THIN FILMS. VOLLJME 24

MicrocontactPrinting of SAMs

JoE TrsN, YoUNRN XIA, AND GgoRce M. WHITeSIDES

Departntent of Chemistry' and Chenical Biologt. Han,ard lJniversiD', Cantbridge. Massat ltusetts

8.1 Introduction.. . . 228 8.2Microcontact Printint...... 130 8.3 PatternedHydrophobic SAMs as Ultrathin Resists . 240 8.4 Two-ComponentPatterned SAMs as Templates. . . . . )11 8.5 Conclusions.. . 250

THIN FILMS Copyright q' l99tt by Academic Press Vol. 24 All rights of reproduction in any form reserved rsBN 0-t2-533024-3 ISSN 1079-4050198$25.00 JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES

8.1 Introduction

Self-assembledmonolayers (SAMs) are used in many fields that require homogeneoussurface modification,e.g., control of wetting,lr control clf bio-compatibility,-s-7lubti.ution.s corrosion inhibition,e'10metal refining.rr adhesion,l2'13and passivation.raThe utility of SAMs is based on their ls 2l characteristics:

1. They areeasy to prepareand form quicklyfrom solutionsof theassembling molecules. 2. They aremolecularly ordered and are robust under many conditions of use. 3. They are thermodynamicminimum structures;thus they form sponta- neouslyand tendto rejectdefects. 4. They permit controlof film thicknessto within - 0.1 nm by varyingthe lengthof their constituentmolecules. 5. They allow surfaceproperties to be controlledthrough tailoring of exposed surfacefunctional groups.

Many typesof function.however. r'equire puttt't'ned surl'accs. In microelec- tronics,metal_patterns are neededto clelineatetransistors ancl other electronic components:--in microelectromechanicalsystems (MEMS). siliconand glass patternsfbrm free-stanclingmicrostructures sensitive to electricalor envirot.t- mental actuation:'''ancl in optics.relief f'eaturesgenerate diffraction gnttinss. waveguides.and microlensarrays.t* In eachof thesetechnologies. the general trendhas been toward smaller devices because a microscopicdevice is usuallv less expensive,more accurate.and more sensitivethan its macroscopic equivalent.Since SAMs are nanometer-sizedelements in one dimension (perpendicularto the plane of the surface).patterning o1'SAMs ancl the subsequentdevelopment of thesepatterns into usefuldevices has the potentialto increasethe performanceof certaindevices. Patterning of SAMs is.then. the lirst steptoward the realization of devicesthat involve this class of nanostructurcsin l-ubrication.processing. or use. Table I lists the techniquesfor patterningSAMs currentlyin use anclthcir resolutions:the mostwiclely used are microcontact printing (lrCP)2s'16 ancl ['\1 lithography.ttTh" latteris primarilya historicalartifact. as UV lithographris thc basisfor photolithographyand is thuswidely farniliarto microfabricators.ln this l" technique.UV radiationand an amplitudephotomask are usedto activatc.l- damage,3('orcross-linkrl'r2 a SAM selectivelyin the illuminatedregions: thc illuminationtherefore generates a patternof SAMs that replicatesthat of the mask.However, patterning of SAMs with UV radiationhas severaldrawbacks. MICROCONTACTPRINTING OF SAMS 22.9

TABLE I TecuNrques or PATTERNTNcSAMs, THErnDEnroNsrR,crEo LerEnel RESOLL]I.IONS.AND AREAS THATCAN eE PATTERNEDIN A SINCI-ESTEp

Technique Lateral Resolution(nm) PattemeclArea (cmr)

Microcontactprinting'rs -s00 -50 UV lithographyr0 -s00 -50 E-beamlithography+t' l0 10 I tl Scanningprobe lithographya I t0 Focuseclion heamlithographyrr l0 l0 l( Micromachininga3 I (X) l0 Neutral metastableatom lithography 1o I

First.its resolutionis not high.The minimum demonstratedlinewidth is 0.5 1rm. and the achievableedge resolution appears to be modest.Seconcl. this "brute force" method destroysmost of the exquisitesurface chemistry that SAMs introduce.The chemistryof UV photopatterningappears to be photooxiclationof the sulfur (when patterningalkanethiolates) unless photolabile groups have intentionallybeen included in the SAM. Thusthe surf-acethat is producedis not well-definedchemically. Third, like mostphotolithographic procedures. it is not applicableto curvedsurfaces. Finally, it is relativelyslow. Microcontactprinting (lrCP). one of thenon-photolithographic techniques that makeup "soft litho-uraphy".tt36 provicles an alternativeto I-JVphotolithography that is chemicallymore versatileand allows certaintypes of patterningto be carriedout more easily.In trrCP.an elastomericpolydimethylsiloxane (PDMS) stampwith a surfacerelief patternis "inked" with a molecularprecursor of a SAM-typically an alkanethiolHS(CH:),,x -and printedto generatea SAM on the stampedregions. With this rlethod, only the regionsthat come into contact with the stampare covered with a (near)monolayer of SAM: unstampedregions remainbare. Because trrCP is inherentlyan additiveprocess. it is compatiblewith a wide rangeof surfacefunctional groups, including,the structurally complex and fragile groupstound in biology and biochemistry."Microcontact printing is a patterningtechnique that can be performedeasily in laboratoriesthat do not have routine accessto photolithographicequipment. and becausetrrCP is a parallel methodof patterning.f'eatures are printedelficiently in a sin-elestep. Moreover. sincePDMS is an elastomer,trrCP can easilybe adaptedto curvedsubstrates.'r7 The smallestfeatures routinely generated with lrCP are 30O-nm-widelines, and 50-nm-widelines may be achievedwith caretul planningof the stamp geo- ltt lL)' metrv.-"'' Alternativesto lrCPand UV lithographyfor patterningSAMs includee-beam lithography,o0scanning probe lithography,t'focused ion beam lithography,a2 JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES micromachining,a3and neutral metastableatom lithography.**While the first three of thesetechniques have demonstratedvery high resolutions( < 50 nm). they require specialized equipment and-as with all serial lithographic techniques-are relatively slow. Micromachining(i.e.. scratchingthe SAM with a probe)and neutralatom lithographyhave alsobeen used to producehigh- resolutionpatterns, but substantialdevelopment needs to be done before their capabilitiescan match that of pCP. This chapterdescribes microcontact printing and its usesin microfabrication. The first sectiondetails how to carry out microcontactprinting: Fabricationof PDMS stampsand the various configurationsof pCP are described.The next sectionreviews the use of hydrophobicSAMs patternedwith trrCPas nm-thick resists.The final sectiondescribes the useof two-componentpatterned SAMs as templatesfor generatingpatterned materials such as polymers. crystals. ancl bioloeicalcells.

