a polymer in its own monomer like PMMA (poly-methylmethacrylate) solved in MMA (methylmethacrylate). At the beginning of the stere-olithography process the platform is located just below the reactive resin’s surface. Anoptical imaging system consisting of different mirrors allowing a wide laser beam deflec-tion in x, y-direction moves the laser beam along the resin’s surface. The first slice contour


FIGURE 2. Stereolithography—principal process scheme.

geometry is written into the reactive resin via polymerization during irradiation. After fin-ishing the first layer, the platform is moved a small distance towards the bottom of thereaction container and the second layer can be written. The sequence can be repeated untilthe physical model is finished (figure 2). Subsequent after part removal out of the con-tainer a post exposure curing via thermal treatment has to be performed for final polymersolidification.

The attainable geometric dimensions, the prototype formation time and the resultingpart properties depend strongly on the following factors:

• Applied reactive resin composition• Photoiniator properties (absorption, radical formation, radical lifetime, a.o.)• Laser source, especially wavelength and intensity• Laser beam penetration and curing depth• Optical path length• Optical imaging system accuracy in x, y-direction• Platform positioning accuracy in z-direction• Repeatability of all positioning systems• Prototype complexity• Necessity of supporting structures.

Besides the acrylate based systems epoxides, unsaturated polyesters and urethanes or com-binations of these reactive molecules can be used. These molecules contain reactive groupslike a vinyl moiety, which can be cured via radical polymerization either thermally or photo-chemically induced [84, 207]. The latter one is suitable for application in stereolithographydue to the simple selective structuring capability using UV-light. The reactive resin con-tains a photoinitiator, which decomposes under irradiation and generates organic radicalsor cations. For certain applications initiator mixtures are often used with respect to differ-ent radical life time, chemical reactivity and sensitivity to the applied light wavelengths.Sometimes photosensitizers are necessary for a precise energy matching of the light sourceemission wavelength to the photoinitiator absorption wavelength.


TABLE 1. Suitable laser sources for stereolithography

Laser source Wavelength /nm

Nd-YAG, Nd:YVO4 fundamental 1064, 532, 355, 266and higher harmonics

Ar-Ion 514.5, 488, 457, 351XeF 352HeCd 442, 325XeCl 308KrF 248ArF 193F2 157

The initiator radicals start the polymer chain grow reaction by attacking a monomerlocally at the irradiated spot. The number of monomer units polymerized per absorbedphoton is defined as the quantum yield of polymerization φm (1), typical values are in therange of thousands [84].

φm = Polymerization rate

Absorbed light intensity= Rpolym

Iabs(1)

Depending on the used monomers, uncrosslinked or slightly crosslinked thermoplasticpolymers or thermosets can be obtained after polymerization reaction.

A powerful light source, mostly a laser with an operating wavelength in the visibleor UV-region, initiates the photoinitiator or photosensitizer decomposition. Mercury highor medium pressure light sources were—with a few exceptions [77, 251]—not used aslight sources as in classical photolithography due to the impracticable emission properties.Important laser sources and the related operation wavelengths are listed in table 1. Espe-cially all wavelengths below 500 nm are of particular interest. A large light absorption canbe achieved by matching the initiator’s absorption spectra with the emission wavelengthof the light and by the use of molecules with large molar extinction coefficient. Both re-quirements can be realized via chemical synthesis, especially a molecular modelling usingsemi-empirical or ab initio calculation methods can be helpful in finding a suitable molecularstructure.

In general the absorption of light can be described using Lambert-Beers law (2). I0

is the intensity at the resin’s surface, ε is the molar extinction or absorption coefficient,c represents the concentration of the absorbing species, here the photoinitiator contentand z the thickness. Iabs,z represents the light intensity as function of the thickness [10].For a realistic description besides the laser intensity the laser scanning speed vs and thelaser beam hatching distance hs has to be considered resulting in the maximum energyEmax entering the resin’s surface at z = 0 (3). The local energy absorption Ez as functionof the distance from the resin’s surface results from the combination of (2) and (3) in amodified Lambert-Beers law (4) using the optical penetration depth Dp which is definedas the intensity decay down to a factor of 1/e of the initial intensity or 1/e2 of the initialenergy respectively. The accessible curing depth z(Eth) under consideration of a thresh-old energy Eth necessary for the initiation of the polymerization reaction can be obtained


(5) [93].

Iabs,z = I0 exp(−εcz) (2)

Emax = PLaser

vshs(3)

Ez = Emax exp

(− z

Dp

)(4)

z(Eth) = Dp ln

(Emax

Eth

)= Dp ln

(PL

vshsEth

)(5)

In addition to the curing depth, the width and the shape of the laser beam at the resin’s surfacedetermines the geometry of the cured structures and the resulting curing time. Pulsed lasersystems generate in the reactive resin a 3D-rotational parabolic structure following thegaussian profile of the applied laser beam. Typical penetration depths are between 0.2 and0.3 mm, the resulting smallest layer thickness is around 0.1 mm. A further reduction towardssmaller geometries in x, y and z-direction can only be achieved by process optimization(scanning speed, laser wavelength, laser beam diameter, positioning control, a.o.) or by achemical modification of the initiator for improved light sensitivity. A chemical tailoringalso allows an improvement of the prototype’s mechanical properties [68].

Table 2 lists the relevant data of two typical commercial stereolithography machineryequipment. The SLA7000 from 3D Systems is designed for the fabrication of large pro-totypes up to a part weight of 68 kg, the Viper si2 from the same company is a dual usemachinery for the generation of standard and precision prototypes using variable beam spotsizes. Figure 3 shows a typical application i.e. the prototyping of watch hands using theviper si2 from 3D Systems. The vendor does not give further details concerning the smallestaccessible geometric features, these data depends among others on the part geometry andthe used photocurable resin.

The complete process sequence of the stereolithography process can be described asfollows:

1. Prototype formation using generative techniques2. Prototype removal and cleaning3. Removal of all supporting structures4. UV and thermal post exposure curing for final solidification.

TABLE 2. Typical values for commercial stereolithography systems [3]

Feature SLA R© 7000 Viper si2TM

Laser type Frequency tripled Nd:YVO4 Frequency tripled Nd:YVO4

Laser wavelength 354.7 nm 354.7 nmLaser power at resin surface 800 mW 100 mWBeam diameter@1/e2 /mm Small spot: 0.23–0.28 Small spot: 0.075 ± 0.015

Large spot: 0.685–0.838 Large spot: 0.250 ± 0.025Scanning speed/(m/s) Small spot: 2.54 n.a.

Large spot: 9.52Smallest layer thickness/µm 25 n.a.Vertical resolution/µm 1.25 2.5Position repeatability/µm ±1 ±7.6


FIGURE 3. Collection of watch hands (by courtesy of Universo and 3D-Systems).

Especially the final processing step is important for the mechanical properties as well asfor the geometric accuracy of the model, hence a complete understanding of the UV andthermal curing characteristics of the applied photopolymers is necessary [88].

In the last few years the process characteristics and capabilities of stereolithographyhave been extended significantly. Some examples are:

• Formation of complex assemblies of different materials by using metal insert struc-tures [133]

• Improvement of accessible accuracies via stereo-thermal-lithography [12]• Stereolithography on silicon substrates for MEMS packaging [235].

2.3. Selective Laser Sintering

Laser sintering outlines a group of rapid prototyping methods where a solid object isbuilt by joining powder particles together via a focused laser beam. Laser sintering methodscan be split into two main categories with Selective Laser Sintering (SLS) and LaserEngineered Net ShapingTM (LENSTM) as its main representatives. In the SLS process,a thin layer of powder is spread across a platform by a blade or a roller mechanism. Amodulated CO2 laser writes the CAD data selectively on the powder bed so that only theparticles in an area with the cross-section of the object are fused by laser energy (figure 4).To facilitate fusion of the particles the powder bed is heated to just below the melting point

FIGURE 4. Selective Laser Sintering (SLS)—principal process scheme.


FIGURE 5. Laser Engineered Net ShapingTM (LENSTM)—principal process scheme.

of the material. The powder bed is then lowered and a new layer of particles is spreadacross the building platform. Both steps, laser joining and particle spreading, are repeateduntil the object is complete. The untreated powder remains loose and serves as a supportduring the building process. After finishing the solid object is removed from the slightlyadhering powder. In the LENSTM process, the particles are also fused together by a laserbeam [6]. Unlike SLS the particles are not spread as a layer but fed through a nozzle intothe laser focus. Nozzles may be mounted on one side of the object or coaxially with thelaser beam. In the focal point of the high power laser a molten pool of material is producedinto which the powder is injected (figure 5). The object to be built is moved below the laserbeam to fabricate the desired cross-sectional geometry. The process is repeated by addingconsecutive layers thereby producing a 3D-part.

Selective Laser Sintering was developed and originally patented by the University ofTexas at Austin and commercialized by DTM Corporation, before the company was boughtover by 3D Systems Corp. (Valencia, CA, USA) in 2001. In Europe laser sintering wasintroduced by EOS GmbH (Krailing, Germany) with the first machine for direct sinteringof a low melting-point bronze-nickel powder [54]. SLS is mainly used for the preparationof visual representation models, for casting patterns and for the manufacturing of injectionmolding tools for small production runs. Materials for SLS are cheaper than the resinswhich are used for stereolithography. They are nontoxic and can be fused with a low powerlaser [187]. In principle, any fusible or thermally softening powder can be used for theprocess [143]. Currently, nylon-based materials are applied as a standard material for SLS.Unfortunately, nylon parts require an appropriate laser setting and a long cooling cycle inthe machine before they can be removed. Another class of SLS materials are acrylic orpolystyrene based powders. They are used mainly for producing casting patterns as theirlower processing temperatures limit shrinkage and enhance the accuracy of the part. For adirect fusing of most metal or ceramic powders the power of the laser beam is not sufficient.In that case the particles are coated with a thermoset polymer. The polymer coating issoftened by the laser to bond the particles together. If dense models are required SLSfabricated parts must be infiltrated with liquid resins or molten metals like copper or brass.The accuracy of the process is mainly affected by the particle size and by the diameter ofthe laser focus. For most materials an accuracy of 0.2–0.4 mm is reached by SLS when apart is built the first time. If the building is repeated, it is possible to reach an accuracy of0.1–0.2 mm by adjusting the shrinkage. However, the accuracy in z-direction very often iscritical and difficult to control as deviations can be caused by warping. Due to adherent


powder particles the surface of laser sintered parts is relatively rough, hence a finishing isrequired.

The LENSTM technique was developed by Sandia National Laboratories and com-mercialized by Optomec Inc (Albuquerque, NM, USA) since 1997 [54]. It is the mostimportant representative of laser cladding, in which a laser is used as a heating source tomelt a powder to be cladded onto a substrate [31]. LENSTM is able to produce fully densemetal parts. Materials processed include steels, aluminum, titanium alloys, nickel basedalloys and metal matrix composites [110]. It is used for functional prototypes but it canalso be used for repairing and modifying existing parts and tools. The extremely rapidcooling creates a fine grained microstructure, resulting in a high tensile strength and highductility for most deposited metals [101]. Parts fabricated by LENSTM are near net shapebut generally will need surface finishing. The accuracy of the process is also in the range of0.1–0.2 mm.

2.4. Inkjet Printing

Inkjet printing is a well-known technique for printing text and images on paper ortransparencies and has also increasingly been used in a variety of methods for non-graphic-art applications with different materials. Trends and applications of inkjet printing werereviewed by Le [144] and Calvert [39]. A comprehensive overview is given in [108]. Abriefer overview of selected applications in the area of displays [211], microelectronics [38]and biology [261] focused on the deposition of functional materials. The inkjet printing ofpolymers was recently reviewed by de Gans et al. [91]. Hayes et al. also reported variousapplications of inkjet printing in MEMS packaging [109]. Besides these manifold 2D-applications, where typically only one layer of drops is deposited, inkjet printing is alsoused for the rapid prototyping of 3D-objects, where the parts are built up by the depositionof multiple layers of single drops. There are two main types of printers, the continuous inkjet(CIJ) and the drop-on-demand (DOD) printers [50], that are both used in non-graphic-artapplications. In CIJ printers a continuous stream of droplets is formed, whereas in DODtechnology the droplets are only generated when required (see chapter 3.2.2).

Direct inkjet methods for the rapid prototyping of thermopolymers were commer-cialized by Sanders Prototype (now Solidscape Inc., Merrimack, NH, USA) [216] and 3DSystems (Valencia, CA, USA) [3]. In both systems the DOD technique is used to build3D-models from low-melting thermoplastics. According to the CAD data the models arebuilt by multiple layers of deposited droplets. In the systems T612TM and T66TM, distributedby Solidscape, a thermoplastic is used to fabricate the model, and a wax-based material isused to build the support structures, which are later removed with a solvent (figure 6). Inthe ThermoJetTM (3D Systems) a thermopolymer is used to build the model as well as thesupport structures in the so-called Multi-Jet Modeling (MJM) technique (figure 7). Thelateral resolution of both systems is similar (table 3). As a first approximation it is definedby the spreading of the deposited droplets and the placement accuracy. In both techniqueseach printed layer is milled to a preselected layer thickness, to level the deposit for the nextlayer to be printed. The systems of Solidscape can build parts with a smaller layer height,which leads to a very good approximation of inclined walls.

In the recent past, both Objet Geometries Ltd. (Rehovot, Israel) [1] and 3D Systemslaunched systems, which combine the printing technique with UV curing. In the case of


FIGURE 6. Schematic drawing of the printing process used in concept modellers by Solidscape.

FIGURE 7. Schematic illustration of the MJM process of 3D Systems.

EdenTM (Objet) two different photopolymers are used for building: one material is used forthe actual model, while a second photopolymer material is used for support. Immediately af-ter printing, each layer is treated by UV light, curing and hardening each layer subsequently.The support material is either mechanically separated from the model or by using a waterjet. The InVisionTM (3D Systems) combines an acrylic photopolymer for the model witha wax-based thermopolymer for support. In the recently launched InVisionTM HR printer(3D Systems) the resolution in x/y is increased by a factor 2 and the minimum layer isreduced to ca. 30 µm compared to the data given in table 3.

TABLE 3. Properties of concept modellers.

Type T612TM

or T66TM

ThermoJetTM

InVisionTM

EdenTM

260/330

Manufacturer Solidscape 3D Systems 3D Systems Objet GeometriesBuild material thermoplastic 1 thermopolymer acrylic acrylic photopolymer 1

photopolymerSupport material thermoplastic 2 thermopolymer thermopolymer acrylic photopolymer 2minimum build 13 42 42 16

layer /µmDrop size / µm 76 ca. 85 ca. 80 80–100Resolution in:x/y/z /dpi n/a 300/400/600 328/328/606 600/300/1600

x / y / z /µm 85/64/42 77/77/42 42/84/16Number of 1 build-jet, 352 jets 448 jets 8 jetsprintheads 2 support-jets


FIGURE 8. Schematic representation of the 3DPTM process (MIT).

Concept modellers are the best selling RP machines. The models built on these mod-ellers are mainly used for design verification, but especially the thermopolymer modelsmay also be used in molding or casting applications. The overall resolution is in the rangeof a few hundred micrometers. Different concept modellers were compared by Pham andDimov [187] and Hackney [103], representing the available equipments up to about 2001.Thus far, there are no commercialized systems for the direct 3D-printing of functional ma-terials like ceramics or metals. The ongoing research in this field of application will bereviewed in chapter 3.2.2.

The 3D Printing (3DPTM) process was invented by the Massachusetts Institute of Tech-nology [164, 203]. In contrast to the above-mentioned techniques, it is an indirect process,as a binder is printed onto a powder bed. After spreading a thin layer of ceramic, metallic orpolymeric powder by dry dispersion or in a liquid vehicle, the powder is selectively joinedby the deposition of binder droplets, printed according to the CAD data (figure 8). Thepowder bed, which also acts as a support structure, is lowered and the process is contin-ued with the spreading of a new layer of powder. Following a heat treatment, the excesspowder is finally removed from the finished part. As printing methods both DOD and CIJprinting were explored. The drawback of this technique is the restricted density of the parts,which is due to the limited density that can be achieved in the powder layers. Thereforemost parts are infiltrated after heat treatment. The resolution of the process mainly dependson the penetration behavior of the binder droplets in the powder bed, which may rangefrom 150 to more than 500 µm [168]. Comparing the resolution and the surface finish, thistechnique is inferior to the direct printing processes of thermopolymers. An advantage of3DPTM is its fast process and it may be used for a great variety of materials and almost anygeometry.

The 3DPTM technology is licensed to several companies for different applications.The printers of Z Corporation (Burlington, MA, USA) [262] use the 3DPTM technique tobuild models from plaster-, composite- or starch-based materials. To enhance the properties,like strength, durability or temperature resistance, the models can be infiltrated with waxor a range of epoxy or acrylic resins. These models are used as design models or formetal castings. The plaster-based material is also suited for color printing, which can beused e.g. to output finite element analysis data. Other applications of this technology aree.g. the fabrication of metal parts (Extrude Hone—ProMetal, Irwin, PA, USA), which aresubsequently infiltrated by low-melting alloys, the fabrication of e.g. silica-based ceramic


FIGURE 9. Schematic representation of FDMTM process (Stratasys).

molds for metal castings (Solingen, Northridge, CA, USA), or the fabrication of implantablebio-structures (Therics, Princeton, NJ, USA, www.therics.com).

2.5. Extrusion Techniques

In contrast to inkjet printing, where single drops are deposited, in extrusion techniquesa material is extruded from a nozzle and deposited onto a platform to form a layer. FusedDeposition Modeling (FDMTM) was commercialized by Stratasys Inc. (Eden Prairie, MN,USA, [219]). In this technique phase-change materials like ABS (acrylonitrile–butadiene–styrene) plastics or polycarbonate are used. A thermopolymer filament passes through aliquefier heated to a temperature slightly above the melting point of the polymer (figure9). A continuous bead, or road, is extruded through a nozzle and deposited on a platform.A different material from a second nozzle is used for support structures, which are laterremoved by solving or breaking off. Depending on the material, a layer thickness down to127 µm can be achieved. Due to the elliptical cross-sections of the bead, the lateral resolutionis only in the range of 250–1000 µm [54]. The parts offer a low strength in vertical directionand a relatively poor finish that is mainly caused by the rippled walls, characteristic for thisprocess.

Based on the FDMTM process the Rutgers University (Piscataway, NJ, USA) developedthe Fused Deposition of Ceramics (FDC), where ceramic loaded polymer filaments areused to fabricate green ceramic components. This direct approach and the indirect method(i.e. via lost mold technique of FDMTM fabricated negative polymer molds) were particu-larly investigated to fabricate piezocomposites [7] (see also chapter 3.2.3). The FDMTM pro-cess has also been used for tissue engineering to directly fabricate bioresorbable polymerscaffolds [263], polypropylene-bioceramic composites with intricate architectures [132] orfor alumina scaffolds via the lost mold technique [33].


The Fraunhofer-Institute for Applied Materials Research (IFAM, Bremen, Germany)developed the so-called Multiphase Jet Solidification (MJS), which is related to theFDMTM process and is able to build parts from a mixture of thermoplastic binders and metalor ceramic powders with solid contents of up to 50 vol.%. The powder-binder mixture isheated above the melting temperature and squeezed out through a nozzle. Using differentnozzles with diameters between 0.5 and 2 mm, the process is suitable for producing mediumsized parts [98].

Other extrusion techniques are extrusion freeforming [148], robocasting and mi-cropen writing. While the first two are capable of fabricating 3D-ceramic parts (seechapter 3.2.3), micropen writing has been used to produce integrated multimaterial electro-ceramics thick film devices [138, 170, 233] and is considered to be beyond the scope of thisoverview.

