Micromolding of Polymers in Gapillaries:Applications in Microfabrication

Younan Xia, Enoch Kim, and George M. WhitesidesDepartment of Chemistry, Harvard University,

Cambridge, Massachusetts 021 38

ChEmlsrnyofMATERIAIS'

Reprinted fromVolume 8, Number 7, Pages 1558-1567

1558

Introduction

Micromolding in capillaries (MIMIC) is a convenienttechnique for generating patterned microstructures oforganic polymers on the surfaces of solid substrates.lIn this technique, an elastomeric block (typically madefrom poly(dimethylsiloxane), PDMS) having a patternedrelief structure in its surface is prepared by casting theelastomer against a master using procedures developedfor use in microcontact printing (pCP);z,s the master canbe prepared by photolithography or by a range ofothertechniques.a's The PDMS mold that is placed on thesurface of a substrate makes conformal contact withthat surface; as a result, a network of channels is formedbetween the mold and the substrate. The substratescan be either planar or curved. When a low-viscosityIiquid prepolymer is placed at the open ends of thenetwork of channels, the liquid spontaneously fills inthe channels by capillary action. After filling thechannels and curing the prepolymer into a solid, cross-linked polymer, the PDMS mold is removed, and anetwork of polymeric material remains on the surfaceof the substrate. A variety of liquid prepolymers6 (boththermally and ultraviolet curable) such as polyurethane(PU), polyacrylate, and epoxy can be used in this

Chem. Mater. 1996' & 1558-1567

Micromolding of Polymers in Capillaries: Applicationsin Microfabrication

Younan Xia, Enoch Kim, and George M. Whitesides*

Department of Chemistry, Haruard Uniuersity, Cambridge, Massachusetts 02138

Receiued April B, 19968

This paper describes the use of micromolding in capillaries (MIMIC) to produce com-plex polymeric microstructures supported on different substrates and the applications ofthese microstructures in microfabrication. Patterned microstructures of several organicpolymers-polyurethane, polyacrylate, and epoxy-were formed by molding in enclosed,continuous channels formed by conformal contact between a solid support and an elastomericmold whose surface had been patterned with a relief structure having micrometer-scaledimensions. A liquid prepolymer frlled these channels by capillary action and was allowedto cure photochemically or thermally. The mold was then removed. Polymeric microstruc-tures formed on films of Saran Wrap could be folded into different shapes, while thesemicrostructures retained their forms; they could also be stretched uniaxially to generatemicrostructures having distorted forms. The patterned polymeric microstructures formedon SiOz, glass, and metals (Au, Ag, and Cr) could be used directly as resists in the selectiveetching of underlying substrates. Free-standing polymeric microstructures fabricated bylift-off were used as disposable masks to generate patterned microfeatures of metals on thesurfaces of both planar and nonplanar substrates in two different procedures: (a) evaporationof gold through the polymeric mask supported on a substrate; (b) formation of patternedself-assembled monolayers (SAMs) by exposure of a silver film covered by a polymeric maskto hexadecanethiol (HDT) in vapor, followed by selective etching of the regions that werenot exposed to HDT (that is, the parts of the surface protected by the mask) in an aqueoussolution containing KzSzOs and I&Fe(CN)o/IQFe(CN)e.

process. These polymeric microstructures can be re-leased by dissolving the underlying substrates to formfree-standing polymeric membranes (with an area of upto several cm2). MIMIC has also been used to formpatterned microstructures of inorganic materials, met-als, and polymeric microbeads.?

Photolithography,8 micromolding, and related tech-niquese-l3 are currently used to form patterned micro-structures of polymeric materials. Microfabricationbased on MIMIC is remarkable for its simplicity, for itseconomy, for the effectiveness with which the channelsfrll by capillary action, and for its fidelity in transferringthe patterns from the mold to the polymeric structuresthat it forms. MIMIC is applicable to patterning abroader range of organic polymers-especially structuraland functional polymers 14 - 16-than is photolithography,

(7) Kim, E.; Xia, Y.; Whitesides, G. M. Adu. Mater. 1996, 8,245.Kim, E.; Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc., in press.

(8) Moreau, W. M. Semiconductor Lithography: PrincipLes andMaterials; Plenum: NewYork, 1988.

(9) Injection molding: Dijksman, J.F. Philips Tech. Reu. 1989,44,2 t2 .

