Bilayer, nanoimprint lithography

Bilayer, nanoimprint lithographyBrian FairclothNuvonyx, Inc., Bridgeton, Missouri 63044

Henry RohrsWashington University, St. Louis, Missouri 63130

Richard TiberioCornell University, Ithaca, New York 14853

Rodney RuoffWashington University, St. Louis, Missouri 63130

Robert R. Krchnaveka)

Rowan University, Glassboro, New Jersey 08028

~Received 3 May 1999; accepted 21 April 2000!

Nanoimprint lithography has been shown to be a viable means of patterning polymer films in thesub-100 nm range. In this work, we demonstrate the use of a bilayer resist to facilitate the metalliftoff step in imprinter fabrication. The bilayer resist technology exhibits more uniform patterns andfewer missing features than similar metal nanoparticle arrays fabricated with single layer resist. Thebilayer resist relies upon the differential solubility between poly~methyl methacrylate! andpoly~methyl methacrylate methacrylic acid copolymer!. Evidence is presented that shows thetechnique has a resolution of better than 10 nm. ©2000 American Vacuum Society.@S0734-211X~00!03104-8#

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I. INTRODUCTION

Due to the inevitable transition from the microelectronto the nanoelectronic age, the demand for sub-100 nmture sizes in lithographic techniques will increase greatly.current devices rapidly approach the 100 nm barrier, thecroelectronics industry is considering several technologieovercome this hurdle. Clearly there is an urgent need for freliable, and cost-effective nanolithography. Nanoimprintthography shows promise in meeting this need.

Nanoimprint lithography is a relatively new lithographtechnique that has received considerable attention in reyears.1–3 It is relatively straightforward. First, an imprinte~mask! is made with the desired sub-100 nm features raifrom the surface. Next, the imprinter is coated with a vethin layer of mold release compound to protect it and prevsticking during the imprinting process. The substrate topatterned is then coated with a thin film of thermoplaspolymer. The elastomer is heated above its glass transtemperature (Tg) where it becomes viscoelastic. The imprinter is then pressed into the polymer and the systemcooled back down below the polymer’sTg , freezingthe pat-tern into the polymer. The mask is then removed andtrenches are cleaned with either an O2 plasma or with sol-vents to remove any residual polymer that may remain intrenches. The imprinter can be reused numerous tithereby providing cost-effective, sub-100 nm lithography

There are several challenging problems associatednanoimprint lithography.4 One critical area is to perform thnext level of processing while maintaining the same reso

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1866 J. Vac. Sci. Technol. B 18 „4…, Jul ÕAug 2000 0734-211X Õ200

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tion demonstrated in the polymer resist layer. The formatof patterned metal layers is one application. Finely pattermetal layers are used as interconnects in integrated circThey can also be used as catalysts for subsequent lgrowth. If the subsequent metal layers cannot readilyetched, e.g., due to crystalline dependent etching ratesadditiveapproach such asliftoff is desirable. However, singlelayer resists are problematic when transferring the patternmetal liftoff. The nonvertical sidewalls resulting from thimprinting process result in tearing and detachment ofmetal film during liftoff. Nonvertical sidewalls occur in imprint lithography if the imprinting element does not havertical sidewalls. Even if the imprinter does have verticsidewalls, nonvertical sidewalls are formed in the imprintfilm due to a descumming step necessary to remove resipolymer from the bottom of the imprinted feature. Therefoa technique is required that minimizes the problems assated with metal liftoff using single layer resists.

In this work, we address this issue by consideringfabrication of nanometer-scale metal arrays. We use a bilaresist technique to provide undercutting that dramaticallyproves the metal liftoff. We demonstrate high resolutionrays of particles, lines, and crosshatches fabricated fmetal liftoff after nanoimprint lithography using abilayerresist.5

II. EXPERIMENT

Figure 1 shows a schematic drawing of the imprintiprocess. The experimental steps involve imprinter~mask!preparation, sample preparation, imprinting, postprocessmetalization, and liftoff.

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A. Sample preparation

After suitably cleaning a silicon wafer, the bilayer resistspun on. There are a wide range of thermoplastic elastopolymers available that should work well for bilayer resisWe chose poly~methyl methacrylate! ~1% 495k PMMA inchlorobenzene! and poly~methyl methacrylate methacryliacid copolymer! @2.2% 73k P~MMA-17.5%MAA! in glycolether# for this work. These polymers are used as electbeam lithography and x-ray lithography resists and have csistent formulations that spin on uniformly.

