Nanotechnology7 (1996) 452457. printedin theUK Microcontactprinting of self-assembledmonolayers: applicationsin microfabrication

JamesL wilbur,Amit Kumar, Hans A Biebuyck,Enoch Kim and GeorgeM Whitesidesi

Departmentof chemistry,Harvard University, cambridge, MA 02.138,usA

Abstract. Thispaper describ_es applications in microfabricationusing patterned self-assembledmonolayers (SAMs) formed by microcontactprinting."Microcontact printing(7rCP) is a flexiblenew technique that forms patterned SnMs with regions terminated by differentchemical functionalities (and thus different 'SAMphysicat and chemicalproperties), in patternswith pm dimensions. 'tnk', Patternsof areformed usingan alkanethiolas an andprinting 'stamp'. thealkanethiol on a metalsupport with elastomeric We fabricatethe stamp by mouldinga siliconeelastomer usrng a masterprepared by opticalor x-raymicrolithography or by othertechniques. SAMs of long-chainalkanethiolates on goldand other hetats can act as nanometerresrsts by protectingthe supporting metal from corrosion by appropriatelyformulated etchants:the fabrication of microstructuresof gold and silicon demonstrates the utilityof patternedSAMs (formed by lrCP)as nm resists.Patterned SAMs formed by 4CP canalso control the wettability of a surfaceon thetrrrn scdle. The organization of liquidsin patternedarrays with 1rm dimensions, and the patterned deposition of microcrystalsand microcrystal arrays illustrate the use of controlledwetiabilitv for microfabrication.

1. Introduction andnucleation phenomena ll2. 1,5i9l. prorein[]0. I I I and cellularadhesion [22]; and in analvticalstudics inr olr ing Self-asserlbledmonolavers (S,{\.4s) of- organic compounds scanningelectron microscopv 123l.and scanning1-rrobc on inorganicor nretalsurlaces are becorningincreasingrr, rnicroscopies[2;l]. In this paper.w'e tbcus on applic;.rrior.ts rrnpofiantin many areas of' rnaterialsscience il 3] relatedto the fabricationof structures*,ith micrornc-rr.rand Although there are many diff-ercntsvstems of SAMs sub-micrometerdimensions. based on different oruanic c()lt'rponentsand supp{-)ns. the best de'clopedsvstems arc thoseof alkanethiolates. 2. Results and discussion ,\'(CH:),,1'(CH:),,S-.on gold films f.i 61. .{lkanethiols chemisorbspontaneouslv on a gold sLrrrircetiorn solutron 2.1. Microcontactprinting (pCP) transfersbv conract and fbrrr adsorbedalkanethiolates * ith loss 'ink' of hvcirogen. alkanethiol from an elastomeric.stamp' to a gold A r.videvarietv of organicfunctional groLlps (.\'. )') canbe surface:if the stamp is patternedna patternedS.{}I rncorporatedinto thc surliiccor inleriorof the rnonolayer. forms 17,8l SANIscan therefbrebe tailoredto pro'ide a u'ide'anerv of materialproperties: wettability and protectionagainst The stampis f-abricatedby castingpolydirneth'r'isilorane cclrrosionbv chernicaletchants are especiallvrelevant tcr (PDMS) on a masterhaving the desiredpattern. !{asrr.rs manyapplications. are preparedusing standard photolithographic techniqucs. We dcscribedpreviously [7 l0] a techniquethat forms or constructedfrom existingmaterials hal'ing microscale patternedSAMs w'ith geometricallyr.vell defineci regions surf-acefeatures. of dilferent chernical functionality and thus different In a typical experimentalprocedure (figure I ). \\.e phvsicaland chemicalproperties. This technique,pCp. placeda mastertn a glassor plasticpetri dish, and poured 'stamp' uses an elastomeric and alkanethiol.ink' to a l0:l ratio (',r':u'or r,':r.')mixture of SYLGARD silicone form pattcrnedSAMs of alkanethiolateson gold films elastomer18,1 and SYLGARD siliconeelastomer 184 u'ith dirnensionsranging from 200 nm to severalcm. curingagent (Dow' Ciorning Corporatron) ovcr thc rnastcr. The PatternedSAMs formcdby aCP havemany applications. elastomerdegassecl fbr approrimately30 rrin at roonl temperature.cured fbr I includingrnicrofabrication [7,8. I l-14]. studiesof wetting 2 hr at 60 C. and u,as peeled gently fiom the master. The resultrngstarnp replicatecj ',r'honl i ,.