(10) Micromolding/replication: Van Rijsewijjk, H.; Legierse, P. E.J.; Thomas, G. E. Philips Tech. Reu. 1982,40,287. Hutley, M. C.Diff-raction Gratirrys; Academic Press: New York, 1982.

(11) Polymerization in the pores of membranes: Martin, C.M.Acc.Chem. Res. 1995, 28,6I . Hoyer, P. ;Baba, N. ;Masuda,H.Appl . Phys.Let t . 1995,66,2700.

(12) Capillary frll encapsulation: Shaw, J. E. A. Sensors Actuators(A) 1993, s7-38,74-76.

(13) Embossing: Emmelius, M.; Pawlowski, G.; Vollmann, H. W.Angew. Chem., Int. Ed. Engl. 1989, 28,7445. Chou, S. Y.; Krauss, P.R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114.

(14) Yu, L.; Chan, W.; Dikshit, S.; Bao, Z.; Shi, Y.; Steier, W. H.Appl . Phys. Let t .1992,60, 1655.

(15) Shi, Y.; Steier, W. H.; Chen, M.; Yu, L.; Dalton, L. R. Appl.Phys. Lett. 1992, 60, 2577 .

(16) Hanemann, T.; Nodl, C.; Haase, W. Adu. Mater. L995,7,465.

O 1996 American Chemical Societys0897-4756(96)00219-0 CCC: $12.00

Micromolding of Polymers in Capillaries

in which only certain specialized polymers are used asphotoresists.8

In this paper, we report representative applicationsof MIMIC in microfabrication. These applications in-clude (a) the formation of patterned polymeric micro-structures on the surfaces of various solid substrates,(b) the use of these polymeric microstructures as resiststo protect the underlying substrates from chemicaletchants (for example, to prevent etching of SiOz or glassby aqueous HFA.{H+F solutions), (c) the use of MIMICand selective etching to fabricate chrome masks for usein photolithography, and (d) the use of free-standingpolymeric microstructures formed using MIMIC and lift,-off as disposable masks in the deposition of patternedmetals, and in the formation of patterned self-assembledmonolayers (SAMs)1? on both planar and nonplanarsubstrates. Most of these applications have not beenoptimized: they are intended to demonstrate principlesthat might underlie detailed development work.

Experimental Section

Materials. Poly(dimethylsiloxane) (Sylgard 184) was ob-tained from Dow Corning. Illtraviolet-curable polyurethanes(J-91 and NOA 60, 71, 72,73,88) were obtained from SummersOptical (Fort Washington, PA) and Norland Products (NewBrunswick, NJ), respectively. llltraviolet-curable polyacrylate(SK-g) was obtained from Edmund Scientifrc Co. Thermallycurable epoxies (F113 and F114) were obtained from TRA-CON(Medford, MA). Au (99.999Vo), Ag (99.9999Vo), and Ti (99.997o)were obtained from Aldrich. Hexadecanethiol (HDT) wasobtained from Aldrich and was purifred under nitrogen bychromatography through silica gel.

Substrates. Si(100) wafers (Cz, phosphorous-doped, testgrade, SEMI Std. flats, covered by native or thermal oxide)were obtained from Silicon Sense (Nashua, NH). Silver films(50 nm thick) were prepared by e-beam evaporation onto Siwafers (primed with a 2.5 nm thick layer of Ti). Siliconsubstrates having V-shaped grooves in their surfaces wereprepared using published procedures.ls'le Films of Saran Wrap(Stretch-tite) were obtained from Polyvinyl Films (Sutton, MA).

Quartz plates covered by chrome were obtained from Hoya Inc.(Tokyo, Japan).

Micromolding in Capillaries (MIMIC). Elastomericmolds were fabricated from PDMS (Sylgard 184, A:B : 1:20)using the procedures described for pCP.3 The thickness of thePDMS blocks used in MIMIC was usually -2 mm. The PDMSmold was cut (using arazor blade) in such a way that it formeda network of channels having open ends when it was placedon the surface of a support (Figure 1). When a low-viscosityliquid prepolymer was placed at the open ends of the channels,the liquid spontaneously filled the channels by capillary action.The initial rate at which a liquid prepolymer of PU (NOA 73,11 x 150 cP) frlled a channel with cross-sectional dimensionsof -3.0 x 1.5 flm, over a SVSiOz surface, was -0.9 mm/min.Ultraviolet-curable polymers were cured with an ultravioletlight (Canrad-Hanovia 450 W medium pressure, mercuryvapor lamp, type 7825-34) for -20 min, with the lamppositioned at a distance of -3 cm from the sample. Thermallycurable epoxies were cured in an oven heated at -60 'C for-6 h. All the polymeric microstructures in this paper weremade of PU (NOA 73) unless mentioned in the text.