8.2 MicrocontactPrinting

Microcontactprinting (pCP) extendsconventional pattern printinu to the ,rrm- scale dimension.In conventionalprinting. stampin-gwith a raised surface transfersan ink, usuallya viscoussuspension of a dye or carbonparticles. to the printedsurface: the stampcan be fabricatedby. a rangeof techniques.including polymerreplica molding and manualscribing."' Only the raisedportions of the stamp come into contact with the stampedsurf-ace; the raiseclpattern of the stampis thereforereplicated on the stampedsurface. In pCP.stamps are molded from much more preciselypatterned masters, and the ink is a SAM precursor. The key diff'erenceis that in 1rCP,the ink solvent is forced to evaporatebefore stamping;assuming a precursorconcentration of - 1 mM andan ink volumeof -0.1 mL, only - 1016molecules or l0 monolayersare distributedonto the stamp.This amountis more than enoughto delivera monolayerto the surface. yet is tiny enoughthat excessink doesnot "squirt" out the sidesof the stamp when printing.Thus. with 1rCP,far smallerfeatures can be generatedthan is possiblewith conventionalprinting. We havebeen able to stamp300 nm lines and spacesof SAM over N 50 cm2 areas.*oand it is possibleto print -50nni featuresover - 10 pm2 areas.tt Microcontactprinting consistsof two principalsteps: f-abrication of stampsancl printing.These steps are outlinedschematically in Figs. 1 and 2, respecti'n,ely'. Most stampsused in lrCP are castfrom photolithographicallygenerated resist patterns26(as theseprovide excellentresolution) but stampsmay also be cast from other types of masters,such as TEM grids.l6 commerciallyavailable diffractiongratings,33 etched metal or siliconpattems,4s commercially available MICROCONTACT PRINTING OF SAMS

SiOz,SisN+,metals, photoresistsor wax

Silanize;pour PDMS prepolymer overthe master

PDMS

Cure,peel off PDMS

PDMS t, ->l I l+ +l d 1+ T,

FIG, 1 Schematicdiagram of fabricationof PDMS stamps. polymer relief structures(such as a polyurethanecorner cube reflector),aeand polymerbead patternr.'t' Before casting stamps from any of thesemasters, it is importantto functionalizethe masterwith silanevapor; any hydrophobicsilane such as the fluorosilaneClrSi(CH2)2(CF2).rCF:, octadecyltrichlorosilane, or hexamethyldisilazanewill do. This treatmentcaps any reactive-OH groupson the masterwith inert -CH: or -CF.r groupsso that the castpolymer doesnot adhereto the master. Once the masterhas been prepared and silanized,a stampmay be castfrom it (Fig. I ). In general,a thermosettingprepolymer is pouredover the master,cured, and then peeledoff the master(Fig. 3). If the mastercontains sub-4m-sized features.or if the prepolymeris highly viscous,removal of air bubbleswith vacuum before curing of the polymer may be necessaryto ensurefilling of the submicron-widechannels. Materials successfullyused as stamps include Novolac resin,z6polyimide, polyurethane, and polydimethylsiloxane(PDMS). and we anticipatethat otherpolymers would be suitableas well. (Even patterned Cr on glasshas been used as a stamp,although the relativesoftness of polymers makesconformal contact between the stampand surfaceeasier to obtain.-)';For PDMS stamps,we usethe two-componentSylgard 184elastomer available from JOE TIEN. YOUNAN XIA. AND GEORGEM. WHITESTDES

A PDMS 71)4-"inf" Au/Ti

PrintSAMs

1-sAM

C

FIG. 2. Methodsof carryineout nricrocontactprinting. (a\ Pluntu':The PDMS starr.rpis placcclon the surf'ace.(b) ktllin,q:The thin PDMS stampis rolledover the surface.(cl Curt'ed:A cylinclrical substrateto be stampedis rolledin betweenan inkedPDMS stampand a clcanflat pieceof PD\1S.

Dow Corningin the specifiedI : l0 ratioof curinga-sent to prepolymerancl cure at 60"C for > 2 hrs.It is alsopossible to tailorthe tackiness/softness of the stanrp bv varying the ratio of the two components-lesscuring agentresults in PDMS stampsthat are softerand stickier.re We have focused on stampsmade of PDMS fbr several reasons.First. in contrastto polyimideand polyurethane, PDMS is highly elastomeric,i.e.. PDMS MICROCONTACT PRINTING OF SAMS Master

m t fi{} g,dntt

FIG. l. SEM rmagesof a masterand patternedSAM made w'ith a PDMS stamp cast l-ronlthe

