llon-Photol ithographic Methods forFabrication of Elastomeric Stamps for Use

in Microcontact PrintingYounan Xia, Joe Tien, Dong 0in, and George M. Whitesides

Department of Chemistry, Harvard University,Cambridge, Massachusetts 021 38

The ACS Journal of Surfaces and Colloids

Reprinted fromVolume 12, Number 16, Pages 4033-4038

Langmuir 1996, 12, 4033-4038 4033

Non-Photolithographic Methods for Fabrication ofElastomeric Stamps for Use in Microcontact Printing

Younan Xia, Joe Tien, Dong Qin, and George M. Whitesidess

Department of Chemistry, Haruard Uniuersity, Cambridge, Massachusetts 02138

Receiued April 22, 19968

This paper describes several methods that generate patterned relief structures for casting the elastomericstamps required in microcontact printing kCP). The relief structures include the following: (1) patternsetched in thin frlms of silver (200 nm thick) with printed SAMs of hexadecanethiolate as resists; (2)patterned pol5rmeric structures assembled on the surface of a thin film of siiver (or gold) that has beenpatterned with SAMs ofhexadecanethiolate; and (3) poiystyrene microspheres assembied on a flat surface.Commercial diffraction gratings were also useful as masters. Microcontact printing with poly-(dimethylsiloxane) stamps cast from these relief structures was used in combination with selective wetetching to generate patterns in thin films of siiver and gold. Although some of these techniques usedphotolithography once in fabricating the initial masters, they could produce patterns more complex thanthose on the initial masters. These procedures provide a convenient route to moderately compiex patternswith feature sizes ranging from -100 nm to -100 pm.

Introduction

Microcontact printing (1zCP)1'2 is a non-photolitho-graphic technique for generating p atterned self-as sembledmonolayers (SAMs)3 on the surfaces of a variety ofsubstrates, for example, gold,1,2 silver,a copper,5,6 andsilicon dioxide.T-e In this technique, an elastomeric stamp(typically, made from poly(dimethylsiloxane), PDMS) isprepared by casting against a master that has a reliefstructure on its surface. Patterned SAMs with featuresizes larger than 0.5 prn can be produced routinely usingthis technique; and in some cases, features with sizes downto -100 nm have also been fabricated.l0-12 PatternedSAMs formed using pCP have been used as nanometer-thick resists in selective chemical etching of underlyingsubstratesl,2'4-e and as templates for selective adsorptionof proteins and selective attachment of cells.13,la BecauseprCP involves conformal contact between an elastomerstamp and a substrate, it can be used to print patternedSAMs on curved surfaces.ls

The utility of pCP is limited mainly by the availabilityof masters used to cast PDMS stamps. These mastersare usually made by photolithography in thin frlms ofconventional re sists. 1'2 Photolithography is a wonderfullyversatile and precise technique that generates submicronfeatures routinely.lo It has, however, a number of

E Abstract published inAduance ACS Abstrocfs, July 15, 1996.(1) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993,63,2002.(2) Kumar, A.; Biebuyck, H.;Whitesides, G.M.Langmuir 1994,10,

1498.(3) Dubois, L. H.;Nuzzo,R.G.Annu. Reu. Phys. Chem. L992,43,437 .(4) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996,

143, 1070.(5) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater.

1996. 8. 601.(6) Moffat, T. P.; Yang,H.J. Electrochem. Soc. 1995, 142,L220.(7) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem.

Soc . 1995 , 117 .9576 .(8) Jeon, N. L. ;Nuzzo, R. G.;Xia, Y. ;Mrksich, M.;Whitesides, G. M.

Langmuir 1995, 1 1, 3024.(9) John, P. M. St . ;Craighead, H.G.AppI. Phys. Let t .1996,68,1022.(10) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117,3274.(11) Xia, Y. ; Whi tesides, G. M. Adu. Mater.1995,7,471.(12) Wilbur, J.L.; Kim, E.; Xia, Y.; Whitesides, G. M. Adu. Mater.