Evaporation of Metals and Fomation of SAMs throughPolymeric Masks Supported on Solid Substrates. Free-standing polymeric microstructures that were used as dispos-able masks for the deposition of patterned gold and for theformation of patterned SAMs were prepared using a two-stageprocedure: (i) a continuous polymeric microstructure having

(17) Dubois, L. H.; Nuzzo, R. G. Annu. Reu. Phys. Chem. 1992,43,437.

(18) Peterson, K. E. Proc. IEEE 1982,70,400-424.(19) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996,

143, t070.

Chem. Mater., Vol. 8, No. 7. 1996 1559

1) P lace l iqu id prepolymeraround the PDMS mold

2) F i l l channels bycapil lary action

@Frame

II Cut away the frameV

-^--

W

PDMSmord

I Ptace on a support

| 1 )Cure : remove PDMS

| 2) Lift-off

Figure l. Schematic illustration of MIMIC. After filling thenetwork of channels with a liquid prepolymer, the prepolymerwas cured either thermally or photochemically. The PDMSmold was then peeled away carefully. The patterned polymericmicrostructure remained on the support and could be releasedfrom the surface of the support by dissolving the support.

micrometer-scale features in it was formed using MIMIC ona Si wafer covered with 0.2,rzm-thick thermal oxide; (ii) thepolymeric microstructure was freed from the support bydissolving the sacrifrcial layer of SiOz in an aqueous HF/NH4Fsolution (250 mL of HzO, 165.5 g of NHaF, and 40 mL of 48VoHF) for -20 min.

In the present procedure, a liquid prepolymer was placedaround the PDMS mold. The liquid filled the channels fromall sides. After curing, the excess poiymer that remainedoutside the PDMS mold formed a "frame"-that is, a relativelythick, strong polymeric structure around the thinner, morefragile, patterned microstrcuture (Figure 1);leaving this frameattached made it easier to handle this free-standing polymericmicrostructure: it could be placed on the surface of a planarsubstrate using tweezers and used subsequently as the maskin the deposition of patterned gold. The frame could aiso becut away using a razor blade. This free-standing polymericmicrostructure without a frame was allowed to float on thesurface of water and then picked up by a nonplanar substrate(for example, a glass capillary or a contoured silicon surface)from underneath.

Instrumentation. Gold was evaporated through polymericmasks supported on solid substrates using argon plasmasputtering (Hummer II, Technics Inc.). SEM was done on aJEOL JSM-6400 scanning electron microscope. An accelerat-ing voltage of 3 kV was used for glass substrates and 15 kVfor silicon wafers. Polymeric microstructures were sputteredwith gold (-100 nm thick) before imaging by scanning electronmicroscope.

1560 Chem. Mater., Vol. 8, No. 7, 1996

PDMS MOId

Xia et al.

PU Complement

2pm

micromachined silicona or wax5 structure, etc.); theelastomeric character of PDMS enables it to make aconformal contact with the substrate; the relatively lowinterfacial free energ'y of the surface of PDMS (yeousz.i.: 2L.6 dyn/cm)2o gives adequate adhesion between themold and the substrate but allows it to be separatedfrom both the support and the polymeric microstruc-tures formed by MIMIC. The surface of cured PDMS

i:

i {dr,$ F:iiir i,$f: ii:tI: ii

-ll ij: ft'i:i. $.+!t $i$ $,H $ ,

*#: s:

l ; f f i '$

(f)

2pm

75 pm

Figure 2. Comparison of a PDMS mold with a topologically complex surface and ttre complementary PU structure made againstit using MMIC. (a, b) SEM images of the PDMS mold at two ditrerent magaificationo. (c) Atr'M inage of the PDMS mold. (d, e)SEM images of the corresponding PU microstructures formed on a Si/SiOz eurface. The patterned PU layer ha6 features {'ithdifferent ttricknesses; the thinnest parts ofthe layer (indicated by the arrow in (e)) are less than 100-nm thick. (f) AFM image ofthe PU complement. (g) Fracture view by ecarming electron microscopy (at 60") of the PU layer.