deformselastically, not plastically,over a wide rangeof strain.The elastomeric propertiesof PDMS allow stampsof PDMS to achieveconformal contact with the substrateto be stampedwith little or no appliedpressure. This situation contrastssharply with lrCPwith a rigid stamp;in the caseof lrCP with a hardCrl glass"stalnp," substantialpressure was neededto stampan areaof only a f-ew /tm2.stBecauselrCP with PDMS stampsoccurs with little externalpressure. a stampcan be reusedrepeatedly without de,eradationof the pattern-a problem thatplagues contact printing in photolithography.'-As an elastomer.PDMS can be deformedmechanically to large( - l0o/o)strains without damage to thestamp. Thus, it is possibleto changefeature sizes by simply squeezingthe-.stamp: as discussedlater. this idea forms the basisof c'ontpressivestuntltirtg.-'Second, PDMS hasdesirable chemical^properties: [t presents an inert surfacewith a low surfaceenergy of 22 dyn/cm'. This low energyallows easy releaseliom the substrateafter stampingand resultsin little attractionof the stampfor dust and contaminants.5tDust can also be removedeasily by washingor with Scotchtape. If a hydrophilic surfacewith the elastomericproperties of PDMS is desired, treatmentwith an 02 plasmaeasily oxidizes the PDMS to a poroussilica. PDMS resistsdissolution in most solvents,is only partially swollenby hydrocarbons. and is attackedchemically only by concentratedKOH or HF/HzO2.Finally, for reasonsnot well understood,PDMS appearsto be one of the few polymersinto which alkanethiolscan dissolveat a concentrationthat allows the stampto act as JOE TIEN. YOUNAN XIA, AND GEORGE M. WHITESIDES

a reservoir.'toThis propertyallows the stampto act as a reservoirof ink during pCP and may be one reasonwhy prCPof alkanethiolateshas been so effectiveat printing densemonolayers. Of course,the elastomericproperties of PDMS require certain trade-offsin performance.For one, nanometer-scaleregistration, already difficult to achieve with rigid materialssuch as quartz,is presentlyimpossible with PDMS. The difficulty in accomplishinghigh-resolution registration of PDMS is compounded by the sensitivityof its shapeto temperature:It has a high thermal expansion coefficient.The softnessof PDMS resultsin saggingof the stampin areaswhere the stampis suspendedover the surface.If the stampsags far enoughtoward the substrate,a SAM may be transferredinto regionsthat were intendedto be bare. This undesiredside effect can sometimesbe avoidedby constructingstamps that possessrelief depths that vary with f'eaturesize. However. fabricating such stampsis not trivial. Appropriatedesign-for example,placing "posts" in the designto preventsagging-may be effectivebut may also compromiseother functionsof the pattern. For now,these difficulties have not beensignificant. We havenot attemptedto achieve nanometer-scaleregistration. and unwanted sagging ntay ofien be avoided with careful stampingtechnique. The ability to achieveconformal contactover largeareas with PDMS outweighsany drawbacksarising from the unwanteddeformation of PDMS. this characteristicexplains why our grouphas workedalmost exclusively with stampsmade of PDMS for thepast several years. Microcontactprinting with a PDMS stamphas been performeclrvith three distinctgeometric configurations of the stamp:planar,26 rolling.a(' ancl curvedrT (Fig.2). In all of thesearrangements. the stampis inked,either by spin-coaringor by usinga Q-tip,with millimolarsolutions of a SAM precursor.The inkedstamp is then dried with a streamof N2 for 30-60 sec..and the stampis broughtinto contactwith the appropriatesurface (usually a thin gold or silver film). Upon contactof somepart of the stampwith the substrate.a "wetting front" is usualll, seen as the rest of the PDMS stamp comes in conformal contact with the substrate.If thestamp is very large,or if thestamp is verv soft(e.g., incompletelr curedor castusing a l:20 mix of curingagent to monomer).gentle tapping on the stampmay be neededto ensurethat air bubblesare not trappedbetween the starlp andthe substrate.After 5-10 sec.,the stampis removedfrom the substrate.ancl the stampedsurface may be washedwith anothersolution containinga seconcl SAM precursorto derivatizethe unstampedregions. Where theseconfigurations (planar, rolling, and curved) differ is in ho'uithe stampis appliedto the substrate.In the planararrangement, the stampis sirnply placedon the substrate.Planar stamping has achieved- 500 nm lines/spaces over a - l0 cm2 area, larger areastend to result in trappingof air bubbles betweenthe stampand the substrate.In the rolling configuration,a thin ( - I mm) MICROCONTACT PRINTING OF SAMS

stamp mounted on a plastic rolling cylinder is rolled over the surfaceof the substrate.This method has the advantageof being able to pattern large areas ( > 50 c*t; in oneprinting. In the curvedconfiguration, the stampis broughtinto contactwith a curvedsubstrate, and gradually the substrateis rolled on the stamp. For simple patterntransfer, rolling by hand is sufficient,37but generatingmore complexfeatures such as microcoilsrequires mounting the stampmore carefully using an alignmentjig.s5 The curved configurationcan also be extendedto pattern the inner surfacesof curved shapes(e.g., the inside of a capillary) by rolling an inked stampon the inner surface;this capabilityis uniqueto 1rCP.'s6 In theseconfigurations, the stampedfeatures have exactly the same dimensions asthose in the stamp.We can,however, use the elastomericproperties of PDMS to reducethe spacingbetween stamped ureas." By compressingthe stampin a vise andstamping with sucha stampwhile undercompression. the widthsof the recessedregions of the stampcan be decreasedby as much as 50o/aand trrCPcan be extendeddown to - 200 nm spacet.3nAlro. by squeezingone side of the stampmore than anotherside. a stampof parallellines can be usedto procluce chirpedpatterns of a SAM.3r The ability to changespacings and f-eaturesof a patternby mechanicaldistortion is uniqueto theseelastomeric stamps. At firstglance, it would seemunlikely that distinct f'eatures could be generatecl with ptCPsince the SAM precursormight spreadreactively all overthe surf'ace.57 However,distinct featuresare generatedwhen printing with CHr-terminated SAM precursorsbecause CH:-terminated SAMs areoutophobit,.5s That is, the SAM precursorwill dewetfrom the alreadyformed SAM; this dewettinglimits theextent to whichthe SAM canspread during stamping and thus -qenerates sharp f-eaturesof the SAM (Fig. a). This property of SAMs lends anotherapproach to reducingfeature sizes achievablewith lrCPby stampingunder water.-se When a hydrophobicSAM such as hexadecanethiolateon Au is stampedunder water, the SAM tendsto spread laterallyalong the edgesof the stampedfeatures. This effect is not an artifact resultingfrom excessmaterial being squeezed out alongthe siclesof the stamp. for it is only observedwhen stampingunder water. Reactive spreading occurs becausethe SAM of hexadecanethiolateon gold is hydrophobicand drivesthe spreadingof the SAM precursoracross the gold surfaceas the high-surface- energywater retracts. In general.the speedof spreadingranges from l0-100 nm/ min, dependingon the concentrationof the appliedsolution. By controllingthe time in which thestamp is in contactwith thesubstrate, one can reach dimensions for certaintypes of featuresof - 100 nm. Three important aspectsof lrCP need to be considered.The first question concernsthe structureof the PDMS stamp:What geometriesare allowed in the stamp?One can imaginethe extremecase of ,riCPwith a stampwith I prmlines, I mm spacings,and I ,um relief depth. In this case,pCP will not generateI pm JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES

