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DOI: 10.1002/adfm.200700140

3D Micro- and Nanostructures viaInterference Lithography**

By Ji-Hyun Jang, Chaitanya K. Ullal,Martin Maldovan, Taras Gorishnyy, Steven Kooi,CheongYang Koh, and Edwin L. Thomas*

1. Introduction

The importance of structure and its relation to function andproperties is a central tenet of materials science. As interest instructures on the “nano” and submicron scale continues togrow, the promise of the ability to “dial-in” particular symme-tries and geometrical elements into structures on these lengthscales raises the prospect of rational design. Optical lithogra-phy has been uniquely successful in the ability to create 2D

patterns spanning the range of ten nanometers to several mi-crons and is the key technology enabling the extension ofMoore’s law and the continued revolution in computationalpower. The fabrication of three-dimensional structures on thislength scale is a much more challenging prospect.

A number of techniques have been proposed to address thischallenge, each with its own set of advantages and disadvan-tages. We can broadly classify the fabrication approaches into“self-assembly based” techniques and “construction based” as-sembly. Self-assembly relies on the use of thermodynamicforces to spontaneously pattern components into stable struc-tures[1–6] whereas construction based techniques require thepiece by piece creation or placement of the components intothe appropriate structure. Self-assembly usually holds thepromise of achieving large area coverage in a short time and inan inexpensive fashion. However, self-assembly approachesare frequently plagued by the presence of defects. In addition,the number of configurations that are easily accessible by thistechnique is limited and determined by the subtle interplay ofmany relatively weak types of forces. Construction based ap-proaches allow for the fabrication of arbitrary structures. How-ever, the need to construct the structures in a serial manner,either point-by-point or layer-by-layer, make these time con-suming processes[7–16] and require attention to registrations andjoining of the component parts. The technique of holographicinterference lithography is unique inasmuch that it allows for

Adv. Funct. Mater. 2007, 17, 3027–3041 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3027

Interference lithography (IL) holds the promise of fabricating large-area, defect-free3D structures on the submicrometer scale both rapidly and cheaply. A stationary spatialvariation of intensity is created by the interference of two or more beams of light. The pattern that emerges out of theintensity distribution is transferred to a light sensitive medium, such as a photoresist, and after development yields a3D bicontinuous photoresist/air structure. Importantly, by a proper choice of beam parameters one can control thegeometrical elements and volume fraction of the structures. This article provides an overview of the fabrication of3D structures via IL (e.g., the formation of interference patterns, their dependence on beam parameters and severalrequirements for the photoresist) and highlights some of our recent efforts in the applications of these 3D structuresin photonic crystals, phononic crystals and as microframes, and for the synthesis of highly non spherical polymerparticles. Our discussion concludes with perspectives on the future directions in which this technique could bepursued.

–[*] Prof. E. L. (Ned) Thomas, Dr. J.-H. Jang, Dr. C. K. Ullal,

Dr. M. Maldovan, Dr. T. Gorishnyy, Dr. S. Kooi, C. KohDepartment of Materials Science and EngineeringInstitute for Soldier NanotechnologiesMassachusetts Institute of Technology77 Massachusetts Ave., Bldg 6 Room 113Cambridge, MA 02139 (USA)E-mail: [email protected]

[**] M.M., C.K.U., and J.-H.J. contributed equally to all the research men-tioned in our Feature Article. We thank Prof. Christopher Ober of Cor-nell University and Prof. Shu Yang of the University of Pennsylvaniafor helpful discussions. This work is supported in part by the Institutefor Soldier Nanotechnologies of the U.S. Army Research Office (un-der contract DAAD-19-02-0002) and the National Science FoundationGrant No DMR-0414974.

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control over the detailed geometry ofthe structures being fabricated, whileit has inherent registration and com-ponent joining resulting in the rapidcreation of large area single crystalstructures.

IL is a technique that allows oneto create 1D, 2D, and 3D periodicpatterns very simply using coherentbeams of light.[17–21] The inherent peri-odicity present in the light is exploitedto create structures using this tech-nique. Essentially IL involves the for-mation of a stationary spatial variationof intensity created by the interferenceof two or more beams of light. Thepattern that emerges out of the inten-sity distribution is transferred to alight sensitive medium, such as aphotoresist, to yield structures. A de-tailed comparison of the self-assemblybased, construction based and IL fab-rication capabilities is summarized inTable 1.

2. Controlling Symmetries andStructure in 3D Using HIL

The electric field associated with amonochromatic plane wave can be described mathematicallyas:

!Em !r" t! " # !E0"mei !k$!r%xt&!m! "

where m is the index identifying the particular beam, !E0 is thewave amplitude and direction of polarization, !k is the wavevector, x is the angular frequency, and ! is the phase. The in-tensity distribution created by a set of beams is proportional tothe square of the magnitude of the resultant vector sum. Sincethe polarization associated with an electromagnetic wave neednot be linear, but can be circularly or elliptically polarized aswell, the intensity is arrived at by the inner product of the elec-

tric field with its complex conjugate. From this equation we seethat the interference pattern has only a spatial and time-invari-ant variation.

I !r! " #!n

m#1!n

l#1!El!E'

mei !kl%!km! "$!r&!l%!m! "

Note that I(!r) is simply the sum of sinusoidal terms. Thus, inorder to understand the extent to which IL allows control ofthe geometry and the nature of the resultant patterns, it is use-ful to view the intensity equation as a discrete Fourier sum.The intensity distribution has its translational periodicity deter-mined by the difference between the wave vectors !kl –!km ofthe interfering beams while the polarizations, represented by a

3028 www.afm-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 3027–3041

Ned Thomas’ research interests include polymer physics and engineering of the mechanical and op-tical properties of block copolymers, liquid crystalline polymers, and hybrid organic-inorganicnanocomposites. Currently he serves as the Department Head of Materials Science and Engineer-ing and as Founding Director, Strategic Planning for the Institute for Soldier Nanotechnologies atMIT. He and others from MIT co-founded OmniGuide Inc., in Cambridge. Before coming toMIT, he founded and served as co-director of the Institute for Interface Science and was head ofthe Department of Polymer Science and Engineering at the University of Massachusetts. Thomas isthe recipient of the 1991 High Polymer Physics Prize of the American Physical Society and the1985 American Chemical Society Creative Polymer Chemist Award. He was elected a Fellow of theAmerican Physical Society in 1986 and a Fellow of the American Association for the Advancementof Science in 2003. He has written the undergraduate textbook The Structure of Materials, hascoauthored over 350 papers and holds twelve patents.