FIG. 1. Schematic illustration of the imprint process.~a! A substrate andimprinter prior to imprinting. The substrate is coated with the bilayer reand the imprinter is coated with a thin mold-release polymer.~b! Imprinting.Both substrates are heated and cooled during the imprinting process.~c! Theimprinter has been pulled back. The residual polymer remains at the boof the imprinted features.~d! The residual polymer is removed duringdescumming step. This step also undercuts the lower resist layer.~e! Metalis evaporated over the entire surface.~f! After metallization, the polymerlayers are dissolved thereby lifting off the undesired metal. The resultpatterned metal layer.

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To produce the bilayer resist the PMMA is first spcoated onto the wafer~2000 rpm, 60 s! and soft baked todrive out the solvent~170 °C for 60 min!. Next, the P~MMA-MAA ! copolymer is spin coated~6000 rpm, 60 s! onto thePMMA film and soft baked in the same manner. This resuin each layer being approximately 50 nm thick for a tofilm thickness of approximately 100 nm. It has been shothat these two layers mix neither during the soft baking pcess nor when heated above theirTg’s.6,7

B. Mask preparation

Nanoscale imprint lithography requires a suitable maskimprinter. This is a crucial component of the imprinting prcess and the fabrication of the imprinter is covered in RefIt is sufficient to note that our imprinters have been fabcated in both silicon and silicon dioxide, using e-beamthography and plasma etching. For the work presented hthe imprinter is fabricated in a 500 nm layer of thermagrown silicon dioxide on a silicon wafer.

Prior to use, the imprinter must be coated with a morelease polymer such as the hyperbranched perfluorinpolymer8 used in this study. A 2 mg/ml solution of the moldrelease polymer dissolved in chloroform is drawn into apette and enough solution is expelled onto the imprinterform a meniscus~i.e., the entire surface of the imprintercovered with mold release polymer solution!. Since the moldrelease solution evaporates rapidly, the imprinter is immeately spun at 6000 rpm for 30 s before the concentrationthe imprinter has had time to increase significantly. Atomforce microscopy measurements of the resulting film hashown that the thickness is 5–10 nm. The chloroform solvis removed by placing the imprinter in a vacuum chamband pumping the chamber down below 50 mTorr.

C. Imprinting

The imprinting apparatus consists of aluminum platguide pins, and alignment structures. See Fig. 2. We ucarbon tape to fix the sample with respect to the imprinThis tape serves the dual purpose of holding the samplplace and providing leveling during imprinting. The soplastic nature of the tape helps ensure that the imprintersample are parallel by deforming to compensate for any mroscopic area pressure gradients. The tape was replacedevery five imprintings due to gradual degradation at theevated temperatures.

The next step is patterning the bilayer resist polymeThe bilayer-coated substrate is attached to the bottom pof the imprinting apparatus via the double-sided carbon taThe entire apparatus is placed into an oven heated to 180Then the sample and imprinter are brought into intimate ctact. A specific amount of force is applied to achieve a dsired imprint depth. The force is applied until the ovenmosphere~air! reaches 170 °C. The apparatus is flash cooand the imprinter is removed. Figure 3 shows a set of scning electron microscope~SEM! micrographs taken after imprinting. The imprinted sample is now ready for postproceing.

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D. Postprocessing

After the imprinter is retracted, a small amount of residupolymer remains at the bottom of the trenches. This musremoved if quality reproductions are to be obtained. Thesidual polymer can be removed by means of a short oxyplasma etch or by a wet solvent etch. It should be notedeven if the elements of the imprinter have perfectly vertisidewalls resulting in trenches with perfectly vertical sidwalls, this descumming step will round the edges of thedented structure, resulting in slightly beveled sidewalls. Tcan lead to the errors already mentioned for single layersists. The use of an anisotropic oxygen plasma as thecumming agent will reduce this problem somewhat. Thelayer resist is much less sensitive to this effect andfunction even when descumming is done using isotrotechniques.