2.6. Rapid Tooling

An objective, which is inherent to all rapid prototyping methods, is the intention toobtain a physical model or a functional prototype as quickly as possible. To achieve thisgoal, the materials, which are used for RP, are normally different from the production parts,and less importance is attached to factors like accuracy or mechanical properties. If modelsare intended for visual presentation or conceptualization only, polymer-made parts, e.g. fromSLA or SLS, may be adequate. However, if a functional analysis is required, the materialchoice plays an important role. The material should be similar or even identical to that of thefinal production part. Although there have been significant developments within RP, thereis still a relatively limited number of suitable materials. To overcome these limitations andto get prototypes in production materials, Rapid Tooling methods were developed. RT usesRP methods for the making of a primary model or a mold that can be used for the replicationof prototypes or pilot lots in a casting or molding process. The sequence of various stepshas also given rise to the term “Rapid Prototyping Process Chain” (RPPC) [178]. Thiscombination of several processes, the fabrication of a primary model and the replicationto the final material, creates a large and unmanageable variety of possible process chains(figure 10). Beside the examples described below, a large number of combinations existfor the rapid manufacturing of polymer, ceramic or metallic microdevices. The term RapidTooling bears two meanings. It can be defined as a process where a RP-made tool is useddirectly or after some finishing as an insert for die-casting or injection molding. RT can alsobe called indirect when a pattern is prepared by RP (a so-called RP master), from which a

CAD / NCRapid

PrototypingMold

Fabrication

Powder-metallurgical

Shaping

ThermalProcessing

FIGURE 10. Rapid Prototyping Process Chain (RPPC) for the powder metallurgical shaping of ceramic or metallicparts.


sacrificial mold for the investment casting of metals or a silicone rubber (room temperature-vulcanizing, RTV) mold is made. Due to the last one, these processes are usually known as“soft tooling”, in contrast to the “hard tooling” direct RT methods.

Direct Rapid Tooling methods enable the production of molds that survive productionlots from a few dozen up to ten of thousand parts. As high temperature and pressureresistance is required, often sintered metal molds are manufactured by SLS and infiltratedwith copper or bronze. Examples of such direct RT methods are RapidToolTM (RapidSteel)from 3D Systems [62, 188] or DirectToolTM from EOS [187]. The main disadvantage ofsuch RP methods is, that they come along with a high surface roughness, which requires asurface finishing. Because the finishing must be done on the internal shapes of the mold,the process becomes more difficult with increasing complexity of the mold. For the finedetails of microdevices an additional surface finishing is nearly impossible. Large surfaceroughness and the typical layer structure on vertical sidewalls produce an interlocking due toundercuts between the details of the mold and the molded part. This effect generates a highrisk that fine details are damaged during the ejection of the part. Using a mold release agentmay not be advised for microdevices due to the film thickness of the release agent whichcan no longer be neglected for micropatterns and which leads to inaccurate reproductionof edges. For that reason, “hard tooling” plays a proper role only for the prototyping ofmaterials with sufficiently high strength.

In contrast to the direct fabrication of a mold by RP, indirect Rapid Tooling, involvingthe replication of a RP master model, is intended for the fabrication of smaller series.However, at present only the indirect methods have the potential for the shaping of finestructures down to the micron range. This is mainly due to the availability of strategies toremove micro patterned structures from the mold with a reduced risk of damage. The mostappropriate routes for these parts are:

• Making a re-useable soft tool, e.g. silicone rubber tool, from a master model for thecasting or molding of plastics, ceramics or metals.

• Making a plaster or ceramic mold, which serves as a lost mold for microinvestmentcasting of metals.

In a lost mold technique, a temporary mold is made which is separated from the shapedpart by dissolving the mold in a suited liquid or by pyrolysis. Examples are polymer or waxmolds, that are made e.g. by FDMTM and used in casting application for the fabrication ofpiezoelectric or biomedical components [33, 204]. Lost mold techniques have the poten-tial to produce complex shaped parts with less effort. However, lost molds can also haveconstraints for the manufacturing of fragile microdevices, as they are known to producedamage on fine patterns by the swelling of the polymer in a solvent or by the turbulentpyrolysis process.

3. RAPID PROTOTYPING IN MICROSYSTEMS TECHNOLOGY

The beginnings of rapid prototyping in microsystem technologies can be dated to theearly nineties of the last century related to certain papers published during a Japaneseconference [119, 120]. One year later the topic was recognized outside Japan during theMEMS conferences 1993 and 1994 [121, 122, 224]. First applications of the new technique


like an integrated microfluidic system [122] or a microclamping tool were presented 1993and 1994 [224, 225]. In all cases polymer resins as photocurable materials using either apowerful UV discharge lamp [122, 222] or an Ar-Ion-laser [225] for solidification wereinvestigated. The following quite impressive specifications were published by the authors[122].

• UV-light source and spot size: xenon lamp, 5 µm• Fabrication method: maskless photocuring of reactive resins• Positioning accuracy: 0.25 × 0.25 × 1.0 µm3 in x, y, z-direction• Smallest geometric feature: 5 × 5 × 3 µm3 in x, y, z-direction• Accessible aspect ratio: >10• Maximum structure size: 10 × 10 × 10 mm3

• Fabrication speed: one hour.

The described method allows the fabrication of polymer based actuators and metal com-ponents as well. In the following, a short overview about the different applied techniques,further developments like the fabrication of ceramic and silicon prototypes and the resultingapplications will be given.

3.1. Polymers

Two basic approaches have been realized for a suitable rapid prototyping of polymers:The further miniaturization of the stereolithography process and the application of repli-cation techniques. Both technologies use polymer based reactive resins as photoformingmaterials. In the first case the microcomponent is generated layer by layer, in the secondcase the microstructuring occurs using a prestructured mold followed by a bulk curing ofthe prepolymer (see chapter 5.1).

3.1.1. MicrostereolithographyThe further miniaturization of the aspired structures has resulted in a redesign of the

stereolithography apparatus. In all cases only laser sources guarantee a parallel laser beamwith reduced spot size. For the generation of smallest structures with complex geometriesand dimensions in the µm-range the scanning mechanism described in chapter 2.2 has tobe modified using a dynamic pattern generator (spatial light modulator) in combinationwith a more accurate optical imaging system [19]. Additionally, a chemical modificationof the reactive resin system allows for a reduction of the polymerization depth and width.In certain cases e.g. the generation of simple 3D-geometries, the use of physical masks forlayer patterning are sufficient enough. Applying e.g. an unreactive absorber the realizationof acrylate-based microstructures of the same size like the laser spot (30 µm) has beendescribed [271].

A spatial light modulator can be realized in two basic approaches: A liquid crystaldisplay (LCD) in combination with a beam expander and reducer system allows for thedirect imaging of pixel structures. The scanner equipment can be omitted, the only movablepart is the z-stage for platform positioning. Depending on the resolution of the LCD in the on-state of the display a selective polymerization of one volume element (voxel) is possible notaffecting other areas on the resins surface [19]. LCD-displays are available in two differenttypes: in the transmissive mode the light travels through the cell with a low attenuation


FIGURE 11. Digital light processing device—working principle and SEM image of an micromirror array (bycourtesy of Texas Instruments DLPTM Products, images taken from [66].

around a few percent, in the reflective mode a metal layer on one side of the display reflectsor blocks the light back depending on the actual switching state (on or off) [105]. Thesecond approach uses the Digital Light Processing Technology (DLPTM), invented 1987 byTexas Instruments [66, 135]. The via CMOS technique fabricated MEMS bistable digitalmicromirror device (DMD) switches via deflection the incoming light between the on (lightis travelling to the display or to the reactive resin) or off (light absorber) state (figure 11).Both techniques allow for a microstereolithography of polymer parts with structural featuresdown to several micrometers. One main disadvantage of the LCD approach is the UV-lightsensitivity of the liquid crystalline material resulting in a long-term decay of the accessiblecontrast ratio.

The microstereolithography apparatus developed by Bertsch and coworkers [19, 20]shows the following specifications:

• Layer thickness: 5 µm• Projected pixel size: 5 × 5 µm2

• Initial maximum irradiation field: 2.5 × 2.5 mm2

• Maximum part size: 5 × 7 × 30 mm2

• Number of layers: >1000• Polymerization time: 1 s per layer.

Figure 12, left side, shows a typical prototype fabricated via the microstereolithographyprocess. The microactuator consists of approximately 1000 layers, each with a thickness of5 µm. The diameter is 500 µm, the overall length 2 mm. The fabrication time took about5 hours. The very smooth surface results from the precise platform control and the smalllayer thickness. The holes at the top and at the front side possess a diameter of 40 µmand a depth of 50 µm. The insertion of small shape memory metal stripes in the holesallows the generation of a micro actuator [20]. More examples are shown in [21]. Further


FIGURE 12. Left: Flexible polymer microspring. (Reprinted from [20], Copyright 1999, with permission fromElsevier). Right: Polymer demonstrator micropart (by courtesy of Laser Zentrum Hannover [151]).

enhancement of the microstructure’s surface quality can be achieved with improved slicingalgorithms adapted for the integral irradiation process [243] or accuracy analysis of thefabricated prototype surface properties [11]. Further developments focus on the combinationof microstereolithography with established MEMS fabrication methods for the realizationof multilevel multimaterial microcomponents avoiding assembling steps [20].

Leaving pure polymers, the rapid prototyping of polymer based composites, especiallythe microfabrication of ceramic components, is under investigation [23, 24, 264]. Withinthe framework of a EU-funded project at Laser Zentrum Hannover the so-called MIPRO-technique (Micro-Rapid-Prototyping) allowing the fabrication of microcomponents using afrequency tripled Nd:YAG at 355 nm with polymer layers smaller than 15 µm and structuraldetails of a few microns was developed (figure 12, right) [151].

With respect to the microfabrication of microfluidic components an ultra rapidprototyping (URP) processing a photomask was presented by Khoury et al. [137].A prestructured polymer or glass slide cartridge carrying holes can be filled with aacrylate-based prepolymer mixture and irradiated with UV light through a photomask for30s. After removal of the uncured resin the device is ready for use. Microfluidic channelswith a diameter around 25 µm were obtained within minutes; cartridge fabrication takesaround one hour including different processing steps. Press on fluidic connectors made ofpolydimethylsiloxane (PDMS) for the use of gauge needles can be fixed at the predrilledholes. Time and cost determinant factor is the realization of a mask library for the prototyp-ing of various fluidic microcomponents [137]. The direct comparison of the applied resinmixture to SU8 shows reasonable material and process properties for rapid prototyping(table 4).

Current research efforts concentrate on the further miniaturization of the accessiblesmallest structural features towards the nanoworld. Exploiting the nonlinear response ofmodified photopolymerizable resins in combination with a HeCd laser (cw-mode, 442 nm,100 mW) a further miniaturization towards the fabrication of nanosized structures is pos-sible. Maruo and Ikuta describe a lateral and depth resolution of 1.3 and 2.9 µm usingsingle-photon absorption allowing the generation of movable microstructures [154], furtherdetails are given later.


TABLE 4. Comparison of the URP process with UV lithography [137]

Feature URP resin SU8

Resist type acrylate mixture epoxideResist orientation negative negativeAccessible side wall angle 80◦ ± 10◦ 90◦Contact angle to water 60◦ 75◦Young’s modulus/GPa 0.45–0.6 0.6Accessible aspect ratio 4–8 15Polymer shrinkage/% 10 7.5

3.1.2. Commercialization of MicrostereolithographyAt present only a few companies are capable of micro rapid prototyping, namely in

microstereolithography. The swiss company Proform AG offers rapid prototyping servicein close cooperation with the microstereolithography group at EPFL [78, 191] applying thementioned microstereolithography apparatus.

The Perfactory R© process from Envisiontec Inc. is quite similar to microstereolithogra-phy but the light source, here a flood exposure system avoiding an expensive laser system,is assembled below the resin’s reservoir. The inverse construction (illumination from thebottom, movable platform on top, upwards part extraction out of the resin) allows a reducedequipment size. A transmissive LCD, supported with an optical imaging system, serves asdynamic mask transferring the slice information into the polymer. The smallest accessiblestructural details are in x, y-direction 41 µm, in z-direction 30 µm [77].

The german microTec company, founded 1996 in Duisburg, invented different variantsof the so called Rapid Micro Product Development Process (RMPD R©), which are in generalclosely related to the microstereolithography process. The use of special mask techniquesand adjustable optical imaging systems allows in contrast to the mentioned techniquesthe fabrication of small-scale series instead of single prototypes [96, 162, 249]. Polymer-based microstructured components with an overall size of 50 × 50 × 50 mm3 and smalleststructural dimensions of 1 × 10 × 10 µm3 can be realized [32]. A typical microcomponent,a gear wheel, is shown in figure 13, left side. Of certain importance is the accessible sidewallroughness (figure 13, right). Another prototype fabricated via the RMPDTM-process is a

FIGURE 13. SEM-images of a microgear wheel at different magnifications fabricated by the RMPD R© process.


FIGURE 14. Nozzle prototype fabricated by the RMPD R© process (by courtesy of microTEC GmbH).

nozzle plate designed for a microturbine (figure 14). The inner slit of the nozzle plate hasa width of 25 µm and a height of 150 µm.

A short review covering the status of microstereolithography was published in 2003[22], a detailed description of the theoretical background concentrating on mask-basedtechniques and direct focused beam writing methods is given in [256].

3.2. Ceramics

Besides numerous review articles and books on rapid prototyping in general, thereare also several reviews that focus on Solid Freeform Fabrication (SFF) of ceramics,demonstrating the interest in and the requirement to keep an overview of this growing fieldof activity.

Edirisinghe reviewed the development of SFF of ceramics and presented the ongoingresearch on printing techniques in 1998 [73]. Wang and Krstic gave a qualitative overviewof different available techniques in the same year [246]. Cawley focused on the similaritiesbetween SFF of ceramics and conventional manufacturing technologies [40]. In a reviewprovided by Halloran, the possibility to use SFF for high volume manufacturing was pointedout [104]. Sigmund et al. [212], emphasizing on colloidal suspensions and gelation pro-cesses, and Heule et al. [113], focusing on microfabrication techniques, both included SFFof ceramics in their reviews on powder-based fabrication processes. Recently, Tay et al.provided an excellent and detailed review on SFF of ceramics that quotes more than 300references [228].

The main difference to the RP of polymers is the fact that SFF techniques of ceramicsare only capable of shaping, i.e. to produce a green powder compact [40]. To achieve denseceramic parts with properties similar to those fabricated by established mass productiontechniques like ceramic injection molding or pressing, a thermal treatment is necessary asin conventional shaping techniques. The debinding and sintering usually take more timethan the shaping process and are the time-determining steps.

There are several processing issues in SFF of ceramics to be faced with, which aresimilar to those involved in conventional (ceramic) powder processing.


• Like in any fluidic or plastic ceramic shaping process, the powder has to be dispersedin the solvent or binder to obtain a stable suspension without agglomerates.

• Most SFF techniques are even more sensitive to the viscosity of the fluid than con-ventional shaping techniques. In both cases the upper range of viscosity will limitthe solid content to be processed.

• A certain amount of density in the green compact is needed to prevent warping orcracking during debinding and sintering and to achieve a sufficient densification ofthe part.

• Like in conventional shaping processes, inhomogeneities in the green part will leadto anisotropic shrinkage, and the dimensional accuracy will be improved by a lowshrinkage, i.e. high green density.

Thus, the major challenge will be to increase the ceramic loading of a suspension as muchas possible by simultaneously keeping the viscosity in the window of operability. Thisdichotomy becomes obvious considering the increase of viscosity with increasing solidcontent. The dependence of high solid loadings on the viscosity of suspensions can bedescribed by the Quemada equation (6) [196], which is a simplified version of the wellknown Krieger-Dougherty equation (7) [142].

Quemada ηrel =(

1 − �

�max

)−2

(6)

Krieger-Dougherty ηrel =(

1 − �

�max

)−kE�max

(7)

Einstein ηrel = η

ηM= 1 + kE� (8)

ηrel is the relative viscosity (apparent viscosity η divided by the liquid matrix viscosityηM), � and �max are the apparent volume fraction and the maximum accessible volumefraction of solids in the suspension, respectively. kE is the Einstein-constant derived fromthe origin Einstein equation (8) [75] describing the change of the relative viscosity with thefiller amount for diluted dispersions. Figure 15 illustrates the increase of the viscosity withincreasing solid volume fraction, assuming a maximum solid volume fraction of 0.6. Usingfiner particles, the problem is even more aggravated, as the viscosity of the suspension willusually increase with decreasing powder grain size.

Nearly all RP techniques have been investigated for the 3D-fabrication of ceramics.As the resolution of SFF of ceramics is mainly dominated by the applied method, onlystereolithography and printing techniques are basically relevant for microfabrication. How-ever, extrusion techniques will be included in this scope since they have been extensivelyinvestigated and used e.g. for the fabrication of finescaled piezoelectric composites.

Microstructured prototypes made of glass are rather seldom. A recent approach de-scribes a micro-powder blasting technique using a polyester-polyurethane polymer fillerwith nanoscaled gold particles as mask and SiC microparticles as erosive material for theprototyping of a glass chip for DNA separation [255].


FIGURE 15. Relative viscosity of a suspension versus solid volume fraction according to the Quemada equation(�max = 0.6).

3.2.1. StereolithographyFor the 3D-fabrication of ceramics via stereolithography the liquid monomer is replaced

by a suspension of ceramic powder dispersed in a UV-curable resin, first demonstrated byGriffith and Halloran [100]. To achieve a sufficiently high green density in the part, the solidvolume fraction should be in the range of 0.50–0.65. On the other hand, a low viscosity isnecessary for a proper flow during recoating of the next layer.

The curing kinetics of ceramic suspensions and the variation of the depth of polymer-ization as a function of different parameters has been studied intensely [34, 47, 53, 100,116, 127]. The cured depth, z(Eth), can be described by a equation derived from Beer’slaw (9).

z(Eth) = 2

3

d

�ln

(Emax

Eth

)Q (9)

Where z(Eth) is proportional to the average particle size, d, and the logarithm of the exposure,Emax, while inversely proportional to the solid volume fraction, �. Eth is the minimumintensity required to achieve photocuring. The factor Q represents the capability of a matterto diffuse radiation and is proportional to �n2, the square of the difference of the refractiveindex between ceramic and resin [100].

In experimental and numerical investigations, Sun and Zhang [221] showed that thelight scattering in ceramic suspensions influence the shape of the fabricated part. After asingle exposure, the solidified parts exhibit a larger diameter and a smaller depth comparedto pure polymer resin. To reduce the lateral scattering and to improve the resolution, theyused an UV dopant with a high absorption coefficient.

Corbel and coworkers [69–71] at CNRS (Nancy, F) investigated the properties of PZTsuspensions based on acrylate or epoxy-acrylate resins. While pure epoxy-acrylate resins


FIGURE 16. PZT transducer array assembled by stereolithography with an aspect ratio >7 and a pitch of 100 µm.(Reprinted from [69], Copyright 2002, with permission from Elsevier).

had a viscosity too high for the use in PZT (lead zirconate titanate) suspensions, a mixture ofacrylates and epoxy-acrylates could be used for filler contents of up to 23 vol.%. In contrast,suspensions based on pure acrylates could be used up to 45.5 vol.%, but exhibited smallerpolymerization depths than mixed resins. A sufficient polymerization depth, however, isessential to ensure a good connection and cohesion between layers. Like Hinczewski et al.before on alumina suspensions [116], they also studied the influence of temperature on theviscosity of suspensions. By an increase of temperature from 20 ◦C to 38 ◦C an increase ofthe PZT content from 10 vol.% to 45.5 vol.% could be compensated. The influence of oxygenon the polymerization process was also studied in PZT suspensions [70]. As oxygen inhibitsthe polymerization process, they observed an increase of the cured width with decreasingoxygen pressure. In addition, due to a decrease of the diffusion coefficient, a decrease oftemperature or an increase of the viscosity led to an increase of the polymerization widthalso indicating a loss of lateral resolution. A PZT transducer, fabricated with a layer heightof 25 µm from a suspension with a solid volume content of 0.45 is shown in figure 16.During sintering a homogeneous rate of shrinkage in the range of only 11% was observed,which suggests a certain amount of residual porosity in the final parts. Most recently Corbelet al. published the first results of an approach to use stereolithography for the fabricationof metallic parts made of aluminum [59].

To gain a higher resolution, Zhang et al. [264] used an Argon-Ion Laser for the poly-merization of alumina suspensions with a solid loading of 33 vol.%. The microgears with adiameter of 400–1000 µm consisted of only one layer of 20 µm and reached a density of only56% during sintering at 1400◦C. Monneret and coworkers [167, 192] at ENSIC (Nancy, F),working on the stereolithography of alumina suspensions, used a dynamic mask generatorand a Hg-light source to cure a complete layer in one step. Microparts with outer dimen-sions of a few millimeters and tiny details were fabricated with layer heights less than 20 µmfrom suspensions with solid contents of 24 vol.%. Due to the low filler content, however, theparts could not be sintered without cracks. By an increase of the solid content to 50 vol.%they were able to prevent cracks and distortion during sintering and obtained parts with92.4% of theoretical density, but also observed a slightly higher shrinkage in z-direction[193].


