Extending Microcontact Printing as aMicrol ithograph ic Technique

Younan Xia and George M. WhitesidesDepartment of Chemistry and Chemical Biology, Harvard University,

Cambridge, Massach usetts 021 38

The ACS Journal of Surfaces and Colloids

Reprinted fromVolume 13, Numb er 7 , Pages 2059-2067

Introduction

Microcontact printing (lCP)l is a very convenient, non-photolithographic technique that can generate patternedfeatures of self-assembled monolayers (SAMs)2 on bothplanar and nonplanar surfaces.s The concept of pCP isquite straightforward. An elastomeric stamp (usuallymade from poly(dimethylsiloxane), PDMS) is fabricatedby casting a prepolymer ofPDMS against a master whosesurface has been patterned with a complementary reliefstructure using photolithograph/ or micromachinings orfrom a commercially available relief structure such as adiffraction grating.o When carrying out prCP on Au, aPDMS stamp is wetted with an *ink" (typically, arrt -2mM solution ofhexadecanethiol in ethanol) and is broughtinto contact with the surface of gold for b-10 s. Thehexadecanethiol (HDT, CH3(CH2)IsSH) transfers from thestamp to the gold, forms a hexadecanethiolate (CHs-(CH2)1ES-), and generates patterns of SAMs on the surfaceof gold.

One of the important applications of pCP has been informing patterned SAMs to be used as ultrathin resists.TSAMs are remarkably effective as primary resists incontrolling the etching of the underlying substrates.Patterning of gold with a hydrophobic, long-chain SAM(typically, derived from HDT), followed by selectivedis solution of the underiva tized,gold in chemical etchants(usually an aqueous solution containing IGSzOy'IeFe-(CN)6/IqFe(CN)o or KCN/O2),8 produced patterned fea-tures of Au on the surface; these gold features could be

617-495-9857. E-mail:

Lithography: Prirrciples and

2059

subsequently used as secondarymasks to define and directthe etching ofthe underlying substrates of SiO2 and Si.e,10More recently, microcontact printing has been extendedto form patterned SAMs of alkanethiolates on silverlo andcopper, I 1, 12 SAMs of alkyl sil oxane s on hydroxyl-terminatedsurfaces,ls-16 and Pd colloids on SilSiOr.t1

Features of patterned SAMs with dimensions largerthan I pm can be routinely produced using pCP. It ismore difficult to generate features with sizes less than 1prm, primarily because fabrication of the conespondingmasters in this range critically depends on the availabilityof advanced microlithographic techniques (for example,e-beam writing, deep IfV, and X-ray photolithography)that are still in development.ls-2O Below 100 nm, materialproperties-espeeially deformation of the elastomericstamp-may limit feature size (or at least require devel-opment of new materials optimized for this regime).

We are developing procedures that can extend thecapabilities ofprCP into the submicrometer range withoutrequiring masters having submicrometer-sized featuresthat have to be generated by less readily availableadvanced lithographic equipment. Basically, our strateryis to start with an elastomeric stamp havingfeature sizesof 2-4 pm (that is, easily made using routine photo-lithography) and to frnd non-photolithographic methodsto produce patterned SAMs with feature sizes srnaller

L220.Chem.

G.; Xia, Y.; Mrksich, M.; Whitesides, G.

Craighead, H. G. Appl. Phys. Lett. 1996, 68,

Langmuir 1997, 13, 2059-2067

Extending Microcontact Printing as a MicrolithographicTechnique

Younan Xia and George M. Whitesides*

Department of Chemistry and Chem.ical Biology, Haruard Uniuersity,Carnbridge, Massachusetts 02 1 3 8