Table 1. A comparison of different types of 3D microfabrication techniques

Technique Advantages Drawbacks

Self-assembly-based approach

Block copolymerself-assembly [1,2]

Small size scaleFast

Large areas

Limited volume fraction rangeUnintentional defects

Difficult to control geometricalparametersColloidal self-assembly [3,4] Large areas

Wide range of sizescale

Low cost

Construction-basedapproach

Layer by layer conventionalsemiconductor

manufacturing [7,12]

Large areafabrication

Low defect density

ExpensiveTime intensive

Structures are built by layersto approximate a 3D structure

Limited material platforms

Two-photon lithography [13,14] Arbitrary structure SlowSmall area coverage problem

3D printing / Direct writing [11] Arbitrary structure Resolution issue at smaller lengthscaleSlow

Small area coverage problem

Micromanipulation [15,16] Low defect density Very slow

Interferencelithography

Multibeam interferencelithography [17–21]

Large areaDefect free

Control overgeometry

Can not make arbitrary structureComplex optical setup

Phase mask lithography [22,23] Large areaDefect free

Simple setup

Can not make arbitrary structureCorrelation between mask element anddesired structure not fully established

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J.-H. Jang et al./3D Micro- and Nanostructures via Interference Lithography

set of complex electric field vectors, determine the pattern ormotif placed within the unit cell. The combination of the motifand the translational periodicity determines the full set of sym-metries associated with, and hence the space group, of the re-sultant structure.[24,25]

Any arbitrary structure can be expressed as a sum of Fourierterms each of which could correspond to a grating arising fromthe interference of two beams. For each grating the magnitudeof the relative phase amplitudes fixes the position of the originalong the direction of its periodicity. One of the current pri-mary challenges in IL is to simply and reproducibly achieveregistration between independent gratings. There are two ap-proaches currently used to tackle this registration problem.The first is to restrict the number of beams used so that regis-tration is assured. The other approach is to use a phase mask togenerate the set of beams and, if necessary employ multiple ex-posures.

In general, the interference of two, three, and four beams ofcoherent light results in one, two and three dimensionally peri-odic patterns. A change in phase of one of the interferingbeams results in a shift in the pattern commensurate with thedimensionality of the pattern. Since the use of five beams ingeneral would create a pattern that is periodic in four-space, ashift in phase would result in a translation in four-space, andcreate a completely new pattern in three dimensions. Thus ifwe do not ensure registration between beams through anyother means, we are restricted to the use of four beams of light.The interference of four beams of light with arbitrary direc-tions, amplitudes and phases results in a maximum of thirteenterms (4 terms are fixed constants, so can be combined as a sin-gle constant) of which there are six terms along distinct direc-tions (some terms only differ from each other by p/2 in phase).The set of thirteen terms clearly places a restriction on the sortof structures one can form through a four-beam approach. Giv-en this restriction it is useful to try to develop a crystallo-graphic approach that allows one to target particular symme-tries and space groups.

We first turn our attention to the translational periodicity as-sociated with these structures. As with all translationally peri-odic structures the minimum set of vectors that we need to con-sider is the set of basis vectors, since the remaining vectors canbe achieved by linear combinations. In two dimensions thereare five lattice nets, while in three dimensions there are four-teen Bravais lattices. Thus in order to obtain a desired transla-tional periodicity it is necessary to equate the difference be-tween the wave vectors with basis vectors associated with thattranslational periodicity. One way to do this is to set the firstwave vector, !k0 to the reciprocal lattice vector that is equidi-stant from the origin and the basis vectors !bm. The remainingwave vectors are then simply given by !km =!bm +!k0. Since thereare an infinite number of choices of basis vectors for a particu-lar type of translational symmetry, we can vary the size of theunit cell simply by changing the angle between the beams. Ob-viously the control of the lattice size is limited by the span ofthe mutual angles between the beams and is not continuous. Alist of wave vectors for the translational periodicities associatedwith the fourteen Bravais lattices can be found in ref. [26].

To guarantee a particular space group, we need to make asuitable choice of !k and !E and to impose the set of symmetryoperators for that space group, as listed in the International Ta-bles of Crystallography. The generalized intensity equation canbe taken and the symmetry operations corresponding to thegenerators (the minimum subset of symmetry operators) im-posed on it to yield a set of conditions for a final target equa-tion. This target equation must be closely examined to verifythat it belongs to the desired space group and does not possessadditional symmetry elements that could place it in anothersuper group. This target equation is then compared to the gen-eralized intensity equation and the resultant set of simulta-neous equations solved, to yield the beam parameters that arerequired to obtain a structure with the desired space group.

A simpler way to obtain the target equation to which we cancompare the intensity equation is to remember that we aredealing with Fourier transforms of the space group in consid-eration. An obvious place to look for such target equations isthus the diffraction patterns associated with these groups. Sucha ‘level-set’ approach[27,28] uses the structure factor of the cho-sen space group to obtain candidate functions that possess therequisite symmetries. The structure factor describes the ampli-tudes and phases of the three-dimensional diffraction patterndue to the scattering of incident radiation from the planes (hkl)of atoms in the crystalline structure. Not only does the struc-ture factor equation save us the trouble of building the targetequation, but such an approach also has an important physicalinsight. The addition of higher order structure factor terms fillsWyckoff sites of greater multiplicity and decreasing point sym-metry and leads to structures of increasing topological com-plexity.[27]

Two important structures our group has successfully fabri-cated along with the particular beam conditions to achievethem are summarized in Table 2.[29]

The ability to create structures using an arbitrary number ofbeams is particularly exciting. If multiple gratings, each with adifferent periodicity can be reproducibly registered with re-spect to one another in space, then we could potentially accessany arbitrary structure by writing it out as a sum of its Fourierterms. The fundamental challenge in this case is an engineeringone, viz. controlling the relative phases of a large number ofbeams. A first approach would be to control the phase of eachindividual beam using a setup such as a delay stage or a phaseretarder that does not introduce a change in polarization. Sucha scheme can be used to ensure that all the gratings share thesame origin. Alternatively, if the phase of each of the individualgratings is known, a stage can be used to move the samplearound to achieve the appropriate registration. These are how-ever not straightforward tasks since once the optical path is set,all the elements in the optical path must remain stationary andthe sample placed reproducibly with very high precision. Onesimple approach to circumvent resorting to such a setup is touse a phase mask.[23] A phase mask is a surface relief diffrac-tion grating from which a set of outgoing beams (at specific an-gles related to the geometry of the mask) is created from a sin-gle incident beam (plane wave).[22] It has favorable processconditions compared to multibeam interference lithography

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for integrated optics platforms since the beams all come fromthe same half space. Importantly the phase difference betweenthe different beams as they leave the grating is determined bythe grating. Currently, while it is straightforward to computethe resultant 3D structure for a particular phase mask, there isno simple and rigorous means to design the phase mask in or-der to target a particular desired structure.

3. Practical Issues in IL

While one of the primary draws of IL is the simplicity asso-ciated with being able to obtain a particular set of symmetries(space group), while retaining control over the detailed geome-try, there are practical issues related to the fabrication tech-nique that are worthwhile to keep in mind. We briefly examinesome of the issues related to the optics and materials platformsthat are employed.

