Microfabrication,
Microstructures
andMicrosystems
Dong Qin'Younan Xia .|ohn A. Rogers.RebeccaJ.|ackman .Xiao-MeiZhao .
GeorgeM. Whitesides*
Department
of Chemistry
andChemical
Biology,
HarvardUniversity,
MA 02138,
Cambridge,
USA. E-mail: gwhitesides@gmwgroup.harvard.edu
This reviewgivesa brief introduction to materialsand techniquesusedfor microfabrication.
Rigid materialshave typically been used to fabricatemicrostructuresand systems.Elastomeric materialsare becomingattractive,and may haveadvantages
for certain typesof applications.Photolithographyis the most commonlyusedtechniquefor the fabricationof structures for microelectroniccircuits,microelectromechanical
systems,microanalyticaldevices
and micro-optics.Soft lithographyrepresentsa set of non-photolithographictechniques:it
formsmicropatterns
of self-assembled
monolayers(SAMs)by contactprintingand generates
microstructuresof polymersby contactmolding. The aim of this paper is to illustratehow
non-traditionalmaterialsand methodsfor fabricationcan yield simple,cost-effective
routes
to microsystems,
and now they canexpandthe capabilities
of thesesystems.
Keywords:
Microfabrication,microsystems,
microlithographictechniques.
Introduction
2
Requirements on Materials for Microsystems
2.I
2.2
Rigid Materials for Microsystems
ElastomericMaterials for Microsystems
3
4
3
Microlithographic Techniques
5
3.r
Photolithography
3.3
RapidPrototyping
3.4
Other Methods for Microfabrication
5
6
6
10
t2
t4
Conclusions and Future Directions
16
References
T7
3.2 Soft Lithography
3 . 2 . r Microcontact Printing
3.2.2 Molding of Organic Polymers
* Correspondingauthor.
Topicsin Current Chemistry,Vol.194
@SpringerVerlag Berlin Heidelberg1998
D. Qin et al.
1
lntroduction
Microfabrication is increasingly central to modern science and technology.
Many opportunities in technology derive from the ability to fabricatenen'tvpes
of microstructures or to reconstituteexisting structures in down-sizedversions.
The most obvious examples are in microelectronics.Microstructures should
also provide the opportunity to study basic scientific phenomena that occur at
small dimensions: one example is quantum confinement observed in nanostructures [1].Although microfabricationhas its basisin microelectronicsand
most researchin microfabrication has been focusedon microelectronic devices
[2], applicationsin other areasare rapidly emerging.Theseinclude systemsfor
microanalysis[3 -6], micro-volumereactors[7, 8], combinatorialsynthesis[9],
microelectromechanicalsystems (MEMS) [10, 1l], and optical comPonents
[r2-r41.
One particularly exciting use for microanalytical devicesis in the separation
and analysisof chemicaland biologicalsubstances[3 - 6]. Thesedevicesrequire
only small quantities of reagents,have relatively short analysis times and can
show efficienciesin separationsthat are better than larger counterparts.In the
past few years,a number of miniaturized total chemical analysissystems(pTAS)
[15] have been developedthat perform all sample-handlingstepsin an integrated fashion. For example,capillary electrophoresis(CE) devices using microchannels et c hed i n to p l a n a rg l a s s s u b s tra te shaveattractedattenti
on for
[16-18]
their use in applied molecular genetics.Systemsfor free-flow electrophoresis
capillary elec[19], gas chromatography[20],liquid chromatography121,221,
trochromatography 123) and micellar electrokinetic capillary chromatography (MECC) l24lhave been developed.Rapid and efficient capillary zone
electrophoretic separationsin micromachined channels(open, gel-filled or
polyacrylamide-coated)on quartz substrateshave also been demonstrated[3,
25,26).
Devices for performing not only analysis,but also chemical synthesis in
miniaturized systems(or microreactors)are also being developed17,27-341.
An array of chemical tools on a chip would make it possible,for example,to synthesize,analyzeand characterizeextremely small amounts of product (- tO-tz;;
[6]; thesesystemswill be useful for combinatorial synthesisas part of new proceduresfor the parallel synthesisand screeningof large numbers of compounds.
Development of severalprototypical microreactors suggestthat microfabrication will play an important role in biology and chemistry: examples include
DNA chips for high-speedDNA sequencing[35-38]; microchips for carrying
out the polymerasechain reaction (PCR) [39]; microchip-basedapparatusfor
the synthesisand characterizationof libraries of peptides and oligonucleotides
[a0]; and microchip-based drug discovery using electronic addressingfor the
investigationof small objects such as single biological cells [41].
