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