There are two broad approaches currently employed in orderto achieve beam configurations that yield 3D interference pat-terns. We introduce both, and discuss the practicalities of themultibeam IL technique we have previously been more in-volved with in more detail.

3.1. Phase Mask IL

An elegant implementation of the phase mask approach de-scribed at the end of the last section is to use a conformableelastomeric phase mask,[23] which ensures that the beams al-ways travel a repeatable optical path for different exposures. It

has favorable process conditions for inte-grated optics platforms compared to multi-beam interference lithography in that thebeams all come from the same half space. Theelastomeric phase mask is placed in contactwith a film of photoresist and light is incidenton the phase mask. The diffracted beams theninterfere within the volume of the photoresistto create the structure. Figure 1a shows thefabrication process of elastomeric conformalphase masks and Figure 1b is the schematicdiagram of the lithographic setup showing the3D interference pattern generated when abeam is incident on the phase mask. A reali-zation into SU-8 of a 3D structure createdusing a 2D phase mask is shown in Figure 1c.

3.2. Multibeam IL (Free Space IL)

In free space IL the required interferencepattern is created by first assembling colli-mated, coherent laser beams with beam pa-rameters appropriate for the desired targetedstructure and then interfering them within thevolume of a photoresist. In such a setup onelaser beam is typically divided into multiplebeams using beam-splitters. The beams are

then recombined by mirrors to obtain the desired geometry.The polarization and intensity of the beams are controlled bywave plates and polarizer beam splitters. Given the difficultyof controlling phase in a free space setup the configurationsemployed are usually restricted to four beams or less. An over-view of the optical components for the multibeam IL setup, ex-perimental setup with prism and the realization into SU-8 areshown in Figure 2.

The preservation of beam directions and polarizations is animportant consideration in the experimental setup. Upon en-tering the photoresist the beams are refracted and the polariza-tion can change due to variations in the reflectivity of the TEand TM components that make up the incoming wave. The uti-lization of an appropriately shaped refractive index matchedprism with surfaces normal to the incoming beams can easilycircumvent this issue,[29,30] with the caveat that every new struc-ture would require a new light-coupling prism as shown in Fig-ure 2b. There has been some experimental simplification tothis, for example, by using specially-angled prisms to generatethe required beam directions for the desired interference pat-tern.[31,32] In such fabrication procedures, only a single incom-ing beam is utilized and the requisite multiple beams are thengenerated via refraction through the prism. This approach of-fers several advantages from a practical mass production stand-point, as it reduces possible aberrations due to external vibra-tions, compared with having individually controlled beams aswell as reducing experimental errors involved in the alignmentof individual beams. However, this approach has drawbacks interms of control. It is known that the polarizations of the emer-gent beams depend on the incident polarizations and on the an-


Table 2. Beam parameters for 3D IL structures.

Space Group/Structure

R3̄m/3 term “Diamond-like” structure

Pm3̄m(221)/Schwarz P surface

Intensity Equation I(r)= 12 + cos(2p(-x+y+z)/k) +cos(2p(x-y+z)/k)+ cos(2p(x+y-z)/ k)

I(r)= 7.4 + cos(2px/a)+cos(2py/a)+cos(2pz/a)

kvectors

k0 = 2p/k [0.577 0.577 0.577]k1 = 2p/k [0.761, 0.614, 0.209]k2 = 2p/k [0.209, 0.761, 0.614]k3 = 2p/k [0.614, 0.2094 0.761]

k0 = 2p/k [–0.970 –0.243 0]k1 = 2p/k [–0.970 0.243 0]k2 = 2p/k [0 –0.970 0.243]k3 = 2p/k [0 –0.970 –0.243]k4 = 2p/k [–0.243 0 –0.970]k5 = 2p/k [0.243 0 –0.970]

Evectors

E0 = 3[0.789, –0.211, –0.577]E1 = [0.530, –0.404, –0.746]E2 = [0.94291, 0.009, –0.3329]E3 = [0.722, –0.538, –0.434]

E0 = [0 0 1] E1 = [0 0 1]E2 = [1 0 0] E3 = [1 0 0]E4 = [0 1 0] E5 = [0 1 0]

Perspective viewof structure 2 × 2 × 2with unit cells

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gles of the angled prism faces.[33] In other words, the emergentpolarizations, which determine the motifs of the structure, andthe lattice parameters, which depend on the angles betweenthe beams, are coupled and cannot be varied freely. Phase

shifts between the beams are also not controllable, nor remov-able in this approach. Thus, this approach is suitable for struc-tures in which the motif complexity is not as crucial and losesthe ability for the rational design of structures.


Figure 1. Schematic illustration of phase mask lithography: a) General fabrication process of conformal phase mask; b) Lithographic setup; c) Realiza-tion into SU-8. Upper left is a SEM image of a PDMS phase makes comprised of an array of circular holes, 280 nm in diameter and 450 nm high on a750 nm square lattice. SEM of the resultant 3D structure created from the mask and the computed theoretical intensity distribution for a 2 × 2 × 2 array ofunit cells.

Figure 2. Schematic illustration of multibeam IL: a) A review of several optical components for multibeam IL; b) 6 beam lithographic setup with prism;c) SEM image of P surface fabricated in SU-8 with the conditions described in Table 2. Lower left is a computed theoretical intensity distribution for a2 × 2 × 2 array of unit cells. Reproduced with permission from [92]. Copyright 2007 American Chemical Society.

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A similar consideration is the back reflection of the exposinglight from the substrate. This can be eliminated by using atransparent refractive index matched substrate. In the con-straint that the substrate is opaque, an anti-reflection coatingcan be utilized to minimize the back reflection. Since it is notnecessary that the beam configuration be such that the beamsmake equal angles of incidence with the substrate it might benecessary to optimize the anti-reflection coating, and or use anabsorbing layer to minimize back reflections. A further consid-eration in the case of an opaque substrate is the requirementthat all the beams come from the same half-space. In order tobe able to fabricate some promising structures such as the dia-mond structure, it is necessary to use beams with k vectors thatdo not come from the same half space. This precludes the useof a non-transparent substrate and thus restricts the number ofstructures that are accessible. This problem can be circum-vented in many cases by accessing structures that retain mostof the Fourier terms, and properties, of the desired structure,which at the same time can be achieved by beams which arelaunched from the same half space.[29] For example, if we dropthe sinusoidal term along the <111> direction from the equa-tion of the diamond structure, the resultant structure still re-tains its bandgap. The beams used to make this structure bysingle exposure now all come from the same half space as de-scribed in Table 1 as the 3-term “Diamond-like” structure.