In addition to their biological and chemical applications,microstructures
used in MEMS have evolved rapidly to integrate electronicswith monitoring,
actuating, and controlling tools (including chemical, optical, and mechanical
sensors)for use in engineering.In the past few years,the scope of fabrication
Microfabrication,
Microstructures
and Microsystems
techniques and the types of devices categorizedas MEMS have widened
dramatically [10, 17,42,43]. Most are fabricatedfrom silicon using standard
microlithographic techniques (silicon bulk micromachining and polysilicon
surfacemicromachining) 144-461.With thesemethods,thousandsof mechanimasselements
cal elements(for example,cantileveredbeams,springs,linkages,
and joints) can be batch-fabricatedon a single silicon substrate[9, 11]. Microactuators,micromotors and microengines[46] havebeen fabricatedfor optical
switches,fluid pumps [a7], systemsfor drug deliveryand microchemicalanalysis and high-speedrotational gyros for navigation [48-51].More recently,surface micromachining techniques have been used to fabricate miniaturized
optical components [12-14]. A free-spacemicro-optical system comprising
three-dimensionalmicrogratings, micromirrors and microlensesis one example [14,52]. Theseand similar miniatrrrzed optical devicesare attractivefor
free-spaceoptical interconnects,displaydevices,
applicationsin spectrometers,
sensors,optoelectronicpackagesand data storagedevices.
These current and potential applications motivate the development of
techniquesfor fabricating and manipulating objectswith nanometer and micrometer feature sizes.This review gives a brief introduction to materials and
techniquescommonly used for microfabrication;its focus is on those currently
being explored in our laboratory.Our aim is to illustratehorv non-traditional
materialsand methods for fabricationcan yield simple,cost effectiveroutesto
microsystems,and how they can expand the capabilitiesoi these svstems.In
a c onc luding s ec ti o n w e p ro v i d e b ri e f d e s c ri pti onsoi a number of other
mav providevariable
techniquesfor fabricationthat,likethose\\'eare der.eloping,
alternativesto photolithograph,v.
2
for Microsystems
Requirements
on Materials
Miniaturized systems for performing chemical/biochemicalreactions and
analysisrequire cavities,channels,pumps, valves,storagecontainers,couplers,
electrodes,windows and bridges [53]. The typical dimensionsof thesecomponents are in the range of a few micrometersto severalmillimeters in length or
width, and between100nm and 100pm in depth and height.An extensiveset of
techniquesfor fabricatingthesemicrostructuresis discussedin Sect.3.
2.1
RigidMaterialsfor Microsystems
Microsystemscan be built on various substrateswith a range of materials:
crystallinesilicon,amorphous silicon,glass,quartz,metals,and organic polymers. Single-crystalsilicon substrateshave been used in most areasof microfabrication for a range of excellentreasons:
1. Silicon processingitself can be carried out using thin films of organic
photoresistsas resistsagainst etching, and this technology is very highly
developed.
D. Qin et al.
2. TWo- and three-dimensional shapes and patterns can be reproduced in
silicon with high precision using bulk and surface micromachining techniques 146,54)
3. Silicon devicescan be batch-fabricatedusing the technology currently used
for fabricating integrated circuits.
4. Silicon/silicondioxide is stablechemically and thermally.
Crystalline silicon also has severaldisadvantages:it is expensive,brittle, and
opaque in the UV/visible regions, and its surface chemistry is complicated to
manipulate.As alternativesto silicon,glass,quartz and some rigid organic polymers (for example, epoxy, polyurethane, polyimide, polystyrene and polymethylmethacrylate) have properties that make them useful as materials for
microsystems[55, 56]. In particular, they are transparent in the visible and UV
regions and so can be easily adapted to optical detection in microanalytical
devices.Microstructures of thesematerials can be replicatedreadily using lowcost methods such as replicamolding and embossing[55-61].In addition, the
surface properties of these substrates(e.g. wetness ability, adhesion, surface
adsorption and surfacereactivity) can be modified rationally using a variety of
surface chemistries. Techniques involving the formation of self-assembled
monolayers(SAMs) [62] and the attachmentof complex macromolecules[63]
(e.g. immunoglobulins and nucleic acids) provide new routes to tailoring surfaceproperties.Polymers are,however,not stableat high temperaturesand their
low thermal conductivities may limit their use in applications requiring high
local power dissipation(e.g.in CE).