1995 .7 .649 .(13) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. P.;

Wang, D. I. C.; Whitesides, G. M.; Ingber, D.E. Science 1994,264,696.(14) Mrksich, M.; Whitesides, G. M. TIBTECH 1995, 13,228.(15) Jackman, R. J.; Wilbur, J. L.; Whitesides, G. M. Science 1995,

269 ,664 .(16) Moreau, W. M. Semiconductor Lithography: Principles and

Materials; Plenum: New York, 1988.

disadvantages, especially for potential users of micro-technolory who do not have routine access to microfab-rication facilities. Only specialized polymers can be usedas resists. It is not applicable to nonplanar surfaces. Itrequires a different chrome-mask for each pattern to beproduced. The cost of making a chrome-mask and thecost of maintaining clean room facilities make thistechnique less than accessible to the chemistry community.

In this paper, we describe several methods that usephotolithography seldom or not at all to generate themasters required for casting PDMS stamps. 17 The mastersthat we used for the present work include the following:(1) commercial diffraction gratings;(2) patterns etched inthin frlms of silver (200 nm thick); (3) polymeric reliefstructures assembled on surfaces ofsilver orgold patternedwith SAMs; and (4) polystyrene microspheres assembledon flat surfaces. For those techniques that used photo-lithography once in fabricating the original PDMS stampused in 4CP, they were able to produce patterns morecomplex than those on the original PDMS stamp. Thesmallest features that have been generated so far weretrenches and cavities etched in silver or silicon withdimensions of -100 nm.

Experimental Section

Materials. Au ( 99. 99 9Vo), Ag ( 99 .9999 7o), Ti(99 .99V"), IGSz Os,KOH, I(jFe(CN)e, KaFe(CN)6, and CHs(CHz)rsSH were obtainedfrom Aldrich. Poly(dimethylsiloxane) (PDMS, Sylgard 184) wasobtained from Dow Corning. Polished 5(100) wafers (Cz,N/phosphorus-doped, 1-10 f2cm, test grade, SEMI Std. flats)were obtained from Silicon Sense (Nashua, NH). Hexade-canethiol was purified under nitrogen by chromatographythrough silica gel. Gold films (20 nm) and silver films (50 nm)were prepared using e-beam evaporation onto titanium-primed(-2 nm) silicon wafers. Uitraviolet-curable polyurethanes (PU)were obtained from Summer Optical (J-91, Fort Washington,PA) and Norland Products (NOA-73, New Brunswick, NJ).Blazed diffraction gratings were obtained from Edmund Scientific(Stock # 843206, epoxy replicas from diamond-ruled masters,600 grooves/mm). Polystyrene microspheres (dispersions inwater) were obtained from Polysciences (Warrington, PA).

(17) We have reported a non-photolithographic method, that is,micromachining in thin frlms of wax. See: Xia, Y.; Zhuk, A.; Whitesides,G. M. Chem. Mater., submitted.

O 1996 American Chemical SocietvS0743-7463(96)00387-3 CCC: S12.00

4034 Langmuir, Vol. 72, No. 16, 1996 Xia et al.

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Microcontact printing kCP). PDMS stamps were fabri-cated according to the published procedure.l,2 A solution ofhexadecanethiol in ethanol (-2 mM) was used as the uink" forprCP. After the hexadecanethiol solution was applied (by cottonQ-tip) to the PDMS stamp, the stamp was dried in a stream ofNz for -1 min and then brought into contact with the surface ofsilver (or gold). Afber -5 s, the stamp was separated carefullyfrom the silver surface. In some cases, tr;CP was carried outunder water:10 a piece of siiver substrate was put in a petri dishthat was half-filled with deionized water, and the inked stampwas then brought into contact with the silver surface. After -5s, the stamp was separated from the silver substrate while underwater, the water was replaced with several volumes of cleanwater to remove alkanethiol floating on its surface, and the siiversubstrate was then removed from the water and dried in a streamof Nz.

Assembly of Organic Pol5rmers on SAIVI-Patterned Sur-faces.18'1e After the surface of a silver (or gold) film was patternedwith hexadecanethiol, a drop ofprepolymer of polyurethane wasplaced on the patterned area; the substrate was then carefullytilted to drain off the excess prepolSrmer. The remainingprepolymer selectively wetted and covered the hydrophilicregions(that is, bare regions of Ag or Au) of the surface and was curedwith a medium-pressure mercury vapor lamp (type 7825-34,Canrad-Hanovia) for -20 min, with the lamp positioned at adistance of I-2 cm from the sample. This simple procedure ishighly reproducible and can form patterned polymeric reliefstructures over an area of several square centimeters.