Results and Discussion

Micromolding in Capillaries (MIMIC). Figure 1shows the procedure used in MIMIC schematically.r Thechoice of PDMS as the material to fabricate the mold isimportant for the success of this process;2O it faithfullyreplicates the features of the master structure (photo-Iithographically formed pattern,3 diffraction grating,2l

I

10 pm:

(20) Silornne Polymers; Clarson, S. J., Semlyen, J. A., Eds.; PrenticeHall: Engleword Cliffs, NJ, 1993.

Micromolding of Polymers in Capillaries

Figure 3. SEM images (at three different magnifrcations) ofa free-standing polymeric (PU, J-91) microstructure that wasformed between two PDMS molds. Each mold has a reliefpattern of parallel lines in its own surface. The free-standingmicrostructure was placed on a piece of Scotch tape (there wasa hole in the tape, just under the polymeric microstructure,to demonstrate that the microstructure had the structuralstability to stand without support) using tweezers. The sizesof features in regions I, II, and III are different, as a result,the electron interference patterns (Moir6 effect) from thesethree regions are different. The two blowups were taken fromregion I.

has very low reactivity toward the organic prepolymers

under the curing conditions used here-another require-

Chem. Mater., Vol. 8, No. 7, 1996 1561

Spacer+

PDMSJ t-J lJ l-J r-r l-

I J I J I J U

PDMS

Spacer

Fill; cure; separatePU microstructure

ts Pattern I

{-r- Pattern ll

Figure 4. SEM images of a free-standing microstructure ofPU that was formed between two PDMS molds that haddifferent relief patterns in their surfaces and were separatedby a certain distance using a spacer (Scotch tape) whencarrying out MIMIC. After filling the channels and curing thePU prepolymer into a cross-linked polymer, the two PDMSmold were separated;the PU microstructures remained in oneof the molds and couid be released using tweezers.

ment for the separation of the mold from the formedpolymeric microstructures. The liquid prepolymer usedin this process should be curable either thermally orphotochemically; it should have low viscosity (<400 cP);it should wet the support at least partially;r,22-24 and itshould not contain compounds (such as certain organicsolvents) that swell the PDMS mold.2o The liquidprepolymer can fiII the channels from all sides of thePDMS mold; the air trapped in the center seems toescape by diffusing through the PDMS mold and/or theliquid prepolymer.

Figure 2 compares a PDMS mold (with a topologicallycomplex surface) and the complementary PU micro-structure made from it using MIMIC. The PDMS moldwas made by casting against a microelectronic circuitand shows the topology of that circuit. The PU micro-structure is an accurate negative replica of the reliefstructure in the surface of the PDMS mold. MIMICallows quasi-three-dimensional processing (that is, pat-

(22) Kim, E.; Xia, Y.; Whitesides, G. M., unpublished results.(23) Dong, M.; Chatzis ,I. J. Colloid Interface Sci. 1995, 172, 278.(24) Meyer,D. Surfaces, Interfaces, and Colloids; VCH Publishers:

New York, 1991; pp 87-109.

166F

1562 Chem. Mater., Vol. 8, No. 7, 1996

terning layers with different thicknesses) in a singlestep. Complex arrays of micrometer- and submicrome-ter-scale channels filled completely: in some regions ofthese structures, features are connected to one anotherby channels with thicknesses less than 100 nm.

The support used in MIMIC could have relief patternsin its own surface. Figure 3 shows SEM images (atthree different magnifications) of a free-standing poly-meric microstructure that was formed between twoPDMS molds. Each PDMS mold has a relief pattern ofparallel lines in its own surface. Afber filling with liquidprepolymer and curing it into a solid polymer, the twoPDMS molds were separated. The cross-linked poly-meric microstructure remained on the surface of oneof the two PDMS molds and could be easily removedfrom the mold using tweezers. The two layers of thepolymeric lines formed one inlerconnected polymericmicrostructure. This type of free-standing micro-structure-two layers with an independent relief struc-ture in each-cannot be fabricated by photolithographyin a single step. In carrying out MIMIC, the surfacesof the two PDMS molds could also be separated by acertain distance using a spacer (for example, Scotchtape). In this case, a free-standing polymeric film witha different patterned relief structure on each side of thefilm was formed (Figure 4).