Integration of various components of a miniaturized device requires
methods for bonding materials to substrates[6a]. The most commonly used
technologiesfor joining glassand silicon componentsinclude:
l. Anodic bonding [0S], which uses electrostaticattraction to bring a glass
wafer into contact with a silicon wafer and to form covalentbonds between
them;
2 . S ilic on f us ion b o n d i n g [6 5 ],w h i c h e m p l o ysa si mi l ar mechani smto bond
together the surfaces of two slightly-oxidized silicon wafers at high
temperatures;and
3. Thermal bonding [66],which joins two glasswafersby locally melting them
under controlled conditions.
Bonding technologies for organic polymers will probably be based on
adhesivesand other ones that are still being developed.
2 .2
Elastomeric
Materialsfor Microsystems
Rigid materials have typically been used to fabricate MEMS and optical components (for example,diffraction gratings,lensesand mirrors). Thesematerials
have the advantagesof structural rigidity and strength.In certain applications,
however,rigid materials may not provide the only solution to structural problem: adaptive optics is such an application [67]. Adaptive optics have com-
Microfabrication,
Microstructures
and Microsystems
monly been constructed by assemblingsmaller, rigid elements that can be
moved independentlyto modify characteristics[68]. Elastomersare well suited
as materials for similar systems.We have created a new route for fabricating
optical elements and optical systems from elastomeric materials such as
poly(dimethylsiloxane),PDMS [69]. These types of optical components and
deviceshavecharacteristics
that can be controlledby changingtheir shapeswith
mechanicalcompressionor extension.There are severaladvantagesto using
PDMS as a materialfor adjustableoptical elements:
1. It can be deformed reversiblyand repeatedlywithout permanent distortion
or relaxationof features[70].
2. It canbe molded at a scalesuitablefor optical applications(with featuresizes
in the rangeof 0.I - l0 pm) with high fidelity IZt ] .
3. It is opticallytransparentdown to - 300 nm [72].
4. It is durable and chemically inert.
5. It is non-toxic, commercially availableand inexpensive.
We have demonstrated the concept of elastomeric optics by fabricating
elastomericlenses,corner cubes,mirrors and diffraction gratings [69].We have
u sed t hes e c om po n e n tsa s p h o to th e rma ld e te c tors[73]; devi cesfor measuri n g dis plac em ents, tra i n , s tre s s ,fo rc e , to rq u e , and accel erati on[70]; opti cal
mo dulat or sand dis p l a yd e v i c e sl T a l .In th e s ed e v i ces,the acti veopti calel ement
i s a bloc k of P DM Sw i th th e re l i e f o f a b i n a rv d i tf racti ongrati ngon i ts surface.
Me c hanic alc om pre s s i o n /e x te n s i ocno n tro l s th e rel ati veopti cal path of l i ght
passing through the grating; this change in path causesthe patterns of diffraction to be coupledto the compression/extension.
Fabricationof elastomeric
light valvesrepresentsadditional examplesof elastomericoptical devices[75].
One such exampleis an array of retroreflectivecorner cubes [76]. The amount
of light transmitted through the valve is controlled by mechanicalcompression
and extensionagainstthe surfacesof the PDMSblocks in the out-of-planedirection. Thesedeformable elastomericlight valvesmay have applicationsin display
devices,energy savingwindows, sensors(accelerometers
and pressuregauges)
and photolithographicsystems(such as photomasks)[77].
3
Microlithographic
Techniques
In the following sections,we begin with a descriptionof photolithography,then
focus on a number of methods developedin our laboratory and concludewith
some other non-traditionaltechniques.More extensivedescriptionsof traditional approachesare reviewedelsewhere[78].
3.1
Ph o t olit hogr aphy
Photolithographyis the most commonly used microlithographictechnique [2,
78l. In photolithography,a substrate,spin-coatedwith a thin layer of photo-
D. Qin et al.
sensitivepolymer (photoresist),is exposed to a UV light source through a
photomask. The photomask is typically a quartz plate coveredwith patterned
microstructuresof an opaque material (usually chromium). The photoresist
exposed to UV light becomes either more (positive resist) or less (negative
resist) soluble in a developingsolution.In either case,the pattern on the photomask is transferred into the film of photoresist;the patterned photoresist can
subsequentlybe used as the mask in doping or etching the substrate.
In addition to conventional photoresist polymers, Langmuir-Blodgett (LB)
films and SAMs 179- 811have been used as resistsin photolithography.In such
applications,photochemical oxidation, cross-linking, or generation of reactive
groups are used to transfer micropatterns from the photomask into the monol ay er s[ 82- 84] .