Utilization of both continuous and pulsed lasers has beendemonstrated. Continuous wave systems are generally easierto work with in setups, where the matching of path lengths,such as with the use of a phase plate is not guaranteed. How-ever, the fabrication of thicker samples with a continuous wavesystem requires the use of a photoresist platform that does notundergo a refractive index change during the relatively longerexposure time (e.g., 5 min). For this reason a sufficiently shortpulse width high intensity laser would potentially allow accessto a wider variety of materials systems.[34,35]

Most laser systems have Gaussian beam profiles, which leadsto a significant variation in the light intensity between the cen-ter and the periphery of the beam. This intensity variationcauses a spatial variation in the volume fraction of the resultantstructure. There are two approaches to eliminate this issue:(1) expand the beam sufficiently such that the variation in vol-ume fraction is within tolerable limits (this unfortunately weak-ens the intensity and increases exposure time), and (2) convertthe Gaussian profile into a top hat function via such means as arefractive beam shaper.[36]

Material platform related issues are currently of greater con-cern and interest and we will deal with them more in-depthhere. In general, photoresists that are used in conventional li-thography can also be used for interference lithography. Theyare usually divided into two groups: negative resists and posi-tive resists.

A negative resist is a photoresist in which the regions ex-posed to light become insoluble to the developer. This insolu-bility can be achieved by either 1) an increase in molecularweight, or 2) photochemical rearrangement to form new insol-uble products. To increase molecular weight, photo-initiatorsare generally used that can generate free radicals or strong

acids to facilitate polymeric cross-linking or the photopolymer-ization of monomeric or oligomeric species. Without an in-crease in molecular weight, negative patterns can be achievedby the photochemical formation of hydrophobic or hydrophilicgroups which provide differential solubility between the ex-posed and unexposed regions of the resist film. One commonnegative photoresist for interference lithography is SU-8, anepoxy based monomer that undergoes cationic photopolymeri-zation. It has many advantages, such as chemical amplification,which increases the sensitivity; mechanical robustness, whichallows access to high aspect ratio structures; and wide process-ing latitude with respect to radiation wavelengths.[37,38] Formany optical and mechanical applications it is desirable to in-filtrate IL patterned polymeric structures with other materials(for example high refractive index materials for photonic crys-tals) and then to remove the original polymer structure. How-ever, removal of highly cross-linked polymers templates (whichare usually obtained if negative resists are used) is difficult andmay require rather extreme processing such as resist burningor plasma etching.[39,40] This can result in damage to the materi-al that was back filled into the template and hence negative re-sist removal has been a bottle neck for the fabrication of 3Dtemplates.

A positive resist is a photoresist in which regions exposed tolight become soluble to the developer, while unexposed regionsremain insoluble. Many positive photoresists for 365 nm(I-line) and 436 nm (G-line) radiation can also be consideredas candidates for IL with UV irradiation.[41,42] A commonlyused positive resist is composed of diazonaphthoquinone(DNQ) and novolac resin (a phenol formaldehyde resin). Thephenolic resin is highly soluble in basic solution and has excel-lent film forming properties. DNQ acts as both photosensitizerand dissolution inhibitor. Upon exposure, DNQ undergoes mo-lecular rearrangement generating a carboxylic acid, and the ex-posed area becomes soluble in basic developers, resulting in apositive image in regions of high light intensity. Positive re-sists[43] have the advantage over negative resists in that they donot undergo shrinkage from cross-linking and can be easily re-moved after an infiltration step by dissolution in a 2nd develo-per treatment. A comparison of the fabrication process withpositive and negative resists that we have used in our group’sresearch is shown in Figure 3a and schematic diagrams of thebasic chemical mechanisms responsible for the function of twodifferent types of photoresists are shown in Figure 3b and c.

Next some important considerations on material platformsthat must be taken into account while choosing appropriatephotoresists for interference lithography are discussed. SU-8and DNQ-novolac resists are again used as examples of com-mon negative and positive resists.

3.2.1. Transparency at the Exposure Wavelength

The strong absorption of photoactive compounds (PACs) inphotoresist formulations can prevent the light from reachingthe bottom of the resist film, leading to variations in volumefraction of the developed structure in the direction of the thick-ness of the film. The transparency of the resist at the wave-


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length of exposure is thus an important consideration in ensur-ing a uniform structure through the thickness of a resist filmand determines the practical exposable thickness of the resist.Strong absorption also results in diminished photoresist sensi-tivity.

Since SU-8 is a chemically amplified resist (CAR), in whicha single incident photon can catalyze many chemical eventsleading to very high resist sensitivity, therefore only a smallamount of highly absorbing photoinitiator or sensitizer isneeded. Figure 4a shows the absorbance of commercial SU-8


a

b c

Triaryl sulfonium salt(PAG)

novolac resin DNQ(sensitizer & dissolution inhibitor)

i) Chemical composition of AZ5214-E

i) Chemical composition of SU-8 resist

SU-8 Resin

ii) Photochemical reaction mechanism of SU-8 resist

SU-8 crosslinked SU-8H+

H+SbF6-

Negative Resist

ii) Photochemical reaction mechanism of AZ5214-E

Positive Resist

soluble in base soln

insoluble in base solnWolff

rearrangement

Figure 3. Schematic diagram of the 4 beam interference lithography process that creates the R̄3m 3D pattern a) and general photochemistry schemes forexposure of two different types of common photoresists. b) SU-8 negative resist; c) DNQ-novolac positive resist. Reproduced with permission from [44].Copyright 2006 American Chemical Society.

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resist (Microchem Chemicals) composed of derivatives of theeight epoxy bisphenol-A-novolac base resin with triaryl sulfo-nium salt added as a photoacid generator (PAG).[45] SinceSU-8 is transparent in the near-UV and visible regions of theelectromagnetic spectrum but highly opaque in the deep-UVregion, the use of thick films of SU-8 photoresist in lithograph-ic technologies utilizing < 350 nm light has been hampered bythe absorbance of the resist itself.

Most positive resists based on novolac and DNQ for I-lineand G-line exposure are typically non-CARs. The amount ofDNQ present in the photoresist resin is about 30 %. The ab-sorption of DNQ systems in the UV region decreases upon ex-posure due to the conversion of the compound into indene-car-boxylic acid photoproducts with increasing exposure. This“photobleaching” effect allows for the propagation of the lightthrough the bottom of the resist with minimum loss as the ex-posure proceeds (Fig. 4b).[46] Most DNQ derivatives are poorlyphotobleached in the wavelength region longer than 480 nm.For example in the case of the photoresist AZ5214E, as a con-sequence of this non-bleaching problem, the thick-ness of the film we could successfully use was re-stricted to 3 lm at 532 nm exposure.[44]

Polysilane polymers which possess a backbone con-sisting of silicon atoms are a promising class of posi-tive photoresists. Upon exposure to ultraviolet (UV)radiation in air, photoinsertion of oxygen takes place.The resultant introduction of Si–O–Si and Si–OHbonds induces changes in the nature of the polysilanefilms, such as their solubility and wettability. It hasbeen proposed that the photodecomposition of theorganosilane backbone leads to its photobleachingby UV irradiation and is attributed to the shorteningof a-conjugation of the Si backbone due to the Si–Sibond scission or Si–O bond formation.[47]

3.2.2. Good Adhesion to Substrate

The surface oxide of most substrates (e.g., siliconwafer or glass) forms extensive hydrogen bonds withwater adsorbed from the air. When a resist is spunonto such a surface, it interacts with the water rather

than the surface, resulting in poor adhe-sion. Incorporating some hydrophilic moi-ety such as a hydroxyl group into hydro-phobic resist formulations can improve theadhesion of the polymer to the substrate.