We have used MIMIC to form patterned polymericstructures on a variety of substrates, including Si/SiO2,glass, gold, silver (both bare and covered by SAJVIslT),and films of organic polymers such as Saran Wrap andthe transparencies used for overhead projection. Figure5 shows SEM images of PU microstructures that wereformed on films of Saran Wrap using MIMIC; thesefilms folded subsequently into different shapes. Thepolymeric microstructures roughly retained their formsduring these deformations, since cross-linked polyure-thane is itself modestly flexible. Figure 6 shows SEMimages of PU microstructures that were formed on filmsof Saran Wrap using MIMIC, and were subsequentlystretched uniaxially to different values of strain. As theunderlying frlm of Saran Wrap was elongated, the PUstructure deformed accordingly. The PU structuresbroke and became unrecognizable at an elongation of-500%.

Polymeric Microstructures as Resists in Selec-tive Etching. One of the applications of MIMIC is toform patterned polymeric microstructures that cansubsequently be used as resists in the selective etchingof underlying substrates. Figure 7 shows a series ofSEM images that illustrate the procedure used in thisapplication. Figure 7a shows the SEM image of a teststructure (parallel lines) of PU that was formed on Si/SiOz. Figure 7b shows a SEM image of this samesample, after it had been etched in an aqueous HF/I{H4F solution for -2 min;bare regions of SiOz dissolvedin the aqueous HFA{H4F solution.zs The resultantpattern of SiOz was then used as the secondary resistin the anisotropic etching of Si in a hot aqueous KOIV2-propanol solution.ls'25 Figure 7c shows a SEM imageof the sample of Figure 7b that was subsequently etchedin an aqueous KOW2-propanol solution (400 mL of H2O,92 g of KOH, and 132 mL of 2-propanol) at 65 oC for-15 min.25 The cross-linked PU dissolved or lifted offin the KOlV2-propanol solution at elevated tempera-

Xia et al.

Figure 5. SEM images of patterned PU microstructures thatwere formed using MIMIC on films of Saran Wrap. Thesepatterned frlms folded into different shapes; the PU micro-structures adhere well and roughly retained their forms. Thestrips (A, B) were produced accidentally on certain regions ofthe samples when folding the films mechanically.

tures and could, therefore, not be used directly as theresist in the etching of Si.

Patterned frlms of organic polymers (usually pat-terned using photolithography or e-beam writing) are,of course, ubiquitous as etch resists. The ability to formpatterned films of organic polymers other than photo-resists and to use MIMIC to form polymeric microstruc-tures in a nonphotolithographic procedure provides anew approach to the formation of these resist structures.

Using MIMIC and Seleetive Etching to Fabri-cate Chrome Masks to be Used in Photolithogra-phy. Patterned polymeric microstructures formed us-ing MIMIC can also be used as resists in selectivechemical etching of metals such as Au, Ag, and Cr. Thisability provides a new route for generating chromemasks (that is, masks made of thin films of metallicchromium supported on quartz plates) to be used in

Micromolding of Polymers in Capillaries

Saran WrapI

Chem. Mater., Vol. 8, No. 7, 1996 1563

+- SiOz(0.2pm)

Etch SiOz

Etch Si

PUt

II

1f pm

;;f

:'rr:iirr::;,,;-i;:i;i::::i

-

-10pm

Figure 7. (A) SEM image of a test pattern of PU formed usingMIMIC on a silicon wafer covered by thermally-formed SiOz(0.2 pm thick). (B) SEM image of this sample after etching inan aqueous HFA.IHaF solution for -2 min. The ends of thepolymer strips deadhered on fracturing the silicon substrate.(C) Same sample as (B) after further etching in an aqueousKOW2-propanol solution for -15 min at 65 "C. The PUstruetures dissolved or lifted off in the KOlV2-propanolsolution at this temperature.

formed on chrome substrates using MIMIC; thesepolymeric microstructures were subsequently used as

resists in the etching of chrome in an aqueous solution

10 pmFigure 6. SEM images of a test structure of PU that wasformed on frlms of Saran Wrap using MIMIC and thenstretched along the horizontal direction. The stretched formswere fixed using Scotch tapes. As the underlying film of Saranwrap was elongated, the PU structure deformed accordingly.The PU structures broke and became unrecognizable at anelongation of -500Vo.

photolithography. Figure 8A outlines the procedure

schematically. Patterned microstructures of PU were

A)

1564 Chem. Mater., Vol. 8, No. 7, 1996 Xia et al.

++

Au

+if

Au

++ +

rfT-rT-rT-rn MaSk

Figure 9. Schematic illustration of evaporation of goldthrough a patterned polymeric mask onto a planar (A), curved(B), or contoured surface (C, D). The masks used in thisprocedure were thin (-1.5 pm) free-standing microstructuresof PU fabricated using MIMIC and lift-off.