ffia" + IH I flil

AutophobicPinning ReactiveSpreading

edge_ g 11 _drop il il Il| l_ftl advance \! !_

11 11 dropedge tl ll retreat il [t q ryftsau disordered SAM

- FIG. L schematicdiagrant of tormation of SAMs with 1rCP.The autophobicityof the alrcrrtlr fbrmed SAM causesthe SAM Drecursorto retract.

lines of SAM separatedby I mm spacingsbecause the PDMS will sagdurin-c stampingand make contact with the substratebetween lines. Delamarche et aI."" foundthat to minimizesagging an aspectratio (height to width)of less11tn1 | ;.5 rrr the stampis required.By the sametoken, stamps with unreasonablyhigh aspect ratioare useless in trrCP.If the stamphas, say, 100 nm linesand spacingsbut the relief depthis l0 1rm.the lineswill buckleunder stamping. In fact. stampswith high aspectratio postsare inherently unstable, and the PDMS postscollapse after MICROCONTACT PRINTING OF SAMS

removal from the master.Even if the featuresare mechanicallystable. often the lines will "pair up" from mechanicalagitation during applicationof the SAM solutionor from capillary forcesduring evaporationof the solvent.This effect is equivalentto that of stictionin microelectromechanicalsystems (MEMS)6r and originatesin van der Waalsattractions between components. Here. the attractive forcesare more pronouncedthan in MEMS becausePDMS is soft and compliant: the complianceresults in a largeradhesion area and thus a largerattractive force fbr PDMS thanfbr silicondioxide. Delamarcheet al.62have shown that an aspectratio of 2:l or loweris necessary to eliminatethe problem of line pairing.Since line pairingseems to correlatewith the stickinessof the PDMS stamp-a characteristicthat depends on thedegree of curing-extensive( > 2 days)curing greatly reduces line pairing.Once damaged by line pairing,a stampcan be restoredto nearlypristine condition (from 50% surf'acedamage to < 0.1c/c) by washingthe stamp with 5c/caqueous soclium dodecylsulfate.rinsing with heptane.and drying with supercriticalCO.. Thus.to be usefulin 1rCP.u PDMS stuntp(Svlgurd 181,.f'onnulutedoccrtrdin,q to l)rn, Corning's ittstrLrt'tiorts)should hat'c relie.f'uspect bctv,een 2;l and I :5 untl bt, curetl.for ) 2 davs.Havin-e one aspect ratio throughout the entire stamp is easyttt achieveif the featuresizes are identical, but widely cliff-erentf-eature sizes on the samestamp would necessitatedifferent stamp depths. Fabricating masters that presentsuch varying depthsis technicallydifficult and may require several photolithographicsteps. The secondissue facing lrCP is this: What is the quality of SAMs printedby pCP comparedto that of solution-grownSAMs? This question,addressecl by Larsenet al.,was answered by usingscanning tunneling microscopy (STM) on SAMs of dodecanethiolateon single-crystalAu(lll) thin films.62'63STM can ima-eesurfaces with atomicresolution and is thusappropriate for determinin-ethe structureof SAMs. Single-crystalAu( I I l) films werechosen as substraressince these films are relatively easily prepareclwith trrm2-sizedatomically flat temaces.oa(Polycrystalline Au is less useful fbr STM since the image of the SAM would then be convolutedwith the disorderedbackground ima-ee of the Au.) SAMs of dodecanethiolatewere chosenfor imagingsince molecules of dodecanethiolare long enoughto form close-packed,well-ordered SAMs on Au,l8 yet shortenou-eh to allow an appreciabletunneling current to passfrom the Au surfaceto the probingtip.o't Moreover, there exists a substantialbackground of work on STM of dodecanethiolateon gold. Beforeproceeding to imagesof stampedgold, it is usefulto reviewthe salient featurespresent in STM imagesof solution-grownSAMs of dodecanethiolate on_Au fig. -5a). Solution-grownmonolayers of thiols on gold form a 6/i x /:)R:O' overlayeron top of the reconstructedAu( I I l) face, with the sulfur atomsresiding on threefoldcoordination sites on the Au.'o In STM. the JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES sulfurs are the atoms that are imaged,o'and theseappear on top of the signal given by the Au atoms.Kinetic imagesof solution-phasegrowth of the SAM showsthe nucleationof thesolidl phase,with the thiolatemolecules lying flat on the substrate,followed by gradual coalescenceof the solid2 phase,where the thiolate moleculesproject upward with the usual 30' cant from the Au surface normal.66'6tThe thiols eitheretch the gold surfaceor causeit to reconstructwith time, and the resulting etch pits (and/or pits formed by reconstructionof the surface)are presentin Fig. 5a as dark regionsone atom deep. Larsenet al. examinedthe effectof ink concentration,printing time, andinking "technique" on the quality of stampedSAMs (Fig. 5b).02For an inking concentrationbelow 0.1 mM, a disorderedSAM results;between 0.1 mM and I mM, the solid1 phaseis the product;between I mM and l0 mM. the solid1 phase is replacedby solid2;and for ink concentrationabove l0 mM, the stampedgold is