Photolithography is widely used to fabricate structures for microelectronic
circuits, MEMS, microanalytical devices and micro-optics. It has, however, a
number of disadvantages.It is a relatively high-cost technology and the investment required to build and maintain photolithographic facilities makes this
technique less than accessibleto many chemists and biochemists.It can not be
applied easily to curved surfaces(the formation of micropatterns and microstructures on non-planar substratesis important in the fabrication of certain
types of optical and MEMS devices). It is applicable to only a small set of
materials and it gives little control over the properties of the surfacesthat are
generated.These limitations have motivated the development of alternative,
low-cost microlithographic techniquesfor manufacturing microstructures.
3.2
Soft Lithography
We have developeda set of non-photolithographic techniquesfor microfabrication that are based on the printing of SAMs and molding of organic polymers;
we refer to these techniques,collectively,as soft lithography. Soft lithographic
techniquesinclude microcontact printing [80], micromolding in capillaries [85],
microtransfermolding [86] and replica molding [60, 7t]. Embossing [58], injection molding, and sometechniquesbasedon electrochemicaldepositionand
etching might also be included as soft lithographic techniques.The capability
and feasibility of soft lithography have been demonstratedby the fabrication of
microstructures and systemsin polymers and metals on a variety of substrates.
3 .2. 1
MicrocontactPrinting
Microcontact printing (pCP) is a techniquethat usesan elastomericstamp with
relief on its surfaceto generatepatterned SAMs on the surface of both planar
and curved substrates[87,88]. SAMs are highly ordered molecular assemblies
that form spontaneouslyby chemisorption of functionalized long-chain moleculeson the surfacesof appropriatesubstrates179,891.
Well-establishedsystems
of SAMs include alkanethiolateson coinage metals (Au, Ag, Cu) [90]; alkylsiloxaneson hydroxyl-terminated surfaces(Si/SiOz,glass) [91]; carboxylic and
Microfabrication,
Microstructures
and Microsystems
hydroxamicacidson oxide of metals179,92halkyphosphonates
onZrOrl93,94l;
and alkylphosphonicacidson Indium Tin Oxide [95].The thicknessof a SAM is
about 2-3 nm, and can be varied with an precisionof - 0.1 nm by changingthe
number of methylenegroups in the alkane chain. The surfaceproperties of a
SAM can be controlled by modifying the tail groups (that is, the functional
groups distal to the surface).SAMs have many of the featuresthat are attractive
in microfabrication:
l . They are quasi-equilibriumstructuresthat are at,or closeto, thermodynamic
minima, and that tend, as a consequence,
to be self-healingand defect-rejecting [62].
2. SAMs function as ultrathin resistsagainstcertain types of etches,and patterned SAMs can alsobe usedas templatesto control the nucleationand deposition of other materials(e.g. polymers [96],copper [97] and mammalian cells
Ie8]).
3. They can be handled outside of clean room facilities,and certain types of
fabrication involving SAMs are relatively low in cost compared with conventional photolithographicmethods.
FigureI outlinesthe procedureusedfor pCP.This techniqueis experimentally
simple and inherentlyparallel.The elastomericstamp is fabricatedby castinga
prepolymer of PDMS against a master,which can be prepared using photolithography or other related techniques (for example, micromachining or
e-beamwriting). We have castmore than 50 stampsagainsta single master.To
print hexadecanethiol(HDT) on gold, the "ink" used is a solution of HDT in
ethanol (-2 mM). After the PDMS stamp is inked, it is brought into contact
with a substrateto form patterned SAMs (Fig. 1A). Large area patterning
(10- 100cm2)is possibleeither by using a largeflat stamp or by mounting a thin
PDMS stamp onto a cylindrical rod and then rolling the stamp across the
substrate(Fig. 1B) [99].]rCPalsoallowsgenerationof micropatternsand microstructures on curved substratesby rolling curved substratesacrossan inked
stamp (Fig. 1C) [87]. Conventionallithography lacks the depth of focus to
pattern on curved substrates.