\uthor to correspondence should be acldrcssccl. in reversethe relief of the f-eaturescln the master: raisecJ

0957-44841961040452+06$19.50ig,'1996 tOp pubtishinoLtd pCP of SAMs:applications in microfabrication

<'- Photoresist in thesefeatures was between0.8 and 1.5 /zm, and ga'u'e Si rise to the contrastbetween different regions of the SEM imagesin figures2(a). (b): raisedregions of the master Photolithography is used (in image)than recessedregions to create a master appearedlighter the SEM of the master. <- Photoresistpattern Stampscast from thesemasters had recessedregions Si (1-2pm thickness) that corresponded to raised regions of the master. Microcontactprinting of CH3(CH:)15SHwith thesestamps PDMS is poured over master and cured on a gold film produceda patternedSAM terminatedby CH3 (lighter,figures 2(c), (d); washingthe surfacewith OH PDMS HO(CH2)rrSHformed patterned SAMs terminatedby <- Photoresist pattern in regionsof the gold surfacenot derivatizedby the pCP Si step(darker. figures 2(c), (d)). Sincethe stampedfeatures of the patternedSAM appearbright in the SEM image,the PDMS is peeled away patternedSAMs are directnegatives of the corresponding from the master masters.The complexityand scale of the featuresshon'n in figure2 is typicalof patternedSAMs thatlrCP canproduce PDMS routinely. Closeinspection of figures2(c). (d) (andother patterned PDMS is exposedto a solutioncontaining surfacesin this paper)revealed the srnall defectsin the HS(CH2)1sCH3 patternedsurfaces. At present,stampin-q is conductedby hand under ambient laboratoryconditions: presumably. PDMS <- alkanethiol more sophisticatedmethods of stampingwill improvethe Stamping onto gold qualityof featuresproduced by iiCP. substratetransfers At present,the primary advantageof microcontact thiol to form SANI SAMs(1-2nm) printing is the sirnplicity and ease with which micron- -{ - scalepatterned surfaces can be produced. Microcontact :\- Au (5 2000nm) -,...- printing is experimentallysimple. and can be conducted Si Ti ( 5- l0 nm) in a conventionalchernical laborator,v-no routine access Figure 1. Schematicof the procedurefor pCP. Seetext for to cleanrooms or photolithographicequipment is required details. (althoughsome rnicrolabricationtechnique is requiredto makethe master).Several stamps can be fabricatedfrom a singlemaster and individualstamps can be usedhundreds regions of the stamp coffespondedto recessedregions of of times (while stored under ambient lab conditions) the master. in performance. Pattenied SAMs We 'inked' the elastomeric stamp by exposing the without degradation alkanethiolatesformed by pCP are robust samples stampto a 0.1-1.0mM solutionof alkanethiolin anhydrous of exposed to the laboratory arnbient for ser.'eraltnonths ethanol, usually by rubbing the stamp gently with a Q- and could be u'ashed tip saturatedwith inking solution. The stamp dried until showedno detectabledegradation. without darnage. no liquid was visible by eye on the surfaceof the stamp repeatedlyin organicsolvents (typicallyabout 60 s). eitherunder ambientconditions, or by exposureto a gentlestream of