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Nanopatterning via Pressure-Induced Instabilities inThin Polymer Films

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By Ximin He, Jurjen Winkel, and Wilhelm T. S. Huck*

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[*] Prof. W. T. S. Huck, X. HeMelville Laboratory for Polymer SynthesisDepartment of Chemistry, University of CambridgeLensfield Road, Cambridge, CB2 1 EW (UK)E-mail: wtsh2@cam.ac.uk

Prof. W. T. S. Huck, X. HeThe Nanoscience Centre, University of Cambridge11 J.J. Thomson AvenueCambridge CB3 0FF (UK)

Dr. J. WinkelKodak European Research332 Science Park, Milton RoadCambridge, CB4 0WN (UK)

DOI: 10.1002/adma.200803547

Adv. Mater. 2009, 21, 1–5 � 2009 WILEY-VCH Verlag Gmb

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Polymer thin films are the bedrock of modern electronic devices,

playing key roles in their fabrication (photoresists) and operation(dielectrics, light emitting layers, sensor films, etc.). Conse-quently, enormous efforts have been devoted to understandingtheir fundamental properties and to pattern thin polymer films,especially now that the thickness of the films and the feature sizesare reaching molecular dimensions. The vast bulk of thesestudies has employed films at thermodynamical equilibrium, butas we will demonstrate here, non-annealed films containingresidual stresses from the preparation of the films by spincastingcan be harnessed in new patterning strategies. Nanoimprintlithography (NIL) is an extremely powerful technique for rapidlarge area nanopatterning.[1,2] The basic principle involves placinga master with nanoscale topographic features on top of a thinpolymer film, and subsequently heating the polymer film abovethe glass transitions temperature (Tg) while applying pressure, sothat the polymer flows to fill the relief pattern of themaster. A thinresidual layer usually remains beneath the master protrusions,which needs to be removed by a subsequent etching processbefore the pattern can be transferred into the substrate.Solvent-assisted NIL[3] unexpectedly resulted in pattern replica-tion without any residual film between the features, due todewetting of the plasticized film. Alternatively, films oncompletely non-wettable fluorinated surfaces break up duringimprinting and heating, leading to individual polymer nano- andmicrostructures.[4] Scanning-probe lithography has been usedextensively to produce nanostructures in self-assembled mono-layers on gold by ‘shaving’ the chemisorbed alkanethiols.[5]

Interestingly, residue-free nano- to micrometer-scale trencheswere generated by scribing poly(3-hexylthiophene-2,5-diyl)(rr-P3HT) thin films with an AFM tip, presumably as a resultof the release of residual strain in the films.[6] These examplesopen up interesting opportunities in developing a general strategybased on film rupturing phenomena for creating features withoutresidual films using NIL. Dewetting of thin polymer films at

temperatures above the Tg is a well-understood, although oftenundesirable phenomenon.[7–10] The stability of ultrathin polymerfilms (< 100 nm) is governed by weak surface interactions, andforces acting on the thin film can easily destabilize the continuousfilm leading to dewetting phenomena (film rupture).[7] Depend-ing on the combined effect of the weak forces (van der Waals,hydrogen bonding) films may rupture either by spontaneousdewetting (unstable films) or by nucleation and growth of holes(metastable films). Numerous studies have investigated the roleof film thickness, substrate surface energy on rupture of thinwetting films and nanoscale topography.[8–13] Nanoscale surfaceenergy patterns lead to local variations of the effective interfacepotential and concomitant variations in dewetting kinetics, whichresults in pattern replication in the dewetted films.[14] It should benoted that all dewetting studies are carried out using polymerfilms that are heated above the Tg, below which temperature thepolymer chains are in the glassy state and immobile, henceresisting flow. Here, we present a non-conventional lithographictechnique to fabricate nanometer-scale periodic patterns based onpressure-induced local rupturing at temperatures significantlybelow the Tg. Our approach is based on the presence of residualstresses in non-annealed spincoated polymer thin films. The fastevaporation of solvent during spin-coating leads to frozen-innon-equilibrated chain conformations of the polymer, thusgenerating residual stresses within the film, which can be animportant reason for thin film instability and a driving force forhole-nucleated dewetting.[15,16] Any internal or external distur-bances, e.g., heating, dust particles, film defects, or, as we show inthis communication, local mechanical forces acting on the filmsurface, provide a route for the residual stress to be relieved.