We used two different postprocessing techniques. Infirst method, a high pressure is applied during the imprintprocess such that the imprinting elements create dtrenches in the bilayer resist. The trench is deep enougcause considerable compression of the lower PMMA layBefore the sample can be descummed, any residual mrelease polymer must be removed from the surface ofsample so it does not prevent descumming of the tP~MMA-MAA ! film. This is accomplished with a 5 srinse intoluene. The mold release polymer is very soluble in tolueand a 5 srinse is more than sufficient. Longer times shoube avoided as the toluene will also etch away the lowPMMA layer in areas in which it is exposed. Immediateafter rinsing the imprinted sample it was dried with a streof dry nitrogen.

After removing the residual mold release polymer, theis a thin membrane of P~MMA-MAA ! covering the inside ofthe trench. This thin membrane must be removed otherwundercutting will be impaired. The membrane can bemoved by soaking the imprinted sample in methanol forproximately 5 s and then immediately drying with dry nitr

FIG. 2. A drawing of the imprinting apparatus. Brass weights are placethe walled structure on the top plate to set the applied force. The eapparatus is placed into an oven for the heating process. Cooling is achby pouring liquid nitrogen into the walled structure on the top plaThe sample temperature is assumed to be in equilibrium with the otemperature.

J. Vac. Sci. Technol. B, Vol. 18, No. 4, Jul ÕAug 2000

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gen. The methanol will etch the P~MMA-MAA ! polymerwhile leaving the lower PMMA polymer unaffected. Thisan isotropic etch that not only eliminates the thin P~MMA-MAA ! membrane in the trench, but also reduces the thiness of the top layer of the bilayer resist. The final steppreparing the sample for metal deposition and liftoff isdescum the PMMA from the bottom of the trenches whsimultaneously creating undercut in the bottom layer. Tgoal was accomplished by soaking the imprinted samplechlorobenzene solution. The chlorobenzene attacks the loPMMA layer while leaving the upper P~MMA-MAA ! unaf-fected. Typical soak times are 45–60 s.

The second postprocessing technique is used for destructures in which capillary action upon removal of the imprinter after a deep imprinting results in damage to the po

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FIG. 3. SEM micrographs of patterns imprinted into a bilayer resist. Thsamples have been rinsed with toluene to remove any residual mold repolymer. ~a! An array of 50-nm-diam dots spaced 250 nm apart.~b! Anarray of 250 nm dots spaced 2 mm apart.~c! A set of 50-nm-wide parallellines with a spacing of 500 nm.

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mer films. This damage can be prevented with a shalimprint. This method will also work with less dense patterbut is slightly more complex. This technique uses onenough pressure to make indentations that are from 30 tnm deep. After imprinting, the mold release polymer ismoved with toluene as described earlier. The pattern mnow be transferred deeper into the bilayer resist withoutbenefit of the imprinter. This is achieved with an anisotroplasma etch that cuts the pattern deeper into the resist.crucial that the final trench depth extend into the bottlayer and that 20–30 nm of P~MMA-MAA ! remain. Etchingwas implemented at the Cornell Nanofabrication Facilitying the PlasmaTherm 72 Reactive Ion Etcher. Imprinsamples were etched at a chamber pressure of 15 mTorrW of power, with gas flows of CF4542 sccm and H2515sccm for 65 s~the etch rate of this plasma chemistry wfound to be 29 nm/min!. The choice of CF4/H2 chemistrywas primarily due to its availability in the chamber. Itlikely that the primary etch mechanism is sputtering andinert gas could be used instead. However, the degrewhich any polymer deposition may be influencing the eting process will require further study. Oxygen plasmas walso employed, but the etch rate was much higher, makthe etching of precise depths difficult.

The final step of this postprocessing method is identicathat of the previous method. A slightly longer soak timechlorobenzene is needed to ensure that the lower laPMMA is etched down to the silicon surface. Typical sotimes are around 60 s.

E. Metallization

The final step to validate the results is to transferpattern imprinted into the resist into a second processstep. We chose to metalize the substrate. Metal evaporawas done using a CVC SC4500 thermal evaporator equipwith a crystal thickness monitor~Cornell NanofabricationFacility.! The 16 in. throw distance minimizes heating of tsubstrate which can damage the polymer resist layers.

Once the metal film has been evaporated onto theprinted sample, liftoff must be done to remove all of tunwanted metal and resist polymers. Liftoff is achievedsoaking the metal-coated sample in a 1:1 mixture of acetand methylene chloride for 30 min followed by sonicatifor 3 min. Further sonication in a clean solution of 1:1 aetone:methylene chloride may be required if optical insption reveals incomplete liftoff.