Etchings of Silver, Gold, and Silicon. The etchants usedfor SAM-patterned films of silver and gold were aqueous solutionscontaining thiosulfate and ferrilferrocyanide.a'2o Anisotropicetching of silicon was carried out in an aqueous solution of KOHand 2-propanol at 65 'C (400 mL of H2O, 92 g of KOH, 132 mLof 2-PrOH).? Prior to silicon etching, the native oxide on siiiconwas removed by dipping in an unbuffered aqueous HF solution(-IVo dw).

Instrumentation. SEM was done on a JEOL JSM-6400scanning electron microscope. AFM images were obtained usinga Topometrix TA/D( 2010 scanning electron probe microscope

(18) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10,2790.(19) Kim, E.; Whitesides, G. M.; Lee, L. K.; Smith, S. P.; Prentiss,

M. Adu. Mater. 1996. 8. 139.(20) Xia, Y.;Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater.

1995.7.2332.

1 0 p m

0

Figure 2. (A) SEM of the commercial blazed diftaction gratingused for casting PDMS stamps. (B and C) AFM images of tworepresentative patterns of silver (50 nm thick) that weregenerated usingprCP with a PDMS stamp cast from the blazedgtating, followed by selective etching. The pattern of (C) wasgenerated using double-printing, with the orientation of thelines of the second printing rotated by -90".

(Mountain View, CA); the images were obtained in the contactmode with supertips (Model # 1700-00).

Results and Discussion

PDMS Stamps Cast from Blazed Diffraction Grat-ings. The relief structures (usually generated fromreplicas prepared using diamond ruling or holography)present on the surfaces of commercially available dif-fraction gratings could be used directly to cast PDMSstamps with simple patterns such as parallel lines andarrays of dots. The unusual cross-sectional profile of ablazed diffraction grating provides useful access to featuresizes not easily prepared using relief structures made withthe square shape usually found in conventional resist filmspatterned using photolithography.

Figure l illustrates the procedure for pCP on silverwitha PDMS stamp cast from a blazed difflaction grating. Wespeculate that when the stamp is in contact with the silversurface, the raised areas of the stamp deform as a resultof the attractive force between the stamp and thesubstrate. The lateral dimension of the areas of thepattern in which the SAM is printed is therefore muchlarger than that of the bare areas. Under the conditionswe normally used for pCP,the resultingbare areas on thesilver surface had a width of <300 nm, depending on thesoftness of the PDMS stamp and the period oftime duringwhich the stamp was in contact with the silver surface.

Figure 2A shows an SEM image ofthe blazed diffractiongrating used for the present work. Figure 28 and C showsAFM images of two typical patterns of silver that were

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Figure 3. AFM images of trenches and pyramids anisotro-pically etched in silicon using patterned silver films as resists.(A] The sample of Figure 28 was etched in an aqueous KOFV2-PrOH solution for -20 s at 65 'C; (B) the sample of Figure2C was etched for -1 m. After etching silicon, theiilver miskswere removed by dipping in aqua regia. The dimension of thetrenches in silicon is wider than that in siiver due to undercut-ting.

easily generated usingprCP with a PDMS stamp cast fromthis grating, followed by selective chemical etching in anaqueous ferricyanide solution.2o At present, the smallestfeatures that we have produced using this method haddimensions of -100 nm. The rough edges of the silverlines were caused by the roughness of the features on theoriginal master (Figure 2A). The height of the reliefstructure on the PDMS stamp is important in obtaininghigh-contrast patterns of SAMs. If the height is less than200 nm, no patterns of SAMs form on the silver surface:the SAMs cover the entire area of the substrate that iscovered by the stamp.

Anisotropic etching of silicon with the patterned silverfrlms as secondary masks produced V-shaped trenchesand pyramids in silicon having base dimensions < 800nm (Figure 3). These featured surfaces could be furtherused as masters to cast PDMS stamps for fabricatingstructures ofmetals or polymers with features in the -800nm range of sizes.12'21-23

PDMS Stamps Cast from Relief Structures Etchedin Silver Films. Silver is a relatively inexpensive metaland can be etched rapidly in a variety of aqueous solutionswith printed SAMs of alkanethiolates as resists.a Therelatively high (-290 nm) relief structures formed in silverusing the combination of pCP and selective etching couldbe used to cast PDMS stamps for use in prCP (Figure 4).The silver pattern formed by lrCP with this new PDMS

(21) Xia, Y.; Whitesides, G. M. Unpublished results._-_(22)Xia, Y.; McClelland, J.; Gupia, S.; Zhao, X.-M.; Celota, R.;Whitesides, G. M. Unpublished results.