(A) {E} $AMfromsolution SAIVIformed hy ptCP

FIG. 5. STM imagesof SAMs of dodecanethiolateon single-crystalAu. (a) Solution-grownSAMs. (b) SAMs formed by pCP. (From ref. 60.) MICROCONTACT PRINTING OF SAMS indistinguishablefrom well-packedsolution-grown SAMs on gold. Apparently the quality of stampedSAM is independentof time, providedthat a printing time of ) 0.5 sec.is used.(When stampinga pattern,printing times of ) I min. are inadvisablebecause vapor transpofiof the thiol from the recessedareas of the stampto the surfacewill eventuallyfunctionalrzethe unstamped areas.) To obtain reproducible stamping, the researchersadopted the standardized inking procedureof placing dropsof solutiondirectly on the stamp,letting the solution and stampequilibrate for 30 sec.,and blowing dry with N2 for an additional3t) sec.With suchan inking procedure,stamping becomes highly reproducible,and the only variable affecting stamped SAM quality is ink concentratron.Ink con('entrationof > l0 mM in ethanol and printing time of >0.5 sec'.results in stampedmonolayers indistinguishable b.v STM w,ith solution-,qrot4,tlmono- la ,-ers. The third issueinvolved in lrCP is the questionof stampdistortion: How well canthe patternon the stampbe replicatedon the substrate?The softnessof PDMS is expectedto causesome distortion in the stampduring 1rCP.and substantial distortionmay leadto a limit on the abilityof pCP to registersubmicron leatures. To quantifythis distortion,Rogers et al. examineddistortions in testgrids using Moire patterns.68In theseexperiments, a stampwith a squarepattern is brought into contactwith a substratewith the samesquare pattern on its surface(made by lift-off of Au). Any distortion, whether angular or lateral. leads to the characteristic,visible featuresin the Moire pattern,and thesefeatures can be recorded by a CCD camera for analysis.The distortion was defined as the maximum distancebetween the grid squareon the PDMS stampand that on the surface,and the distortionacross a 0.5 x 0.5 cm2area was found to be - 500 nm. The leastdistortion occurred with thin (- I mm) stampscast from PDMS with a l:5 mix of curingagent to monomer. With carefultechnique, it is possibleto generateSAMs indistinguishablefrom solution--urownSAMs with triCP,with a registrationaccuracy of at least0.-5 trrm over a 0.5 x 0.5 cm2 area.The SAM systemsused in uCP are listedin Table2.

TABLE 2

SAM SySTEMS USED wltH ltCPro'186e7r'7rq8

SAM Precursor Substrate cHr(cH: ),,SH.HOrC( CHr),,SH Au, Ag, Cu. GaAs,Pd cH3(cH2),,siclr SiO2,SiOH, glass.surfaces presenting OH groupsor other polar functionalities cH.r(cH2),,PO3H2 AI cH3(cH2),,co2H AI JOE TIEN. YOUNAN XIA. AND GEORGEM. WHITESIDES

Among the SAM precursorsused in flCP. CH:-terminatedthiols on gold26and silverashave been the most widely used systemsbecause they are the most straightforwardSAMs to form with trrCP.Stamping on Cu6eleads to multilayers. although the formation of multilayers appearsto be a property of SAMs of alkanethiolateson Cu ratherthan any particularoddity of ,uCPon Cu. Stamping with polar moleculessuch as HS(CH2),,CO2Hleads to nonuniformstamping or stampingthat results in morethan a monolayerbeing depositedTo since the SAM precursortends to crystallizefiom solution.Stamping of alkylsiloxanesalso ofien ' leadsto multilayers' (againthis appearsto be characteristicof the SAM system ratherthan peculiarities in pCP).Stamping thiols on Pd andalkylphosphonic and carboxylicacids on Al is in development'':Preliminary results have shown that SAMs do form in the caseof thiols on Pd andacids on Al andthat theseSAMs are orderedenough to act as barriersto etching.Other systems. such as hydroxamic acidson refractorymetals,73 isonitriles on Pt.7aand alcoholson Pd.7'thave not beenexplored and remain targetsfor future developmentof 1rCP.

8.3 PatternedHydrophobic SAMs as UltrathinResists

Homogeneous CHr- and CFr-terminated SAMs have long been knor.r'nto protect the underlying substratefrom corrosion and etching. The abilitl, ol'these SAMs to act as a barrier stems from their inherent low detect densities. nearlv crystalline packings, and hydrophobic natures: these characteristicsrender the lilms relatively impermeable to water vapor ancl oxygen. Thus. a filrn of Ag. when protected by a SAM of hexadecanethiolate.can be kept in ambient air lirr several months with no visible corrosion. Without the SAM. a piece ot' Ag tarnishesin air afier only a few days. This property has naturally motivatecl the application of trrCPtoward patterning thin films by etching or gaseousCVD. ln pattern delineation by etching. a surface patterned by pCP is simply placetl in a wet etchant, and a patternedthin film develops due to the difference in etch ratcs between SAM-coverecl and bare regions. In re-eions covered with a SAI\1. etching proceeds slowly-probably initially at defects and grain bounclariesirr the SAM-while attack on underivatized substrateis unhindered and rapid. This contrast in etch rate can be used to generatea patternedthin film: this film itscll can act as a resist for etching the underlying substratein a subsequentprocess step. For wet etching. the most appropriate SAMs to use are the hydrophohic CH,- and CF:-terminated ones since hyclrophobic interactions betu'eett patterned SAM and surrounding aqueous wet etch seems to enhance the barrier propertiesof the SAM.76 The use of patternedSAMs as ultrathinresists against etchin-s has beert recentlyreviewed by Xia et aI..77and we will only mention general points here. MICROCONTACT PRINTING OF SAMS