We havebeen able to generatepatternedSAMs of
a l kanet hiolat es
on c o i n a g eme ta l s[9 0 ,1 0 0 - 1 0 2 ],patternedS A Ms of al kyl si l oxaneson hydroxyl-terminatedsurfaces[ 103- I 05] . CP has alsobeen extendedfor
p a tter ningar r ay sof c o l l o i d a lp a rti c l e so f Pd o n S i /S i O,[106].l npC P ,eachP D MS
stamp can be used more than 100 times without degradationin performance;
much higher levelsof use may alsobe possiblebut havenot been explored.
pCP followed by selective wet etching allows the formation of arrays of
microstructuresof coinagemetalswith controlled shapesand dimensions[96,
1071.This capabilityhas direct applicationsin the fabricationof sensors[108],
arrays of microelectrodes[109], and diffraction gratings [110]. Moreover,the
patterned metallic microstructures can be used as secondarymasks in the
etchingof underlying substrates(for example,Si, SiO, and gallium arsenide)to
fabricatemicrochannelsand microcavitiesfor microreactorsand microanalytical systems[5a]. Figure 2 shows severaltypical examplesof microstructures
fabricatedusing the combinationof pCP and selectiveetching.Figure2A shows
D. Qin et al.
I Pnotolithooraohv
tSi
Pour PDMS against
Master and Cure
Si
PeelPDMS
otf from Master
@
ffi
I
Microcontact Printing
HDT
't t-t
PDMS
l-l
t-t
I-t
r
Att/Ag--+
Si
with a planarPDMS
Fig.1.Schematic
procedures
for pCP:(A) printingon a planarsubstrate
with a rollingstamp;and(C)printingon a curved
stamp;(B)printingon a planarsubstrate
substrate
with a planarstamp
a scanning electron micrograph (SEM) of patterned microstructures of Ag on
a Si/SiO2surface that were generatedby pCP hexadecanethiolwith a rolling
stamp, followed by selectivewet etching in an aqueous ferricyanide solution
[107].Wecan routinely produce sub-pm (>0.3pm) featuresover an area of
- 50 cm2 in a single impression in approximately 30 s [99]. We are beginning to
define the registration and distortion of the patterns associatedwith elastomeric stamps I I I I ] . Figure 2 B showsan SEM of patterned microstructures of Ag
on Si/SiOr.Figure 2C shows a cross-sectionalSEM of microchannels that were
generatedin a Si(100)wafer using a combination of pCP,shadow evaporation
and silicon micromachining [5a]. Figure 2D shows an SEM of a microtrans-
Microfabrication,
Microstructures
and Microsystems
Fig.2.A, B SEMsof testpatternsof silverfabricated
usingpCP,followedby selective
etching;
C cross-sectional
SEMof microchannels
etchedin Si(100);
and D opticalmicrograph
of a
microtransformer
fabricated
using;rCP,
selective
etchingandelectroplating
former fabricatedusing a combination of pCP,selectivewet etching and electroplating llI2). Conducting microcoils produced in this way on micro-capill a ri eshav ebeen us e d fo r h i g h -re s o l u ti o np ro to n N MR spectroscopyon nanol i ter v olum es [ 113 ] a n d a s c u rre n t c a rri e rsi n m i cro-i nductors[114].S i mi l ar
cylindrical structuresmaybe useful as intravascularstentsand micro-springs
[114].
10
D. Qin et al.
3. 2. 2
Molding of OrganicPolymers
Formation of replicasby molding againstrigid mastershas been widely used to
manufacturecompact disks (CDs) [115] and diffraction gratings [59]. By extending this procedure to include an elastomeras the material for the mold, we
have developeda number of techniquesfor fabricating microstructures of polymers, including micromolding in capillaries (MIMIC) [85, 116, ll7], microtransfer molding (pTM) [86], and replica molding 160,711.An elastomeric
(PDMS) stamp with relief on its surface is central to each of these procedures
(Fig.3).In MIMIC, a liquid prepolymer (for example,a UV curablepolyurethane
or a thermally curable epoxy) wicks spontaneouslyby capillary action into the
network of channelsformed by conformal contactbetweenan elastomericmold
and a substrate.In pTM, the recessedregions of an elastomericmold are filled
Micromoldingin Capillaries
(MtMlc)
Microtransfer
Molding
t
=mR e p l i c aMo l d i n g
PD M S
H
t
Fig.3.Schematicproceduresfor MIMIC, pTM and replicamolding
Microfabrication,
Microstructures
and Microsystems
1l
with a liquid prepolymer,and the filled mold is brought into contact with a substrate. After curing the prepolymer, the mold is removed, leaving a polymer
microstructure on the substrate.MIMIC can only be used to fabricateinterconnectedmicrostructures.Figure 44. showsan SEM of a free-standingmicrostructure of polyurethanethat was fabricatedon an SiO, surfaceusing MIMIC,
followed by lift-off in an aqueousHF solution [85, 116].Figure 4B showspolystyrene beads crystallized in microchannelsfabricated by MIMIC using an
aqueous suspensionof the colloidal beads [118]. Arrays of such crystalline
microbeadsare interestingfor potential applicationsin chromatographyand
applied optics.pTM can be used to generateboth isolatedand interconnected
microstructures;it can also be used sequentiallyto fabricatemulti-layer structures.The smallestfeatureswe haveproduced using theseproceduresare parallel
lines with cross sectionsof -0.1x2 m2. These dimensionsare limited by the
PDMS molds used in our work. Figs 4C and 4D show SEMsof two- and threelayer microstructuresfabricatedusing TM [86]. Figs 4E and 4F show SEMs of
carbonized polymeric structures (an interdigitated capacitor and an optical
deflector)fabricatedby pTM [1 19].