(23) Kim, E.; Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Unpublishedresults.

Langmuir, Vol. 72, No. 16, 1996 4035

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Figure 5. SEMs of some representative patterns of silver (b0nm thick) that could be generated by double-printing using aPDMS stamp having parallel lines on its surface. The masterswere generated in 200 nm-thick films of silver by pCP withhexadecanethiol, followed by selective etching. In generatingthe masters, the direction of the Iines of the second printingwas rotated by (A) -90", (B) -45", and (c) -2". The brighlregions are silver; the dark regions are siiicon, where the silverhas been dissolved.

stamp and selective etching is complementary to theoriginal silver pattern. This method, therefore, providesa convenient route to negative patterns without furtheraccess to the photolithographic facilities. By using

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multiple-impression printing with PDMS stamps havingdifferent relief structures, we can generate moderatelycomplex patterns and structures from simple initialpatterns. The edge resolution ofthe second silver patternis < 100 nm, a value that is good enough for manyapplications.

Figure 5 shows some representative patterns that havebeen fabricated using this procedure with a single PDMSstamp that has parallel lines (2 pmin width and separatedby 2 prn) on its surface. The patterns on the surfaces ofthe masters were generated in 200 nm-thick silver films;

Figure 8. SEMs of silver patterns (50 nm-thick) that weregenerated by pCP using PDMS stamps cast from assembledmicrostructures of polyurethane, followed by selective etching.In making the polymeric microstructures, the silver surfacewas patterned with parallel lines ofhexadecanethiol, the stampwas then rotated by 90", and another printing was made.Polyurethane prepolymer liquid only assembled on those reglonsof silver that were not covered by hexadecanethiolate SAMs.The bright regions are silver; the dark regions are silicon, wherethe siiver has been dissolved.

the frnal patterns fabricated using the newly cast stamps

were in 50 nm-thick silver films. By rotating the orienta-

tion ofthe lines ofthe second print, we could easilygenerate

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1FmFigure 9. Same as Figure 8, except that the direction of lineswas rotatedby -2 in making the second printing and thatpCP with the newly fabricated PDMS stamp was carried outunder water.

microfeatures with different shapes: for example, squares,rhomboids and parallelograms.

Figure 6 shows two examples of complex patterns thatcan be generated using this procedure with two PDMSstamps: one with parallel lines on its surface and theother with a star pattern. In order to generate well-controlled, complex patterns usingthis procedure, we needa high-resolution translational stage to register theposition and orientation of the pattern formed in eachprinting.

PDMS Stamps Cast from Pol5rmeric Microstruc-tures Assembled on Surfaces of Silver (or Gold)Patterned with SAlVIs. When a liquid prepolymer ofpolyurethane is placed on a silver (or gold) surface thathas been patterned with SAMs ofhexadecanethiolate, theprepolymer selectively dewets from the hydrophobicregions (that is, regions covered by SAMs) of the surface.The remaining prepolymer is then cured into a cross-linked, solid polymeric material. This simple procedurehas been used to fabricate polymeric resists used inselective chemical etching,T arrays of microlenses,ls andmultimode waveguides.le Here we demonstrate that suchpolymeric relief structures formed on Ag or Au can alsobe used as masters to cast PDMS stamps for use in aCP(Figure 7).

Figures 8-10 are SEM images of silver patterns thatwere generated using a three-stage procedure. First, asilver surface was patterned with lines of hexadecanethi-olate using pCP; the same stamp was then used to printa second set oflines with their orientation rotated by -90".Second, a liquid prepolymer of polyurethane was as-sembled onto the hydrophilic regions (bare Ag or Au) ofthe surface and cured into a solid polymer. Third, a new

(A) '

10[mFigure 10. Optical micrographs of poiyurethane mastersassembled on silver surfaces that had been double-printed usingtwo different PDMS stamps: parallel lines and a star pattern.SEM images of the corresponding silver patterns (50 nm-thick)generated by rrCP (under water) using PDMS stamps cast fromthese polymeric microstructures. The bright regions are silver;the dark regions are silicon, where the silver has been dissolved,