FIGURE 17. As-assembled miniature teapot with a diameter of 3.6 mm made of 295 layers of 10 µm. (Reprintedfrom [23], Copyright 2004, with permission from IOP Publishing).

Bertsch et al. [23, 24] at the EPFL (Lausanne, CH) also used, as described earlier,an integral microstereolithography machine, allowing the UV irradiation of a completelayer in one step. Using fine grained alumina powder (d50 = 0.3 µm) in an acrylate-basedresin, the resulting viscosity was too high for a self-leveling after recoating. Thereforethe machine is equipped with a blade for spreading, which allows a layer thickness of10 µm. For the fabrication of small objects they suggested to build an additional struc-ture around the object to protect it from potential damage during recoating and spread-ing. In figure 17 an as-assembled component is shown. The miniature teapot is made of295 layers of 10 µm and has a diameter of 3.6 mm. While parts with an alumina contentof 50 wt.% could not be sintered without cracks and deformation, components with asolid loading of 75 wt.% experienced a linear shrinkage of 20% during sintering with-out cracks. Unfortunately, the volume content of the suspension was not reported, butfrom the given residual porosity of 9%, a loading of about 47 vol.% of alumina can beconcluded.

To overcome the limited solid contents of ceramic filled resins, Chartier and coworkers[43, 47, 67] at the CNRS (Limoges, F) used ceramics pastes based on acrylic monomerswith solid contents up to 60 vol.% to investigate the influence of powder concentration andparticle size on the cure depth and width. In a machine developed by Optoform (Pulnoy,F), the paste is delivered through a piston and is leveled to a layer thickness of 25–100 µmby means of a special scraper. As observed by Griffith and Halloran [100], at a givenenergy dose, the cured depth and width decreased with increasing powder concentrationand could be described by a power law. Both increased linearly with increasing particlesize of alumina [47]. Due to scattering effects, the cured depth was always smaller thanthe cured width, which was 5 times larger than the laser beam diameter. By varying thedensity of energy and the layer thickness, a lateral resolution between 170 and 230 µmwas obtained in an alumina system (figure 18, left). After sintering, a density of 97% ofthe theoretical density was achieved in alumina parts, and the mechanical properties of theparts were similar to those obtained by uniaxial pressing (figure 18, right). Besides aluminamicroparts, the technique has also been used for hydroxyapatite implant manufacturing[45].


FIGURE 18. Left: Mesh pattern of a cured alumina paste made of 120 layers with a thickness of 25 µm, right:Sintered alumina component fabricated by stereolithography. (Reprinted with permission from [47]. Copyright2002, with permission from Kluwer).

3.2.2. Inkjet PrintingTwo main types of printheads are used for the inkjet printing: in continuous inkjet (CIJ)

printers a continuous stream of droplets is formed, whereas in drop-on-demand (DOD)technology the droplets are only generated when required. In CIJ printers a stream of anelectroconductive fluid is delivered through a nozzle and is thereby subjected to vibrations bya piezoelectric actuator, which regulates the breakup of the stream into individual, uniformdroplets with uniform spacing. As each drop passes through a charging electrode a controlledvoltage can charge it. By passing high voltage deflection plates, the charged droplets aredeflected in proportion to the charge carried and are used for the printing process, whileuncharged droplets are unaffected and are collected in a gutter to be reused (figure 19). Thedroplets may be charged with a constant charge (binary deflection) or with varying charges(multilevel deflection) to steer the drops to different print positions.

In most DOD printers the droplets are either thermally generated or induced by apiezoelectric actuator (figure 20). In thermal DOD printing, the heating of the wall of theink chamber causes the formation of vapor bubbles and thereby the ejection of a dropletthrough an orifice. Thermal DOD mostly uses water as a solvent and may therefore haverestricted applications. In piezoelectric DOD printers a droplet is expelled from the nozzle

FIGURE 19. Principle of drop generation in continuous inkjet printing in binary deflected mode.

FIGURE 20. Principle of drop generation in (left) thermal and (right) piezoelectric drop-on-demand printing.


by an acoustic pressure wave from a piezoelectric actuator, which may work in differentdeformation modes [36].

The CIJ process is much faster than the DOD process, but the fluid has to be electro-conductive, and the possibility of contamination of the recycled fluid limits the range ofapplication. The DOD process offers a higher resolution, which is achieved by a smallerdrop size and a higher placement accuracy, and a less demanding “ink” preparation. Inkjetprinting can be used at room temperature for aqueous or organic suspensions with ceramicor metal loadings, or at elevated temperatures in the range of about 100◦C to 300◦C forlow-melting thermoplastics, waxes or solders [65, 223, 247]. The major difference is thatin the first case the droplets will dry after printing with a considerable amount of shrinkage,while the so-called “phase-change materials” will solidify. Especially in case of solvent-based inks, a certain amount of time is necessary for drying before the next layer can bebuilt.

The most crucial point of inkjet printing is the interaction between print head andfluid. While at the beginning of non-graphic-art use of inkjet printing the “inks” had to beadapted to the printheads developed for graphic applications, there are now more and moreprintheads available that are tailored to the requirements of special applications [60, 247].Nonetheless, it is necessary to control the rheological behavior of the fluid to match thewindow of the inkjet operability and the performance of the print head [65]. The physicalproperties such as viscosity and surface tension of the fluid strongly influence the formationof the droplets, their size and velocity. These properties are also very important duringthe process of droplet spreading on impact that defines the lateral resolution of the system[208].

Inkjet printing for the SFF of ceramics was first investigated by Evans and coworkersat the Brunel University (Uxbridge, UK) [28]. They used CIJ as well as piezoelectricand thermal DOD printers with nozzle diameters of about 50–75 µm for the deposition ofalcoholic or aqueous suspensions with 5–10 vol.% zirconia or titania [29, 213, 254]. Theceramic “ink”, consisting of a thermoplastic resin, a dispersant, a solvent and the ceramicpowder, had to be adjusted to the requirements of the print head concerning sedimentationbehavior, viscosity, surface tension as well as electrical conductivity in case of CIJ [230].The composition of ceramic “inks” for CIJ printing and the influence of the conductivity wasalso studied in detail, as insufficient charged droplets cause a decrease of print resolution[30, 232].

A proper suspension preparation by milling and filtration to avoid agglomerates isnecessary to prevent blocking of the nozzle and to minimize stray drops, which may reducethe resolution [29, 217, 230]. The spreading and the drying kinetics of the deposited dropsand the time between the deposition of two layers also influence the precision as well asthe smoothness of the top surface. Often doughnut-like droplet relics were observed atthe top surface [171, 254] or after deposition of single drops on a substrate with a highsurface free energy [227]. Smoother surfaces were achieved using aqueous, slow-dryingsuspensions [213] or by reducing the time for drying between two layers. Song et al. used aflow of hot air for drying between printed layers to increase the building speed and observeda better quality of vertical walls [217]. Using alcoholic suspensions with a solid loading of2.5 vol.% PZT powder, Bhatti et al. were able to print pillars with a height of up to 1.3 mm.However, deformation of the pillars occurred during binder burnout and sintering due tothe relatively low ceramic volume fraction of 0.50 in the dried ink [25]. The preparation


FIGURE 21. PZT array assembled by inkjet printing of solvent-based suspensions. The pillars have a diameterof ∼90 µm, a height of 700 µm and a distance of 250 µm (reprinted from [184], Copyright 2005, with permissionof Elsevier).

of jettable PZT suspensions with 20 vol.% solid loading was recently reported by Lee andDerby [145]. PZT arrays designed for ceramic/polymer composites were also fabricatedby Chartier et al. [184] using solvent-based inkjet printing. Figure 21 shows an example ofPZT pillars with a diameter of 90 µm and an aspect ratio >8.

To fabricate parts with cavities and overhangs, Mott et al. used a multinozzle DODprinter to deposit a zirconia suspension concurrently with a carbon suspension that wasused for support structures [171]. The produced parts showed deformations especially atoverhangs that were caused by sagging due to ink flow during drying. An increased yieldstress in the drying suspension was proposed to prevent this effect. Using wax as a binderand an octane-alcohol mixture as solvent, Zhao et al. could increase the ceramic con-tent to 14 vol.% zirconia, corresponding to 63 vol.% zirconia in the dried ink [266–269].The samples, consisting of walls, rods and pillars, could be sintered without deforma-tion, and fracture surfaces did not show any pattern associated droplet relics. Besides thesuitability of the ceramic “ink”, the precision of the table movement turned out to be im-portant. A registration problem of the table travel may lead to a displacement of drops andhence to an adulteration of the geometry. Figure 22 shows a sintered zirconia sample madewith 400 layers for the base and 1800 layers for the walls [267]. Variation in height arecaused by a higher proportion of displaced droplets at walls perpendicular to the table traveldirection.


FIGURE 22. Miniature maze built by direct inkjet printing of a zirconia suspension. The walls and the spacingshave dimensions of ∼350 µm, corresponding to three dots. (Reprinted from [267] with permission of the AmericanCeramic Society, www.ceramics.org. Copyright 2002. All rights reserved).

In a different approach, Mott and Evans investigated the use of preceramic polymersto be used in DOD printing [173]. The printability of polycarbosilane solutions filled withSiC powder contents of up to 12.5 vol.% strongly depended on the used aliphatic solvent.While the use of heptane led to a nozzle blockage by a rapid formation of deposits, and de-cane required unacceptable long drying times before overprinting, octane was successfullyemployed. It also turned out, that unfilled polycarbosilane solutions could not be pyrolyzedwithout distortion because of an insufficient ceramic loading.

To eliminate the drying cycle between the deposition of the layers, Derby and coworkersstudied phase-change materials to be used in commercial concept modellers [5, 145]. Wax-based alumina suspensions with ceramic loadings up to 40 vol.% were investigated for DODprinting at 120◦C [5, 65, 208]. Examples of unfired parts printed from 30 vol.% aluminasuspensions are shown in figure 23. Ainsley et al. observed that the ejected drop size displaysa periodic dependence on the excitation frequency of the droplet generator. This periodicityis also a function of the fluid properties and varies with the ceramic loading of the suspension:The droplet size as well as the resonance frequency decrease with increasing volume fractionof ceramic powder. In order to print objects with high precision it is therefore preferableto control drop size and velocity by adjusting the operation parameter of the print head. Amathematical model for droplet formation in piezoelectric printheads and the drop spreadingon impact has recently been proposed [180]. Chartier and coworkers [209] also developedwax-based suspensions with 30 vol.% piezoceramic loading (PMN-PT, lead magnesiumniobate—lead titanate). By studying the rheological behavior of ceramic suspensions with


FIGURE 23. Examples of green alumina bodies printed from suspensions with a ceramic fraction of 0.3. (Reprintedfrom [208] with permission of The American Ceramic Society, www.ceramics.org, Copyright 2001. All rightsreserved.)

different wax types and dispersant additives, they were able to adjust the properties similarto the ProtobuildTM wax (Solidscape). For the usability of the suspensions not only theadjustment of the viscosity had to be achieved but also the dynamic fluid behavior had tobe adapted to meet the requirements for drop formation in the piezoelectric print head.

To gain a higher resolution in inkjet printing, Edirisinghe and coworkers [128–130, 231]used electrostatic atomization in the cone-jet mode, where the drop size is not determinedby the nozzle diameter. In this technique the nozzle is kept at a high voltage referenceto a ground electrode, thereby the pendant drop of the electroconductive suspension isdrawn to a cone, where a stream of fine droplets is generated. A stable cone-jet mode,however, is not easy to obtain, as parameters such as flow rate and potential have to becarefully controlled [129]. Using suspensions with a zirconia loading of 5 vol.%, dropletrelics between 2 and 15 µm were produced [231]. The influence of the applied voltage onthe resulting relic size was demonstrated with a 20 vol.% alumina suspension [128]. Byincreasing the voltage from 8 kV to 10 kV, the relic size was reduced from ∼375 µm to ∼45µm, but simultaneously the print quality was reduced by an increased amount of scattering.Up to now electrostatic atomization is still far from realizing 3D-objects, but combined withthe experiences gained in inkjet printing, it may have the potential to increase the resolutionof printing methods.

3.2.3. Extrusion TechniquesIn the Fused Deposition of Ceramics (FDC), developed at Rutgers University

(Piscataway, NJ, USA) [4, 7], a ceramic loaded filament with a diameter of about 1.8 mmpasses through a heated liquefier (140–200◦C) and acts thereby as a piston to extrudea continuous bead, or “road” of molten material through a nozzle with a diameter of0.254–1.5 mm. The bead is deposited on a platform that indexes down after the first layeris completed. Bonding of neighboring beads and previous deposited layers takes place dueto adhesiveness of partly remelted material [158]. Under- or overfilled areas within one


layer may occur during deposition because of errors in motion or flow control, incorrectdesign parameter settings or they are inherent to the geometric representation [126].

FDC has been used for a variety of ceramic materials and has also been adapted tometals [252] but is particularly interesting and advantageous for the fabrication of piezoce-ramic/polymer composites with different connectivities, i.e. the ceramic and the polymerphase can be self-interconnected in different dimensions [204]. For a successful applicationof fused deposition, the quality of the feed filament is a very important parameter. If thefilament exhibits a low stiffness or the viscosity of the melt is too high, buckling of the fila-ment will occur in front of the liquefier. Thus, binder development has the goal to optimizeviscosity, strength and flexibility of the feedstock [158, 159]. To be able to predict bucklingbehavior during feedstock development for FDC, rheological and mechanical propertiesof different feedstocks were investigated. It was found that buckling will not occur if theratio of compressing modulus of the filament to the apparent viscosity of the melt exceedsa certain value [197, 242]. Filaments with 50–60 vol.% PZT were used for the fabricationof transducers with 2-2 and 3-3 connectivities [149, 238]. Beads extruded through a noz-zle with a diameter of 508 µm resulted in walls with a maximum and minimum width ofabout 550 and 390 µm, respectively [149]. Due to the elliptical cross section of the beadafter deposition, rippled walls with an overhang width of approx. 80 µm were produced.The parts showed a good layer-to-layer bonding and no delamination after sintering. Witha linear shrinkage of 20% perpendicular to the roads and 16% parallel to the roads, ananisotropic shrinkage was observed. The piezoelectric properties of the samples fabricatedby FDC were very similar to those made by conventional techniques. Figures 24 and 25show images of green PZT preforms for the fabrication of composites with 2-2 and 3-3connectivities, respectively.

For the extrusion freeforming, developed at the Advanced Research Center (Tucson,AZ, USA) a Stratasys Modeler was retrofitted with a high-pressure extrusion head [148,240]. This technique has been used for different ceramic materials dispersed in wax-basedbinders. The fabrication of silicon nitride parts by extrusion of suspensions with a ceramic

FIGURE 24. As-assembled PZT preform for composites with 2-2 connectivity built via FDC. (Reprintedfrom [149] with permission of the American Ceramic Society, www.ceramics.org. Copyright 2000. All rightsreserved).


FIGURE 25. Cross-sectional view of a green 3-3 structure made of PZT by FDC. (Reprinted with permissionfrom [238], Copyright 2003, with permission from Kluwer).

loading of 55 vol.% was reported by Vaidyanathan et al. [240]. Grida and Evans [99]investigated the extrusion of thermoplastic zirconia suspensions with a solid content of 50vol.% through nozzles with diameters from 76 to 510 µm. Using fine nozzles, the suspen-sion had to be thoroughly milled to prevent blocking by agglomerates during extrusion.The solidification of the suspension after extrusion must allow for enough time for foldingand to ensure cohesion of the beads by welding. Figure 26 shows a sintered wall assem-bled by the extrusion of zirconia beads from a 190 µm die. In this case the cylindricalsection of the beads is largely preserved, indication a limited welding. Experiments with a75 µm nozzle failed, because the beads cooled before deposition, preventing welding andeven folding. That means that the use of solid-liquid change materials in extrusion tech-niques is limited for microfabrication, because the filament solidifies too quickly in ambientair.

FIGURE 26. Sintered wall constructed from welded zirconia beads extruded through a 190 µm nozzle. (Reprintedfrom [99], Copyright 2003, with permission from Elsevier).


FIGURE 27. Sintered 3-D periodic PZT structure assembled by robocasting. (Reprinted with permission from[214], Copyright 2002 American Chemical Society).

Another extrusion method, the robocasting [41], developed at Sandia NationalLaboratories (Albuquerque, NM, USA), is a slurry deposition technique that has beenapplied to highly concentrated colloidal suspensions. Morissette et al. [169] used thistechnique for the fabrication of alumina components from aqueous alumina-PVB gelcast-ing suspensions with solid volume fractions of about 0.5. The ceramic suspension anda cross-linking agent were separately pumped into a mixing chamber located directly infront of the nozzle with a diameter of 0.254–1.37 mm. Using optimized, self-supportingcolloidal gels of PZT with solid contents of 47 vol.%, Smay et al. [214] were able tofabricate 3D-periodic structures with spanning elements of up to 2 mm. The depositionof the suspension through nozzles with diameters of 200–400 µm was carried out underoil to prevent drying during assembly. The components were subsequently sintered tonearly full density. Figure 27 shows a 3D-array of interconnected PZT rods fabricated byrobocasting with a nozzle diameter of 150 µm. In a similar approach Smay et al. [215]also used robocasting for the assembly of linear and annular PZT arrays. The depositiontook place on a water-saturated plaster plate to prevent premature drying. During sin-tering the parts experienced a linear shrinkage of ∼17% and attained a final density of98% of theoretical density. The assembled 2-2 composites exhibited satisfying electricalproperties.

In many papers about direct-writing fabrication techniques, both droplet- andcontinuous-based techniques, it is often pointed out that these techniques in principle arecapable of simultaneous deposition of multiple inks and a local composition tailoring maybe achieved [79, 146, 166]. Approaches to modeling and designing functionally gradedmaterials have also been reported [125, 270]. There are, however, only few examples thatdemonstrate the realization of this approach. Mott and Evans [172] used inkjet printing toobtain a 1D continuous graded zirconia-alumina-composite. Due to different densificationbehavior and sintering temperatures, however, they could not achieve full density over thewhole composite at a sintering temperature suited for zirconia, and cracks occurred at ahigher temperature, adequate for alumina. Jafari et al. [126] reported the successful fab-rication of a multi-material PZT transducer by FDC, consisting of soft and hard PZT, i.e.materials only differing in doping composition.


To enable cofiring of 3D-ceramic components or even ceramic-metal-composites, thematerials involved should not undergo any reaction and the sintering behavior as well as thecoefficient of linear thermal expansion has to be quite similar. These preconditions will limitthe possibilities of material tailoring by local composition control or functionally gradedmaterials.

3.3. Metals

Metals are of high interest for microsystem technology, as their high strength and theirelectrical conductivity allow applications that cannot be covered by polymers or ceramics.However, besides silicon only some electroformable metals like nickel or copper are widelyused. This is mainly due to the lack of suitable powder metallurgical or casting processesin microdimensions, but also due to the lack of powders or alloys which are suited forthese shaping processes. This situation also has consequences for the application of rapidprototyping methods on a microscale. Up to now only a small number of RP methods hasbeen developed for metallic microdevices and just a few are commercially available.

Concerning the variety of materials, powder metallurgical methods will offer the largestchoice—as long as suited powders are available. Besides the general problems when dealingwith small particles like poor flowability or low tap density, for small sized metal powdersthere are further obstacles that must be overcome:

• Metal powders with a particle size in the micron range are very expensive because thissize range has to be screened from a coarser fraction. In the submicron range, besidessome noble metals, like silver, only tungsten or tungsten carbide are commerciallyavailable.

• Small metal particles have a large specific surface. Precautions have to be taken toprevent these powders from corrosion or oxidation by humidity or oxygen. Metalpowders with sizes in the nanometer range will even start spontaneous pyrolysis inthe presence of oxygen.

Most developments for the RP of metal microdevices are engaged with laser sinteringmethods. By an optimization of the laser equipment and of the powder spreading processit is now possible to enhance the resolution from a few hundred micrometers down to therange of a few ten micrometers. While laser sintering has a counterpart in the macrorange,some developments like Electrochemical Fabrication (EFABTM) are specially designed formicrodevices.