Receiued September 27, 19968

^tbit papeT defcribes a number of approaches that have been employed to reduce the size of featuresofself-assembledmonolayers(SAMs)generatedusingmicrocontactpriniing(pCP). In4CP,anelastomericstamp.is used to Plnt p^atteryed SAMs of alkanethiolates on the surfaces of coinage metals and SAMs ofalkylsiloxanes on SilSiOz. It is a convenient technique for generating patterned'microstructures withfeature sizes > 500 nm. _The capability of this technique cor-rld be extJnded to produce features smallerthan 500 nm using the following approaches: (L) pCp with mechanical deformation of the elastomericstamp-tha_t is, with lateral c^oqapression or uniaxial stretching in the plane of the stamp and with pressureperpeldicular to the plane-of the stamgi Q) pCP with physical alternation of the elastomeric stamp-thatis, with a stamp that has been swelled with a solvenl or a stamp whose dimensions have been r^educedby extraction of an inert fi.ller;(3) pCP with reduction in the size of?eatures resultingfrom processes takingpla99 on the surface-that is, lateral reactive spreading of hexadecanethiol on fold;

"na (+) pCP witfr

m u ltiple impressions on the same surface. The advantages and disadvantages ofeach"approach are evaluatedand compared in this paper.

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2060 Langmuir, VoL 73, No. 7, 1997

than those ofthe original stamps (preferably, in the range

Microcontact Printing

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lpmFigure 2. SEM images of Ag patterns (50 nm thick) that werefabricated with a PDMS stamp of parallel lines that had beencompressed in the direction perpendicular to the lines. Eachpattern was formed by printing twice with the direction of linesrotated by 90' before making the second printing. The firstnumber indicates the compressive strain in the verticaldi_rection. The bright regions are Ag; the dark regions are Siwhere the unprotected Ag has dissolved in an aqueous ferri-cyanide solution.

Right now, the smallest features that we have generatedusing this procedure are -200 nm in dimension; theaccuracy of this procedure is limited by the mechanics ofthe system used to compress the stamp laterally.

Since PDMS deforms isotropically under mechanical

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Langmuir, Vol. 73, No. 7, 1997 206L

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Figure 3. SEM images of Ag patterns (50 nm thick) that werefabricated using microcontact printing with a PDMS stampthat was under one-dimensional (A-C) and two-dimensional(D) compression. The one-dimensional compressionwas in thevertical direction. The bright regions are Ag; the dark regionsare Si where the unprotected Ag has dissolved in an aqueousfericyanide solution.

compression, both the shapes and sizes of the featurespresent on the surface of the PDMS stamp change in acontrollable manner. This procedure can be easily ex-tended to more complex patterns. Figure 3 shows SEMimages of silver patterns that were fabricated using thisprocedure with a moderately complex pattern that hasfeatures with acute angles and nonuniform sizes. Thestamp was under one- and two-dimensional compressionduring pCP, respectively. It is obvious that the size ofcertain features of a test pattern with such a complexitycan also be reduced using this procedure, while retainingthe regularity in the original pattern.

This procedure for generating submicrometer-sizedfeatures is general. It indicates a new type of microli-thography, in which dimensions of selected regions ofthepattern can be made smaller or larger by deforming thestamp mechanically. It also suggests a potential route tothe fabrication of "smart stamps", that is, stamps whosesurfaces can be deformed locally with certain spatialresolution.

Replica Molding against a PDMS Mold underMechanical Compression. The reconfigurated reliefstructures on the surface of a compressed PDMS stampcould also be replicated into a second polymeric materialand then used to cast new PDMS stamps.2a Figure 4outlines the procedure schematically. The size offeaturesis reduced in the lateral compression of this stamp. Thecompressed features are replicated by molding an ultra-violet-eurable, liquid prepolymeric polyurethane (PU)against the compressed stamp. This procedure is itera-tive: a new PDMS stamp can be prepared, by castingagainst the relief structures on the surface of the PUreplica, and then compressed and replicated. Each time

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2062 Langmuir, Vol. 13, No. 7, 1997

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lpmFigure 6. SEM images ofAu patterns (20 nm thick) that werefabricated using microcontact printing of HDT with PDMSstamps that were stretched alongthe direction of the lines (thestrain was IVo, L00Vo, and 300Vo, respectively), foliowed byselective chemical etching.