TABLE 3 ElcuarursaNo EocEREsor-utroNs ActttEveete By ptCp72.71

SAM System Etchant Edse Resolution(nm) cHj(cH2),,SH/Au Ferricyanide 200 cHj(cH2),,SH/Ag Ferricyanide 5r| cHr(CH:),,SH/Cu Ferricyanide 500 cHr(cH:),,SH/Pd Ferricyanide < 1000 cHr(cH,),,F0rH2/Al Phosphorrcacid < 1000

Table3 liststypical edge resolutions achievable with differentSAM systemsand etchants,typical examples of etchedfilms areshown in Fig. 6. For patterningof coinagemetals by pCP, Ag appearsto be the most suitableelement since its reactivitylies between that of Cu andAu. The high reactivityof Cu causesSAMs of alkanethiolateson Cu to be lessordered and more permeableto wet etchants thanare SAMs on Ag; the relativeinertness of Au necessitatesthe useof harsh etchantsthat tendto pit SAM-coveredregions substantially. From a purelymaterials point of view, Ag is preferabledue to the smallgrain sizeobserved in evaporatedthin films. Sincewet etchingoccurs most rapidlvat grain boundaries,patterning of Ag has a higher potentialedge resolution than patterningof Au. Films of Ag platedby electrolessdeposition can be useclin placeof e-beam-evaporatedAg. However.the rougher metal surf ace produced by electrolessplating results in rapid spreadingof the thiol during 1rCP,and stampingtime must be_carefully controlledin orderto obtaina patternedSAM with distinct features.toWhether electrolessly plated or depositedby e-beam evaporation,over 200 nm of Ag can be etchedbefore etch pits nucleatein the SAM-coveredregions. The patternedAg layeris itself thick enoughto function as a etch resistfor the underlyingsubstrate, be it SiO2or Si. and by alternating etch steps with shadow evaporationit is possibleto generateunusual Si topographiesin the Si bulk7s(Fig. 7). The underlyinglayers can alsobe etched completelyto yield free-standingAg shapes. The key questionwhen etching is: What is the densityof def'ects'lThe density of defectsafier etching-stamped Au or Ag hasnot yet beenascer"tained per se, but a useful approximation-basedon the similarity between STM images of stampedand solution-grownfilms-is the density of defectsobserved afier etchingsolution-grown SAMs on Au anclAg.7e For solution-srownSAMs. the def-ectdensity was measuredby etchingderivatized Au or Ag layersfor various times and then etching with a KOH-basedSi etchant.The hydroxide etch selectivelyattacked the Si layer over the Ag and amplifiedany defectsin the etched metal into the Si layer: these def-ectscould be counted directly by inspectionwith an SEM. The densityof defectsin SAMs of hexaclecanethiolate JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES

i TfrsEfrilr

E

-145 p,rn

FIG. 6. SEM imagesof films of Ag etchedusing SAMs printed with pCP as resists.(a. b) .50-nnt- thick pattems,stamped in a rolling configuration.(c) 5O-nm-thickpatterns, stamped in a planar configuration.(d) 200-nm-thickpatterns, stamped in a planarconfiguration. (e) 100-nm-thickpattems. stampedon a capillary.(f ) 1OO-nm-thickfree-standing pattems, fabricated by pattemingthe Ag and dissolvingthe underlyingcapillary with HF. (From refs. 54 and 5-5.)

on Ag hasbeen found to be much lower thanthat in SAMs on Au: -0.I/mml versus - 100/mm2.Although these values may still be too high for certain applications(e.g., in microelectronics),often all that is requiredis a condu.t]ng metalfilm, andfor thesetypes of applications,the defectdensity of - 0. l/mm' of MICROCONTACT PRINTING OF SAMS

FIG. 7. SEM imasesof shapesetched into Si usinsa combinationof trrCP.selective etching. and shadow evaporationof gold. Gold rvas selectivelyetched using a t-erricyanideetch. Silicon was selectivelyand anisotropically etched using a mixtureof KOH.isopropanol. and water. (From ref. -52.)

SAMs on Ag is smallenough to be useful.We alsobelieve the defect clensiry can be lowered with a judicious choice of componentsfor the SAM-perhaps a branchedthiol wouldbe moreprotective than the straight-chainHS(CH:),,CHr- along with more careful metal film preparation.Both approachesare currently underinvestigation. While wet etching has been successfulat producingfine featuresof Au, Ag, and Cu. sometimesit is desirableto patterna film for which thereis no known SAM chemistry(e.g., tungsten and ceramics) or for which the SAM systemis not sufficientlyrobust to withstandwet etching(e.g., HF attackon alkylsiloxanes).In thesecases. the abilityof CHj-terminatedSAMs to resistnucleation of metaland dielectricsin chemicalvapor deposition (CVD) can be usedto parrernthin films by pCP.8o'*'Inthis application,SAMs of alkylsiloxanesare printed on oxidized surfaces,and nucleationvia CVD occursmost rapidly on surfacesthat are not coveredwith a SAM. In CVD, the driving forcefor nucleationis the temperature- controlled decompositionof a metastablegaseous precursor onto a heated JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES substrate;bonds form betweenthe decomposingorganometallic precursors and the surface during nucleation of a thin fi1m.82Hydrophobic SAMs have no reactive functionalities,so the likelihood of nucleationonto those surfacesis limited to the probabilitythat the gaspenetrates through the SAM andnucleates a film directly onto the underlying substrate.Such penetrationis unlikely with a well-orderedand denseSAM, and there is a substantialdifference in deposition rateson SAM-coveredand bare regions;this differenceallows patternedthin- film deposition.Among the materialsthat have been patternedare Cu, Pt, and ceramicssuch as LiNbOr; representativeexamples of depositedfilms are shown in Fig. 8. Nevertheless,the stickingprobability on SAM-coveredregions. while small, is nonzero, so the passivatingproperties of the CH:-terminated SAM slowly erodewith time. In genersl,- 0.2 pm of materialcan be depositedonto unpassivatedregions before islands nucleate beneath the hydrophobicSAM. This thicknessis sufficientfor using thesepatterned films as wet etch resistsfor the underlyingmaterial. Selective thin-lilm depositionthrough pCP and CVD thus provides a complementaryalternative to wet etching for the generationof patternedmicro structure s.