III. RESULTS AND DISCUSSION

In this work, we have seen that the quality of imprintindepends on the density of the patterns on the imprinter.patterns become more dense, there is an increasing tendfor the polymer layers to adhere to the imprinter uponmoval. This capillary action is reduced with the use ofmold-release polymer, but in dense patterns, it is not elinated. As discussed in Sec. II, the tendency for the polylayers to adhere to the imprinter can be reduced by doinshallow imprint followed by a solvent rinse. The shallo


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imprint is controlled by the pressure applied to the imprintFigure 4 shows the imprint depth as a function of applpressure. In our system, applied pressure is controlledweights. The pressure was calculated using the area ofimprinter elements~assuming perfect patterning and verticetching! and the force due to gravity for the specific weighused. There is a systematic error in these numbers due to

FIG. 4. Imprint depth as a function of applied pressure in a bilayer reconsisting of 50 nm of 495k PMMA~bottom layer! and 50 nm of 73kP~MMA-17.5%MAA!~top layer!.

FIG. 5. Polymer thickness removed from a polymer film as a functiontime for various solvents. Note that the 495k PMMA is quite solublechlorobenzene whereas the 73k P~MMA-17.5%MAA! is essentially in-soluble in chlorobenzene.

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FIG. 6. SEM micrographs of a bilayerresist imprinted with 250 nm dot ar-rays and soaked in chlorobenzene f~a! 0 s,~b! 30 s,~c! 60 s,~d! 120 s, and~e! 240 s. In~f!, the top polymer layerhas collapsed because of the extremundercutting after soaking in chlorobenzene for 240 s.

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friction between the rods and the top plate~see Fig. 2! in theimprinting apparatus. The entire imprinting apparatusbrought up to 170 °C before entering the cooling cycle. Fthe highest pressure shown in Fig. 4, the time to reach 17was approximately 85 min, whereas for the lowest pressthe imprinting temperature was reached in 40 min. Theference between these times is due to the method we usapply the pressure. It was simply a set of weights on theplate of the imprinter apparatus. More pressure reqularger weights, which in turn take longer to heat up. Tmost striking feature in the data is the nearly constant deof imprint given by the first four data points. These poincover a wide range of pressures. This phenomenon is mlikely a surface effect with several contributing factors. Torientation of the polymer at the surface is almost alwadifferent from the interior. In addition, small molecules suas oxygen and water can diffuse deep into the polymerface due to the large free volume of amorphous polymFurthermore, surface oxidation or contamination can exFor example, P~MMA-MAA !, which is the top layer, con


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tains electrolyte side chains which readily attract and rewith water molecules creating a contamination layer. Sinthe imprints are done at a relatively high temperaturediffusion of contaminants into the polymer can be large,sulting in a fairly thick contamination layer.

Very recently it has also been found that the glass tration temperature at the surface of a polymer can be mhigher than that for the bulk polymer.9–11In addition, in mostpolymers the surface skin can be from a few nanomethick to tens of nanometers thick12 and this can be a sizablfraction of the 50 nm P~MMA-MAA ! layer. To complicatematters further, bulk polymers are known to have a skinthe rubbery flow regime due to surface tension effects,12 buthow this skin would manifest itself and what effect it wouhave upon nanoscopic thin films is uncertain.

It is likely that the relatively constant imprint depth of 3nm for pressures less than 1400 MPa is due to some conation of the above-mentioned effects. Above 1400 MPa,surface layer yields to the imprinter elements and they puthrough. The yielding pressure is likely to be highly depe

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FIG. 7. SEM micrographs of 60-nm-diam, 20nm-thick nickel nanoparticle arrays on a 25nm pitch produced through~a! single layernanoimprint lithography and ~c! bilayernanoimprint lithography.~b! and~d! are mag-nified images of the respective arrays.

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dent upon the shape and geometry of the imprinting ements, such that a sharp cone like element would eapuncture the surface layer while a flat cylinder like elem~a pillar for instance! would have more difficulty. After thesurface layer has been compromised the imprinting elemquickly pass through the remaining liquid like P~MMA-MAA ! layer and reach the soft elastic PMMA layer below~itis assumed the PMMA layer is in the soft elastic region dto the slope of Fig. 4 coupled with the observation that sinlayer PMMA films do not show flow upon compression!.The rubbery PMMA layer is then compressed with the aplied stress from the imprinting elements as shown bymonotonically increasing imprint depth with increasing presure after 1600 MPa.