nitrogengas. Following 2.3. Microcontactprinting of alkanethiolson gold and inking, the stamp was applied by hand to a gold surface. wet-chemicaletching produced microstructuresof We used50-2000 A-thick gold films, preparedby electron- gold [7] beamevaporation on Si[100]wafers, with l0-100 A of Ti pCP with CHr(CH:)rsSH as an adhesionpromoter (betweenthe gold and the Si). We formedpatterned SAMs by (prepared Very light handpressure aided in completecontact between on a 2000 A-thick gold surface by electron-beam the stamp and the surface. The stamp was then peeled evaporationof goldon a Si[100]wafer using l0 A of Ti as gently from the surface. an adhesionpromoter). Submergingthis patternedsurface in a basicsolution of cyanideion (0.01MKCN, 2M KOH) with continuous stirring and bubbling of 2.2. Microcontact printing formed patterned SAMs 119.25.261, oxygen gas, removed the gold in regions of the surface with featureshaving pm dimensions[7,8] not protectedby SAMs U). Using this procedure,we Figures 2(a), (b) show SEM images of complex pat- have produced featureshaving dimensionsfrom 0.2 pm terns produced by standard lithographic techniques to severalhundred pm (figure3) [7,8]. Figures3(a), (b) (photolithography, etching, metalhzation and lift-off that suggest the size and complexity of features produced were used as mastersfor pCP. We selectedthese masters routinely by pCP and wet-chemicaletching. A feature to demonstratethe complexityof featuresthat could be pro- profile of 2 pm lines of gold revealedthe depth of the ducedby prCP. The mastershad feafureswith a range of gold features(figure 3(c)). The smallestfeatures of gold sizes(< 1trrmto hundredsof prm),arranged in patternssim- fabricatedto date by prCP are lines of gold 200 nm wide ilar to thoseused in microelectronicsfabrication. The relief that are separatedby 200 nm (figure 3(d)).

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Figure2. Microcontactprinting (trCP) using an elastomericstamp' and alkanethol Inkformed patterned SAMS on gold:the patternof the SAI\.4Sformed by /rCP correspondedto the featuresof the mastersused to caslthe elastomericstamps (a), (b) Scanningelectron micrographs of mastersused to castelastomeric stamps Standardithoqraphrc techniques were used to fabricatethe masters.The reliefin the featuresof the masterswas between0 8-l 5 r,ir. c). (d) Scanningelectron micrographsof patternedSAMS lormed by pCP withstamps cast from maslers n ia. bi. Mrcroconlactprinting with CHs(CHz)rsSH (light regions) formed the initialpattern: washing the surfacewith HOi CHr r. , SH idarkreg ons) forrned SA[.4s (terminatedby OH) on goldnot derivatizedby the i,CP step.

Typically.heraclecanethiol (CH.l(CHr)r.SH) r.r'as thc prcviouslr ir\ ir rc\i\1 lor' \r ctchrns.ligurr' -ltc)). \\'ashin_g "l-iO1 best resist for etching erperiments. Longer chain ri'ith it Ior, :t)iLrlr()n()1 ll[: rcrnorcd thc lay'er. Thc' alkanethiolsprovided cornparable protection against thc. resultingslrrlr.'li,ri'c\ ()1 Sr had tiatrrrcsu itlt sLrb-t.nicrot.neter CN- etch and were lessconvenient to use: alkanethiols dirnensrons(ircurc -+(lj ). Srnallr irnationsirr etchingtinrcs of shorter chain length pro','ideddecreasins protcction. resultccirn .lrglrtll tlrl'tcrcntlcatLrrcs. For best cases. \\,e have obsen'ed t8] (by SEM) -5 etchabledefectsrnrrnl o'u'er reqions of gold protcctcclbv 2.5. Patterned S{\ls ftrrmed bv p,CP organized SAMs of CH:(CHr)rsSH. Thesedefects mav arisefiorn spatiallr' liquids on a gold surf'ace18. 