7A). The raised features on the PDMS stamp deformedin this process of compression: the vertical dimensions ofthe raised features decreased while the lateral dimensionsincreased. As a result, the lateral dimensions of therecessed regions decreased (Figure 78 and C).

This approach does not reduce the period ofthe pattern;it can only be used to reduce the lateral dimensions of thebare regions of gold (that is, regions not derivatized withSAMs). Much effort is required to measure and controlthe vertical pressure; therefore, this procedure cannotgenerate features \Mith the required size reproducibly,unless we invest in a certain amount of mechanicalapparatus.

Microcontact Printing with a Starnp That HasBeen Swelled in a Solvent. Cross-linked PDMS can beswelled in a number of "good" solvents, such as toluene

Langmuir, Vol. 73, No. 7, 1997 2063

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1pmFigure 7. (A) Illustration of size reduction by mechanicallypressing a PDMS stamp against the gold substrate whileconducting microcontact printing. The dashed lines representthe original profrles of the microstructures on the stamp. (Band C) SEM images of Au patterns (20 nm thick) that werefabricated using microcontact printing with PDMS stampswithout and with vertical pressure, followed by selective etchingin an aqueous cyanide solution.

and hexane.26-28 A swelled PDMS stamp still has goodmechanical strength to be used inpCP. For normal PDMSstamps, the dimensions ofboth recessed and raised regionsincrease after swelling in the solvent. We only observedsize reduction for certain features for those thin stampsthat are grafted to a rigid support (Figure 8A). Thesespecial stamps were fabricated using a similar procedureas described for the fabrication of stamps used for pCPunder vertical pressure. Figure 88 and C shows SEMimages ofAupatterns that were fabricatedusinga PDMSstamp without and with swelling in toluene for -30 min.

(26) Bueche, A. M. J. Polym. Sci. 1955, )fi/,97.(27)Hene, L. E. ST.;Dewhurst, H. A.; Bueche, A. M. J. Polyrn. Sci.

1959. -ICCrW. 105.(28) DeBoIt , L. C.; Mark, J . E. Macromolecules lg87 , 20, 2369 .

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2064 Langrnuir, VoL 73, No. 7, 1997 Xia and Whitesides

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lpmFigure 8. (A) Illustration of size reduction by swelling thePDMS stamp in toluene for -30 min before conductingmicrocontact printing. (B and C) SEM images of Au patterns( 20 nm thick) that were fabricated using a PDMS stanr p withoutand with swelling in toluene, followed by selective chemicaletching in a cyanide solution.

After being swelled in toluene, the raised features on thestamp increased in size, and this increase caused areduction in the distance between the raised features.

The swelling process was reversible: the PDMS blockreturned to its original shape after the solvent hadevaporated. The shape of a swelled PDMS stamp couldbe locked into place by using a solvent that can be graftedto the PDMS network or that can be cross-linked into asolid material via thermal or ultraviolet treatment. Ingeneral, this approach to reducing sizes is not as conve-nient as others.

Microcontact Printing with a Stamp Whose Di-mensions Have Been Reduced by Extraction of anInert Filler. In making a PDMS stamp, we added aninert filler into the prepolymer mixture before curing thesystem. After cross-linking (at 65 "C for -10 h) the PDMSprepolymer into a solid material, the inert filler was

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1pmFigure 9. (A) Illustrations of size reduction by extraction ofthe inert filler that was added into the PDMS prepolymer beforecuring. (B and C) SEM images of Au patterns (20 nm thick)that were fabricated using a PDMS stamp before (B) and after(C) removing the filler. T,he bright regions are Au; the darkregions are Si where the unprotected Au has dissolved in anaqueous cyanide solution. In this example, -54Vo (dw) of PS04 Iwas added into the Sylgard 184 mixture (A:B : 1:10).