Figure 4 demonstrates that one can control the deptimprinting to a certain extent by controlling the applied presure. In particular, the two depths~35 and 100 nm! consid-ered in Sec. II are easily obtained by controlling pressuNote that although the bilayer was nominally 100 nm thicthere is approximately 20% uncertainty in this value resing in the high-pressure imprint depths exceeding 100 nm

It is well known that liftoff can be considerably improvewhen a certain amount of undercutting is produced instructure. This proves to be the great advantage of usinbilayer resist for imprint lithography. Electron beam lithoraphy and photolithography naturally produce undercutt~in positive resists! due to diffraction spreading effects fo


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photolithography and electron scattering and secondary etron production in the resist for electron beam lithography.imprint lithography, undercutting must be produced in aother way. We achieve this with a bilayer resist. The twpolymers used for the bilayer resist, PMMA and P~MMA-MAA !, have considerably different chemical propertieThere are a wide range of solvents that will attack PMMbut not P~MMA-MAA ! and vice versa~see Fig. 5!. The 495kPMMA is highly soluble in chlorobenzene and toluene bquite insoluble in isopropanol and methanol. In contrast,73k P~MMA-17.5%MAA! exhibits a high solubility inmethanol and low solubility in chlorobenzene, toluene, aisopropanol. After imprinting, the sample is exposed tosolvent that will attack the bottom PMMA layer but not thupper P~MMA-MAA ! layer. In this way, undercutting can bproduced in the imprinted features and the vapor deposmetal film will not form a continuous film as long as ththickness of the metal is less than that of the bottom layThe degree of undercutting is easily controlled by timing tsolvent rinse. Figure 6 is a series of SEM micrographs shing undercutting in a bilayer resist imprinted with 250-nmdiam holes. The solvent was chlorobenzene and rinse tiranged from 0 to 240 s. Figure 6~a! represents an imprint inwhich no postprocessing has been done other than a tolrinse. The continuous nonvertical sidewalls are apparFigure 6~b! shows a sample that has been soaked in chrobenzene for 30 s. The undercut and discontinuity in

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sidewalls is now apparent. In Fig. 6~c!, the sample has beesoaked for 60 s. The undercut is now very noticeable. Fures 6~d! and 6~e! represent samples soaked in chlorobezene for 120 and 240 s, respectively. In both cases thedercut is extreme. Finally, in Fig. 6~f!, in which the sample

FIG. 8. SEM micrographs of parallel Cr metal lines patterned with bilananoimprint lithography and metal liftoff. The lines are approximatelynm wide and 20 nm thick.


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has been soaked for 240 s, the degree of undercutting iextreme that the upper, unaffected P~MMA-MAA ! layer hascollapsed onto the substrate, possibly due to the capilforces present during solvent evaporation.

One measure of the performance of the bilayer imprinttechnique is how well metal liftoff occurs. There were thrbasic types of patterns originally produced on imprintersdot arrays, parallel lines, and crosshatch patterns. The firsthese patterns is shown in Fig. 7. Figure 7 shows a seSEM micrographs of 60-nm-diam dots arrays of 20-nm-thnickel. The 20-nm-thick nickel was thermally evaporatonto the imprinted sample followed by liftoff. Figure 7 compares the single layer imprint lithography@Figs. 7~a! and7~b!# with the bilayer imprint lithography@Figs. 7~c! and7~d!#. For the case with the single layer of 495k PMMresist, the dots are very nonuniform in both size and shaAdditionally, it can be seen in Fig. 7~a! that many of the dotshave been completely pulled from the substrate duringliftoff process. Finally, many of the dots appear to havetailswhich are created by metal breaking at an intermediate pon the sidewalls of the trenches.

Figures 7~c! and 7~d! show nickel metal array produce

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FIG. 9. SEM micrographs of 20-nm-thick Cr crosshatch lines patterneding bilayer nanoimprint lithography and metal liftoff. The lines are appromately 55 nm wide.