15, 16.28-301 irnperfectionsin the patternedSAlv{ fbrrncd b1,'1r C'P and contamination(dust. hydrocarbons)present in the Microcontact pnntnrr.ll.rrgpxrgcl pattenred SAMs with rvell laboratoryarnbient. dcfinccl hrdnrphobic arrd hvclrophilic rc,sions.crcating a patternin tlrc ncttat'rrlrtrot'thc surfhcc.Wc crploitcd this pattenr in thc uc-ttahilrtrof thc surface to fbrrn arrays of 2.4. Microcontactprinting of alkanethiolson gold and liqLridsorsrrnizcrl on tltc nr scalc. wet-chemicaletching produced microstructures of 7r Figurc 5(u) shon: l SF\l irnagcof a patternedSAM silicon l8-l I I fornrecibr' 7r( P ri itir rcgronstcrnrinated b1 COOH (darker. We used rnicrostructuresof gold fabricatedby lrCP hr,'drophilict or ('il, (lightc'r.hrdrophobic). When the and wet-chemicaletching (see abovc) as resistsfbr the tempcraturcol' thc-:Lrrlacc \\ as lrtncrcd. clropletsof water anisotropicetching of Si. Figure4 shor,i'sour proceduretbr condcnsr'ci:clce trr c'lr ()n thc hvclrophilicregions ! 6,28]. 'conclt:nslf the fabricationof Si microstructures.Microcontact printint Thcsc irrn tjgures' pror ided an image of the formed a patternedSAM (figure 4(a)); exposureto CN- patterncd S'\\{ surtacc []8] Condensation figures on solutioncreated gold microstmcturescorresponding to the patternedS.A\1s rilso actcd as optical diffraction gratings: patternof the SAM formed by pCP (figuresa(b), (c)). follo',rrng filrmation ol'a condc-nsationfigure, the light Anisotropicetching of the Si wafer.in areasexposed by from a lle:\c laser(r.:631.8 nnr. I nrW) reflectedoff the remor,'alof the gold andTi layers,was accornplishedby the surfacegr\rng a dif traction pattcrn (inset. figure 5(a)) submergingthe substratein a stirredsolution of KOH (4 M) Ii6] The patterncd S.,\\1 in figure 5(a) had a lattice in isopropanol( 15%by volume)at 60 C for approximatelv of srnall t'catLrrr'su ith small spacing and a lattice of 30 min (figure4(d)) !0.271. Aquaregia (l:1 HNO:/HCl) larger f-eaturcsri ith large'r spacing. Tl,l'o periodicities ',1'asthen usedto dissolvethe remainincsold laver (used corresDondingto thcsc cliffercnt lattices were evident in

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l'lch Si rrilh K()ll rrsinggold ar l'csisl w Figure3. Microcontactprinting and wet-chemical etchrng producedmicrostructures of gold. (a, b) SEMimages of (l ) microstructuresof gold fabricated by lrCPwith qillr CH:(CHz),sSH on a 2000A-thick gold surface and etchrng I l{,.,0,,,rc'li II}:: witha CN /2M KOHsolution. Light areas of the image l r"n,,,rc qolrl rrillt -t4,* correspondedto goldnot removed by the CN etch:the t rtqtta lt't1iit t. -nr- patternedSAMs protected these regions of gold.Dark areas e) of the imagecorresponded to regionsof thesample where t-F{ theetch removed gold not protected by SAMs (c)SEM imageshowing a fractureprofile of goldlines fabrrcated by lrCPand chemical etching. The width of thegold lines is Figure 4. ltrlrc:rli,liiIi(ii.''lrIr1i'ir,:l arrc w€i-a..ilernICaietchIng - 2 1tm,and the thickness of thegold is 2000A. (d)SEM fOfmeO$r '-;ri.'!,r!,i';,.'.,'|.S vlii- I ,;rl-feaitlres. ial imageof 200 nm widegold lines with 200 nm widespacing 'ti; '1 MicrocortiaCti.rrr'r' !.r'I'r Orl aH , ,.SH iolnecl Datterned fabricatedby TiCPand etching. Features on scalesof 7rm SAMs on a 1,r|; A 1l'r,crqorc' surlace ihe qold surface was areproduced easily through lrCP by hand;200 nm features deposrtec oy i'cr^1ro- LliiarTrevaporatlon or a s'i100] wafer. (d) suchas thoseshown in arethe smallestfeatures wrth 11-rA f i as a,r adheston promoter. rbr lmmersing fabricatedto dateby pCP andare notachieved routinely. the sample ,r-a [ras'c CN