PDMS stamp was made by casting against the assembledrelief structures of polyurethane, and this stamp was thenused to make patterned SAMs of hexadecanethiolate ona silver surface. Those regions of a patterned silver surfaceuncovered by SAMs were subsequently removed by etchingin an aqueous ferricyanide solution.a Figure 8 showspatterns formed using PDMS stamps with paraliel lines(different widths and spacings). As the dimension of thebare silver decreases, the height ofthe assembled PU reliefstructures decreases, and the shape of the polymericfeatures also changes from squares to circles as a resultofthe rounding ofthe corners caused by the surface tensionof the prepolymer liquid. When the dimension is less than5 ptn,the height of the polymeric relief structures becomesso small that it is very difficult to obtain high-contrastpatterns of SAMs by printing with the fabricated PDMSstamps (Figure 8D). In Figure 9, the lines between thefrrst and the second printing were rotated by -45'; thusthe assembled polymeric features have an elliptical shape.In Figure 10, the two PDMS stamps have differentpatterns on their surfaces: one has parallel lines, and theother has a star pattern. Again, in order to form well-controlled, complex patterns using this procedure, precisealignment is required for the second printing step.

PDMS Stamps Cast from Polyst5rrene Micro-spheres Assembled on Flat Substrates. Extensivestudies have been carried out on the formation andpotential applications of two-dimensional arrays of poly-styrene microspheres on flat substrates.2a-z8 The prepa-

(24) Skjeltorp, A. T.; Meakin, P. Nature 1988,335,424.(25) Fulda, K.-U.; Tieke, B. Adu. Mater. 1994. 6. 288.(26) Lenzman, F. ;L i , K. ;Ki ta i , A.H.;Stc iver, H. D. H. Chem. Mater,

1994 .6 . 156 .

'ffiffiffilii|ff$ff

4038 Langmuir, Vol. 12, No. 16, 1996

II

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ration ofthese an'ays usuallyinvolves deposition of a thinlayer of a suspension of monodispersed microspheres ona flat surface, followed by evaporation ofthe solvent. Thesemicrospheres usually form a hexagonal dense packing, asdriven by a laminar fl ow and attractive capillary forces. 27'28The arrays of closely packed microspheres have been usedas masks for metal evaporation to generate hexagonalarrays of nanofeatures.26 Here we use these assembledmicrospheres as relief structures to cast PDMS stampsfor use in uCP.

Figure 11 outlines the procedure schematically. Twotypes of stamps were obtained. In the first, the reliefstructures on the surface of the stamp were made fromPDMS. In the second, the relief structures were madefrom the polystyrene beads. Silver patterns generatedusing these two types of stamps are complementary toeach other (Figure 12). Because the polystyrene is not agood elastomer, there is no conformal contact betweenthe substrate and the second type of stamp. As a result,the quality of the SAMs printed using this kind of stampis not as good as that printed using an elastomeric stamp(Figure 128). Polystyrene beads with diameters as smallas -200 nm were suitable for this process. This procedureis only suitable for generating regular patterns havingareas of -5001m2, since these microspheres usually formdomain structures on large flat substrates.2a

Conclusions

This paper demonstrates several non-photolithographicmethods to fabricate PDMS stamps with feature sizes

1993. 99. 9156.D.; Kralchevsky, P. A.

Figure 12. SEM images of gold patterns generated by pCPwith the PDMS stamps (the insets) cast from assembledpolystyrene microspheres, followed by seiective etching. Thebright regions are gold; the dark regions are silicon, where thegold has been dissolved.

ranging from -100 nm to -100 pm for use in pCP.Microcontact printing is an extremely convenient tech-nique for generating patterned SAMs on a variety ofsubstrates. Its utility has been limited mainly by theavailability of PDMS stamps that have various patternsand features on their surfaces. We believe that themethods described here will be useful in providing PDMSstamps with certain types ofmoderately complex patterns,especially for those who want to use pCP but have noaccess to the clean room and photolithographic facilities.

Acknowledgrnent. This work was supported in partbv ONR and ARPA. This work made use of MRSECShared Facilities supported by the National ScienceFoundation (Grant DMR-9400396). We would like tothank Mr. Yuan Lu and Mr. Stephen Shepard for theirassistance in using SEM and photolithographic instru-ments. J.T. is grateful to the NSF for a predoctoralfellowship.

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