8.4 Two-ComponentPatterned SAMs as Templates

HydrophobicSAMs patternedwith lrCP may be convenientresists ugainst wet etchantsand gaseousattack but do not take full advantageof the unmatched controlover surfacechemistry that pCP offers.In this section.we reviewthe use of nlo-componentSAMs-those containingnot only CHj-terminatedSAMs bLrt also SAMs with otherfunctionalities-as templates for patterningof polvnters. crystals,and biologicalcells. Two-component SAMs are madeby printins one SAM (typicallythe most hydrophobic)followed by solution-phasederivutrzu- tion of the unstampedregions with another SAM. The versatility o1' tlris approachallows diffbrent functional groups to be localized on a surface ancl enablesinteresting applications not accessiblewith othertechniques. The simplestapplication involves the ideaof selectivewetting and dewettinil on surfacespatterned with CH:- and CO2H-terminatedSAMs (Fig. 9). In this approach,a drop of hydrophilicprepolymer is placedon top of the patternecl waf'er,and the excessliquid is removedeither by tilting the substrateand sloulr decantingthe prepolymerfrom the surfaceor by blowinggently on the substratc with a streamof N2. If the surf'acefree energy of the prepolymeris higherthan tlic surf'acefree energyof a CHj-terminatedsurt-ace, the liquid will retractfronr tlte hydrophobicregions and wet only the hydrophilicparts. Many prepolymershar e sufficientlyhigh surfacetensions to be usedin this procedure,and amongthe polymerssuccessfully patterned are polyurethaneand polymethylmethacrylate MICROCONTACT PRINTING OF SAMS

100p{,rrr

FIG. 8. SE,Mimages of patternedthin films grown by selectiveCVD on surl'acespatternecl with alkylsiloxanes.(From ref. 80.)

(PMMA). Theseprepolymers can thenbe curedto makethe patternperrnanent. Among the structuresmade with this techniqueare waveguides.s3 microcrystals. diffraction gratings.and microlenses.saBy performing the dewetting under water,it is possibleto invertthe roles of theCH:- andCO2H-terminated SAMs so that the polymer remainsonly on the hydrophobicpatches. In this way. it is JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES

2pm 5Spm 50 prm

0 pm

(B)

75 pm S pnr

FIG. 9. Atomic fbrce microscopeimages of patternedpolvurethane fabricatecl bv dcucttins ttt't'rt surfacewith hydrophobicand hydrophilicregions. (a) An arrayof squares.(b) A uarcguidc.(Frorrr ref.83.)

possibleto make PDMS shapeswith constantGaussian curvature.s5 as uell as patternedfilms of hydrocarbon.8a A representativeapplication of polymer dewetting with lrCP is in the fabricationof waveguides.Although we now preferto makewaveguides b1' otlier soft lithographictechniques such as micromoldingin capillaries(MIMICIr+s.' and microtransfermolding (1rTM),36dewetting and 4CP still remain a i'iahlc technique for waveguide fabrication. Waveguideschannel light; the one prerequisitefor waveguide operation is that the index of refraction of the waveguide is larger than that of the surrounding material (often another transparentmaterial, known as c'ladding).The wave-euideshown in Fig. 9b u as pulled from a polyurethane-basedprepolymer and measures- 5 /tm hi-shancl - 45 pm wide.When a He-Nelaser with rotationallysymmetric Gaussian output was coupledinto the end of one of thesewaveguides, a multimodeasymmetric MICROCONTACT PRINTING OF SAMS output was emitted from the other end of the polymer. This asymmetryresults becausethe waveguide used was itself highly asymmetric.By usingslightly more hydrophobicprepolymers for the waveguide,it is possibleto changethe cross- sectionalshape of the waveguideand thus tailor its waveguidingproperties. Anotherapplication of selectivewetting with pCP is in the useof condensation figures,the array of water dropsthat appearwhen water vapor is passedover a cooled substrate.8TSince water vapor condenseson hydrophilic substratesat a lower humidity than on hydrophobic regions, it is possible to use a surface patternedwith CHj- and CO2H-terminatedSAMs as a humidity sensor.At low humidities,there is no condensation,and at near-saturation.water condenses regardlessof surfacetermination. But at intermediatehumidities, a condensation figure forms that replicatesthe hydrophilic regions.If a periodicpattern is used. then the affay of water drops fbrm a diffraction grating, and the drffracting intensityof this gratingis highly sensitiveto theamount of waternucleated. Thus the patternedSAM can act as a simplehumidity sensor. Surfacespattemed with hydrophobicand hydrophilic SAMs can,however. be used for more than dewetting. In particular, selectivenucleation on the hydrophilicregions can be appliedto obtain patternedgrowth of organicand inorganiccrystals. Here, we take a cue from studiesof ice nucleationunder Langmuir-Blodgettfilms of long-chainedcarboxylic acids: The hydrophilic areas act as "seeds" to nucleatecrystal growth, with crystalorientation determined primarily by the confbrmationof the hydrophilic end group. In our work with inorganiccrystals, we havefocused on the growth of CaCO: on COzH-terminated SAMs. Singlecrystals grown on CO2H-terminatedSAMs aretilted with respect to the surfacenormal. This findingindicates that crystals of CaCOrnucleate in a specificconformation with respectto thetemplating acid groups. and has enabled the predictionof the cantangles for SAMs of alkanethiolateson othermaterials suchas Pd andAu/Ag alloys.As a result,crystal growth on SAMs hasemerged as a convenient tool for obtaining molecular-scaleinformation about the conformationof a SAM using simple "macroscopic" techniques.We can also changethe polymorphismof the grown crystalsby varyingpattern sizes. As for organic crystals, diketopiperazinewill selectively crystallize from solutionon regionswith the sametermination when presentedwith a surface patternedwith Me-terminatedand diketopiperazine-terminatedalkanethiolates. Similar results are obtained for crystallizationof benzonitrileon similarly terminatedSAMs.88 The originalintention was to usepatterned SAMs to assistin the growth of large organic crystalsfor X-ray diffraction studies.This goal has provenelusive, the organiccrystals appear under SEM to be amorphouswith no apparent crystal faces. Nevertheless,pCP provides a convenient route for patterningorganic compounds,which is a capability not readily obtainedwith other techniquesfor patterningSAMs. JOE TIEN. YOUNAN XIA. AND GEORGE M. WHITESIDES