solutton for approxtmalely 15 mtn removedthe qotc f rnr tri regtons not derrvatrzec by SAMs. (c)SEM rmaqe of a patternedSAM gold forrned oy riCPwith CHTTCH:r..SH and etched selectively. Darx regrons rn the rmagecorresponded to regrons unprotected by SAMsand etchedaway to exposethe urtderlyrng Si: lrght reglons rn this imagecorresoonded io gold derrvatized wrth SAMs and nrntpctpr-i clrrrrnn ctcl.,nn,v. rcir Gnlcl qtrr rntrtroc fnn-rorl hr7 the diflractionpattern. Quantitati\/erneasurelrcnts of the vrv(vvLUV vu: rIv \/rv' 'u ,i Qp intensitiesof the drffiaction spots nreasuredthe tirrrt--- andetching were reststs for anrsotroprc etching of Sr. Submergingthe substrates in a stirredsolutron of KOH(4 M) ()n evolution of conderlsationfigures fbrmed patterned rnisopropanol t 15% by volume) at 60 C for- 15min etched SAMs. Sincethe conditions (ternperature. hurnrdit.v") o1-the anisotroprcallySrnot orotected by qold (e)The remarnrng surroundingenvironment influenced directly the evolution goldlayer was removed wrth aqua regra (.1 .1 HNOT.HCI):the Ti layerwas removed by washing with a 1% HF solution^(f) of a condensationfigure. thrs type of analysisproduced a SEMimage of Srmrcrostructures formed by etchingthe simplehurnidity sensor [16]. samplein ic) TheSE[/ rmaqesin (c)and ff) were obtained 70 improve Passingpatterned SAMs throughan interfacebetueen withthe sa.nole trlted by approxrmately to imagrngof thesurface relief of thesample The trlt of the \vaterand an irnmisciblehvdrocarbon fluid firnred droplets sampledistorts the rmagethe rectangularpatterns in (c) of hydrocarbonlocalized on hydrophobicregit-rr"rs ot' the andtf t wouldappear as squaresrf the sample were not tilted. surface112,29.30]. Figure 5(b) shou's drops of heradecane organizedon hydrophobicregions of a SAM terminated 2.6. PatterncdSA\ls u'ith regionsof different by CHr (lighter, inset figurc 5(b)) and COOFI (darker. wettabilih directedthe formation of microcrvstalsand insetfigure 5(b)). The shapeof thesedrops rellected the microcn'stalarrar s ltlI shapeof the hydrophobicregions in the pattcrnedSAM We dcmon:tmtctithc prirrciplcof patterrtedcr1'stallization surface. We formed permanentstructurcs a using UV- bv producingiin arrar ol'drople'tsthat contained dissolved curableoptical-quality polyurethane (NOA 60.'Norland') salts.and crrstlllizingthcsc saits b\ evaporationof the as the hydrocarbonliquid. Thesepolyr.rrethane structLlres droplets. \lrcrtrcorrtactpnntrng lbrnred pattcrnecl SAMs actedas microlensarrays (figure 5(c)) [ 2.29]. w'ith regionstcrntrrtatcd br hrdrophrlic(COOtl ) and

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Withdrawsample from dilule solutionof salt SAM of -s(cH2hscH3 Droplelsof -10 yrtn @{,3;,t",1'"T" ,..::''" '' .';1'' Figure5. PatternedSAMs formed by pCP organizedliquids - 't.,,-t'l into simplethree-dimensional structures: the soatial Evaporate water arrangementof thesestructures on the surfacereflected the SAMo'1ruur of v patternin the wettabilityof the surface.(a) SEM imageof a -s(cH2hscH3 Salt ''\\ l1;.( r') patterned !}- SAM formedby pCP. Darkregions corresponded .,/' /--, ..| (OJ to SAMsterminated by COOH:light regions corresponded r'-_r SAM of "'-. (r) -s(cH2hscooH to SAMsterminated by CH3.Drops of watercondensed ' ''':''_- '/' selectivelyon the hydrophilic(COOH terminated) regions of .-r.