Commercially available SU-8 is suffi-ciently hydrophilic to allow film formationon common substrates by spin coating.However, even though the resist has suffi-cient hydrophilicity to be spin-coated, po-larity changes and physical stresses asso-ciated with shrinkage after crosslinking aswell as thermal stress created during thefabrication process may cause delamina-tion of the film during the developing step.To improve the adhesion between the po-

rous film of crosslinked SU-8 and the substrate, a buffer layercan be spun on the substrate first, flood exposed and post-expo-sure baked. This is followed by spin coating the imaging layerfor the 3D structure, which prevents delamination by inducingchemical grafting of the patterned layer onto the buffer layer.

Novolac-DNQ phenolic resin has excellent film formingproperties on polar substrates because of the hydroxyl group.Additional treatments, for example the utilization of adhesionpromoters such as hexamethyldisilazane (HMDS) or trichloro-phenylsilane (TCPS) can improve the adhesion of some rela-tively hydrophobic resists onto the substrate by increasing thehydrophobicity of substrate surface. This is a result of the for-mation of siloxane linkages (Si–O–Si) between the polar sur-face and chemically active group within the silane primerwhich is described in Figure 5. The newly formed terminationon the substrate renders the surface more hydrophobic in char-acter and hence, affords greatly improved adhesion by thephotoresist, making the silicon substrate surface highly com-patible with both positive and negative photoresists.


Figure 4. UV absorbance spectra: a) SU-8 resist [93]; b) DNQ-novolac system. Reproduced withpermission from [42]. Copyright 1994 American Chemical Society.

Figure 5. Schematic diagram demonstrating surface changes with treatment byHMDS. Reproduced with permission from [41]. Copyright 1993 SPIE.

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3.2.3. High Contrast of I(r)

In multi-beam interference, an inherent DC offset in thelight intensity can result in insufficient contrast which canmake it experimentally challenging to find exposure conditionsfor which the resultant developed structure is bicontinuous.Contrast can be maximized by two independent approaches.The first is to optimize beam polarizations to minimize the DCoffset while preserving the desired symmetry of the final struc-ture. The second approach is by the addition of an appropriateamount of organic base into the SU-8 photoresist formula-tion.[48] Since the negative photoresist platform is a photoini-tiated cationic polymerization system, the base neutralizes acontrolled amount of the photoacids generated during expo-sure in spatially homogeneous manner, thereby reducing oreliminating the DC offset and hence enhancing the contrast.

3.2.4. Refractive Index

A change in refractive index of the photoresist during the ex-posure in continuous wave (cw) mode is not desirable since thiscan result in the perturbation of the incident light and causedeviations from the desired interference pattern. SU-8 doesnot exhibit any detectable change in refractive index even afterthe hard-baking process (n = 1.59 for crosslinked and uncross-linked films at 633 nm).[49] The refractive index of the DNQ/novolac resists is n = 1.66 at 532 nm and is not changed detecta-bly during exposure as well.[44] As discussed earlier, pulsed la-ser systems are expected to allow access to a wider window ofmaterials because the short duration of the exposure (! 8 ns)occurs at time scales much shorter than for PAG generation viadiffusion.

3.2.5. Thermal Stability

Normally three types of bake steps are applied during thelithographic process: Prebake, post exposure bake (PEB) andhard bake. The recommended temperature and time of eachstep depends on the photoresist and the properties of the resistresulting from the bake step. Prebake is usually low tempera-ture bake step after spin-coating of photoresist on the sub-strate. This is done to 1) evaporate the solvent from the spun-on resist; and 2) anneal out the stresses resulting from the spin-coating. PEB is one of the critical pattern transfer steps anddone to 1) drive diffusion of the photoactive compound; and2) drive the acid-catalyzed reaction that changes the solubilityof the polymer in many CARs. Hard bake is an annealing stepperformed after resist developing to strengthen and stabilizethe patterned structure.[42]

When choosing a photoresist platform, the thermal stabilityof the photoresist is important.[50] A high degradation tempera-ture is desirable in order that the photoresist be able to with-stand standard process conditions such as prebake and PEB.Further, a relatively high glass transition temperature of thephotoresist is needed in CAR to minimize, for example theacid diffusion during the exposure step. The glass transitiontemperature of uncrosslinked SU-8 is approximately 50 °C

which minimizes acid diffusion during the exposure which isperformed at room temp and allows for subsequent acceleratedacid diffusion during the PEB (75 °C). As a consequence of itsaromatic functionality and highly cross-linked matrix, the finalSU-8 structure is thermally and chemically stable. Fully cross-linked SU-8 is thermally stable and exhibits no flow up to220 °C. This is an important consideration from the perspectiveof further treatment of the final structure.

The novolac resin in novolac/DNQ positive resist systemsprovides the physical properties required in the photoresist,such as etch resistance and thermal stability (150–190 °C formultifunctional and orthocresol novolac). In general DNQs areoften modified to improve their resolution. In addition, theirthermal stability characteristics are also modified so as to pre-vent potential break down at the prebake or PEB temperature.

3.2.6. Mechanical Stability

Mechanical stability of the final structure is needed to makethick samples with high aspect ratio patterns and low overallpolymer volume fraction, which are essential for many optical,acoustical and mechanical applications. The elastic modulus offully crosslinked SU-8 film is around 4 GPa with the ultimatestrain reaching 8 % and can be modulated by changing the ex-posure or post-exposure bake time.[51] The fully crosslinkedSU-8 is, like most thermosets, a rather brittle material. How-ever, both the submicron framework and an intermediatecrosslink density could push the strain-to-break value up to300 %, demonstrating that high plastic deformation can beachieved in an otherwise rather brittle material by suitable mi-croframe structures. This suggests an interesting new pathwayfor making ultralight, mechanically dissipative structures.[52]

We will discuss length-scale dependent mechanical propertiesof SU-8 in detail in Section 5.3.