formed a frame around the patterned polymeric micro-structure, Figure 1); (ii) release the patterned polymericmicrostructure by dissolving the support-for example,dissolving glass or SiOz frlms in aqueous HFA'IHaFsolutions, single crystals of NaCl and KCl in water, andphotoresist frlms in the developing solutions. Free-standing polymeric microstructures having micrometer-scale features in them and with macroscopic dimensionsup to several cir;r2 have been prepared. The free-standing polymeric microstructures could be transferredto the surfaces of other substrates using two differentprocedures: (a) polymeric microstructures \{rith framescould be placed on the surfaces of solid substrates usingtweezers (Figure gA,C); (b) polymeric microstructureswithout frames (the frame could be removed using arazor blade) were allowed to float on the surface ofwater, and were subsequently picked up by other solidsubstrates from underneath (Figure 9B,D).

Figure 10a shows a SEM image of the free-standingpolymeric microstructures that were subsequently trans-ferred onto various solid substrates and then used asthe disposable masks in the deposition of patternedmetals and in the formation of patterned SAMs. Figure10b shows SEM images of a planar Si surface that wascovered with a free-standing polymeric mask and thathad been patterned with gold using evaporation throughthe mask. Some portions of the mask still remainedon the surface. Figure 10c shows a SEM image ofpatterned silver features that were formed on Si/SiOzusing a three-stage procedure: (i) a piece of free-standing polymeric mask was placed on a fu film (50

Xtt ,/Polymer

+\Au

+ + + l+

Au

l++1) Etch Cr2) Remove polymer

ffiMask

c)

Commercial mask

-10 pm

Figure 8. (A) Schematic procedure that was used to fabricatechrome-masks using MIMIC. (B) Optical micrographs of thechrome-masks that were produced using MIMIC and theconventional method, respectively. (C) SEM images of a testpattern that was generated in positive-tone photoresist films(Microposit 1813, Shipley) by photolithography using the newlyfabricated mask (made using MIMIC) and the commercialchrome mask, respectively.

containing HNO3 and NHaNOs'Ce(NOs)s (400 mL ofHzO, 24 mL of 63Vo HNO3, 62 g of NHaNOs'Ce(NOe)s;the etching time was -1 min).26 Figure 88 comparesthe optical micrographs (in transmission mode) of thechrome masks prepared using MIMIC and the conven-tional procedure2T (obtained commercially), respectively.Figure 8C shows SEM images of the resulting micro-structures in photoresists (Microposit 1813, Shipley,MA) fabricated by photolithography using the above twochrome masks, respectively. The chrome-mask fabri-cated using MIMIC has edge resolution as good as thecommercial one.

Free-Standing Polymeric Stmctures as Dispos-able Masks in Patterning the Surfaces of Planarand Nonplanar Substrates. Free-standing polymericmicrostructures can be easily fabricated using a two-stage procedure (Figure 1): (i) form patterned polymericmicrostructure on a solid support (after curing, theexcess polymer remained outside the PDMS mold

(26) Thin FiIm Processes; Vossen, J.L., Kern, W., Eds.; AcademicPress: New York, 1978.

(27) Larrabee. G. B. CHEMTECH 1985. March.768.

Si

Mask by MIMIC

Micromolding of Polymers in Capillaries

Figure 10. (a) SEM image of a free-standing PU microstruc-ture that was used subsequently as the disposable mask forpattering different substrates using evaporation of metals orformation of SAMs. (b) SEM of a planar SilSiOz surface thathad been patterned with gold by evaporation through themask. In this picture, half of the mask still remained on thesurface. The inset is a blowup to show the edge resolution ofthis pattern. (c, d) SEMs of patterned features of silver (brightregions) that were generated on a Si/SiO2 surface by selectiveetching using patterned SAMs of hexadecanethiolate as resiststhat were formed using free-standing PU frlms as the masks(see text for detail).

nm thick) evaporated on Si/SiOz; (ii) regions of the Agsurface not covered by the polymeric mask were deriva-tized with SAMs of hexadecanethiolate by exposing thesample to the vapor of HDT in a closed container atroom temperature for -30 min; (iii) the polymeric maskwas then removed, and the sample was etched in anaqueous solution containing KzSzOs and I&Fe(CN)orIq-Fe(CN)o (0.5 IW0.01 n/V0.001 M) for -20 s;1e this