-/ 'condensation thesample: these figures'actedas optical I I lmaoe bv LFM diffractiongratings when a He:Nelaser reflected off the Y- surface(inset). (b) Dropsof hexadecaneformed on a patternedSAM withspatial arrangement and shapes correspondingto the patternof the SAM.The inset in (b) showsan SEM imageof the patternedSAM usedto assemblethese hexadecane drops: dark regions correspondedto SAMsterminated by COOHand light regionscorresponded to SAMsterminated by CH3.(c) Cu$04 Polyurethanestructures, similar to the hexadecanedrops shownin (b),assembled on a patternedSAM on transparent gold,acted as microlenses.The arrayof thesestructures - lQ pg1 showedlensing properties; three plates correspond to the imagesfocused at (0),above (+10 pm), and below Figure6. PatternedSAMs formed by lrCPdirected spatially (-10 pm) thefocal plane. thedeposition of inorganicsolids on a SAMsurface, forming microcrystalsand microcrystalarrays. Microcontact printing withCH3(CHz)rsSH and washing with HOOC(CHz)rsSH formeda patternedSAM with regions terminated by CH3 hydrophobic(CH:) groups. By slowly withdrawingthese (hydrophobic,dark crosses in LFMimage) or COOH patterned surfaces lrom aqueous solutions of inorganic (hydrophilic,light ovals with channels in LFMimage). salts,liquids containing the saltsorganized on hydrophobic Withdrawingthe sample slowly from a dilute(1 M) solutionof regions of the pattern: by varying the pattern in the CuSO+organized microdroplets of watercontaining dissolved the patterned Evaporation wettabilityof the surface(lines. circles, etc) we controlled CuSOaon surface. of thesedroplets formed an arrayof CuSOamicrocrystals. the structuresof the liquids on the surface(microtubules, lmagingthis sample by lateralforce microscopy (LFM) microdrops,etc). revealedcontrast between regions of the SAM terminatedby Figure 6 showsthe procedurefor producingmicrocrys- CH3(dark) or COOH(light) and between the CuSO+and the SAMsurface. Individual microcrystals had a diameterof tals and microcrystalarrays, and showsan image(obtained '1 - and heightof - 250 nm (measuredby atomicforce by lateralforce microscopy (LFM)) of a represenra- /rm [31] microscopy(AFM)). tive sample. Microcontactprinting with CH:(CH:)rsSH formedthe initial hydrophobicpattern (dark crossesin LFIV{ image\aD; washingwith HOOC(CH2)15SHformed a hy- tals on the surface. The CuSOacrystals were localized drophilicSAM on regionsof the gold not derivatizedby the approximatelyin the centerof the hydrophilicregions: ev- piCP step (light ovals with channelsin LFM image l24l). idently, the dropletswithdrew from the edgesof the hy- This samplewas withdrawn slowly (lessthan 2 cm s-l) drophobicregions (during evaporation)before the CuSO+ from a I M CuSOa solution, forming an array of micro- deposited.The crystalsin figure 6 were - | pm in diam- dropletscontaining dissolved CuSO+. Evaporation of these eterand - 250 nm high (relativeto the planeof the SAM) dropletsat room temperature(27 "C) and ambienthumidity by AFM [32]. Reducingthe concentrationof the CuSO+ (- 4M0oA) formed a periodic arrayof CuSOamicrocrys- solutionreduced the size of the crystals:a 0.1 M solution

456 ttCP of SAMs: applicationsin microfabrication of CuSO+produced 0.5 pr.m-diameterCuSO+ crystals in an [4] Bain C D and WhitesidesG M 1989 Angew. Chem.Int. Ed. 28 506-12 extended(several mm) patternedarray [32]. Using a similar Engl. WhitesidesG M and Laibinis P E 1990Langmuir 6 87-96 procedure,we formed patternsof microcrystalswith other [5] [6] Ulman A l99l An Introduction to Ultrathin Organic Films salts(KI, LiCIO+, K:Fe(CN):) [8]. For thesesalts, changes (New York: Academic) in the temperatureand relative humidity of the environment [7] Kumar A and Whitesides G M 1993 Appl. Phys. Leu. 63 during evaporationresulted in