3.2.7. Preventing Pattern Collapse

Pattern collapse due to high surface tension after wet devel-oping (e.g., PGMEA for SU-8 and basic solution in water fornovolac/DNQ) during the drying process is another importantissue in the fabrication of high quality 3D structures. Supercrit-ical (sc) CO2 drying of SU-8 is a well known technique to im-prove quality of the resultant 3D pattern by avoiding the de-structive effects of surface tension.[48,53] sc CO2 developmentfor the negative type molecular-glass photoresist has been sug-gested as a new process step to alleviate pattern collapse indensely packed, high-aspect-ratio structures.[54] However, su-percritical drying of some positive photoresists resulted in theformation of cracks. Replacement of water with pentane orhexane, which have a lower surface tension (73.05 mN m–1,13.72 mN m–1, and 18.43 mN m–1 for water, pentane, and hex-ane respectively at 20 °C[44,55]) has been proposed as an alterna-tive method. Another approach to eliminate the surface ten-sion problem is to take advantage of the etch selectivity byoxygen for silicon-containing polymers that can form a protec-tive oxide layer when exposed to an oxygen plasma. It hasbeen reported that a critical threshold concentration of above


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10–15 % silicon in the polymer significantly improves O2-RIEresistance by formation of a SiO2 layer.[56,57] An alternative re-sist system is a dry developable resist in which silicon contain-ing areas remain after oxygen plasma treatment and non-sili-con containing regions are removed. Silicon containing groupscan either be removed/deprotected by an acid catalyzed reac-tion or they can be selectively formed by a silylating reac-tion.[58] This resist system can either act as a positive or nega-tive resist.

3.2.8. Shrinkage Issues

One of the other main problems incurred after exposure andpost-exposure processing of a negative resist is shrinkage of the3D polymer/air structure.[59,60] This can arise from a variety offactors, among them shrinkage due to the cross-linking inher-ent from the polymerization process and from post-exposuredevelopment in developers that plasticize the structure andmay even leach out low molecular weight components causingfurther shrinkage during subsequent drying, which was ad-dressed in the previous section. The stresses associated withthe change in volume can sometimes even lead to either crack-ing or severe distortion of the template and dimensions canchange further if the structure is heated above its glass transi-tion temperature during use or during other high temperatureprocess steps if the structure is part of a system. Detailed mod-eling, as well as experimental characterization of the fabrica-tion processes have reported shrinkage values of mostly be-tween 7 % to 15 %,[51,60–63] the reason for the variation beingthe well-established fact that the precise shrinkages vary, de-pending on the exact details of the dose, post-exposure andhard bake cycles, developer and development times as well asthe amount of constraint of the structure by substrates, super-strates etc.[51] Anisotropic shrinkage can result in a breaking ofthe symmetry associated with the structure being fabricated.[59]

In photonic bandgap materials, for example, this alteration insymmetry of the template structure can result in the loss of theinitial targeted complete bandgap. Thus, in the fabrication pro-cess flow, it is important to either account for or eliminatethese distortions. One recent demonstration of pre-compensa-tion for the known distortion directions by modifying the initialstructure caused by the interference pattern, has been carriedout by Meisel et al.,[59] where they did a comprehensive experi-mental study on shrinkage and compensated their beam expo-sure configuration to allow retention of the targeted simplecubic structure after development induced anisotropic distor-tions. An alternative approach to alleviate distortion fromshrinkage is to provide additional rigid boundary conditionsthat can help support the stress.[9] SU-8 is one of the better neg-ative resists since it has low shrinkage compared to other nega-tive resists (we find about 7–10 % under our normal processingcondition). Further decrease in shrinkage has been achieved byadding nano-size filler.[64] Another means to mitigate theshrinkage problem is via the use of a positive photoresist plat-form. In a positive photoresist system one starts with a cross-

linked highly insoluble polymer that is subsequently brokendown into a soluble form in the regions exposed to higher in-tensity of light. This process is not accompanied by the volumechange that is seen in the negative resist platforms.

4. IL Structures as Templates

While control over the geometry of the structures allows oneto target a number of applications, the material platforms opti-mum for each of these uses can vary widely. The polymeric 3Dstructures created can be used as Interference LithographicTemplates (ILT) for infiltration so as to access a wide varietyof additional material platforms. There have been several priorefforts to synthesize 3D air/inorganic material through the in-filtration method.[65,66] The structures can be infiltrated withother materials followed by the removal of the original poly-mer template to obtain geometrical complementary 3D struc-tures with superior, for example, mechanical or optical proper-ties than the original polymer template. An ideal materialplatform would thus be one, which in addition to fulfilling theconsiderations listed in the previous section, would allow forthe facile removal of the template material after filling. Posi-tive photoresist platforms tend to be better suited from thisperspective than their negative resist counterparts. For exam-ple, a positive resist was used as an ILT for infiltration with aPDMS precursor to create an elastomeric three dimensionalstructure. The positive resist could be readily removed via awater-based basic solution which circumvented PDMS swellingor pattern collapse during the ILT removal process. The resul-tant structure was shown to have a mechanically tunable pho-nonic dispersion relation (discussed later in the phononic crys-tals applications section). However, in this case, due to alimitation posed by the absorbance of the resist, the structurewas restricted to a thickness of only 3 lm.

In addition to easy removal of the resist in order to facilitatean exchange of materials, the ILT must retain its integrity dur-ing the exchange process. Taking advantage of its mechanicalrobustness, SU-8 also can be used as an ILT using a high tem-perature removal process. For example, titania was depositedinto a SU-8 ILT from titanium propoxide (IV) under atmo-spheric pressure at room temperature. The ILT was removedduring calcination at 500 °C. SEM images of the initial poly-meric ILT and the final TiO2 inverse structure are shown in Fig-ure 6. X-ray diffraction measurements confirmed the phase ofthe annealed TiO2 as rutile. Because of shrinkage from the sol-gel reaction, the period of the structure was reduced about15 % from 980 nm to 830 nm. The stresses associated with thisprocess often result in fracture and an attendant loss of long-range order. Alternatively, atomic layer deposition (ALD) canbe used as one of the well-controlled TiO2 infiltration tech-nique for the inversed structure of ILT.[67] The bicontinuousTiO2 structure can be used as a photonic crystal as well as adye-sensitized solar cell with a large internal surface area forthe adsorption of light-harvesting dye molecules.[68]


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5. Applications of 3D Structures Fabricatedfrom IL

Clearly the ability to make three dimensional, porous, bicon-tinuous large area, single crystalline micro and nanostructureswith control over the geometry and volume fraction with accessto other materials platforms has important implications for anumber of fields. Here we briefly survey recent progress fromour group in three areas: photonic crystals, phononic crystalsand microframes.