3.3.1. Laser Based MethodsLaser Microsintering: A technique, based on SLS which overcomes these problems,

was developed at the Laserinstitut Mittelsachsen e.V. (Germany) [81, 82]. The technologycalled “microSINTERING” is now commercialized by 3D-Micromac AG (Chemnitz, Ger-many) [2]. It enables the fabrication of features with a structural resolution < 30 µm andaspect ratios > 10 [72]. High melting metals like tungsten can be shaped as well as lowermelting metals like copper, aluminum, silver, titanium and others. For an expansion of thewell known SLS process to the production of metal microdevices on a micrometer scale,it is important to control the gas environment, powder layer homogeneity and the sinteringregime. The complete process takes place in a vacuum tight chamber (vacuum SLS) where


FIGURE 28. Photograph of the powder deposition unit of a laser microsintering machine (Courtesy of H. Exneret al., University of Applied Sciences, Mittweida, Germany, 2003).

sintering is performed by a Q-switched Nd:YAG laser in the pulse regime at a gas pressurein the range of 10−5 Pa up to 4 × 105 Pa. Using a vacuum chamber or an inert gas underreduced pressure has the benefit that oxygen sensible powders can be used for the process.It also enables an effective drying of the powders as the flow ability of the particles willbe improved if adsorbed water is removed from the particle surface. Finally, this approachprevents the blowing of the powder by air turbulence resulting from a fast moving scraper.

For the laser microsintering process a specially designed scraper was developed whichhas the potential for spreading the powders with a layer thickness of less than 5 µm. Thescraper has the shape of a ring with a sharpened blade (figure 28). One or more scraperssweep the powder from the reservoir onto a probe piston that is withdrawn into a verticallymovable cylindrical bore. Thereby a first layer with an oversized thickness is produced bythe scraper, which is then reduced to the final thickness by gradually lifting the piston andscraping from opposite directions. After spreading, the density of a layer is still very low.For a tungsten powder with a mean particle size of 0.3 µm a density in the range of 15% th.D. only was measured [72].

Laser pulses are applied to the powder bed with powers from 0.5 kW to 2 kW at pulserates in the range of 5 to 20 kHz. The laser spot diameter can be minimized to less than20 µm. The beam is steered by a beam scanner with a positioning resolution of 0.1 µm.This enables the fabrication of structure features with a contour of less than 30 µm and alateral resolution of 3 to 5 µm. As is customary for SLS processes, all surfaces show roughand porous qualities (figures 29). In the process, the particles are not completely compactedby the laser but a densification zone with voids and a minimal thickness of app. 5 µm isformed. In contrast to standard SLS machines, by laser microsintering a roughness of onlyRa = 1.5 µm is realizable for suited powders without post processing.

A survey of already used metal powders can be found in table 5. It was found thatwith appropriate mixtures of metal powders a higher sintering density could be obtained


FIGURE 29. Blow up images of a tungsten test pattern made by laser microsintering (Courtesy of H. Exner et al,University of Applied Sciences, Mittweida, Germany, 2003).

than with single-component powders. Highest densities of 90% and above were found fortungsten/copper powder mixtures. The process can also be applied for the selective sinteringof ceramics and composite materials.

Micro Selective Laser Sintering (µ-SLS): The treatment of a fine metal powder in alaser sintering process is also investigated at the Ecole Polytechnique Federale de Lausanne(EPFL, CH), where a laboratory sintering machine for metal or ceramic prototypes is indevelopment [95]. Objective of the work is to obtain a resolution and a roughness in therange of the grain size and the size of the focused laser spot. Therefore powders with a grainsize in the micrometer range and blends thereof and a laser wavelength in the near infraredat 1064 nm instead of a CO2 laser are applied [83]. To avoid fusing of other powder regionsand to reduce thermal stresses and shrinkage, pulsed radiation with pulse lengths that limitthermal penetration depths in the range of the grain size, is used. To overcome the problemsarising from the spreading of fine powders in a thin layer, a method is developed whichdeposits the powders from a liquid suspension. In this case, the liquid has to be evaporatedprior to the sintering step to ensure that the laser interacts with the dry and compactedpowder.

Selective Laser Micro Sintering (SLMS): The Selective Laser Micro Sintering pro-cess (SLMS) which was developed at the National Center of Laser Technology at BeijingPolytechnic University provides an improved resolution by a smaller laser beam spot. Inorder to fabricate microdevices, they use a Q-switched ND:YAG laser at 1064 nm and apower of 50 W. In addition to that, an equipment was developed which performs frequency

TABLE 5. Survey of metals used for laser microsintering (Courtesy of H. Exner et al. 2004)

Metal W Al Cu Ag Mo Steel Au Ti

Particle size/µm 0.3–10 3 1–10 2 7 20 1 20


doubling by the external resonant ring cavity [48]. With this equipment, the wavelength wasshifted to the green light (532 nm). Compared to a laser spot diameter of 28.3 µm with thefundamental frequency of 1064 nm, at the second harmonic generation of 532 nm a laserspot of only 13 µm was measured. The maximum average power output was more than10 W with a conversion efficiency of 31%. For demonstration, micro Chinese characterswere sintered from a Pb powder with a wall thickness of less then 100 µm and a height ofabout 2 mm [49].

3.3.2. Layer Deposition MethodsSpatial Forming was developed to enable the formation of complex metallic microde-

vices using offset printing techniques [229]. Like in other RP methods, the CAD model issliced into thin cross-sections, which are used for the patterning of a chrome mask by anE-beam pattern generator. The mask is then imaged to a lithographic printing plate, whichis used for the printing of a UV curable organic ink. After printing a series of approximately0.5 µm thick layers, in which each layer is individually cured with UV light, the resultingreverse structure is filled with a powder containing ink by a knife. This material is also UVcured and the complete layer planarized. The entire process is then repeated until the desiredthickness is reached (figure 30). Finally the resulting green body is debindered and sinteredin controlled atmospheres to obtain the final microdevice. The process was demonstratedon stainless steel microdevices with overall sizes from 100 µm × 150 µm × 150 µm up to250 µm × 1 mm × 20 mm. Minimum feature sizes of nearly 10 µm were achieved by a17-4 PH steel powder with a mean particle size of 3 µm (figure 31). The as-prepared inkcontained about 50 volume percent of powder, therefore a linear shrinkage of approximately20% took place during sintering.

Electrochemical Fabrication (EFABTM): The deposition of material in layers forbuilding up complex microdevices is also the basis for a technology called ElectrochemicalFabrication (EFABTM). It was developed at the Information Sciences Institute (Marina delRey) at the University of Southern California (USC) and commercialized by Memgen Inc,now Microfabrica Inc., Burbank, CA [161]. EFABTM is a repeated sequence of electroplatinga patterned layer, deposition of a support material and planarization of the layer (figure 32).With EFABTM, it is possible to fabricate very complex 3-D microdevices by stacking thinlayers which can have a thickness of only 2 to 10 µm in a high deposition rate (figure 33,left side) [8, 57, 58, 80].

Each layer consists of a structural material e.g. copper and a sacrificial material e.g.nickel. The structural material is imbedded within the sacrificial material, which provides

FIGURE 30. Schematic of the Spatial Forming process [229] ( c©1995 IEEE).


FIGURE 31. Examples of stainless steel microdevices fabricated by Spatial Forming [229] ( c©1995 IEEE).

mechanical support for the microdevice during the fabrication steps. It also allows thenext layer to be deposited over the entire area without constraint. Structural and sacrificialmaterial are deposited by electroplating. For the structural material a selective depositionprocess is used called “Instant MaskingTM”. In this process an initial photomask is created bystandard lithographic methods. The photomask is then used to make the “Instant maskTM”formed by a metal anode and a conformable insulator material. Depending on the size ofthe substrate it is possible to prepare many single layers on one mask. The Instant MaskTM

is pressed against the substrate within an electrochemical bath. Now the structural material

FIGURE 32. Principle steps of the EFABTM process (by courtesy of Microfabrica Inc, Burbank, CA, USA).


FIGURE 33. Examples of microstructures made by the EFABTM process (by courtesy of Microfabrica Inc,Burbank, CA, USA).

is selectively deposited on the substrate by an applied current. The mask is removed andthe substrate is placed into a second bath where the sacrificial material is blanket-depositedover the whole substrate. Then both materials are polished to a planar layer with the desiredthickness and the process is repeated as many times as required. Finally, after finishing thecomplete stack, the sacrificial material is removed in an etching bath.

Timescale of fabrication is in the range of a few weeks. It takes up to a week to producethe “Instant maskTM” from the CAD drawing. As the fabrication of a layer only lasts lessthan one hour, it is possible to build the stack at a rate of more than 30 per day. However,due to the small size of microdevices it is possible to build a high number and a large varietyof devices simultaneously on the same substrate (figure 33, right side).

An interesting aspect of EFABTM is that it allows a monolithic integration with ICs toprovide a system-on-a-chip solution. This is possible due to the low processing temperaturesthat are below 60◦C. The features that can be built by EFABTM can have a minimal sizeof 20 µm. Although planar sides show a good surface after polishing, sidewalls or beveledstructures have a higher roughness than LIGA parts due to the layerwise deposition. Adrawback is that EFABTM is limited to electrodepositable materials, primarily metals, likecopper, nickel, silver or gold and alloys.

3.3.3. Particle Deposition MethodsMetal jet is a method where patterned layers are formed by droplets of a molten

metal [257, 258]. This mechanism is similar to an ink jet printer. But unlike the 2D-printing,a 3D-structure is formed by the repetition of a layerwise deposition of patterns. The moltenmetal drops are jetted out of a metal nozzle by a piezoelectric actuator which moves adiaphragm. Drops have a speed of 1 m/s and a frequency of ejection of 4–20 Hz. Witha 50 µm nozzle, droplet sizes of 80 µm were realized. This size is comparable with theink droplet size of commercial ink jet printers. However the potential of the method hasbeen demonstrated by the realization of drops with a size of only 8 µm (figure 34). When


FIGURE 34. Ultra fine metal drops with a size of 8 µm (Reprinted from [257] Copyright 2000, with permissionfrom Elsevier).

the droplets collide with the surface, they are still liquid. This reduces the accuracy of theparts but enhances the packing density of the deposited layer. Therefore maximum packingdensities of 98% th. D. could be reached.

Figure 35 shows examples of models made by the metal jet method. As standardmaterials, low melting metals have been used like a fusible alloy (Bi-Pb-Sn-Cd-In) with amelting point at 47◦C or a solder with a melting point at 183◦C. A nozzle for high meltingmetals is in development. With this nozzle it shall also be possible to produce drops of goldor nickel. The speed of part formation can be enhanced by multiple nozzle print heads. Withadditional nozzles support material for overhangs (figure 35, right side) as well as differentmetals can be provided for the manufacturing of 3D-functional gradient materials.

FIGURE 35. Pyramids made by metal jet with different drop size (left: 300 µm, right: 80 µm) and model withoverhangs (Reprinted from [257] Copyright 2000, with permission from Elsevier).

3.4. Silicon-Based MEMS Devices

Although huge efforts have been made concerning alternatives, silicon is still one ofthe most prominent materials used in MEMS. Its fully elastic and fatigue-free deformationbehavior is an undeniable advantage in many micromechanical applications, especiallywhen a reliable long-term operation under harsh environmental conditions is required. Amultitude of microfabrication technologies allow an unsurpassed, highly precise structuringof silicon e.g. by anisotropic wet chemical etching in alkalines or by dry plasma etchingprocesses like DRIE.


While highly advantageous for mass fabrication, the high precision and reproducibilityavailable from standard technology is a high goal to be achieved with a rapid prototypingprocess for silicon. Research on this subject is still in its beginning. Today, “rapid” proto-types have to be fabricated with the same standard technology as used for the latter massfabrication, which is cost and time intensive. Thus, a complete new set of photomasks mayhave to be provided for each prototyping run, although only a few devices are requiredfor testing purposes. Apart from additional costs, this hinders rapid design iterations dueto the inevitable time lag caused by photomask design and fabrication. Also a multitudeof expensive machinery has to be kept operational for fabricating only small numbers ofdevices. In this situation the demand for a true and genuine rapid prototyping process ishigh.

Rapid fabrication of silicon and other monocrystalline materials has been started around1990—with first publications in the 1980s—mainly by adopting laser micromachining tech-niques. The advantages are obvious: By using a laser the time and cost-intensive photomaskfabrication can be bypassed. Conventional photolithography is replaced by a—more or lessdirect—laser micromachining, laser-assisted etching or laser-assisted deposition, which caneither mimick the result of a standard MEMS fabrication process or, alternatively, generatemicrostructures that are not obtainable with standard fabrication technology. Moreover, themachining process is compatible with CAD, i.e. a CAD drawing of the latter device can bedirectly concerted into control commands for the laser system.

3.4.1. Laser Assisted EtchingIt is useful to have at least a brief look on the widespread research on laser microfabri-

cation, although this research did not have rapid prototyping in mind. A substantial part ofthis work has been designated to laser-assisted etching in reactive gases [17, 74, 174]. Forthis purpose a laser beam—frequently a green, blue or UV emission line from an Argon-Ionlaser—is focused onto a silicon wafer through an atmosphere of e.g. molecular chlorine(figure 36, left). The laser focus heats the silicon to near-melting or melting temperatures.In the following, the chlorine gas reacts with the heated silicon to form volatile compoundslike SiCl2 and SiCl4. This complete transition of silicon into the gas phase allows a rapid,easy and complete removal from the wafer. The process takes additional benefit from theexceptionally high etch rate of molten silicon (up to 1000 µm s−1) [234] in comparison toa very low etch rate of the unaffected material. Only the locally molten silicon will reactwith the chlorine thus providing a microfabrication technique for silicon microstructures.

Cl2Cl2

Cl2

Cl2

Cl2

Cl2

Cl2

Cl2 Cl2 Cl2

Cl2SiCl2SiCl4

focused laser beam

Si

FIGURE 36. Laser-assisted etching of silicon: principle (left) and example of a 3D-microstructure (right, takenfrom [245]).


Many research groups have used this concept for their laser-assisted microfabrication.Even complete microstructures have been fabricated by these techniques, as, for instance,shown in a publication by Minami et al. [163]. They describe a YAG-laser-assisted etchingat a relatively long wavelength of 1064 nm in atmospheres containing HCL, SF6 or NF3.Also a usefull short review is given on the state-of-the-art in laser-assisted microfabricationat that time. A very illustrative example of the capabilities of laser-assisted etching was givenrecently by Walker et al. As shown in figure 36, right, a very flexible 3D-structuring of siliconcan be performed that would be impossible with standard planar silicon micromachining[139, 245].

Despite its excellent technological capabilities and a widespread research around themid 1990s laser-assisted etching has not found its way into commercial application. Onegeneral disadvantage may be the relatively complex machinery that involves a vacuumchamber with optical access and a complicated handling apparatus for the highly reactiveand toxic gases in use. Moreover, the serial nature of focused-beam laser machining resultsin a time lag in comparison with the parallel wafer-level approach of standard lithography.For rapid prototyping, however, this latter criterion is not significant, as only small numbersof test samples are required, which keeps the laser machining time at a reasonable level. Thisis demonstrated with more recent research work on genuine laser-assisted rapid prototypinginstead of a rapid microfabrication (see below).

With the appearance of suitable laser sources, the direct laser micromachining of siliconand other materials has come into the focus of research. Today, deep UV lasers as well aspicosecond and femtosecond lasers are available for a highly precise machining of a vastvariety of materials, either by using the high energy density of the short laser wavelengthor the non-thermal ablation processes occurring in ultrashort laser pulses.

3.4.2. Rapid Generation of Etch Resistant Masks on SiliconMost of the laser-based technologies outlined above have been developed as genuine

microfabrication processes and not as a rapid prototyping technology compatible with stan-dard MEMS fabrication. For that reason a laser-machined MEMS device will usually notbe functionally equivalent to a device fabricated by photolithography and chemical etching.To solve this problem, attempts were soon made to obtain a process-compatible rapid pro-totyping by combining laser-assisted microfabrication with standard MEMS technologies.The methods described in the following target the rapid fabrication of etch-resistant maskson silicon by a replacement of mask-based photolithography with a laser process. This ap-proach is favourable as it combines standard microstructuring techniques, like anisotropicetching or DRIE, with a rapid etch pattern generation. Lithography masks are no longerrequired. This gives a time gain of at least one or two weeks compared to standard lithog-raphy and allows rapid iterations in the design process of a MEMS device. Nevertheless,the prototyping result is close to a “standard” device, as the microstructuring techniquesare identical in both technologies.

Laser lithography is the most straightforward approach to a maskless lithography[205, 248]. The lithography process itself remains the same; only the photomask exposureis replaced by a scanned exposure with a switched focused laser beam. The advantage ison the one hand, a high compatibility with standard MEMS and IC fabrication processesand materials. On the other hand, the lithography and patterning procedure is not reducedin its complexity. Still, a photoresist has to be spun onto the wafer, prebaked, illuminated


layer deposition(PECVD, LPCVD)

laser-assistedploy-Si etchingin chlorine gas

RIE etch of SiO2

RIE etch of thesillicon substrate

removal ofetch mask

focused laser beamSiCl2 SiCl4 Cl2

Cl2Cl2

Cl2Cl2 Cl2

poly-Si or a-SiSiO2

Si

FIGURE 37. Mask generation by laser-assisted etching of polysilicon [175].

by the laser, developed and postbaked. For most wet chemical etching processes, the resistpattern has subsequently to be transferred into a previously deposited etch resistant mask.This leaves the maskless fabrication as the only benefit for laser lithography. Moreover, theexposure time is directly correlated with the desired optical resolution. The generation ofsub-micron features on a single 4 inch wafer may take up to 1 day of scan time on a systemwith an x,y-table.

An alternative to laser lithography is the attempt to pattern the etch resistant mask itselfvia laser machining. Early work on this subject has been done in the mid 1990s. The groupof Mullenborn has combined laser-assisted etching and wet-chemical etching of silicon forthis purpose [175] (figure 37). Their approach is based on laser-assisted chemical etching ofa thin amorphous or polycrystalline silicon film. For this, a relatively thick (10 µm) siliconoxide layer was deposited first by PECVD, followed by an LPCVD deposition of a thin (typ.2 µm) silicon film. A focused laser beam was used to locally heat the polysilicon layer upto the melting point at which it is rapidly etched away in a molecular chlorine atmosphere.The silicon oxide layer is serving as an etch stop during this melting and etching procedure.The high etch rate of the molten silicon allows a rapid single scanning with the laserbeam thus preventing any reflow and diffusion of the silicon top layer. Consequently, theauthors report on a relatively short laser machining time of 15 min for a 4 inch waferwith 50% coverage and a spot size of 8 µm. Later on the laser-fired pattern is transferredinto the SiO2 layer by RIE resulting in a common etch mask for the silicon substrateunderneath.


FIGURE 38. Mask generation by laser-assisted oxidation of silicon: [175] (left) and [141] (right).

An even simplier alternative is the direct generation of etch-resistant SiO2 layers bylocal laser irradiation [141, 176] (figure 38). For this purpose the silicon wafer is passivatedwith hydrogen by applying hydrofluoric acid (HF). Subsequently the hydrogenated surfaceis locally treated by laser irradiation until the hydrogen desorbs due to optical and thermaleffects. The unprotected silicon surface is immediately oxidized in air thus forming an etch-resistant SiO2 layer. This layer is extremely thin (several nm) and will resist to chemicaletching for a short time only thus allowing only shallow features to be etched [177].

While the technological capabilities of these laser-assisted procedures are excellent,they still rely on a relatively complicated set-up, comprising a laser system with vacuumchamber and gas handling facilities. Moreover, LPCVD, PECVD and RIE are required asauxiliary processes to generate and etch the different layers.

Easier pattern generation methods are possible by the use of metals instead of polysil-icon or SiO2. An interesting concept for a laser patterning of metallic etch masks wasdescribed by Chapman et al. in 2001 [44]. Their original layer system consisted of 0.5–1.5 µm thick amorphous carbon hydrogen and a sputtered bimetallic layer of indium andbismuth, each between 15 nm and 150 nm thick. Alternatively, the bimetallic layer wasapplied without a third layer. Upon laser irradiation the Bi/In double layer is melted into aneutectic alloy at particularly low temperatures (around 112◦C for a 53% bismuth content).This allows the use of low laser power and rapid scan speeds for the pattern generation. Theconversion effect is purely thermal i.e. the wavelength and the operation mode (pulsed orcw) of the laser are uncritical in a wide parameter range. The original intention was to usethis bimetallic system as a thermally generated photomask, as the alloy shows a relatively


FIGURE 39. Mask generations by laser processing of In/Bi.

high optical transmission, wheras the unaffected bilayer is optically dense. However, itwas also found that the alloy can be used as an etch mask for wet chemical etching afterchemically removing the unexposed areas (figure 39). The remaining Bi/In alloy shows etchrates comparable to SiO2 in TMAH (0.5 nm/min), EDP (0.9 nm/min) and KOH (1 nm/min),is, however, chemically weaker than the commonly used Si3N4 masks [237]. This meansthat relatively thick resist layers are required to achieve high etch depths in silicon. Con-sequently, only shallow etched features are demonstrated with this resist up to now. Ina recent publication plasma-assisted etching has also been demonstrated with this layermaterial [46].