crystalswith different shapes 20024 Kumar A, Biebuyck H A and Whitesides G M 1994 and morphologies[8]. [8] Langmuir10 1498-51I [9] Wilbur J L, Kumar A, Kim E and WhitesidesG M 1994 3. Conclusions Adv. Mater. T 60o-4 [10] Kumar A, Abbott N L, Kim E, Biebuyck H A and M 1995Acc. Chem. Res.28 219-26 Microcontact printing, which uses a patternedelastomeric WhitesidesG 1] Kim E, Kumar A and WhitesidesG M 1995J. to a gold surface, is [ stamp to deliver alkanethiols Electrochem.Soc. 142 (2) 62V33 flexible technique for forming at present the most [2] Biebuyck H A and WhitesidesG M 1994Langmuir I0 patternedSAMs of alkanethiolateson gold. Although the 2790-Z technique is experimentally simple--with the exception [13] Xia Y, Mrksich M, Kim E and WhitesidesG M 1995-r. Soc. 117,9576-7 of the photolithographic processing of the masters, the Am. Chem. Wilbur J L, Kim E, Xia Y and WhitesidesG M 1995Adv. this paper were conducted [14] experiments described in Mater. T 649-52 laboratory-patterned SAMs in a conventional chemical [5] Biebuyck H A and WhitesidesG M 1994 Langmuir l0 with complex features having micrometer dimensions are 4581-7 producedroutinely. [16] Kumar A and WhitesidesG M 1994 Science263 6V2 WhitesidesG M 1996J. Electrochem. We have describedseveral applications for pCP in the [17] Xia Y, Kim E and Soc. 143 1070-8 area of microfabrication. The capacity to form patterned I S] Xia Y and WhitesidesG M 1995J. Am. Chem.Soc. ll7 SAMs, and the ability of SAMs to act as nm resistsagainst 327+-5 selectedwet-chemical etchants, allowed the formation of [19] Xia Y, ZhaoX. Kim E and WhitesidesG M 1995Chem. microstructuresof gold and Si [7,8, 10,11]. The ability Mater. 12 2332-7 and WhitesidesG M l99l Science252 1164-1 to pattern the wettability of surfacesby pCP allowed for [20] Prime K L Mrksich M and WhitesidesG M 1995 Trendsin Biotech- gold surfaces:applications 12r) the organizationof liquids on 13 22V3s included condensationfigures of water [16], microlenses l22l Singhvi R, Kumar A, Lopez G P, StephanopoulosG N, and microlensarrays formed from organicliquids 112,291, Wang D I, Whitesides G M and Ingber D E 1994 and pattemedarrays of microcrystals[32]. Science 264 690-8 123)LopezG P, Biebuyck H A and WhitesidesG M 1993 Langmuir 9 1513-6 Acknowledgments 124lWilbur J L, Biebuyck H A, MacDonaldJ C and Whitesides G M 1995Langmuir 11 825-31 The Chemistryof Gold (Amsterdam: This research was supported in part by the Advanced 12slPuddephattR J 1978 Elsevier) Research Projects Agency and the Office of Naval 126lKumar A, Biebuyck H A, Abbott N L and Whitesides G M Research. JLW gratefully acknowledges a postdoctoral 1992J. Am. Chem.Soc. ll4 9188-9 fellowship from the National Institutes of Health (grant t27l Clemens D P 1973 Electrochem.Soc. Extend. Abstr. 79-2 numberl-F32 GMl65I l-01). 407 [28] Lopez G P, Biebuyck H A, Frisbie C D and Whitesides G M 1993 Science260 647-9 References l2el Kim E and Whitesides G M 1995 Chem. Mater. 12 125744 M 1995 [] NuzzoR G andAllara D L 1983J. Am. Chem.Soc. 105 [30] Gorman C B, Biebuyck H A and Whitesides G 44811 Chem.Mater. T 2524 on [2] ChidseyC E D, PorterM D andAllara D L 1986-/. [31] For reviews of scanningprobe microscopy studies Electrochem.Soc. 133 130 organic surfaces,see Fuchs H 1993 J. Mol. Struct. 292 [3] WhitesidesG M andGorman C B 1995Handbook of 29 SurfaceImaging and Visualizationed A T Hubbard Frommer J 1992Angew. Chem.Int. Ed. Engl.31 1265 (BocaRaton: CRC Press)pp 711-33 132l Wilbur J L and Whitesides G M unpublishedresults

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