5.1. Photonic Crystals

One of the first applications suggested for 3D IL structureswas in the fabrication of photonic crystals.[17,19] Photonic crys-tals can be described as dielectric composites with periodicallyvarying refractive indices, which allow for the control of the in-teraction of light and matter. This functionality depends bothon the materials parameters as well as the geometry of the sys-tem employed. The idea, first proposed by Yablonovitch,[69]

centers around the concept that full three dimensional spatialperiodicity of k/2 in the refractive index can result in a range offrequencies in the electromagnetic spectrum near the wave-length k not being able to propagate, irrespective of direction.This is an extension of the principle behind Fabry–Perot reso-nators into three dimensions. The use of such photonic crystalsholds the promise of numerous applications in integrated opti-cal circuits such as the control of the spontaneous emission oflight,[70] bending of light around sharp corners for wave-guides,[71] and all on-chip optical transistors.[72]

The use of IL to create photonic crystal structures is a physi-cally intuitive one, since we are exploiting the inherent period-icity of light to create periodic structures that in turn interact ininteresting ways with electromagnetic waves. Strong physicalinsight into the existence of bandgaps can be obtained by view-ing these low order Fourier term structures as having sinusoidalmodulations along principal directions of the Bravais lattice.[73]

When the gaps associated with each of these sinusoidal modu-lations overlap, one can obtain a structure with complete pho-tonic bandgap, that is, a structure which for a given band offrequencies has no propagating modes regardless of the polar-ization and propagating direction of the light.[74]

The P structure is of particular interestsince it is easily size-scalable without resort-ing to a multiple exposure technique and theassociated problem of registration betweengratings. This is possible since the three grat-ings that make up the P structure are perpen-dicular to each other.[29] Realizations of the Pstructure with different spacings via the singleexposure of multiple laser beams of 532 nmare shown in Figure 7a and b. Here the peri-odicity is simply controlled by varying theangle between pairs of beams from 7.2° (forFig. 7a) to 16.3° (for Fig. 7c).

To produce a structure with strong photoniceffects, it is necessary to enhance the refrac-

tive-index contrast by exchanging the polymeric ILT with high-er index materials. The sol-gel method for infiltration was usedas previously described. Since the P surface is a self-comple-mentary structure, the inverse P from the infiltration of TiO2

into the SU-8 ILT is also a member of the P surface family. Fig-ure 7c is the SEM image of a TiO2 inverse P and its associatedreflectance spectrum. P structures have only a partial photonicbandgap at the dielectric contrast of 4.4:1 which corresponds tothat of TiO2/air. The reflectivity peak of Figure 7c measuredfor the (100) plane is shown in Figure 7d. In order to open acomplete photonic bandgap, we are currently exploring highindex material deposition into ILT via double inversion tech-nique which has been suggested by Ozin et al.[75]

5.2. Phononic Crystals

Phononic crystals are the acoustic counterpart of photoniccrystals. They allow control over propagation of mechanical(acoustic and elastic waves) by forming gaps in a phononic dis-persion relation of a medium.[76] As a result, mechanical waveswith frequencies within the gap are completely forbidden frompropagation. Phononic crystals have potential for many inter-esting applications. For example, by creating bandgaps in sonicfrequency range (20 Hz–20 kHz) one can make sound andnoise isolation structures.[77,78] This feature is of interest forstructural and architectural acoustics. Higher frequency ultra-sonic waves (100 kHz–100 MHz) are widely employed in ultra-sonic imaging and medical diagnostics. Recently, ultrasonicphononic crystals were suggested for use as acoustic superlensesthat operate based on negative refraction effect.[79] Such lensescan dramatically improve resolution and efficiency of the cur-rent acoustic imaging technology. Finally, at very high hyperso-nic frequencies (1 GHz–1 THz) phononic crystals may influ-ence electronic, optical and thermal properties of materials.[80]

To fabricate phononic crystals one must create structureswith mechanical properties (density and elastic constants)varying periodically in space. IL is ideally suited for such fabri-cation. Currently phononic structures have been fabricatedvia IL to operate at frequencies ranging from 500 MHz to5 GHz[81] which is ideal for high resolution acoustic imaging.IL fabricated structures consist of two bicontinuous networks,one of which is a solid material (most often polymer resist) and


Figure 6. SEM images of a) a SU-8 ILT with symmetry R̄3m and b) the inverse TiO2 structureafter infiltration and calcination to remove the SU-8 and convert the TiO2 to the rutile phase.

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the other is often air. In contrast with pho-tonic crystals, mechanical contrast be-tween polymers and air is very large(qepoxy/qair ! 103). As a result, completephononic bandgaps can be opened di-rectly in polymer-air structures, withoutneed for additional pattern transfer to adifferent (e.g., inorganic) material sys-tem.[82] Another advantage of IL polymersis their optical transparency, which allowsoptical techniques to be employed to char-acterize the phononic dispersion relationof a structure. In particular, Brillouin lightscattering (BLS) has been used to directlymeasure phononic band diagrams of anumber of polymer phononic crystals.[83]

It is often desirable to tune the banddiagram of phononic (and photonic) crys-tals after their fabrication. For example, tuning would allowcreating lenses with focal distance that can be modified dynam-ically during operation or filters and resonators with an adjust-able frequency of operation. Our group has recently used IL togenerate 3D patterns in elastomeric materials, such as PDMS.These materials can then be deformed reversibly and repeat-edly leading to mechanical tuning of their dispersion relation.Figure 8 shows an SEM image of a 3D phononic crystal fabri-

cated in PDMS (a) and its dispersion relation before (red lines)and after (blue lines) it was subjected to 30 % strain along the[101̄0] direction. One can clearly see that the deformation re-sults in distortion of Brillouin zone and leads to a shift inthe band diagram. In addition, one of the modes that is presentin the unstrained sample disappears after the deformation. Thisis likely to be the result of the change in symmetry of the lat-tice.


d

Figure 7. SEM images of the P surface level set structure: a) P surface structure with periodicity of 2.7 um in SU-8; b) P surface structure with periodicityof 1.2 um in SU-8 fabricated using the same wavelength of light as (a) but with a smaller the angle between pairs of beams; c) the inverse structure of (a)via TiO2 infiltration; d) Reflectivity data of TiO2 sample in (c).

Figure 8. a) SEM image of elastomeric PDMS 3D network/air structure. Insets are AFM image ofthe sample before (i) and after deformation (ii); b) BLS spectrum measured along [101̄0] direction.Black dots, substrate; red dots, before deformation; blue dots, after deformation. Reproduced withpermission from [44]. Copyright 2006 American Chemical Society.