Chem. Mater., VoL 8, No. 7, 1996 1565

100 pm

Figure 11. (a) SEM images of a glass capillary (the radius ofcurvature is -400&m) that was covered by the polymeric maskand was exposed to gold by evaporation for -5 min. The imageis of the mask resting on the capillary, with both mask andglass covered with gold. (b) SEM images of the glass capillaryof (a) afber removing the polymeric mask by dipping thecapillary in an aqueous KOW2-propanol solution at 65 oC for-2 min. Gold patterns (-100 nm thick) were formed on theglass capillary. The inset is the blowup of the correspondingSEM image.

procedure dissolved those regions of the silver film thatwere not covered with the SAM. Figure 10d shows aSEM image of patterned silver features that wereproduced on Si/SiO2 using a procedure similar to thatof Figure 10c, except that the HDT was transferred tothe unmasked regions of the silver surface using a flatPDMS stamp "inked" with a solution of HDT in etha-nol.2'3 When an "inked" flat PDMS stamp was placed(in this case, for -1 min) in contact with a polymericmask that was in turn resting on the surface of anevaporated silver film, SAMs formed on those regionsnot covered by the mask by processes that probablyinclude reactive spreading and the diffusion of HDT tothe silver surface through the vapor phase.28 Since theSAM formed by reactive spreading is more ordered thanthat formed by exposure to HDT vapor for the shortperiod of time used here (<5 min), the edge of the silverpattern has a lower density of defects than the centerpart of the pattern (FigUre 10d).

Figure 11a shows a SEM image of a glass capillary(the radius of curvature is -400 um) whose surface was

,,rMask1

1566 Chem. Mater., Vol. 8, No. 7, 1996

covered with a free-standing polymeric mask and thensputtered with gold. In this procedure, a patterned free-standing PU film (without a frame) was transferred tothe surface of the capillary by floating the frlm on thesurface of water and then picking up the fiIm carefullywith the capillary from underneath. The free-standingpolymeric microstructure was thin enough (-L.5 pmthick) to bend and to make conformal contact with thesurface of the capillary (see the inset of Figure 11a).Gold was sputtered onto this system for -5 min (thethickness of evaporated Au was -100 nm). Afterevaporation of gold, the mask was dissolved by dippingin an aqueous KOW2-propanol solution for -2 min(Figure 11b). The defects seen in the pattern wereprobably caused by the removal of gold from thecapillary surface during the stripping of the polymericmask, since no adhesion promoter (for example, Cr orTi)2e was used in this demonstration. This procedureprovides a new approach to forming patterns havingmicrometer-scale dimensions on curved surfaces.sO Pat-terning curved surfaces is difficult by current photo-lithographic techniques.

Using a free-standing polymeric microstructure as themask, it was also possible to form patterned features ofmetals on a contoured surface. Figure 12a shows a SEMimage of a contoured silicon surface that was maskedby a free-standing polymeric microstructure (see Figure9C; the mask had a frame and was placed on the surfaceusing tweezers) and was patterned by evaporation ofgold through the mask for -6 min. A small portion ofthe polymeric mask was removed (most of the maskremained on the surface) by blowing a stream of N2 overthe sample for -2 min. The polymeric mask madeconformal contact with the surfaces of the plateaus butremained suspended over the trenches. Evaporation ofgold through this structure produced patterned featuresof gold only on the surfaces of the plateaus; the surfacesof the trenches were completely covered with gold as aresult of the random diffusion of gold and the spread inthe beam.3l When a free-standing polymeric mask(without a frame) was placed onto the contoured surfaceusing a different method-floating on the surface ofwater and picking up using the substrate from under-neath-the polymeric mask folded into the trenches andmade partial conformal contact with the surfaces of thetrenches while retaining conformal contact with thesurfaces of the plateaus (see Figure 9D). Under theseconditions, patterned features of gold were formed onthe surfaces of both plateaus and trenches (Figuret2b).32

These results illustrate the use of these free-standingpolymeric microstructures as masks to fabricate pat-terned microfeatures on both planar and nonplanarsurfaces. They represent only initial, proof-of-conceptexperiments and substantial development will be neededto defrne the full utility of this method of forming

(29) Bain, C. D.;Troughgton, E. B.;Tao, Y.-T.; Evall, J.;Whitesides,G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111,321.

(30) Jackman, R. J.;Wiibur, J. L.;Whitesides, G. M. Science 1995,269 ,664 .