In most of theseexamples, the templatedmaterial grows on the hydrophilic surface;the hydrophobicsurf-ace is inert toward chemicalreaction and generally resistsattachment of materialsfrom vapor or organic solution.The oppositeis true in the case of protein adsorption from water; in these adsorptions. hydrophobicinteractions dominate. Proteins adsorb from aqueoussolution onto any surfacewith a moderatehydrophobicity, presumably through interactions betweenrecessed hydrophobic patches on the protein and hydrophobicmoieties on the surface. The surfaces that best resist protein adsorption expose oligo(ethyleneglycol) moieties:the mechanismof this inertnessto adsorption is still controversial.When the initial trrCPof Me-terminatedthiols is combined with a subsequentwash with oligo(ethyleneglycol)-terminated thiols. the adsorptionof proteinscan be directedonto only the CH:-tenninatedregionst"'"t'

HG60hfl #ffiu/PnoteEn

t ffiS#drYT ffi" ffi.11; m,''ffi ffi:;:ir:W ::ii: tffim

FIG. 1(). SEM images of adsolption of fibronectin on surl'aces patterned with hvdrophohie irntl oligo(ethylene glycol)-temrinated ( (EG)6OH) SAMs. (From rcf. 91.) MICROCONTACT PRINTING OF SAMS

(Fig. l0). This procedure is very general and has been clemonstratedfor fibronectinel'c)2and laminin.6other hydrophilicSAMs, such as the oligo(pro- pylene sulfoxide)-terminatedones, also resist protein adsorption.e3With patternedproteins,one can direct the growthof cells(Fig. l1ler'o+'u-s'this ability to control cell growth has affordedinvestigations into basicquestions about cell spreadingand apoptosis.et'e2'e6

Me/FN (EG)s

100p,nr 50rum ffilIlliltffi!!!!ff!fi!!!!!!!!ffiHH ffi ffi FIG. ll. Optical micrographsof cells grown on surfacespatterned with adsorbec'lprotein. (a. h) Bovine endothelialcells plated on patternedfibronectin. (Me - CH3-terrninatedSAM, FN: fibronectin.(EG)r : HO(CH:CHzO).3-terminatedSAM) (c) Primary rat hepatocytesplared on pattemedlaminin. (d) Hepatoclitesplated on unpattemedlaminin. (From refs.6 and 91.) JOE TIEN. YOUNAN XIA, AND GEORGE M. WHITESIDES

8.5 Conclusions

Microcontactprinting hasemerged as the techniqueof choicefor patterningself- assembledmonolayers. Its ability to patternSAMs with pm-scalelinewidths and nm-resolutionperpendicular to the plane over an entire 3-inch waf'er in one imprint, its ability to pattem curved surfaces,its easeof use,its unique control over surfacechemistry-all thesequalities make trrCPa powerful new technique for micro- and nanolithography.and highly complementaryto photographic, lithographic,and mechanicalpatterning. We believethat trrCPwill form the basis for many applicationsin biology and biochemistrythat use functional groups more complex than the usualCH3 or CO2Hterminations. SAMs formed by pCP are indistinguishableby STM from SAMs adsorbedtrom solution; their high degreeof order makesit possibleto use pCP to patternthin metal films and to use thesefilms as resistsfor wet etching.Indeed, if the defectdensity could be reduced by one or two more orders of magnitude,trrCP would immediately becomea seriouscontender for fabricationof sensors,SAV/ devices.and similar simple microstructuresin a relatively low-resolution single-layer fab. The capital cost of trrCPis also low. All that is required is PDMS (commercially availablein quanriry)and alkanethiol ($so buys - 100mL of alkanethiol.which is enoughto make - 0.5 L of inking solution).The disadvantagesof lrCP- problems with registration,deformation of the stamp, saggin-u.ancl det-ects- becomesignificant only for featuresizes less than I 1rmand for certaintypes of patterns that encouragesagging and buckling; we have not come close ttl reachingthe "theoretical" limit of this technique.Our experiencewith lrCPhas relied mainly on one system-PDMs and alkanethiols-and optintizationof each componentwill surely lead to even greatercapabilities fbr 1rCP. In the cutting edgeof microelectronics,where linewidths are approaching100 nm and future fabricationfacilities may require in excessof a billion dollars in capital investment, it would be presumptuousto believe that pCP will be competitive with photolithography.Even if pCP were able to pattern 100 nm lines and spacesover largeareas with ease(and we believethis will be achieved in the next few years),the existinglithographic technology is so well-established it is doubtful pCP will ever be adoptedthere. However, for systemsrequiring featuressubstantially smaller than 100 nm, which projection photolithographl' (asit is currentlypracticed) cannot reach, there may be a role for pCP or for other forms of soft lithography. In the fledgling arena of microelectromechanicaland microfluidic systems. however, alternativesto microlithography such as embossing and prCP are seriouslybeing considered. Here, it is not so crucialto haveever-smaller features; in fact, most feature sizesrange between 10 pm and 100 prm,well within the capabilitiesof pCP.Using a silicon-basedfab for making suchdevices is a bit of