An alternative attempt was made with a direct local ablation of a metallic etch mask incombination with anisotropic etching in KOH (figure 40) [9, 111, 250]. For first experimentsplatinum layers were deposited on silicon with a thickness of 100–500 nm. The ablation wasperformed with a pulsed Nd-YAG laser and an x-y-scanner at 1064 nm. Examples of thefirst ablation experiments are shown in figure 41. In general, a direct ablation of platinumhas proven to be critical. Due to the high melting temperature of the metal a heat-affectedzone is generated in the substrate, where the silicon changes from its monocrystalline intoa polycrystalline structure. In the subsequent wet chemical etch this polycrystalline zoneis rapidly etched away, thus generating an undesired broadening of the etched features.Also shown in figure 41 is the laser machining result for bismuth, which was taken forcomparison as a material with a much lower melting temperature. In contrary to the platinumlayer, the bottom of the laser-ablated line in bismuth is flat indicating a non-affected siliconsubstrate. Unfortunately, bismuth—as most of the other low-melting metals available—isnot chemically resistant in KOH.


FIGURE 40. Mask generation by laser ablation [9, 111, 250].

Based on the results described above another study has been performed recently to testa larger number of metals and alloys as ablative masks. As a result a novel mask materialhas been found with high KOH etch resistivity and low energy threshold for laser ablation.Layers with a minimum thickness of 70 nm are etch resistant in KOH (30% at 85◦C) formore than 10 hrs, which is much higher than the etch rate of the Bi/In alloy described above.

Figure 42 shows, as a preliminary result, the rapid prototyping of a v-groove byanisotropic etching in KOH. In comparison to figure 41 the higher ablation quality is

FIGURE 41. A 10 µm wide laser ablation line of platinum (left) and bismuth (right) on silicon.


FIGURE 42. Rapid prototyping of a 25 µm wide v-groove on silicon.

clearly visible. It has also turned out that no thermal influence zone is generated in thesilicon substrate due to the low ablation threshold of the mask material.

As only wet chemical etching and laser machining are required, the whole process canbe done in a standard laboratory and with only a few pieces of equipment. In particular, nocleanroom is required. The smallest feature sizes achieved up to now are in the range of 7 µmwithout thermal broadening. While the practical usability of this concept is demonstrated,uncertainty still exists about the true physical and metallurgical effects occurring within thenew mask material.

3.4.3. Rapid Prototyping of Silicon Microfluidic DevicesMicrofluidic devices are among the premier candidates for micro rapid prototyping.

Many microfluidic elements, e.g. channels, mixing chambers, microreactors, filters or actu-ation membranes are fabricated in a quite simple planar technology which is easily providedby the rapid prototyping methods described above. Moreover, silicon rapid prototypes canbe used as masters for a polymer replication by polymer casting, polymer hot embossingor injection molding as described in the rapid tooling section of this review.

Figure 43 demonstrates the capabilities of the second generation rapid prototypingprocess described above with a microfluidic test chip. On an area of 20 × 20 mm2 thisdevice contains trapezoidal microchannels with two different channel widths (1025 µmand 250 µm at the wafer surface) and a common depth of 260 µm, realized by anisotropicetching of (100) silicon. The middle channel contains etch compensation structures at theconvex corners. The laser-exposed area is 80.2278 mm2 which is about 20% of the totalchip surface. The chip layout was designed with AUTOCAD and translated into the laser’s


FIGURE 43. Rapid prototyping of a microfluidic test chip: mask layout (left) and photograph of the fabricatedchip (right) with the etch mask still on top. Chip size is 20 × 20 mm2.

control language via a custom-made software processor. Using the laser system describedin [9, 111], the mask pattern was ablated with a focus diameter of 25 µm and within a scantime of about 9 min. The following wet chemical etch took 220 min to achieve the desiredchannel depth. After a wet chemical removal of the etch mask, which would take about 5min, the whole design is transferred from the PC drawing into silicon within 4 hrs.

4. RAPID PROTOTYPING OF NANOSTRUCTURES

Mainly three basic approaches were pursued for the realization of NEMS struc-tures. Nanoimprinting lithography (NIL) techniques use flexible stamps for replication.A further miniaturiazation of microstereolithography towards polymer nanostructures ex-ploits nonlinear absorption properties. Focus Ion Beam gives access to metal and ceramicnanostructures.

4.1. Nanostereolithography

Besides the application of the later described nanoimprinting techniques, the directstereolithographic generation of submicron or nanosized structures is of particular interestespecially for photonic or life science applications. The use of vapour pressure arc lampsor standard laser sources limits a further reduction of the accessible structural details dueto the diffraction limit. Another pronounced disadvantage of the established microstere-olithography is the layer-by-layer generation of the solid part.

The use of multi-photon absorption processes for an improvement of the resolution limitwas discussed intensely with respect to laser scan fluorescence microscopy by Nakamura[181]. Early work of Maruo and coworkers proposed a real 3D-microfabrication using apulsed femtosecond high power Ti:sapphire laser with an operating wavelength of 790 nm[153]. The laser beam is focused inside a commercial UV-sensitive photocurable resin that


FIGURE 44. Microbull (scale bar 2 µm], venus and Sydney opera sculptures generated via two-photon-absorption(left image taken from [134], with permission from Nature Publishing Group; middle image taken from LaserZentrum Hannover e.V., www.lzh.de); right image taken from [220], Copyright 2004, with permission fromElsevier).

is polymerized only in a small volume element locally in the depth of the focus due tothe squared point-spread function of the two-photon-absorption process [181]. At the focuspoint the photon’s spatial density becomes significantly high, the UV-sensitive photoinitiatorabsorbs two NIR-photons at 790 nm, which is energetically equivalent to one UV-photon at395 nm. The photocuring speed is proportional to the square of the spatial photon densitydistribution, with respect to a polymerization threshold, energy polymerization occurs onlyin the focus voxel. The first realized microstructures showed details around 1 µm [153].Further research efforts concentrated on the synthesis of photoinitiators especially for two-photon-absorption [61].

Kawata et al. published movable nanosized structures with a spatial resolution of 120 nmapplying two-photon-absorption [134]. Quite impressive are the realization of a microbullsculpture (length 10 µm, height 7 µm, fabrication accuracy 150 nm, figure 44, left) and avenus sculpture with nanometer resolution (figure 44, middle) [115, 134, 151]. Recently ananosculpture of the Sydney Opera House was published using siloxane-based photoactiveresins [220] (figure 44, right). Future application will be the prototyping of 3D organic pho-tonic crystals. Quite recently the two-photon-absorption process has been commercialisedby Georgia Institute of Technology and Focal Point Microsystems [85, 92].

Using the same polymerization technique, light driven submicron tools or microma-chines like microtweezers [157], rotors [89], microgears and turbines [155] with resolu-tions around 200 nm [156] have been realized. Similar results were described by Miwa andcoworkers [165].

The above described pinpoint solidification can be achieved also by using a focussedHeCd laser (442 nm, 100 mW) in single-photon polymerization [154]. In opposite to stan-dard polymerization processes, the resin absorbs only at higher laser intensities due tothe nonlinear absorption behaviour. The achieved resolution around 1 µm was reduced incomparison to the capabilities of two-photon-microstereolithography.

As an alternative technique near-field optical lithography on prepolymers allows aprototyping of 2D-photonic crystals consisting of conjugated polymers like poly-phenyle-vinylene (PPV) [201]. A near-field optical microscope (SNOM) with a HeCd-laser (325 nm)generates smallest feature sizes around 160 nm. Smaller features e.g. nanopillars made on a


PMMA sphere with a size around 100 nm or PMMA based nanofilters with hole diametersaround 20–30 nm can be realized via electron beam lithography [259].

4.2. Nanoimprint Lithography (NIL) and Related Techniques

Different types of imprint or embossing techniques have been used as nanofabrica-tion techniques. Chou et al. [52] developed an imprint method for the realization of viaswith a dot size around 25 nm and trenches with a width of 60 nm and 100 nm height inPMMA on silicon. The master pattern on the silicon dioxide on silicon mold was fab-ricated by electron beam lithography with subsequent reactive ion etching. The patterntransfer was implemented at a temperature of 200◦C and a pressure of 13 MPa. A cur-rent paper by Sotomayor and coworkers [218] demonstrates the application possibilitiesof the different nanoimprint techniques in optics and organic optoelectronics like organicthin film transistors. Electron beam lithography allows a prototyping of nanosized wireswith diameters around 25 nm [218]. Quite recently the NIL-technique was extended to non-thermoplastic organic films, which consist of electron rich aromatic molecules like someoligothiophenes [190]. As a further improvement the replication step was established underambient conditions avoiding the time consuming temperization steps. As a demonstrator a400 nm period grating structure was generated.

All mentioned nanoimprint lithography techniques apply the process sequence startingwith a rigid master generation via electron beam lithography and subsequent replicationin organic materials. As an alternative, flexible molds like elastomeric stamps as in soft-lithography and derived methods developed by Whitesides and coworkers in the ninetiesof the last century can be used [253]. Bulthaup et al. combine a liquid molding step—nanostructuring with an elastomeric stamp made of PDMS—with a high thermal curing ofnanocrystalline gold colloids at 300◦C for the prototyping of electrostatic actuators [37].The same material can be nanostructured via AFM. A similar approach, a photoresist filledwith ceramic nanoparticles, is of certain interest for the development of high resolutionresists for electron beam lithography [97].

4.3. Focused Ion Beam

Focused ion beam (FIB) is a versatile technique which enables imaging, masklessmilling and the deposition of conducting and insulating material with high local precision(figure 45). It has the ability to fabricate structures which have a feature size below 1 µm

FIGURE 45. Principles of Focussed Ion Beam (FIB) (a) imaging, (b) milling and (c) deposition (Reprintedfrom [200], Copyright 2001 with permission from IOP Publishing).


FIGURE 46. Diamond 3D nano-sculptured structure, milled by FIB. (b) Close-up of the hand (Reprinted from[152] Copyright 2003, with permission from Springer).

(figure 46). FIB can therefore be used for the prototyping or manufacturing of high precisionmicrodevices with a large number of materials [136, 152, 179, 194, 241, 244]. A FIB systemoperates similar to a scanning electron microscope (SEM). Both instruments use chargedparticles from a source, focus them into a beam through electromagnetic/electrostatic lensesand scan them across small areas of the sample. With both instruments a high resolutionimaging is possible by collecting the secondary electron emission produced by the beam’sinteraction with the sample surface. FIB differs from an electron microscope by usingcharged ions from a field emission liquid metal ion (FE-LMI) source. The most popular ionspecies are Arsenic, Beryllium, Gallium and Silicon. Since the ions are orders of magnitudemore massive than electrons, a FIB’s ion beam rather mills the sample surface than it imagesit. In most applications this micromilling feature is emphasized [236]. The subtraction ofmaterial by the impact of an ion beam has been used e.g. for the manufacturing of gaps fortunneling devices [182], for diffractive optical elements [86] for scanning probe microscopetips [136] or for microcutting tools (figure 47) [189]. It is possible to mill high-precisiondevices with a resolution in the order of 1 nm by removing ultra small amounts of materialwith beam diameters in the order of 10 nm [236].

FIB can also be used for the deposition of material to produce patterns in an almostarbitrary shape. In the deposition process, a precursor gas is sprayed onto the surface ofthe substrate by a fine nozzle, where it adsorbs. The adsorbed layer is hit by the ion beamwhich locally decomposes the precursor. The desired reaction product remains at the surfacewhile volatile reaction products are removed by the vacuum system. By scanning the ionbeam across the surface a layer of material with defined shape is created and by depositinglayers on top of each, a 3D-object is produced. FIB deposition also enables the deposition ofseveral layers forming complex shapes with overhanging features (figure 48) [200]. Lateralresolutions and thickness of the deposited layers can be well below 100 nm. Recent FIBsystems are even capable of producing patterns with a dimension of 10 nm [94]. Aspectratios between 5 and 10 are obtained, at a typical deposition rate of 0.05 µm3/s [200].Due to the relatively slow deposition and the long processing times for large struc-tures, realistic dimensions for deposited structures are in the order of a few tenths ofmicrometers.


FIGURE 47. (a) Low magnification view of a single crystal diamond tool shank and junction with mandrel. Thetool cutting edges are fabricated on the last ∼30 µm near the tip. (b) micrograph of the FIB-shaped facets. (c) Leftside cutting edge of same microtool. This image shows the intersection of three FIB-sputtered facets. (d) End viewof the tool (Reprinted from [189] Copyright 2003, with permission from Elsevier).

In case of the deposition of metals, FIB is primarily used for making connectionsin integrated circuits. On commercially available machines mainly metals like platinum(Pt) and tungsten (W) are deposited [87, 123], however, deposition has also been achievede.g. for gold, tantalum, aluminum or copper [27, 64, 90, 102, 195, 198, 226]. Althoughthis application is of a 2D-nature, the deposition of structures with high aspect ratio is inprinciple also possible (figure 49) [124].

5. RAPID TOOLING FOR MICROREPLICATION

In non-silicon microtechnology microcomponents were mostly fabricated via replica-tion techniques using a master mold carrying the negative of the aspired microstructure. Inthe last few years various mold making techniques like mechanical microengineering, laserassisted micromachining or UV- or deep x-ray lithography in combination with electroplat-ing (e.g. LIGA: german acronym for lithography with synchrotron radiation, electroplatingand molding) have been developed and established [35, 160]. In the following, a few exam-ples shall demonstrate some efforts to establish rapid tooling in microsystem technologies.

5.1. Direct Tooling

In general microreplication techniques may not be classified as rapid prototyping tech-niques due to the necessity of using prestructured mold inserts. In the last few years, different


(a)

(b)

FIGURE 48. 3D-shaped SiO2 depositions with overhanging features, made by FIB (Reprinted from, Copyright2001, with permission from IOP Publishing).

rapid reaction molding techniques in combination with a photocuring of reactive resins asin microstereolithography have been developed.

The UV reaction molding process can be used for the fabrication of sensitive lensstructures or deflecting prisms with low aspect ratios on silicon wafer substrates using amodified mask aligner [63]. The replication tools were fabricated either via photolithographyin combination with electroplating or reactive ion etching into silicon or glass. The lensdiameters range from 5 up to 300 µm, lens sag from 1 to 100 µm and with a smallestdistance around 2 µm [63]. A similar process uses silicon wafers with a non-stick coatinglayer on the microstructured surface avoiding a pronounced sticking of the reactive resin atthe mold insert. Structural details around 2 µm and a tip radius of a few hundred nm canbe realized [76]. Microstructured prototypes with an aspect ratio up to 5 can be realized


FIGURE 49. Micrographs of a FIB-deposited tungsten pillar after cleaning the sidewalls by FIB-milling. Sideview (a) and top view (b) are SEM and FIB-induced SEM images, respectively (Reprinted from [124], Copyright2003, with permission from AVS, American Institute of Physics).

with UV embossing technique using modified curable acrylates [42]. Thin foils carryingmicrostructures with typical heights around 50 µm and a thickness around 9 µm weregenerated within seconds using a flood exposure system. The applied nickel on siliconmaster was fabricated via SU8-based UV lithography in combination with electroplating.

A combination of a rapid mold insert fabrication via laser-assisted micromachiningand rapid replication (photomolding) technique [106, 185, 186] allows a fast generation ofmicrostructured polymer parts and a rapid redesign implementation. The laser fabricationof metallic mold inserts is preferable in case of microstructures with small aspect ratiosaround 1-2 and structural details between 5 and 50 µm. In addition polymer based moldinserts made of polysulfone (PSU), polyetheretherketone (PEEK) or polyimide (PI) can bemicrostructured using excimer lasers [185]. The processing time for tooling a new moldinsert is around 24 h or less depending on the aspired microstructure. A quite recent workdescribes the rapid prototyping of microstructured PEEK carrying protrusion structuresvia excimer laser ablation [131]. The use of these PEEK microstructures for tooling wasdemonstrated.

The replication of mold inserts applying the photomolding process using photocurablereactive resins as in microstereolithography enables the rapid fabrication of microstructuredparts made of PMMA, polyesters or composites. A typical photomolding processing timefor the fabrication of one microstructured part is between 2 and 5 minutes using pure


FIGURE 50. Examples of fluidic prototypes (PMMA) fabricated by laser assisted micromachining and photo-molding (Reprinted from [106], Copyright 2002, with permission from Springer).

photocurable resins, in case of metal or ceramic filled resins the curing time is increasedto 20 up to 30 min. In all cases the aspect ratio of the microstructures on the mold inserthas no influence on the curing time. Aspect ratios up to 20 have been realized, typicalmicrostructures of interest possess aspect ratios up to 2 and structural details in the rangeof 20–100 µm e.g. for microfluidic applications (figure 50). A rapid prototyping of ceramicmicrocomponents in combination with reactive resin based feedstocks similar to thosedescribed in the chapters 3.2.1 and 3.2.2 is being evaluated [107].

Another process combination was described by Liao et al. [147], the so called imprintlithography. The used mold was fabricated via optical lithography and reactive ion etchingof a silicon wafer, the resulting smallest structural feature is around 2 µm with an aspectration of 2.5. A solution of PMMA in chloroform was spin-coated on the mold. Afterdrying and softbake a flexible PP substrate was attached on the back, stabilizing the thinPMMA film. Due to the simple spincoating technique complex molding technique can beavoided.

Silicon dicing, silicon etching processes and microstereolithography as representativetechniques for the fabrication of microstructures were compared for master generationusing soft lithography techniques for replication [55]. More detailed aspects on soft moldreplication are described in the following chapter.

5.2. Soft Mold Replication Techniques

The use of rigid molds as mentioned above sometimes causes demolding problemsas e.g. microstructure destruction due to enhanced mold surface roughness. Therefore forevaluation purposes the application of elastomeric silicone tools was found to be a betterchoice for the manufacturing of micropatterned structures. Flexible silicone tools are notonly easy and fast to fabricate, the material enables the fabrication of precise replicas evenfor details in the nanosized range. It has been demonstrated that structures with lateraldimension smaller than 50 nm can be replicated [26]. A typical representative is the oftenused elastomeric PDMS [56], which e.g. can be used in hot embossing for the fabricationof low aspect ratio PMMA microstructures [183].

The methods that use a patterned elastomer as a mold, stamp or a mask are often summa-rized as “soft lithography” [253, 260]. Although mainly applied for polymers, soft lithogra-phy can also be used for the patterning of ceramic suspensions [18, 112, 114, 206, 210, 260].There are several variations available, using e.g. capillary forces for the spontaneous filling


FIGURE 51. SEM images of microstructures (star pattern), formed on a silicon substrate using MIMIC froman ethanol solution of the precursor polymer to ZrO2: (a) before firing, (b) after firing in air at 460◦C for ∼4h(Reprinted from [18], Copyright 1999, with Permission from MRS).

of the cavities (Micromolding In Capillaries, MIMIC [253]). These methods work withlow viscous suspensions, suffering high shrinkage stresses during drying and sintering(figure 51). To obtain defect-free patterns, they are limited to patterns with a width of a fewmicrometers and maximum aspect ratios of 2–3. By using highly concentrated suspensionsit is not possible to structure the very fine patterns, but this enables the fabrication of partswith larger dimensions and high aspect ratios, e.g. by using molds which were cast frompatterned SU-8 photoresist masters (figures 52 and 53). Such suspensions can be of anaqueous [265] or non-aqueous type [202] or they can be a powder containing wax melt, likeit is used for Low-Pressure Injection Molding (LPIM) [13, 14]. With the application of a

FIGURE 52. SEM image of a freestanding Al2O33 micropiston (Reprinted from [265], Copyright 2003, withpermission from Wiley-VCH).


FIGURE 53. Crossectional SEM images of a microstructured PDMS mold and sintered lead-zirconate-titanate(PZT) microridges (Reprinted from [202], Copyright 2002, with permission from Elsevier).