In replicamolding,microstructuresare directly formed by castingand curing
a UV curablepolymer againstan elastomericmold. This method is effectivefor
replicatingfeaturesizesranging from severalcentimetersto - 30 nm. Figure4 G
l b j e c t w i th 1 00m corner-cubeson i ts sursh o wsan S E M of a h e m i s p h e ri c a o
facethat was fabricatedusing replica molding againsta deformed PDMS mold
[60]. Replica molding also provides a convenient route to microstructures
w i th high as pec tra ti o s . F i g u re 4 H s h o w s a n SE M of one such structure replicatedfrom a master generatedin a thin film of heat-shrinkablepolystyrene
[120].
First,
Molding againstan elastomericPDMS master has severaladvantages.
releait
can
also
be
form
contact
with
a
substrate;
conformal
the elastomercan
sedeasily,evenfrom complexand fragile structures.Secondly,PDMS providesa
surfacewith low interfacialfree energy (-21.6 dyn cm ') and low in reactivity.
As a result,the polymers being molded do not adhere irreversiblyto or react
with the surfaceof PDMS.Thirdly, the deformation of PDMS can be controlled
easilyby mechanicalcompression,bending and stretching.Taking advantageof
this flexibility, we have been able to fabricatemicrostructures of polymers with
controlledshapeson both planar and non-planarsurfaces[60].
Microfabrication based on molding is remarkable for its simplicity, for its
economy,and for its fidelity in transferring the patterns from the mold to the
polymeric structuresthat it forms. MIMIC, pTM and replicamolding havebeen
used to fabricatemicrostructuresof a wide range of materials,including polymers,inorganic and organic salts,sol-gels,polymer beadsand precursorpolymers to ceramicsand carbon.The feasibilityof thesemolding techniqueshas
been demonstratedby the fabrication of chirped, blazed diffraction gratings
l tZZl ,
[6 0] , poly m er ic wa v e g u i d e s[1 2 1 ],w a v e g u i d ei nterferometers/coupl ers
interdigitatedcarbon capacitors[118],and suspendedcarbon microresonators
[123).Without steps for transferring patterns,photolithography can only be
used to generatemicrostructuresin the classesof polymers that havebeen developedas photoresists.
t2
D. Qin et al.
Fig.4.SEMsof microstructuresfabricatedusing A, B MIMICi C, D, E, F pTM and G, H replica
molding. Seetext for details
3. 3
RapidPrototyping
An elastomericstamp or mold with relief structures on its surfaceis the key to
soft lithographic techniques (see Sect. 3.2). As a result, the utility of these
techniquesis often limited by the availability of appropriatemasters.In general,
the mastersare fabricatedusing photolithography.Chrome masks are available
commercially from custom fabricators,but the time required for vendorsto produce a chrome mask from a design presentedin a computer aided design (CAD)
file can be weeks,and they are expensive(-$300 per square inch for features
larger than 20 pm, and -$ 500- $ 1000per squareinch for featuresbetween I and
Microfabrication,
Microstructures
and Microsystems
13
20 pm). The time and expenseinvolvedin generatingchromemasksarebarriers
to the use of photolithography by chemists and biologists and have limited the
use of microfabrication in thesefields.
Recently,we [124)and others [125] havedevelopeda systemthat has enabled
us to fabricate masters having feature sizes) 20 prmrapidly and at low cost. In
this technique[124],wedraw patternsusing computerprogramssuchasMacromedia Freehandor AutoCAD and print them directly onto polymer films using
image-settingsystem(for example,HerkulesPRO,
a commercial,laser-assisted
resolution of 3387 dpi, Linotype-Hell Company,Hauppauge,NY). Using this
method,photolithographicmasks- transparentpolymeric films patternedwith
microstructuresof black ink - can be made in a few hours at a cost of - $ 1 per
squareinch. Although thesemasks do not have the durability and dimensional
stability required for use in the manufacturing of microelectronic devices,they
are suitablefor the rapid production of limited numbers of prototype microfluidics,sensors,micro-optics,and microanalyticalsystems.They alsohavetwo
other attractive features:
1. They are flexible,and can be used to pattern non-planar substrates.
2. They are thin, and can be stackedon top of one anotherto generatenew types
of patterns.