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by imprinting completely through the bilayer resist. It is important to note the high uniformity in size and shape of60-nm-diam dots. An additional point can be seen from F7~c!. Our imprinter had an error in the dot array caused bpattern generator error in the Leica VB-6 electron beamthography machine. This error produced a subset of theprinting elements with a spacing of less than 10 nm. Asbe seen in the right half of Fig. 7~c!, bilayer nanoimprintlithography is capable of distinguishing the two imprintinelements, suggesting that bilayer nanoimprint lithographas a resolution of 10 nm or better.

Two other patterns were evaluated. Figure 8 showsnm-wide parallel lines spaced 500 nm apart. Imprinting wdone using the shallow imprinting method and liftoff wperformed after thermal evaporation of chromium~20 nm!.Figure 9 shows a crosshatch pattern with 50-nm-wide lispaced 500 nm apart. Shallow imprinting and chromium loff were used to fabricate this pattern. In both of theseures, a perspective view is provided to orient the readeran overhead view is then used to measure the width oflines. For both the parallel lines and the crosshatch pattvery few errors can be seen in the SEM images.

IV. CONCLUSIONS

In this work, we demonstrated the advantages of a bilaresist for nanoimprint lithography. Metal nanoparticle arracan be fabricated using thermal evaporation and metal lifwhen an appropriate bilayer resist is used. In comparisonsingle layer resist, the metal arrays exhibit more unifopatterns and fewer missing features. The bilayer resist reupon the differential solubility between PMMA anP~MMA-MAA !. While gross errors are associated wsingle layer nanoimprint lithography, bilayer nanoimprintthography produces much higher quality metal patterns wusing liftoff.

We have also demonstrated a shallow imprinting tenique in which the bilayer resist is used in conjunction wshallow imprinting to reduce film damage when imprintindense feature patterns.


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Finally, there is evidence that the bilayer resist techniqhas a resolution of better than 10 nm if an appropriateprint mask is fabricated. This high resolution coupled wliftoff technology opens up new opportunities for nanoscresearch.

ACKNOWLEDGMENTS

The authors would like to thank Lynn Rathbun, MichaRooks, Jerry Comeau, Mike Skvarla, Jerry Drumheller, Dtin Carr, Jim Clair, and Gary Bordonaro of the Cornell Nanfabrication Facility. This work was performed in part at thCornell Nanofabrication Facility~a member of the NationaNanofabrication Users Network!, which is supported by theNational Science Foundation under Grant No. ECS-97312Cornell University, and industrial affiliates. In addition, wthank Stephen Irons, Nikolay Nemchuk, Anja MuelleKaren Wooley, and Tomek Kowalewski for numerous tecnical discussions. This work was supported by a grant frthe National Science Foundation~No. DMR-9704644, Peri-odic Nanotube Arrays!.

1S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Science272, 85 ~1996!.2J. Haisma, M. Verheijen, and J. van den Berg, J. Vac. Sci. Technol. B14,6 ~1996!.

3X. Sun, L. Zhuang, W. Zhang, and S. Y. Chou, J. Vac. Sci. Technol16, 6 ~1998!.

4H.-C. Scheer, H. Schultz, T. Hoffmann, and C. M. Sotomayor TorresVac. Sci. Technol. B16, 6 ~1998!.

5B. Faircloth, Master’s thesis, Washington University, St. Louis, 19986R. C. Tiberio, J. M. Limber, G. J. Galvin, and E. D. Wolf, Proc. SP1089, 124 ~1989!.

7M. Hatzakis, J. Vac. Sci. Technol.16, 6 ~1979!.8A. Mueller, T. Kowalewski, and K. Wooley, Macromolecules31, 3~1998!.

9G. B. DeMaggio, W. E. Frieze, D. W. Cidley, M. Zhu, H. A. Hristov, anA. F. Yee, Phys. Rev. Lett.78, 1524~1997!.

10Y. C. Jean, R. Zhang, H. Cao, J.-P. Yuan, C.-M. Huang, B. Nielson,P. Asoka-Kumar, Phys. Rev. B56, 8459~1997!.

11Y. M. Boiko and R. E. Prudh´omme, J. Polym. Sci., Part B: Polym. Phy36, 567 ~1998!.

12L. H. Sperling, Introduction to Physical Polymer Science~Wiley Inter-science, New York, 1992!.

Bilayer, nanoimprint lithography

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