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)). 453 J L Wilburet a/ (c t Sr\l\'l qql. ?;i; i!,s r.*l i# :,ll i (tl) SAI\'1: ffiffi $l ffiti 'r1 ttcP ffi "ffi + ffii ,;ffi ffi$ :t- ffiffi il ffi# "ffi wPBqw ffi WJ ----"--:*-11''r.i, ffil ll ffi$l ;;lffiffiffi#ffiffiffiffiwwffi*w 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