PDMS Mould

Add suspensionand centrifuge

Remove excessivesuspension

Ceramic Suspension

Invert ontosubstrate

Photoresist coatedalumina substrate

Dry and removethe mouldDissolve the photoresist

and release components

FIGURE 54. Schematic steps of soft molding for the fabrication of freestanding microparts (Reprinted from [265],Copyright 2003, with permission from Wiley-VCH).

sacrificial photoresist layer, which is dissolved before the thermal treatment, freestandingparts can be realized (figure 54). Examples for the manufacturing of ceramic micropartsby rapid tooling, i.e. starting from a rapid prototyping model, can be seen at figure 55. Forthe samples, a master model was fabricated by a polymer based RP method, in this caseby RMPD R© (see chapter 3.1.2). Shaping was performed with LPIM, which permitted the

FIGURE 55. SEM images of details from a zirconia microturbine, replicated from a RMPD R© master model.Parts were shaped by Low-Pressure Injection Molding (LPIM).


FIGURE 56. Inverse parts of a microturbine, cast in gold base alloy Stabilor G R© (Reprinted from [16], Copyright2004, with permission from Springer).

manufacturing of small series with a few hundred parts. In spite of the soft nature of thesilicone, it is possible to ensure dimensional accuracy by LPIM. Therefore it is necessarythat the wax-powder-mixture, the so-called “feedstock”, has a sufficiently low viscosityto enable a mold filling at an injection pressure below 0.5 MPa. The mean particle sizeof standard ceramic powders like alumina or zirconia is in the range of 0.3 µm to 2 µm.Therefore these powders are fine enough to replicate most details which can be made byRP methods. The used RP method and the quality of the master model, however, are ofdecisive importance for the quality of ceramic components replicated by LPIM [140].

In principle, the formerly described procedures for the manufacturing of ceramics canalso be adapted to metallic microdevices using metallic instead of ceramic powder. Yet,because of the variety and the particle fineness, which is much more superior for ceramicpowders, ceramics are preferred.

5.3. Microinvestment Casting

The manufacturing of sacrificial models by RP for investment casting was one of thefirst applications for RP. To produce the casting cavity, the model is embedded in plaster orcalcium-silicate-based castable (CBC) ceramics. The model is removed via pyrolysis andthe resulting cavity filled with the molten metal by die casting or centrifugal casting. Afterthe solidification of the metal, the ceramic shell is removed by mechanical means or bythe use of solvents. The method can also be adapted to the manufacturing of microdevices.This microcasting process was demonstrated on the gold base alloy Stabilor G R© and anAl-bronze for the manufacturing of fine cavities and high flow lengths [15, 16]. Cast partsshow fine details with a size of a few tens of micrometers (figure 56).

6. CONCLUSION

In the last few years a large variety of different rapid prototying techniques havebeen adapted to microsystem technologies. In many cases the experiences obtained frommacroscopic technologies were transferred in a top-down-approach towards the microscale.Quite new technologies like FIB or the exploitation of exotic physical processes like


two-photon-absorption allow the access to the nanoworld. Rapid Prototyping of compo-nents made of polymer, metals or ceramics is possible on the lab scale applying differenttechnologies, in certain cases a commercialization happened and microstructured parts canbe obtained as service.

Rapid prototyping of silicon-based MEMS devices is still in its beginning. Researchactivities are rarely found today and mostly based on different laser machining techniques.Nevertheless, it has already been demonstrated, that the combination of laser machiningwith the whole capability of today’s silicon micromachining technology provides functionalprototypes with essentially the same features as a latter mass-produced device. It is obviousthat such a technology could have a tremendous impact on MEMS design.

The realization of a microstructured part or a microdevice starting from an idea tillmassfabrication is generally spoken time and cost effective. The implementation of rapidprototyping in MEMS, regardless whether polymers, metals, ceramics, silicon or compositesare used, should result in a significant cost reduction and reduced time-to-market cycle.

REFERENCES

1. http://www.2objet.com, July 2004.2. http://www.3d-micromac.com, July 2004.3. http://www.3dsystems.com, June 2004.4. Agrawala, M.K., Bandyopadhyay, A., van Weeren, R., Safari, A., Danforth, S.C., Langrana, N., Jamalabad,

V.R., and Whalen, P.J., FDC, Rapid Fabrication of Structural Components, Am. Ceram. Soc. Bull.,1996;75(11):60–65.

5. Ainsley, C., Reis, N., and Derby, B., Freeform Fabrication by Controlled Droplet Deposition of PowderFilled Melts, J. Mater. Sci., 2002;37:3155–3161.

6. Atwood, C.L., Griffith, M.L., Schlienger, M.E., Harwell, L.D., Ensz, M.T., Keicher, D.M., Schlienger, M.E.,Romero, J.A., and Smugeresky, J.E., Laser Engineered Net Shaping (LENS R©): A Tool for Direct Fabricationof Metal Parts, Proc. of ICALEO ’98, Orlando, FL, 1998, pp. 1–7.

7. Bandyopadhyay, A., Panda, R.K., Janas, V.F., Agrawala, M.K., Danforth, S.C., Safari, A., Processing ofPiezocomposites by Fused Deposition Technique, J. Am. Ceram. Soc., 1997;80(6):1366–1372.

8. Bang, C., EFAB: A New Approach to MEMS Fabrication, Sensors, 2002;19(11):14–25; Published by:Advanstar Communications. http://www.sensorsmag.com/articles/1102/14/.

9. Bange, S., Herding, M., and Woias, P., Evaluation of a Rapid Prototyping Process for Silicon Microstructures,Proc. of SPIE, Vol. 4979, 2003, pp. 593–600.

10. Banwell, C.N., Fundamentals of Molecular Spectroscopy, McGraw-Hill Book Company Ltd., London,1983.

11. Bardell, R., Balendran, V., and Sivayoganathan, K., Accuracy Analysis of 3D Data Collection and Free-formModelling Methods, J. Mat. Proc. Techn., 2003;133:26–33.

12. Bartolo, P.J. and Mitchel, G., Stereo-thermal-lithography: A New Principle for Rapid Prototyping, RapidPrototyping Journal, 2003;9(3):150–156.

13. Bauer, W. and Knitter, R., Development of a Rapid Prototyping Process Chain for the Production of CeramicMicrocomponents, J. Mat. Sci., 2002;37:3127–3140.

14. Bauer, W., Knitter, R., Bartelt, G., Emde, A., Gohring, D., and Hansjosten, E., Replication Techniques forCeramic Microcomponents with High Aspect Ratios, Microsys. Techn., 2002;9(1–2):81–86.

15. Baumeister, G., Muller, K., Ruprecht, R., and Hausselt, J., Production of Metallic High Aspect Ratio Mi-crostructures by Microcasting, Microsys. Techn., 2002;8(2–3):105–108.

16. Baumeister, G., Ruprecht, R., and Hausselt, J., Microcasting of Parts Made of Metal Alloys, Microsys. Techn.,2004;10(3):261–264.


17. Beeson, K.W. and Houlding, V.H., Laser Etching of LiNbO3 in a Cl2 Atmosphere, J. Appl. Phys.,1988;64(2):835–840.

18. Beh, W.S., Xia, Y., and Qin, D., Formation of Patterned Microstructures of Polycrystalline Ceram-ics from Precursor Polymers Using Micromolding in Capillaries, J. Mater. Res., 1999;14(10):3995–4003.

19. Bertsch, A., Zissi, S., Jezequel, J.Y., Corbel, S., and Andre, J.C., Microstereolithography Using a LiquidCrystal Display as Dynamic Mask-generator, Microsyst. Techn., 1997;3(2):42–47.

20. Bertsch, A., Lorenz, H., and Renaud, P., 3D Microfabrication by Combining Microstereolithography andThick Resist UV Lithography, Sensors and Actuators, 1999;73:14–23.

21. Bertsch, A., Bernhard, P., Vogt, C., and Renaud, P., Rapid Prototyping of Small Size Objects, Rapid Proto-typing Journal, 2000;6(4):259–266.

22. Bertsch, A., Jiguet, S., Berhard, P., and Renaud, P., Microstereolithography: A Review, Mat. Res. Soc. Symp.Proc., 2003;758:L.L.1.1-L.L.1.1-11.

23. Bertsch, A., Jiguet, S., and Renaud, P., Microfabrication of Ceramic Components by Microstereolithography,J. Micromech. Microeng., 2004;14:197–203.

24. Bertsch, A., Jiguet, S., Hofmann, H., and Renaud, P., Ceramic Microcomponents by Microstereolithography,Proc. MEMS, 25–29.01.2004, Maastricht, The Netherlands, 2004, pp. 25–29.

25. Bhatti, A.R., Mott, M., Evans, J.R.G., and Edirisinghe, M.J., PZT Pillars for 1–3 Composites Prepared byInk-jet Printing, J. Mater. Sci. Lett., 2001;20:1245–1248.

26. Biebuyck, H.A., Larsen, N.B., Delamarche, E., and Michel, B., Lithography Beyond Light: MicrocontactPrinting with Monolayer Resists, IBM J. Res. Dev., 1997;41(1–2):159–170.

27. Blauner, P.G., Butt, Y., Ro, J.S., Thompson, C.V., and Melngailis, J., Focused Ion Beam Induced Depositionof Low-resistivity Gold Films, J. Vac. Sci. Technol., 1989;7(6):1816–1818.

28. Blazdell, P.F., Evans, J.R.G., Edirisinghe, M.J., Shaw, P., and Binstead, M.J., The Computer AidedManufacture of Ceramics Using Multilayer Jet Printing, J. Mater. Sci. Lett., 1995;14(22):1562–1565.

29. Blazdell, P.F. and Evans, J.R.G., Preparation of Ceramic Inks for Solid Freeforming Using a Continuous JetPrinter, J. Mater. Synth. Proc., 1999;7(6):349–356.

30. Blazdell, P.F. and Evans, J.R.G., Application of a Continuous Ink Jet Printer to Solid Freeforming of Ceramics,J. Mat. Proc. Techn., 2000;99:94–102.

31. Boddu, M.R., Landers, R.G., and Liou, F.W., Control of Laser Cladding Processes for Rapid Prototyping-A Review, Proc. of the 12th Annual Solid Freeform Fabrication Symposium, Austin, TX, 2001, pp. 460–467.

32. Bohlmann, H. and Goetzen, R., High Aspect Ratio Components through RMPD, Proc. HARMST’01, 17.-19.06.2001, Baden-Baden, FRG, 2001, p. 49.

33. Bose, S., Darsell, J., Hosick, H.L., Yang, L., Sarkar, D.K., and Bandyopadhyay, A., Processing and Charac-terization of Porous Alumina Scaffolds, J. Mater. Sci.: Materials in Medicine, 2002;13:23–28.

34. Brady, G.A. and Halloran, J.W., Stereolithography of Ceramic Suspensions, Rapid Prototyping Journal,1997;3(2):61–65.

35. Brueck, R., Rizvi, N., and Schmidt, A., Applied Microtechnology, Hanser Verlag, Munchen, 2001.36. Bruenahl, J. and Grishin, A.M., Piezoelectric Shear Mode Drop-on-demand Inkjet Actuator, Sensors and

Actuators, 2002;A101:371–382.37. Bulthaup, C., Wilhelm, E., Hubert, B., Ridley, B., and Jacobson, J., Direct Fabrication of All Inorganic

Logic Elements and Microelectromechanical Systems from Nanoparticle Precursors, Mat. Res. Soc. Proc.,Materials Research Society, 2001;636(D10-4).

38. Burns, S.E., Cain, P., Mills, J., Wang, J., and Sirringhaus, H., Inkjet Printing of Polymer Thin-film TransistorCircuits, Mat. Res. Soc. Bull., 2003;28(11):829–834.

39. Calvert, P., Inkjet Printing for Materials and Devices, Chem. Mater., 2001;13(10):3299–3305.40. Cawley, J.D., Solid Freeform Fabrication of Ceramics, Current Opinions in Solid State and Material Science,

1999;4:483–489.41. Cesarano III, J., Segalman, R., and Calvert, P., Robocasting provides Moldless Fabrication from Slurry

Deposition, Ceram. Ind., 1998;148(4):94–102.42. Chan-Park, M.B. and Neo, W.K., Ultraviolett Embossing for Patterning High Aspect Ratio Polymeric

Microstructures, Microsys. Techn., 2003;9:501–506.


43. Chaput, C., Doreau, F., and Chartier, T., Stereolithography of Ceramic Part Manufacturing, in R.I. Campbell(Ed.), Proc. 8th European Conf. on Rapid Prototyping and Manufacturing, 05.-07.07.1999, Nottingham,UK, 1999, pp. 291–297.

44. Chapman, G.H., Sarunic, M.V., and Tu, Y., A Prototype Laser Activated Bimetallic Thermal Resist forMicrofabrication, Proc. of SPIE, 2001, Vol. 4274, pp. 183–193.

45. Chaput, C., Doreau, F., Loiseau, M., and Chartier, T., Complex Ceramic Parts Manufacturing by Stereolithog-raphy Process, in J. Luyten and J.-P. Erauw (Eds.), Proc. 2nd Conf. on Shaping of Advanced Ceramics,24.-26.10.2002, Gent, Belgium, 2002, pp. 293–298.

46. Chapman, G.H. and Tu, Y., Bi/In Thermal Resist for both Si Anisotropic Wet Etching and Si/SiO2 PlasmaEtching, Proc. of SPIE, 2004, Vol. 5342, pp. 192–203.

47. Chartier, T., Chaput, C., Doreau, F., and Loiseau, M., Stereolithography of Structural Complex CeramicParts, J. Mater. Sci., 2002;37:3141–3147.

48. Chen, C. and Zuo, T., Microfabrication of Micron Metallic Powder with Laser Sintering, MicronanoelectronicTechn., 2003;40(7–8):170–172.

49. Chen, C., Wang, X., and Zuo, T., The Micro Fabrication Using Selective Laser Sintering Micron MetalPowder. Smart Sensors, Actuators and MEMS, Proc. of SPIE, 2003, Vol. 5116, pp. 647–651.

50. Cherlin, E. (Ed.), Ink Jet Printing: 1994 Overview and Outlook, BIS Strategic Devisions, Norwell, MA, USA,1993, p. 140.

51. Choi, S.H. and Samavedam, S., Visualisation of Rapid Prototyping, Rapid Prototyping Journal,2001;7(2):99–114.

52. Chou, S.Y., Krauss, P.R., and Renstrom, P.J., Imprint of sub-25 nm vias and Trenches in Polymers, Appl.Phys. Lett., 1995;67(21):3114–3116.

53. Chu, T.-M.G. and Halloran, J.H., Curing of Highly Loaded Ceramic Suspensions in Acrylates, J. Am. Ceram.Soc., 2000;83(10);2375–2380.

54. Chua, C.K., Leong, K.F., and Lim, C.S., Rapid Prototyping—Principles and Applications, 2nd Ed., WorldScientific Publishing, New Jersey, London, Singapore and Hong Kong, 2003, p. 420.

55. Chung, S., Im, Y., Choi, J., and Jeong, H., Microreplication Techniques Using Soft Lithography, Microelec-tronic Eng., 2004;75:194–200.

56. Clarson, S.J. and Semlyen, J.A., Siloxane Polymers, Prentice Hall, NJ, 1993.57. Cohen, A., Zhang, G., Tseng, F., Frodis, U., Mansfeld, F., and Will, P., EFAB: Rapid, Low-cost Desktop

Micromachining of High Aspect Ratio True 3-D MEMS, Proc. of the 12th IEEE MEMS ’99, Orlando, FL,1999, pp. 244–251.

58. Cohen, A., Frodis, U., Tseng, F., Zhang, G., Mansfeld, F., and Will, P., EFAB: Low-cost AutomatedElectrochemical Batch Fabrication of Arbitrary 3-D Microstructures, Proc. of SPIE, 1999, Vol. 3874,pp. 236–247.

59. Corbel, S., Dufaud, O., Mauzon, A., and Le Gall, H., Microfabrication of Metal Components by DirectStereolithography, 10th European Forum on Rapid Prototyping, 14.-15.09.2004, Paris, France, 2004.

60. Creagh, L.T. and McDonald, M., Design and Performance of Inkjet Print Heads for Non-graphic-arts Appli-cations, Mat. Res. Soc. Bull., 2003;28(11):807–811.

61. Cumpston, B.H., Ananthavel, S.P., Barlow, S., Dyer, D.L., Ehrlich, J.E., Erskine, L.L., Heikal, A.A., Kuebler,S.M., Sandy Lee, I.-Y., McCord-Maughon, D., Qui, J., Rockel, H., Rumi, M., Wu, X.-L., Marder, S.R.,and Perry, J.W., Two-photon Polymerization Initiators for Three-dimensional Optical Data Storage andMicrofabrication, Nature, 1999;398:51–54.

62. Dalgarno, K. and Stewart, T., Production Tooling for Polymer Moulding Using the RapidSteel Process, RapidPrototyping Journal, 2001;7(3):173–179.

63. Dannberg, P., Erdmann, L., Bierbaum, R., Krehl, A., Braeuer, A., and Kley, E.B., Micro-optical Elementsand Their Integration to Glass and Optoelectronic Wafers, Microsys. Techn., 1999;6:41–47.

64. Della Ratta, A.D., Melngailis, J., and Thompson, C.V., Focused-ion Beam Induced Deposition of Copper, J.Vac. Sci. Technol., 1993;11(6):2195–2199.

65. Derby, B. and Reis, N., Inkjet Printing of Highly Loaded Particulate Suspensions, Mat. Res. Soc. Bull.,2003;28(11):815–818.

66. http://www.dlp.com, July 2004.67. Doreau, F., Chaput, C., and Chartier, T., Stereolithography for Manufacturing Ceramic Parts, Adv. Eng,

Mater., 2000;2(8):493–496.


68. http://www.dsmsomos.com, June 2004.69. Dufaud, O., Marchal, P., and Corbel, S., Rheological Properties of PZT Suspensions for Stereolithography,

J. Eur. Ceram. Soc., 2002;22:2081–2092.70. Dufaud, O. and Corbel, S., Stereolithography of PZT Ceramic Suspensions, Rapid Prototyping Journal,

2002;8(2):83–90.71. Dufaud, O. and Corbel, S., Oxygen Diffusion in Ceramic Suspensions for Stereolithography, Chem. Eng. J.,

2003;92:55–62.72. Ebert, R., Regenfuss, P., Hartwig, L., Klotzer, S., and Exner, H., Process Assembly for µm-Scale SLS, Reac-

tion Sintering, and CVD, 4th International Symposium on Laser Precision Microfabrication, 21.-24.06.2003,Munich, Proc. of SPIE, 2003, Vol. 5063, pp. 183–188.

73. Edirisinghe, M.J., Solid Freeform Fabrication Methods for Engineering Ceramics, British Ceramic Trans-actions, 1998;97(6):283–286.

74. Ehrlich, D.J., Osgood Jr., R.M., and Deutsch, T.F., Laser Chemical Technique for Rapid Direct Writing ofSurface Relief in Silicon, Appl. Phys. Lett., 1981;38(12):1018–1020.

75. Einstein, A., Uber die von der molekularkinetischen Theorie der Warme gefordete Bewegung von in ruhendenFlussigkeiten suspendierten Teilchen, Ann. Phys., 1906;17:549–560.

76. Elsner, C., Dienelt, J., and Hirsch, D., 3D-microstructure Replication Processes Using UV-curable Acrylates,Microelectronic Eng., 2003;65:163–170.

77. http://www.envisiontec.com, July 2004.78. http://dmtwww.epfl.ch/ims/micsys/projects/prtop.html, July 2004.79. Evans, J.R.G., Edirisinghe, M.J., Coveney, P.V., and Eames, J., Combinatorial Searches of Inorganic Materials

Using the Ink-jet Printers: Science, Philosophy and Technology, J. Eur. Ceram. Soc., 2001;21:2291–2299.80. Evans, J.D. and Bang C., A Demonstration of EFABTM as a Fundamental Shift in the Way Microdevices are

Manufactured, ASME International Mechanical Engineering Congress and Exposition, New Orleans, LO,2002, pp. 351–354.

81. Exner, H., Regenfuss, P., Hartwig, L., Klotzer, S., and Ebert, R., Selective Laser Micro Sintering with aNovel Process, 4th International Symposium on Laser Precision Microfabrication, 21.-24.06.2003, Munich,Proc. of SPIE, 2003, Vol. 5063, pp. 145–151.

82. Exner, H., Regenfuss, P., Hartwig, L., Klotzer, S., and Ebert, R., Microsintering of Miniature and PreciseComponents and Tools, 2003, Proc. of Euro-µRapid, Frankfurt/Main, 01.-02.12.2003, B/3.

83. Fischer, P., Karapatis, N., Romano, V., Glardon, R., and Weber, H.P., A Model for the Interaction of Near-infrared Laser Pulses with Metal Powders in Selective Laser Sintering, Appl. Phys. A, 2002;74(4):467–474.