removed by extraction with a solvent to reduce the volume(and, therefore, the dimension in each direction) of thePDMS block.26-28 We have used a number of linear, lowmolecularweight oligomers ofPDMS such as silicone fluids(PS039, PS040, PS041, Hiils) as the fillers. These fluidsmix well with the Sylgard silicone prepolymer and can beextracted easily with toluene. After extraction ofthe flller,the volume of the PDMS block as well as the dimensionsofthe microfeatures on its surface decreased isotropicallyin all three directions (Figure 9A). We have been able toload as much as -687o (dw) of PS041 to the mixture ofSylgard prepolymer. The calculated reduction in the sizeof features was inconsistent with that measured using alight diffraction method. Figure 9B shows lines of goldthat were fabricated using microcontact printing with aPDMS stamp that has -54Vo (w/w) PS041 in it. Figure9C shows gold lines that were fabricated using this samestamp after the PS041 has been extracted by immersingin toluene for -30 min. The period of the pattern and thedimensions of both lines and the spacings between lineswere reduced in this process. However, afber the extrac-

Microcontact Printing

tion of the frller, the stamp became very soft. When webrought the stamp into contact with the gold surface forprinting, the raised areas of the stamp deformed againand resulted in the characteristics observed in the Snntphotographs shown here, that is, less reduction in thedimensions of the lines than in the dimensions of thespacings between lines.

This approach can only be used to achieve size reductionby a factor of -1.4. It was the only procedure that allowedus to reduce the size of the microfeatures in all threedirection. we can design an iterative procedure (such asthat shown in Figure 4) that will allow us to achieve moresignificant reduction (by a factor of >b) in the size of thefeatures.

Microcontact Printing Using Controlled ReactiveSpreading. This process is shown in Figrrre 1 schemati-cally. The lateral spreading ofHDT liquid (predominatelyfrom the edge of the patterned SAM beyond the area ofthe stamp in contact with the substrate) on the surfaceof gold reduces the dimensions ofthe bare regions (Figure10A and B). The alkanethiol used in this p"ocess mustbe insoluble in water;thus, the water acts ai a barrier tothe diffirsion ofHDT from the recessed regions ofthe stampto the surface of gold. The distance d (pm) over which thledge of the SAM advances through reactive spreading isrelated to the concentration of HDT tCl (mM) and theprinting time t (min) by the empirical equation d.2 =(0.L6lcluz)t. The dependence of d on r is consistent witha model proposed for the spreading of a liquid on a solidsur{'ace.29,30

Conducting pCP under water is critical for the successof this procedure. If printing was carried out in air, HDTliquid did not spread on gold: it formed an autophobicsystem-a liquid in contact with a surface modifies thechemistry of the surface and lowers its solid-vapor andsolid-liquid surface tension, and the liquid ietractsspontaneously.sl In this case, the stamp cannot be keptin contact with the gold surface for intervals longer than30 s; otherwise, disordered SAMs also formed on thoseregions not in contact with the stamp by the diffusion ofHDT from the recessed regions of the stamp to the goldsurface through the vapor phase.

This method only reduces the dimensions of certainfeatures-bare regions of gold underivatizedwith SAMs.The dimensions of the whole patterned area cannot besh_runk by using this technique alone. In combining withselective etching of gold, this method can be used tofabricate arrays of nanometer-sized trenches or grids ingold that are separated by several micromete., (Figur"10C and D).

Microcontact Printing with Multiple Impressionson the Same Surface. Muttiple printings ol SAMs onthe same surface were also possible: we could use thisapproach to reduce the size of certain features of SAMsorto generate more complex patterns from a single stamponly having simple patterns on its surface. Figure 11Aand B illustrates the use of this procedure for sizereduction. The dimensions ofthe printed regions (that is,regions derivatized with sAMs ) increased aft er the secondprinting while the dimensions of the bare regions de-creased. Figure llC shows the application of multipleprintings in formingnew patterns. The gold patterns (20nm thick) were generated by a double-printing proce-dure: a PDMS stamp having parallel lines on its surfacewas first contacted with the Au surface for -10 s: it wasthen removed, cleaned with ethanol, rewetted with HDT

, Lz-g) Lapez, J.; Miller, C. A.; Ruckenstein, E. J. CoIIoi.d,Interface Sci.1979,56, 460.