(31) The mean-free-path (,1) of gold atoms is -0.3 trzm under theconditions used for gold evaporation. The mean free path of gold wasestimated using the expression: 1: (UJ2o)kTlP;herc o is the collisionsection (-10-18 mz), and P is the pressu.re (-80 mTorr). The distancebetween the mask and the substrate must be less than 0.3 pm in orderto form patterned features ofgold using this procedure.

(32) Note that forming patterned features on recessed regionshaving dimensions of < 100 pm can not be achieved by photolithographyand microcontact printing (aCP).

Xia et al.

100pm

10 pmFigure 12. SEM images of contoured surfaces anisotropicallyetched in Si that were patterned with microfeatures of gold(-L20 nm thick) by evaporation using free-standing polymericstructures as the masks. (a) Mask with a fame was placed onthe surface using tweezers; (b) mask without a frame wasplaced on the surface by floating the mask on water andpicking up by the substrate from underneath.

microstructures. It is, however, immediately clear that

for certain applications-for example, limited areas ofcomplex structures on nonplanar surfaces (see Figure

11 and the in-trench patterns of Figure lzb), it would

be much easier to use this type of procedure than anyother method currentlv available"

pm1

os*:si

ffiHHsxs

ffiHrug

Micromolding of Polymers in Capillaries

Conclusions

Micromolding in capillaries (MIMIC) is a remarkablyversatile technique for fabricating patterned micro-structures of various organic polymers on a variety ofsolid substrates. The smallest features that we haveproduced using this procedure were parallel lines withcross-sectional dimensions of -1 x I pm. These dimen-sions were set by the PDMS molds that were availablefor use with this work; we have not tried molds withsmaller features. and the lower limit to the definitionthat can be achieved by this technique is not establishedby the present work. The liquid prepolymer used in thisprocess should have low viscosity (<400 cP) and shouldbe curable either thermally or photochemically'6,33'34 i1could be a structural or functional pol5rmer3s or aprecursor polymer to a ceramic material.7,36 The sub-strate used in this procedure could be any solid material(planar or nonplanar, rigid or flexible) as long as itprovides a reasonably flat surface and this surface canbe partially wet by the liquid prepolymer.

The patterned polymeric microstructures formed us-ing MIMIC could be used as resists in the wet etching

(33) Ultraviolet-curable polyphosphazene: Nelson, C. J.;Coggio, W.D.: Allcock. H. R. Chem. Mater. 1991. 3. 786.

(34) Ultraviolet-curable poly(dimethylsiloxane): Silicon Com-pounds: Register and Reuiew, Sth ed.; Anderson, R., Larson, G. L.,Smith, C., Eds.; Hi.rls America: Piscatway, NJ, 1991; p 285.

(35) Zhao, X.-M.; Stoddard, A.; Smith, S. P.; Kim, E.; Xia, Y.;Prentiss, M.; Whitesides, G. M. Adu. Mater., in press.

(36) Ultraviolet-curable polycarbosilane: Thorne, K. J.; Johnson,S. E.; Zheng, H.; Mackenzie, J. D.; Hawthorne, M. F. Chem. Mater.1 9 9 4 . 6 . 1 1 0 .

Chem. Mater., Vol. 8, No. 7, 1996 1567

of underlying substrates (for example, SiO2, Au, Ag, andCr). The patterned polymeric microstructures formedusing MIMIC could also be released by dissolving thesupporting substrates to produce free-standiog micro-structures. These free-standing polymeric microstruc-tures could be transferred to the surfaces of other solidsubstrates (both planar and nonplanar) and subse-quently used as disposable masks for the evaporationof patterned metals and the formation of patternedSAMs.

Microfabrication based on the present procedure hasat least three advantages over conventional photolitho-graphic techniques: (a) MIMIC allows a wider range oforganic polymers to be patterned; (b) metal evaporationthrough flexible polymeric masks provides an easier wayto form patterned features on curved surfaces; (c)MIMIC is less expensive and capital intensive thanphotolithography. At its current stage of development,it also has a range of disadvantage and undefinedcharacteristics in important areas such as dimensionalstability and registration in multilevel fabrication.

Acknowledgment. This work was supported in partby ONR and ARPA. This work made use of MRSECShared Facilities supported by the National ScienceFoundation under DMR-9400396. We thank Mr. YuanLu and Mr. Stephen Shepard for their assistance inusing the SEM and photolithography facility.

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