84. Fouassier, J.-P., Photoinitiation, Photopolymerization, and Photocuring: Fundamentals and Applications,Hanser Publisher, Munich, 1995.

85. http://Fpmicro.com, July 2004.86. Fu, Y.G. and Ngoi, B.K.A., Investigation of Direct Milling of Micro-optical Elements with Continuous Relief

on a Substrate by Focused Ion Beam Technology, Opt. Eng., 2000;39(11):3008–3013.87. Fu, Y.G., Ngoi, B.K.A., and Shing, O.N., Characterization of Focused Ion Beam Induced Deposition Process

and Parameters Calibration, Sensors and Actuators A, 2001;88(1):58–66.88. Fuh, J.Y.H., Lu, L., Tan, C.C., Shen, Z.X., and Chew, S., Curing Characteristics of Acrylic Photopolymer

used in Stereolithography Process, Rapid Prototyping Journal, 1999;5(1):27–34.89. Galajda, P. and Pal, O., Complex Micromachines Produced and Driven by Light, Appl. Phys. Lett.,

2001;78(2):249–251.90. Gamo, K., Takehara, D., Hamamura, Y., Tomita, M., and Namba, S., Maskless Ion Beam Assisted Deposition

of W and Ta Films, Microelectron. Eng., 1986;5(1–4):163–170.91. de Gans, B.-J., Duineveld, P.C., and Schubert, U.S., Inkjet Printing of Polymers: State of the Art and Future

Developments, Adv. Mater., 2004;16(3):203–213.92. Georgia Institute of Technology, Research News, 3D Chemistry: Fabrication Technique Uses Light-activated

Molecules to Create Complex Microstructures, February 16, 2004.93. Gebhardt, A., Rapid Prototyping, Hanser Publisher, Munich, 2000.94. Gierak, J., Mailly, D., Faini, G., Pelouard, J.L., Denk, P., Pardo, F., Marzin, J.Y., Septier, A., Schmid, G.,

Ferre, J., Hydman, R., Chappert, C., Flicstein, J., Gayral, B., and Gerard, J.M., Nano-fabrication with FocusedIon Beams, Microelectron. Eng., 2001;57–58:865–875.


95. Glardon, R., Private Communication, 2004.96. Goetzen, R., Tool-less Batch Production of MST Products, Proc. MICRO.Tec 2nd VDE World Microtech-

nologies Congress, Munich, 13-15.10.2003, 2003, pp. 119–122.97. Gonsalves, K.E., Wu, H., Hu, Y., and Merhari, L., High Resolution Resists for Next Generation Lithogra-

phy: The Nanocomposite Approach, Mat. Res. Symp. Proc., Materials Research Society, 2001, Vol. 636,D6.5-1.

98. Greul, M., and Lenk, R., Near-net-shape Ceramic and Composite Parts by Multiphase Jet Solidification,Industrial Ceramics, 2000;20(2):115–117.

99. Grida, I. and Evans, J.R.G., Extrusion Freeforming of Ceramics through Fine Nozzles, J. Eur. Ceram. Soc.,2003;23(5):629–635.

100. Griffith, M.L. and Halloran, J.W., Freeform Fabrication of Ceramics via Stereolithography, J. Am. Ceram.Soc., 1996;79(10):2601–2608.

101. Griffith, M.L., Ensz, M.T., Puska, J.D., Robino, C.V., Brooks, J.A., Philliber, J.A., Smugeresky, J.E., andHofmeister, W.H., Understanding the Microstructure and Properties of Components Fabricated by LaserEngineered Net Shaping (LENS), Proc. Of Materials Research Society, 2000, Vol. 625. Symposium Y(2000), online available at http://mfgshop.sandia.gov/1400 ext/MRS00mg.pdf.

102. Gross, M.E., Harriott, L.R., and Opila Jr., R.L., Focused Ion Beam Stimulated Deposition of Aluminum fromTrialkylamine Alanes, J. Appl. Phys., 1990;68(9):4820–4824.

103. Hackney, P.M., The Development of Three-dimensional Printing Techniques for Concept Modellers to Com-petitive Rapid Prototyping Systems, in A.E.W. Rennie, D.M. Jacobson and C.E. Bocking (Eds.), 3rd NationalConference on Rapid Prototyping, Tooling, and Manufacturing, Professional Engineering Publishing Lim-ited, Bury St Edmunds and London, UK, 2002, p. 165 (ISBN 1 86058 374 1).

104. Halloran, J.W., Freeform Fabrication of Ceramics, British Ceramic Trans., 1999;98(6):299–303.105. Hanemann, T. and Haase, W., Liquid Crystals Containing X = Y Groups, in S. Patai (Ed.), The Chemistry

of Functional Groups, Supplement A: The Chemistry of Double Bonded Functional Groups, John Wiley &Sons Ltd. Chichester, 1997, pp. 423–471.

106. Hanemann, T., Pfleging, W., Hauβelt, J., and Zum Gahr, K.-H., Laser Micromachining and Light InducedReaction Injection Molding as Suitable Process Sequence for the Rapid Fabrication of Microcomponents,Microsys. Techn., 2002;7:209–214.

107. Hanemann, T., Boehm, J., Henzi, P., Honnef, K., Litfin, K., Ritzhaupt-Kleissl, E., and Hausselt, J., FromMicro To Nano: Properties and Potential Applications of Micro and Nano Filled Polymer Ceramic Com-posites in Microsystem Technology, IEE Proceedings Nanobiotechnology Special Issue, 2004, Vol. 151,pp. 167–172.

108. Hanson, E. (Ed.), Recent Progress in Ink Jet Technologies II, The Society for Imaging Science and Technology,Springfield, VA, USA, 1999, p. 614. (ISBN / ISSN: 0-89208-220-8).

109. Hayes, D.J., Cox, W.R., and Wallace, D.B., 2001, Printing system for MEMS packaging, Proc. of SPIE, 22.-25.10.2001, San Francisco, CA (available at http://www.microfab.com/about/papers/spie MEMS printer01.pdf).

110. Hedges, M. and Kiecher, D., Laser Engineered Net ShapingTM—technology and Applications, Proc.of 3rd National Conf. On Rapid Prototyping, Tooling and Manufacturing, High Wycombe, UK, 2002,pp. 17–23.

111. Herding, M., Bange, S., Goldschmidtboing, F., and Woias, P., Rapid Micro Prototyping of MEMS in Silicon,Proc. of the Micro System Technologies Conference, 07.-08.10.2003, Munich, Germany, 2003, pp. 556–558.

112. Heule, M. and Gauckler, L.J., Gas Sensors Fabricated from Ceramic Suspensions by Micromolding inCapillaries, Adv. Mater., 2001;13(23):1790–1793.

113. Heule, M., Vuillemin, S., and Gauckler, L.J., Powder-based Ceramic Meso- and Microscale FabricationProcesses, Adv. Mater., 2003;15(15):1237–1245.

114. Heule, M., Schell, J., and Gauckler, L.J., Powder-Based Tin Oxide Microcomponents on Silicon SubstratesFabricated by Micromolding in Capillaries, J. Am. Ceram. Soc., 2003;86(3):407–412.

115. Hill, P., Femtosecond Pulses Generate Microstructures, Opto & Laser Europe, December 2002;22–23.116. Hinczewski, C., Corbel, S., and Chartier, T., Ceramic Suspensions Suitable for Stereolithography, J. Eur.

Ceram. Soc., 1998;18:583–590.117. Hull, C.H., Apparatus for Production of 3D Objects by Stereolithography, 3D Systems, US Pat. 4,575,330;

1984.


118. Hunt, E., Towards Rapid Design and Manufacturing-An Evolution of Rapid Prototyping, Proc. 10th EuropeanConference on Rapid Prototyping and Manufacturing, 07-08.06.2001, Paris, France, 2001.

119. Ikuta, K. and Hitowatari, K., Photo Fabricated Three Dimensional Micro Fabrication, Proc. of Robotics andMechatronics Conference of Japanese Society of Mechanical Engineer, 1992, pp. 545–546.

120. Ikuta, K. and Hitowatari, K., Study of Three Dimensional Micro Fabrication (No. 1), Proc. of Robotics andMechatronics Conference of Japanese Society of Mechanical Engineer, 1992, pp. 113–1216.

121. Ikuta, K. and Hirowatari, K., Real Three Dimensional Micro Fabrication Using Stereolithography and MetalMolding, IEEE Proc. Micro Electro Mechanical Systems MEMS’93, 07.-10.1993, Fort Lauderdale, FL, USA,1993, pp 42–47.

122. Ikuta, K., Hirowatari, K., and Ogata, T., Three Dimensional Micro Integrated Fluid Systems (MIFS) Fab-ricated by Stereo Lithography, IEEE Proc. Micro Electro Mechanical Systems MEMS’94, 25.-28.01.1994,Oiso, Japan, 1994, pp. 1–6.

123. Iliadis, A.A., Andronescu, S.N., Yang, W., Vispute, R.D., Stanishevsky, A., Orloff, J.H., Sharma, R.P.,Venkatesan, T., Wood, M.C., and Jones, K.A., Pt and W Ohmic Contacts to p-6H-SiC by Focused Ion BeamDirect-write Deposition, J. of Electronic Mater., 1999;28(3):136–140.

124. Ishida, M., Fujita, J., Ichihashi, T., and Ochiai, Y., Focused Ion Beam-induced Fabrication of TungstenStructures, J. Vac. Sci. Technol., 2003;21(6):2728–2731.

125. Jackson, T.R., Lui, H., Patrikalakis, N.M., Sachs, E.M., and Cima, M.J., Modeling and Designing Func-tionally Graded Material Components for Fabrication with Local Composition Control, Materials Design,1999;20:63–75.

126. Jafari, M.A., Han, W., Mohammadi, F., Safari, A., Danforth, S.C., and Langrana, N., A Novel System forFused Deposition of Advanced Multiple Ceramics, Rapid Prototyping Journal, 2000;6(3):161–174.

127. Jang, J.H., Wang, S., Pilgrim, S.M., and Schulze, W.A., Preparation and Characterization of Barium TitanateSuspensions for Stereolithography, J. Am. Ceram. Soc., 2000;83(7):1804–1806.

128. Jayasinghe, S.N., Edirisinghe, M.J., and De Wilde, T., A Novel Ceramic Technique Based on ElectrostaticAtomization of a Suspension, Mat. Res. Innovat., 2002;6(3):92–95.

129. Jayasinghe, S.N. and Edirisinghe, M.J., Electrostatic Atomisation of a Ceramic Suspension, J. Eur. Ceram.Soc., 2004;24(8):2203–2213.

130. Jayasinghe, S.N., Edirisinghe, M.J., and Kippax, P.G., Relic and Droplet Sizes Produced by ElectrostaticAtomisation of Ceramic Suspensions, Appl. Phys. A, 2004;78:343–347.

131. Jensen, M.F., McCormack, J.E., Helbo, B., Christensen, L.H., Christensen, T.R., and Geschke, O., RapidPrototyping of Polymer Microsystems via Excimer Laser Ablation of Polymeric Moulds, Lab Chip,2004;4:391–394.

132. Kalita, S.J., Bose, S., Hosick, H.L., and Bandyopadhyay, A., Development of Controlled PorosityPolymer-ceramic Composite Scaffolds via Fused Deposition Modelling, Mater. Sci. Eng. C, 2003;23:611–620.

133. Kataria, A. and Rosen, D.W., Building Around Inserts: Methods for Fabricating Complex Devices in Stere-olithography, Rapid Prototyping Journal, 2001;7(5):253–261.

134. Kawata, S., Sun, H.-B., Tanaka, T., and Takada, K., Finer Features for Functional Microdevices, Nature,2001;412:697–698.

135. van Kessel, P.F., Hornbeck, L.J., Meier, R.E., and Douglas, M.R., A MEMS-based Projection Display, Proc.IEEE, 1998, Vol. 86, No. 8, pp. 1687–1704.

136. Khizroev, S. and Litvinov, D., Focused-ion-beam-based Rapid Prototyping of Nanoscale Magnetic Devices,Nanotechnology, 2004;15(3):R7–R15.

137. Khoury, C., Mensing, G.A., and Beebe, D.J., Ultra Rapid Prototyping of Microfluidic Systems Using LiquidPhase Photopolymerization, Lab on a Chip, 2002;2:50–55.

138. King, B.H., Dimos, D., Yang, P., and Morissette, S.L., Direct-write Fabrication of Integrated MultilayerCeramic Components, J. of Electroceramics, 1999;3(2):173–178.

139. Kirby, P., Papapolymerou, J., D’Aubigny, C., and Walker, C., Silicon Laser Micromachining for the Devel-opment of Planar Waveguide-based Terahertz Structures, Proc. of the 14th Intern. Symp. on Space TeraHertzTechnology, 22.-24.04.2003, Tucson, Arizona, 2003.

140. Knitter, R., Bauer, W., and Gohring, D., Microfabrication of Ceramics by Rapid Prototyping Process Chains,J. Mech. Eng. Science, Proc. Instn. Mech. Engrs., 2003;217 Part C:41–51.

141. Kramer, N., Niesten, M., and Schoneberger, C., Resistless High Resolution Optical Lithography on Silicon,Appl. Phys. Lett., 1995;67(20):2989–2991.


142. Krieger, I.M. and Dougherty, T.J., A Mechanism for Non-newtonian Flow in Suspensions of Rigid Spheres,Transactions of the Society of Rheology, 1959;III:137–152.

143. Kruth, J.P., Wang, X., Laoui, T., and Froyen, L., Lasers and Materials in Selective Laser Sintering, AssemblyAutomation, 2003;23(4):357–371.

144. Le, H.P., Progress and Trends in Ink-jet Printing Technology, J. Imaging Sci. Technol., 1998;42(1):49–62.145. Lee, D.H. and Derby, B., Preparation of PZT Suspensions for Direct Ink-jet Printing, J. Eur. Ceram. Soc.,

2004;24:1069–1072.146. Lewis, J.A., Direct Write Assembly of Ceramics from Colloidal Inks, Current Opinion in Solid State and

Material Science, 2002;6:245–250.147. Liao, W.-C., Hsu, S.L.-C., Chu, S.-Y., and Kau, P.-C., Imprint Lithography for Flexible Transparent Plastic

Subtrates, Microlectronic Eng., 2004;75:145–148.148. Lombardi, J.L. and Calvert, P., Extrusion Freeforming of Nylon 6 materials, Polymer, 1999;40(7):1775–1779.149. Lous, G.M., Cornejo, I.A., McNulty, T.F., Safari, A., and Danforth, S.C., Fabrication of Piezoelectric

Ceramic/Polymer Composite Transducers Using Fused Deposition of Ceramics, J. Am. Ceram. Soc.,2000;83(1):124–128.

150. Lynn-Charney, C. and Rosen, D.W., Usage of Accuracy Models in Stereolithography Process Planning,Rapid Prototyping Journal, 2000;6(2):77–86.

151. Laser Zentrzum Hannover, press report 13.10.2003, LZH Aktuell 47, September 2003, www.lzh.de, July2004.

152. Khan Malek, C., Hartley, F.T., and Neogi, J., Fast Prototyping of High-aspect Ratio, High-resolution X-rayMasks by Gas-assisted Focused Ion Beam, Microsys. Techn., 2003;9(6–7):409–412.

153. Maruo, S., Nakamura, O., and Kawata, S. Three-dimensional Microfabrication with Two-photon-absorbedPhotopolymerization, Optics Letters, 1997;22(2):132–134.

154. Maruo, S. and Ikuta, K., Three-dimensional Microfabrication by use of Single-photon Absorbed Polymer-ization, Appl. Phys. Lett., 2000;76(19):2656–2658.

155. Maruo, S., Ikuta, K., and Hayato, K., Light-driven MEMS made by High-speed Two-photon Microstere-olithography, Proc. of 14th IEEE Intern. Conf. on Micro Electro Mechanical Systems (MEMS2001), 21.-25.01.2001, Interlaken, Switzerland, 2001, pp. 594–597.

156. Maruo, S., Ikuta, K., and Korogi, H., Optical drive of constrained Micromechanisms produced by two-photonmicrostereolithography with 200 nm resolution, Proc. 4th Intern. Workshop on High Aspect RatioMicrostructure Technology, 17.-19.06.2001, Baden-Baden, Germany, 2001, pp. 57–58.

157. Maruo, S., Ikuta, K., and Korogi, H., Submicron Manipulation Tools Driven by Light in a Liquid, Appl. Phys.Lett., 2003;82(1):133–135.

158. McNulty, T.F., Mohammadi, F., Bandyopadhyay, A., Shanefield, D.J., Danforth, S.C., and Safari, A., Develop-ment of a Binder Formulation for Fused Deposition of Ceramics, Rapid Prototyping Journal, 1998;4(4):144–150.

159. McNulty, T.F., Shanefield, D.J., Danforth, S.C., and Safari, A., Dispersion of Lead Zirconate Titanate forFused Deposition of Ceramics, J. Am. Ceram. Soc., 1999;82(7):1757–1760.

160. Menz, W., Mohr, J., and Paul, O., Microsystem Technology, Wiley-VCH, Weinheim, 2001.161. http://www.microfabrica.com, July 2004.162. http://www.microtec-d.com, July 2004.163. Minami, K., Wakabayashi, Y., Yoshida, M., Watanabe, K., and Esashi, M., YAG Laser-assisted Etching of

Silicon for Fabricating Sensors and Actuators, J. Micromech. Microeng., 1993;3:81–86.164. http://web.mit.edu/tdp/www/index.html, July 2004.165. Miwa, M., Juodkazis, J., Kawakami, T., Matsuo, S., and Misawa, H., Femtosecond Two-photon Stereo-

lithography, Appl. Phys. A, 2001;73:561–566.166. Mohebi, M.M. and Evans, J.R.G., A Drop-on–demand Ink-jet Printer for Combinatorial Libraries and Func-

tionally Graded Ceramics, J. Com. Chem., 2002;4:267–274.167. Monneret, S., Provin, C., Le Gall, H., and Corbel, S., Microfabrication of Freedom and Articulated Alumina

Based Components, Microsys. Technol., 2002;8:368–374.168. Moon, J., Grau, J.E., Knezevic, V., Cima, M., and Sachs, E.M., Ink-jet Printing of Binders for Ceramic

Components, J. Am. Ceram. Soc., 2002;85(4):755–762.169. Morissette, S.L., Lewis, J.A., Cesarano III, J., Dimos, D.B., and Baer, T., Solid Freeform Fabrication

of Aqueous Alumina-poly(vinyl alcohol) Gelcasting Suspensions, J. Am. Ceram. Soc., 2000;83(1):2409–2416.


170. Morissette, S.L., Lewis, J.A., Glem, P.G., Cesarano III, J., and Dimos, D.B., Direct-write Fabrication ofPb(Nb,Zr,Ti)O3 devices: Influence of Paste Rheology on Print Morphology and Component Properties, J.Am. Ceram. Soc., 2001;84(11):2462–2468.

171. Mott, M., Song, J.-H., and Evans, J.R.G., Microengineering of Ceramics by Direct Ink-jet Printing, J. Am.Ceram. Soc, 1999;82(7):1653–1658.

172. Mott, M. and Evans, J.R.G., Zirconia/alumina Functionally Graded Material made by Ceramic Ink JetPrinting, Mater. Sci. Eng. A, 1999;271:344–352.

173. Mott, M. and Evans, J.R.G., Solid Freeforming of Silicon Carbide by Inkjet Printing Using a PolymericPrecursor, J. Am. Ceram. Soc., 2001;84(2):307–313.

174. Muellenborn, M., Dirac, H., and Petersen, J.W., Silicon Nanostructures Produced by Laser Direct Etching,Appl. Phys. Lett., 1995;66(22):3001–3003.

175. Muellenborn, M., Heschel, M., Larsen, U.D., Dirac, H., and Bouwstra, S., Laser Direct Etching of Siliconon Oxide for Rapid Prototyping, J. Micromech. Microeng., 1996;6:49–51.

176. Muellenborn, M., Birkelund, K., Grey, F., and Madsen, S., Laser Direct Writing of Oxide Structures onHydrogen-passivated Silicon Surfaces, Appl. Phys. Lett., 1996;69(20),11:3013–3015.

177. Muellenborn, M., Grey, F., and Bouwstra, S., Laser Direct Writing on Structured Substrates, J. Micromech.Microeng., 19977:125–127.

178. Mueller, H. and Schimmel, A., The Decision Dilemma Assessment and Selection of Rapid PrototypingProcess Chains, Proc. of 8th European Conference on Rapid Prototyping and Manufacturing, Nottingham,UK, 1999, pp. 177–192.