After the patterns on these polymer films are transferred into photoresist
films coatedon silicon substratesusing photolithography,the developedphotoresist patterns can serve as a master to make the required PDMS stamps.By
combining this method of rapid prototyping with soft lithographic techniques,
we can fabricate patterned microstructures of polymers and metals within
24 h of the time that the design is completed.Rapid prototyping makesit possible to produce substantialnumbers of simple microstructuresrapidly and
inexpensively.
The rapid prototyping method has been demonstratedby fabricatingstructures representativeof those used in microanalyticalsystems.Figure 5 shows
two examples:a microCE channel and a surfaceacousticwave (SAW) device.
Figure5A showsa schematicdesignof the pattern usedin the microCE.Figs.5 B
and 5 C show optical micrographsof two areasof the pattern that were etched
into a glassslide using the patternedfilm of photoresistas the mask.Figure 5 D
illustratesthe pattern used for a SAW device and Fig. 5 E shows an SEM of a
portion of this device (made of silver on Si/SiOr)that was fabricatedby selective
etchingin an aqueousferricyanidesolution.
At present, the smallest features that can be generated directly using this
procedure are - 20 pm, a size that is limited by the resolution (3387dpi) of the
image-settingsystem.Itshouldbe possibleto generatefeatureswith smallersizes
by using printers with higher resolution.Even with masks made by the current
image-settingsystem,we are able to generatefeaturessignificantly smaller than
20 pm by using some of the techniquesreported previously (e.9. mechanical
compressionof an elastomericstamp) [126].We believethat rapid prototyping
pavesthe way for expanded use of microfabrication (especiallywhen patterns
may be complex but require only modest linewidths) in chemistry and biology.
t4
D. Qin et al.
AO
o,r
B
o
n
T
c
c
.:
!
t!
!!
:l
DSI€
I=
Fig.5.A
Thedesigned
patternusedin microCE;
andB,C opticalmicrographs
of two selected
areas
of thepatternthatwastransferred
intoa glassslide.D Thedesigned
patternusedin SAW
device;
andE anSEMimageof a portionof thepatterngenerated
in silveron Si/SiO,
3. 4
OtherMethodsfor Microfabrication
Embossing (or Imprinting) is a low-cost,high-throughput manufacturing technique that imprints microstructures in plastic materials [127].The manufacture
of CDs based on imprinting in polycarbonate is a good example of a large-
Microfabrication,
Microstructures
and Microsystems
t5
volume commercialapplicationof this techniqueI I 15].Until recently,embossing had not been seriouslydevelopedas a method for fabricatingmicrostructures of semiconductors,metals and other materials used in semiconductor
integrated circuit manufacturing. Work by Chou and coworkers showed that
embossingcan be used to make featuresas small as 25 nm and has attracted
attention to the potential of this kind of patterning [58, 128]. We have also
demonstrateda variant of embossingusing an elastomericmaster [129].In this
technique,the elastomericmaster is wet with an appropriate solvent and is
brought into contact with the surfaceof a polymer. The polymer is softenedby
the solvent,and the resulting (probably gel-like) polymeric material is molded
againstthe relief structureof the elastomerto form a pattern complementaryto
that on the surfaceof the mold.
Injection Molding is an alternativetechnique used for manufacturing CDs
[57].It has been usedto generatemicrostructureswith featuresizesof > 0.5pm.
Injection molding combined with sintering technology provides a potential
route to complex structuresmade of nearly every sinterablematerial or material
combination [130].The capability of this techniquehas been demonstratedby
producingfiber-reinforcedmetal componentsand thosemade of metal-ceramic
co mpounds[ 130] .Ba s e do n th e tw o -c o m p o n e n itnj ecti onmol di ng process,i t i s
possible to produce components with a rigid exterior and a tough core. A
low-pressureinjection molding technique[131] has been used as a method for
fabricatingceramic thread-guidecomponentsused in the textile industry. Injection molding has great promise for fabrication on the < 100nm scale,
although the required technology has not yet been developed.