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5.3. Microtrusses and Microframes

The previous two applications that have been dis-cussed primarily exploit the periodic nature of struc-tures created by IL for interactions with periodicwaves (photons, phonons). The use of 3D IL struc-tures from the point of view of their mechanicalproperties is an interesting prospect for a number ofreasons. The mechanical response of truss-andframe-like arrangements has as much to do with theshape and topology as with the material of construc-tion.[84] Given the length scales over which we cancreate structures, we might expect to encounterlength scale dependent mechanical behavior. IL af-fords control over the geometry of the structures,particularly, the ability to make bicontinuous struc-tures. This means that in addition carrying loads, thestructure can be used to impart additional functional-ities. For example, if one of the phases is air, the con-struct could be used for cooling depending on thesize scale of the connected pore space.[85]

A first attempt to fabricate and mechanically test a3D polymer microframe was done by using IL to pat-tern SU-8.[52] The structure is a four-functional net-work having a basic unit comprised of a thick verticalpost supporting three thinner beams similar to thatmade in ref. [17]. In bulk, crosslinked SU-8 is brittle.A peeling test was performed to make a preliminaryassessment of the deformation behavior of the micro-frame film. Since peeling involves a complex interplay offorces, the mechanical response of the microframe structure ina wide variety of deformational modes is observed. Thus, theeffects of tension, bending, compression, and shearing are seenand reveal a host of interesting morphologies. Some unusualbehavior is observed: this includes portions of microframebridging across wide cracks (Fig. 9a), highly stretched membersin front of arrested cracks (Fig. 9b), and crushed and densifiedareas in regions of compression (Fig. 9c). The diameter of themost highly stretched members decreases dramatically. Bycomparison of the strut length in unperturbed unit cells withthose in deformed regions, member strain is estimated to beupwards of 300 % (see elongated members in Fig. 9b). Thestrain to failure of these features is about an order of magni-tude higher than that observed for uncured (ef ! 30 %) and forfully crosslinked bulk novolac resins (ef ! 8–10 %).[51] The plas-ticity of the fine scale structure is further evidenced in the for-mation of microfibrils in regions of extension, due to pull outand alignment of the struts (Fig. 9d).

The ability to control the topology of the structures bringsfurther interest to make truss and frame like structures via IL.Just as at larger length scales, suitable geometries depend onthe particular application. For example recent interest has fo-cused on topologies that are stiff, strong and bicontinuous.[85]

In particular stretch dominated structures are targeted, inwhich all the members of the structure are in tension/compres-sion, as opposed to structures that are bending dominated. Ithas been demonstrated that such properties can be achieved in

lattice structured materials consisting of rods with node con-nectivities of 12.[86] Further, it has been demonstrated by FEMthat the elastic mechanical response of a structure comprisedof air holes in a matrix with cubic symmetry. Depending on theset of mechanical properties targeted, once a particular geome-try is identified, approximations to these structures can beachieved by IL.[87]

5.4. Multivalent Colloidal Particles

Many of the most successful structured materials made viaself-assembly are based on simple spherical colloids (e.g.,magnetic materials, photonic crystals, microlenses, and tem-plates).[88–91] A particular need in nanotechnology is to createindividual particles with controlled shapes and to have a scal-able process that allows fabrication of useful quantities of suchparticles. The use of more sophisticated shaped 3D particles asthe constituent building blocks, offers increased functionalityin a self-assembled device not only from the constituent mate-rials, but also from variety in packing.

As discussed in the second section, IL affords control overgeometry by filling and connecting high symmetry Wyckoffsites in the lattices that it creates. This suggests the possibilityof creating particles with targeted ‘valencies’ if these Wyckoffsites are ‘disconnected’ in the appropriate 2D and 3D struc-tures. In addition to being able to control shape and symmetryin the particles obtained, this technique has the potential toprovide tight control over size and yield.


Figure 9. SEM images of plastically deformed polymer microframe structure: a) Re-gion with a microframe bridge extends from one side of a crack to the other. The insetshows extensive shear, bending and microplastic deformation of the structure nearthe left terminus of the bridge. b) Evidence of plastic deformation and fracture oftransverse beams with up to several hundred % strain (e.g., circled beam) in the vicin-ity of a crack. c) Portion of the film that was compressed, showing the collapsed mi-croframe region at left. d) (left) Cross section of a region of the film where the struc-ture has been plastically deformed. (right) Microfibrils formed due to peeling ofmicroframe from substrate. Reproduced with permission from [52].

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As a demonstration of this we designed various 3d intensitypatterns that would create such particles. We utilized beam pa-rameters and process conditions such that the intensity alongthe arms between nodes would be so low as to not be able tocreate a stable polymer region, whereupon washing/harvestingof the structure, only the node regions would be solid parti-cles.[92] As a first step to disconnect the structure, a low cross-linked region volume fraction is achieved by lowering the lightintensity and by subsequent removal of the low crosslinked re-gions using a strong development. The particles become sepa-rated at the lightly connected thin arms by either chemicaletching with UV/ozonolysis or by mechanical forces, for exam-ple, from the expansion due to the freezing of water in thecontinuous matrix region. Figure 10 shows “4-line valent”(Fig. 10a) and “6-point valent”(Fig. 10b) polymer particles fab-ricated by this process.

Future studies will explore making these materials respon-sive (for example pH, temperature or magnetic fields) andloading them with sensing particles (for example functionalizedquantum dots). Alternatively, if a diamond network were em-ployed instead, the node regions would be 4-functional andafter pinch off, and 43m point group objects with tetrahedralsymmetry could be harvested. Deposition of materials onto thesurface of the crosslinked polymer lattice followed by the dis-connection may introduce a different chemical functionality atthe vertices of the particles, thereby providing the possibility ofhaving particles with both anisotropic geometry and chemistry.

6. Conclusion and Perspectives

IL has now emerged as a fast and relatively easy techniquecapable for fabricating finite complex 3D structures. In orderto be able to exploit optical or mechanical properties the fabri-cation must result in defect-free, high fidelity single crystallinestructures that cover a large area. Phase mask lithographyusing 2D masks is an important pathway to fabricate very largearea 3D nanostructures in a simple way. Our results also show

that the 3D structures fabricated via multibeam IL promise awide range of applications as photonic crystals, phononic crys-tals as well as microtrusses. We are currently exploring a novelphotonic crystal with complete bandgaps via the double inver-sion method and the mechanical properties of the inverse ce-ramic microtruss structures. We are also investigating 3D hy-drogel structures which are expected to show a number of usesin biological applications such as cell patterning or drug re-lease. The next step in particle synthesis is re-assembly of thecolloidal particles by the introduction of proper functionalgroups at tips of the particles.

Received: February 1, 2007Revised: April 20, 2007

Published online: October 5, 2007

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Figure 10. SEM images of polymer particles synthesized by IL: “4-line valent” (a) and“6-point valent” (b) polymer particle after lowering the light intensity and strong de-velopment followed by UV/Ozonolysis for the 2D square and 3D P level surface fabri-cated in SU-8, respectively. Each upper inset in is the magnified image of the individu-al particles. The scale bar in the upper insets is 300 nm. Each lower inset is the singleunit cell with the calculated light intensity distributions showing good correspondenceto the SEM images. Reproduced with permission from [92]. Copyright 2007 AmericanChemical Society.

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