179. Murakawa, M., Hayashi, M., and Noguchi, H., Fabrication of 3-D Shaped Micro Body Structures of Diamondby use of Focused Ion Beam, New Diamond And Frontier Carbon Technology, 2004;14(3):173–179.

180. Mythili, V.S. and Prakasan, K., Dynamic Model for Flow and Droplet Deposition in Direct Ceramic Ink.jetPrinting, Defence Science Journal, 2004;54(1):85–93.

181. Nakamura, O., Three-dimensional Imaging Characteristics of Laser Scan Fluorescence Microscopy: Two-photon Excitation vs. Single-photon Excitation, Optik, 1993;93:39–42.

182. Nakayama, M., Yanagisawa, J., Wakaya, F., and Gamo, K., Focused Ion Beam Process for Formationof a Metal/Insulator/Metal Double Tunnel Junction, Jap. J. of Appl. Phys. Part 1, 1999;38(12B):7151–7154.

183. Narasimhan, J. and Papautsky, I., Polymer Embossing Tools for Rapid Prototyping of Plastic MicrofluidicDevices, J. Micromech. Microeng., 2004;14:96–103.

184. Noguera, R., Lejeune, M., and Chartier, T., 3D Fine Scale Ceramic Components Formed by Ink-jet Proto-typing Process, J. Eur. Ceram. Soc., 2005;25:2055–2059.

185. Pfleging, W., Bernauer, W., Hanemann, T., and Torge, M., Rapid Fabrication of Microcomponents—UV-laserAssisted Prototyping, Laser Micro-machining of Mold Inserts and Replication via Photomolding, Microsys.Technol., 2002;9:67–74.

186. Pfleging, W., Hanemann, T., Torge, M., and Bernauer, W., Rapid Fabrication and Replication of Metal,Ceramic, and Plastic Mould Inserts for Application in Microsystem Technologies, J. Mechanical EngineeringScience, Proc. Instn. Mech. Engrs, 2003;217 Part C:53–63.

187. Pham, D.T. and Dimov, S.S., Rapid Manufacturing: The Technologies and Applications of Rapid Prototypingand Rapid Tooling, Springer, London, UK, 2001, p. 214 (ISBN 1-85233-360-X).

188. Dimov, S.S., Pham, D.T., Lacan, F., and Dotchev, K.D., Rapid Tooling Applications of the Selective LaserSintering Process, Assembly Automation, 2001;21(4):296–302.

189. Picard, Y., Adams, D.P., Vasile, M.J., and Ritchey, M.B., Focused ion Beam-shaped Microtools for Ultra-precision Machining of Cylindrical Components, Precision Engineering, 2003;27(1):59–69.

190. Pisignano, D., Persano, L., Raganato, M.F., Visconti, P., Cingolani, R., Barbarella, G., Favaretto, L., andGigli, G., Room-Temperature Nanoimprint Lithography of Non-thermoplastic Organic Films, Adv. Mater.,2004;16(6):525–529.

191. http://www.proform.ch, July 2004.192. Provin, C. and Monneret, S., Complex Ceramic-polymer Composite Microparts made by Stereolithography,

IEEE Trans. Electron. Packag. Manuf., 2002;25(1):59–63.193. Provin, C., Monneret, S., Legall, H., and Corbel, S., Three-dimensional Ceramic Microcomponents made

by Stereolithography, Adv. Mater., 2002;15(12):994–997.194. Puers, R., Reyntjens, S., and De Bruyker, D., The NanoPirani-an extremely Miniaturized Pressure Sensor

Fabricated by Focused ion Beam Rapid Prototyping, Sensors and Actuators A, 2002;97–98:208–214.


195. Puretz, J. and Swanson, L.W., Focused Ion Beam Deposition of Pt Containing Films, J. Vac. Sci. Technol.,1992;10(6):2695–2698.

196. Quemada, D., Rheology of Concentrated Disperse Systems and Minimum Energy Dissipation Principle,Rheol. Acta, 1977;16:82–94.

197. Rangarajan, S., Qi, G., Venkataraman, N., Safari, A., and Danforth, S. C., Powder Processing, Rheology,and Mechanical Properties of Feedstock for Fused Deposition of Si3N4 Ceramics, J. Am. Ceram. Soc.,2000;83(7):1663–1669.

198. Ray, V., Antoniou, N., Bassom, N., Krechmer, A., and Saxonis, A., Development of Void-free FocusedIon Beam-assisted Metal Deposition Process for Subhalf-micrometer High Aspect Ratio vias, J. Vac. Sci.Technol. B., 2003;21(6):2715–2719.

199. Regenfuss, P., Hartwig, L., Klotzer, S., Ebert, R., and Exner, H., Microparts by a Novel Modification ofSelective Laser Sintering, Proc. Rapid Prototyping & Manufacturing, Hyatt regency Chicago, Illinois, USA,12.-15.05.2003, RPA/SME, 2003.

200. Reyntjens, S. and Puers, R., A Review of Focused Ion Beam Applications in Microsystem Technology, J.Micromech. Microeng., 2001;11:287–300.

201. Riehn, R., Charas, A., Morgado, J., and Cacialli, F., Near-field Optical Lithography of a Conjugated Polymer,Appl. Phys. Lett., 2003;82(4):526–528.

202. Rosqvist, T. and Johansson, S., Soft Micromolding and Lamination of Piezoceramic Thick Films, Sensorsand Actuators A, 2002;97–98:512–519.

203. Sachs, E., Cima, M., Williams, P., Brancazio, D., and Cornie, J., Three Dimensional Printing: Rapid Toolingand Prototypes Directly from a CAD Model, J. of Engineering for Industry—Transactions of the ASME,1992;114(4):481–488.

204. Safari, A., Novel Piezoelectric Ceramics and Composites for Sensor and Actuator Applications, Mat. Res.Innovat., 1999;2(5):263–269.

205. Schomburg, C., Hofflinger, B., Springer, R., and Wijnaendts-van-Resandt, R., Economic Productionof Submicron ASICs with Laser Beam Direct Write Lithography, Microelectronic Eng., 1997;35:509–512.

206. Schonholzer, U. and Gauckler, L.J., Ceramic Parts Patterned in the Micrometer Range, Adv. Mater.,1999;11(8):630–632.

207. Scranton, A.B., Bowman, C.N. and Peiffer, R.B., Photopolymerization-Fundamentals and Applications, ACSSymposium Series, American Chemical Society, Washington DC, 1997, Vol. 673.

208. Seerden, K.A.M., Reis, N., Evans, J.R.G., Grant, P.S., Halloran, J.W., and Derby, B., Ink-jet Printing ofWax-based Alumina Suspensions, J. Am. Ceram. Soc., 2001;84(11):2514–2520.

209. Senlis, G., Dubarry, M., Lejeune, M., and Chartier, T., 3D Piezoelectric Structures made by Ink-jet Printing,Ferroelectrics, 2002;273:279–284.

210. Seraji, S., Wu, Y., Jewell-Larson, N.E., Forbess, M.J., Limmer, S.J., Chou, T.P., and Cao, G., PatternedMicrostructure of Sol-Gel Derived Complex Oxides Using Soft Lithography, Adv. Mater., 2000;12(19):1421–1424.

211. Shimoda, T., Morii, K., Seki, S., Kiguchi, H., Inkjet Printing of Light-emitting Polymer Displays, Mat. Res.Soc. Bull., 2003;28(11):821–827.

212. Sigmund, W.M., Bell, N.S., and Bergstrom, L., Novel Powder-processing Methods for Advanced Ceramics,J. Am. Ceram. Soc., 2000;83(7):1557–1574.

213. Slade, C.E. and Evans, J.R.G., Freeforming Ceramics Using a Thermal Jet Printer, J. Mater. Sci. Lett.,1998:17(19):1669–1671.

214. Smay, J.E., Cesarano III, J., and Lewis, J.A., Colloidal Inks for Directed Assembly of 3D Periodic Structures,Langmuir, 2002;18:5429–5437.

215. Smay, J.E., Cesarano III, J., Tuttle, B.A., and Lewis, J.A., Directed Colloidal Assembly of Linear and AnnularLead Zirconate Titanate Arrays, J. Am. Ceram. Soc., 2004;87(2):293–295.

216. http://www.solid-scape.com, July 2004.217. Song, J.H., Edirisinghe, M.J., and Evans, J.R.G., Formulation and Multilayer Jet Printing of Ceramic Inks,

J. Am. Ceram. Soc., 1999;82(12):3374–3380.218. Sotomayor, C.M., Zankovych, S., Seekamp, J., Kam, A.P., Clavijo Cedeno, C., Hoffmann, T., Ahopelto,

J., Reuther, F., Pfeiffer, K., Bleidiessel, G., Grutzner, G., Maximov, M.V., and Heidari, B., NanoimprintLithography: An Alternative Nanofabrication Approach, Materials Science and Engineering C, 2003;23:23–31.


219. http://www.stratasys.com, July 2004.220. Straub, M., Ngyuen, L.H., Fazlic, A., and Gu, M., Complex-shaped Three-dimensional Microstructures and

Photonic Crystals Generated in a Polysiloxane Polymer by Two-photon Stereolithography, Optical Materials,2004;27:359–364.

221. Sun, C. and Zhang, X., Experimental and Numerical Investigations on Microstereolithography of Ceramics,J. Appl. Phys., 2002;92(8):4796–4802.

222. Suzumori, K., Koga, A., and Haneda, R., Microfabrication of Integrated FMAs Using Stereo Lithography,IEEE Proc. Micro Electro Mechanical Systems MEMS’94, 25.-28.01.1994, Oiso, Japan, 1994, pp. 136–141.

223. Sczcech, J.B., Megaridis, C.M., Gamota, D.R., Zhang, J., Fine-line Conductor Manufacturing UsingDrop-on-demand PZT Printing Technology, IEEE Transactions on Electronics Packaging Manufacturing,2002;25(1):26–33.

224. Takagi, T. and Nakjima, N., Photoforming Applied to Fine Machining, IEEE Proc. Micro Electro MechanicalSystems MEMS’93, 07.-10.1993, Fort Lauderdale, FL, USA, 1993, pp. 173–178.

225. Takagi, T. and Nakajima, N., Architecture Combination by Micro Photoforming Process, IEEE Proc. MicroElectro Mechanical Systems MEMS’94, 25.-28.01.1994, Oiso, Japan, 1994, pp. 211–216.

226. Tao, T., Ro, J., Melngailis, J., Xue, Z., and Kaesz, H.D., Focused Ion Beam Induced Deposition of Platinum,J. Vac. Sci. Technol., 1990;8(6):1826–1829.

227. Tay, B.Y. and Edirisinghe, M.J., On Substrate Selection for Direct Ink-jet Printing, J. Mater. Sci Lett.,2002;21:279–281.

228. Tay, B.Y., Evans, J.R.G., and Edirisinghe, M.J., Solid Freeform Fabrication of Ceramics, Int. MaterialsReview, 2003;48(6):341–370.

229. Taylor, C.S., Cherkas, P., Hampton, H., Frantzen, J.J., Shah, B.O., Tiffany, W.B., Nanis, L., Booker, P.,Salahieh, A., and Hansen, R., Spatial Forming-a Three Dimensional Printing Process, IEEE Proc. MicroElectro Mechanical Systems MEMS’95, Amsterdam, The Netherlands, 1995, pp. 203–208.

230. Teng, W.D., Edirisinghe, M.J., and Evans, J.R.G., Optimization of Dispersion and Viscosity of a CeramicJet Printing Ink, J. Am. Ceram. Soc., 1997;80(2):486–494.

231. Teng, W.D., Huneiti, Z.A., Machowski, W., Evans, J.R.G., Edirisinghe, M.J., and Balachandran, W.,Towards Particle-by-particle Deposition of Ceramics Using Electrostatic Atomization, J. Mater. Sci. Lett.,1997;16(12):1017–1019.

232. Teng, W.D. and Edirisinghe, M.J., Development o Ceramic Inks for Direct Continuous Jet Printing, J. Am.Ceram. Soc., 1998;81(4):1033–1036.

233. Tohver, V., Morissette, S.L., Lewis, J.A., Tuttle, B.A., Voigt, J.A., and Dimos, D.B., Direct-write Fabricationof Zinc Oxide Varistors, J. Am. Ceram. Soc., 2002;85(1):123–128.

234. Treyz, G.V., Beach, R., and Osgood Jr., R.M., Rapid Direct Writing of High-aspect Ratio Trenches in Silicon:Process Physics, J. Vac. Sci. Technol., 1988;B6:37–44.

235. Tse, A.L., Hesketh, P.J., and Rosen, D.W., Stereolithography on Silicon for Microfluidics and MicrosensorPackaging, 4th Intern. Workshop on High-Aspect-Ratio-Micro-Structure-Technology, Book of Abstracts,17.-19.06.2001, Baden-Baden, FRG, 2001.

236. Tseng, A.A., Recent Developments in Micromilling Using Focused Ion Beam Technology, J. Micromech.Microeng., 2004;14:R15–R34.

237. Tu, Y. and Chapman, G.H., Bi/In as Patterning and Masking Layers for Alkaline-base Si Anisotropic Etching,Proc. of SPIE, Vol. 2003;4979:87–98.

238. Turcu, S., Jadidian, B., Danforth, S.C., and Safari, A., Piezoelectric Properties of Novel Oriented Ceramic-polymer Composites with 2-2 and 3-3 Connectivities, J. Electroceramics, 2002;9:165–171.

239. Upcraft, S. and Fletcher, R., The Rapid Prototyping Technologies, Assembly Automation, 2003;23(4):318–330.

240. Vaidyanathan, R., Walish, J., Lombardi, J.L., Kasichainula, S., Calvert, P., and Cooper, K.C., The Ex-trusion Freeforming of Functional Ceramic Prototypes, JOM—J. Min. Met. Mat. Soc., 2000;52(12):34–37.

241. Vasile, M.L.J., Nassar, R., and Xie, J., Focused Ion Beam Technology Applied to Microstructure Fabrication,J. Vac. Sci. Technol., 1998;16(4):2499–2505.

242. Venkataraman, N., Rangarajan, S., Matthewson, M.J., Harper, B., Safari, A., Danforth, S.C., Wu, G.,Langrana, N., Guceri, S., and Yardimci, A., Feedstock Material Properties—Process Relationships in FusedDeposition of Ceramics (FDC), Rapid Prototyping Journal, 2000;6(4):244–252.


243. Vogt, C., Bertsch, A., Renaud, P., and Bernhard, P., Methods and Algorithms for the Slicing Process inMicrostereolithography, Rapid Prototyping Journal, 2002;8(3):190–199.

244. Walker, J.F., Moore, D.F., and Whitney, J.T., Focused Ion Beam Processing for Microscale Fabrication,Microelectron. Eng., 1996;30(1–4):517–522.

245. Walker, C., Laser Micromachining of Silicon: A New Technology for Fabricating THz Imaging Arrays, takenfrom http://soral.as.arizona.edu/micromachining.html (July 2004).

246. Wang, G. and Krstic, V.D., Rapid Prototyping of Ceramic Components—Review, J. Canadian Ceram. Soc.,1998;67(3):52–58.

247. Wehl, W. and Wild, J., Lemmermeyer, B., A drop-on-demand Metal Jet Printer for Wafer Bumping, 14th Euro-pean Microelectronics and Packaging Conference & Exhibition, 23.-25.06.2003, Friedrichshafen, Germany,2003 (available at http://www.mm.fh-heilbronn.de/wehl/files/EMPC2003-Metal-Jet.pdf).

248. Wijnaendts-van-Resandt, R.W. and Buchner, C., Super Resolution Lithography Using a Direct Write LaserPattern Generator, Proc. of SPIE, 1994, Vol. 2213, pp. 18–23.

249. Wohlers, T., Wohlers Report, Wohlers Associates Inc., 2000.250. Woias, P., Ein Verfahren zur schnellen Herstellung von Mikrostrukturen, German Patent Application 101 54

361.1-33, 2001.251. Wu, M., Zhao, W., Tang, Y., Li, D., and Lu, B., A Novel Stereolithography Technology with Conventional

UV Light, Rapid Prototyping Journal, 2001;7(5):268–274.252. Wu, G., Langrana, N.A., Sandanji, R., and Danforth, S., Solid Freeform Fabrication of Metal Components

Using Fused Deposition of Metals, Materials & Design, 2002;23(1):97–105.253. Xia, Y. and Whitesides, G.M., Soft Lithography, Angew. Chem. Int. Ed., 1998;37(5):550–575.254. Xiang, Q.F., Evans, J.R.G., Edirisinghe, M.J., and Blazdell, P.F., Solid Freeforming of Ceramics Using a

Drop-on-demand Jet Printer, Proc. Instn. Mech. Engrs. Part B, J. of Eng. Manufacture, 1997, Vol. 211,No. 3, pp. 211–214.

255. Yagyu, H., Sugano, K., Hayashi, S., and Tabata, O., Rapid Prototyping of Glass Chip with Micro-powder Blasting Using Nano-particles Dispersed Polymer, IEEE Proc. Micro Electro Mechanical SystemsMEMS’2004, 25-29.01.2004, Maastricht, The Netherlands, 2004, pp. 697–700.

256. Yamagutchi, K. and Nakamoto, T., Micro Fabrication by UV Laser Photopolymerization, Memoirs of theSchool of Engineering, Nagoya University, 1999;50(1/2):33–82.

257. Yamaguchi, K., Sakai, K., Yamanaka, T., and Hirayama, T., Generation of Three-dimensional Micro StructureUsing Metal Jet, Precision Engineering, 2000;24(1):2–8.

258. Yamaguchi, K., Generation of 3-dimensional Microstructure by Metal Jet, Microsys. Techn., 2003;9:215–219.

259. Yamazaki, K. and Namatsu, H., Three-Dimensional Nanofabrication (3D-NANO) Down to 10-nmOrder Using Electron-beam Lithography, IEEE Proc. Micro Electro Mechanical Systems MEMS’2004, 25-29.01.2004, Maastricht, The Netherlands, 2004, pp. 609–612.

260. Yang, H., Deschatelets, P., Brittain, S.T., and Whitesides, G.M., Fabrication of High Performance Ce-ramic Microstructures from a Polymeric Precursor Using Soft Lithography, Adv. Mater., 2001;13(1):54–58.

261. Zaugg, F.G. and Wagner, P., Drop-on-demand Printing of Protein Biochip Arrays, Mat. Res. Soc. Bull.,2003;28(11):837–842.

262. http://www.zcorp.com, July 2004.263. Zein, I., Hutmacher, D.W., Tan, K.C., and Teoh, S.H., Fused Deposition Modeling of Novel Scaffold Archi-

tectures for Tissue Engineering applications, Biomaterials, 2002;23:1169–1185.264. Zhang, X., Jiang, X.N. and Sun, C., Micro-stereolithography of Polymeric and Ceramic Microstructures,

Sensors and Actuators A, 1999;77:149–156.265. Zhang, D., Su, B., and Button, T.W., Microfabrication of Three-Dimensional, Free-Standing Ceramic MEMS

Components by Soft Moulding, Adv. Eng. Mat., 2003;5(12):924–927.266. Zhao, X., Evans, J.R.G., Edirisinghe, M.J., Song, J.H., Ceramic Freeforming Using an Advanced Multinozzle

Ink-jet Printer, J. Mat. Synthesis and Processing, 2001;9(6):319–327.267. Zhao, X., Evans, J.R.G., Edirisinghe, M.J., and Song, J.H., Direct Ink-jet Printing of Vertical Walls, J. Am.

Ceram. Soc., 2002;85(8):2113–2115.268. Zhao, X., Evans, J.R.G., Edirisinghe, M.J., and Song, J.H., Ink-jet Printing of Ceramic Pillar Arrays, J. Mater.

Sci., 2002;37(10):1987–1992.


269. Zhao, X., Evans, J.R.G., Edirisinghe, M.J., and Song, J.H., Formulation of a Ceramic Ink for a Wide-arraydrop-on-demand Ink-jet Printer, Ceramics International, 2003;29(8):887–892.

270. Zhou, M.Y., Xi, J.T., and Yan, J.Q., Modeling and Processing of Functionally Graded Materials for RapidPrototyping, J. Mater. Processing Technol., 2004;146:396–402.

271. Zissi, S., Bertsch, A., Jezequel, J.Y., Corbel, S., Lougnot, D.J., and Andre, J.C., Stereolithography andMicrotechniques, Microsys. Techn., 1997;2(2):97–102.

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