(30) Greenspan, H. P. J. Fluid Mech. lg79, 84,IZS.(31) Biebuyck, H. A.; Whitesides, G. M. Langrnuir lgg4, 10,45g1.

Langmuir, Vol. 13, No. 7, 1997 20GE

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$ffi$ffiFigure 10. SEM images of Au patterns (20 nm thick) thatwere fabricated using microcontact printing of HDT underwater, followed by selective chemical etching in an aqueouscyanide solution. By carrying out microcontacl printing underwater for several minutes, it was possible to acfiieve asubstantial reduction in dimension for the bare Au (that is, notcovered by the SAMs) from -2.5 to -0.1 pm (A and B). Bycro ss - stamping with this proce dure, it was p ossible to fabricategn tlray ofAu gnds with submicrometer-sized features (C andD). The bright_regions are Au covered by sAMs; the dark regionsare si where the underivatized Au has been removed by etJhingin CN-/O2.

ink, and recontacted with the Au surface for -10 s, withthe orientation of the lines rotated by -2. The printedsamples were then etched in an aqueous cyanide solution

2066 Langrnuir, VoI. 73, No. 7, 1997

10 prn 1 pmFigure 11. (R and B) Illustration of size reduction by double-printing. The gold patterns (20 nm thick) were fabricated by(A) stamp once and (B) stamp twice (after the first printing, thestamp was removed from the Au surface, it was translated inthe direction perpendicular to the lines for a certain distance,and the second printing was made). (C) Illustration of makingnew patterns using double-printing. The lines were rotated bya small angle (-2") before making the second printing.

to remove the unprotected gold. Note that the dimensionsof the features etched in the gold film are only -200 nmat the tip.

By rotating for different angles between these twoprintings, we could produce patterns with a variety of

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1 prmFigure 12. TWo more examples to illustrate the applicationof double-printing in making new patterns. (A) SEM image ofa test pattern ofAu (20 nm thick) generated by double-printing,followed by seiective wet etching in an aqueous ferricyanidesolution. (B) SEM image of another test pattern ofAu that wasfabricated using a modified procedure for double-printing (seethe text for details). The bright regions are Au covered by SAMs;the dark regions are Si where the underivatized Au hasdissolved. The stamp used here only had parallel lines on itssurface.

shapes and sizes (for example, Figure 1^2A, rotated by-45). Moreover, we could remove hexadecanethiolateSAMs on Au by immersing the patterned sample inpiranha solution (a mixture ofHzSO+ (987o) and HzOz (30qo)at the ratio of 7:3). This property opens the door togenerate new forms of patterns that cannot be producedsimply by double-printing. Figure 128 show the SEMimage of a gold pattern that was fabricated using a frve-stage procedure: (1) lines of hexadecanethiolate SAMswere printed on a Au surface; (2) this sample was etchedin an aqueous ferricyanide etchant for -7 min; (3) thehexadecanethiolate SAMs on the remaining Au wereremoved by immersing the sample in piranha solution for-5 s; (4) lines of hexadecanethiolate SAIVIs were printedon this Au surface, with the orientation ofthe lines rotatedby -45"; (5) this sample was etched in the fericyanideetchant for another -7 min. The Au pattern generatedin this way was complementary to that fabricated usingthe normal procedure of double-printing (Figure l2A).

At present we conduct microcontact printing by hand,and it is difficult to achieve accurate alignment between

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Microcontact Printing

the stamp and the substrate. Once a good procedure forregistration is available, we believe that this approachcan be used to generate features with sizes of