Excimer Laser Micromachining [132, 133] is a technique based on laser
ablation. Currently, this process can routinely ablate vias as small as 6 pm in
diameter in polymers, glass,ceramics and metals.The minimum size of the
featuresthat this method can produce is limited by diffraction and by heat/mass
transport.Commercialinstrumentsand servicesare availablefrom a number of
companies(for example,Resonetics,
Itek).
Laser Direct Writing is a techniquethat combineslaser-assisted
deposition
and a high-resolutiontranslationalstageto fabricatepatternedmicrostructures
from a wide range of materials [134-1371.For example,laser-assisted
deposition can be used for generatingmicropatternsof seedingmaterials for electroless plating [138]. Laser-assisted
polymerizatronenablesthe fabrication of
patternedmicrostructuresof polymers [ 139] . Stereolithography,
basedon laserassistedprocessing,can be usedto fabricatethree-dimensionalmicrostructures
[r4 0 - 1421
LIGA (Lithography,Electroforming,Molding) [143, 144] is a techniquethat
combines X-ray (or synchrotron) lithography electroplating,and molding for
fabricating microstructures with high aspectratios and relatively large feature
sizes (- 10 pm). Although the standard equipment for UV exposure can be
adaptedfor this application,specialopticsand alignmentsystemsare neededfor
structuresthicker than 200 pm.
Electrochemicalmicromachining(EMM) is a techniquedesignedto generate
patternedmicrostructuresin metalsand alloys[145].Microfabricationby EMM
may involve masklessor through-mask dissolution.MasklessEMM uses the
16
D. Qin et al.
impingement of a fine electrolytic jet for thin film patterning. Through-mask
EMM involves selectivemetal dissolution from those regions unprotected by
a patterned photoresist on the workpiece. The smallest feature that can be
achievedusing this technique is - 1 pm.
Ultrasonic machining, also known as ultrasonic impact grinding, usesultrasonically induced vibration delivered to a tool to create accuratecavities and
channelsof many shapes[146]. It can be used to form deep cavities as small as
250 pm in diameter (with an accuracyof - 50 pm) in both hard and brittle materials such as glass,quartz, polymers, ceramics and metals.This technique may
be useful for fabrication of large masters.
4
and FutureDirections
Conclusions
Microstructures and systemsare typically fabricatedfrom rigid materials,such
as crystalline silicon, amorphous silicon, glass, quartz, metals and organic
polymers. Elastomericmaterials can be used in applicationswhere rigidity is a
drawback. We have demonstrated the concepts of elastomeric systems by
fabrication of photothermal detectors,optical modulators and light valves.We
believe that elastomericmaterials will find additional applications in the areas
of optical systems,microanalytical systems,biomaterials and biosensors.
Microfabrication is growing in importance in a wide range of areasoutside of
microelectronics,including MEMS, microreactors,microanalytical systemsand
optical devices.Photolithography will continue as the dominant technology in
the area of microelectronics for the foreseeablefuture. Photolithography has,
however,a number of limitations for certain types of applications,as discussed
in Sect.3.l.
Soft lithography offers a new strategy for microfabrication. Basedon SAMs
and molding of organic polymers, this set of techniquesrepresentsa nonphotolithographic methodology for forming micropatterns, microstructures
and microsystems of different materials on a range of substrates.Rapid prototyping enhancesthe utility of soft lithographic techniques and enablesthe
generationof numerous microstructures and systemswith featuresizes>20 pm
at low cost. In a researchsetting, soft lithographic techniques have generated
structureswith featuresizesas small as 30 nm. The strengthsand weaknessesof
these developing techniques are still being defined. Their strengths,however,
include low cost (capital and operational),the ability to pattern large areasand
the ability to generatestructures with feature size ( 100nm; limitations include
the difficulty in achievinghigh resolution registration and in controlling distortion of patterns causedby deformation of the elastomers.We are beginning to
addresstheseissuesin our research,and we believethat new materials,designs
and configurations will lead to improvement in theseareas.
andNSF
Theworkin ourlaboratory
wassupported
in partbyONR,DARPA
Acknowledgements.
(GrantPHY9312572),and
by theNSFunder
madeuseof MRSEC
sharedfacilitiessupported
TheauthorsthankDr.|. MichaelRamsey
for designof theCEin Fig.5A
GrantDMR-9400396.
MichaelGrunzefor the designof the SAWdevicein Fig.5D.JARgratefully
and Professor
Microfabrication,
Microstructures
and Microsystems
t7
acknowledgesfunding from the Harvard University Societyof Fellowsand RJJgratefully
acknowledgesa scholarshipfrom NSERC.The authors thank Andrew l. Black and Scott T.
Brittain for proof readingthis manuscript.
5
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