The patterning procedure we have developed here is shownschematically in Figure 1. Firstly, fluorinated sharp (< 10 nm tipradius) stamps, containing V-shaped lines (Fig. 1c) or cones (Fig.1d), are placed on top of a spincoated polymer film (10–60 nmthick) on hexamethyldisilazane (HMDS)-treated Si wafers (toavoid differences in thickness, the edges of the wafers were cut offafter spincoating and only the center area was used forimprinting). Subsequently, the stamps are brought into contactwith the polymer film surface using a pressure of 3–5 bar. After10–50 s, the stamps were lifted away from the film, at which timeperiodic patterns had formed triggered by the local stimulus fromthe tips. It should be noted that films on HMDS-treated Si areeasily patterned at room temperature, whereas films on untreatedSi, require moderate heating, albeit well below the Tg. Below, wewill describe how the pattern size can be tuned by controlling theimprinting temperature and film thickness, and the effect ofsurface energy and polymer molecular weight.

Figure 2a shows the scanning electron microscopy (SEM)image of the line array formed in a 44-nm thick polystyrene (PS,Mw 50k) film onHMDS-Si by using V-shaped stamps as shown in

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Figure 1. a) and b) Schematic procedure of the patterning process. A 10–60 nm thick polymerfilm, spun cast on a HMDS treated Si/SiO2 substrate, is approached by a fluorinated Si stampscontaining sharp tip arrays, with tip radius of < 10 nm, tip length of 0.5–1.5mm, periodicity of3mm. a) 1D V-shaped stamp produce lines; b) 2D conical-shaped tips produce holes. Step 1.Stamp gently touches the film surface by applying a pressure of 3–5 bar. Step 2. Pattern formationoccurs under areas in contact with stamp. After 10–50 s, stamp is lifted up from the film. c) and d)SEM images of stamps used: top-view of c) 1D V-shaped lines and d) 2D conical-shaped dotsstamps. Insert: side-view of stamp.

Figure 2. Scanning ElectronMicroscope (SEM) images of a)1D line and b)2D hole array patterns in 44-nm thick PS (Mw 50K) film on HMDS-Si andsputter-deposited Pt after lifting off PS (c: 1D lines, d: 2D dots). � 10 nmthick Pt was sputter deposited on patterned PS film for SEMmeasurement,and subsequently PS was lifted off to verify that the patterns reached theSiO2 surface. Insert: high-magnification images. Scale bar: 2mm.

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Figure 1a. The pattern was very uniform and complete over thewhole of the imprinted area (3mm� 3mm). Clear narrowtrenches can be seen in the insert close-up image. Rims at theedges of the trenches are observed, which are typical for polymer

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film dewetting processes.[8] Similarly, a holearray was formed in PS films on HMDS-Si,using the stamp containing 2D conical-shapedsharp tips, as shown in the SEM image inFigure 2b. We can observe in the high-magnification insert image that each dot hasa rim surrounding a central pore. In allexperiments, both the trenches and the holesare larger than the size of the tip and hence thepatterns are not exact negative replicas of themaster. This is a strong indication that thesepatterns are not the result of a standard NILimprinting process. To examine the patterns inmore detail, a thin film of platinum (Pt) wassputtered onto the imprinted films followed bya PS lift-off procedure by soaking and sonicat-ing the samples in toluene. SEM images(Fig. 2c and d) clearly show 52nm wide Pt linesand Pt dots of 36 nm diameter, which indicatesthat the patterns formed in the imprintingprocess lead to completely dewetted, residue-free features. In contrast, much thicker films(100 nm) imprinted with the same stamp donot show these completely dewetted patterns;instead, typical imprinted features that repli-cate the features but leave a thin residual filmare observed (in other work, Stutzmann et al.have used similar stamps in the solid stateembossing, or ‘microcutting’, of semiconduct-ing polymers[17]). We therefore propose analternative route to pattern formation in thesefilms, based on a rupturing instability triggeredby a local mechanical stimulus, which nucle-ates a hole in the film. The ‘holes’ then coarsen

by releasing the internal tensile stress of the film. The key to thismechanism are residual stresses within the polymer filmsresulting from spincoating coupled with fast evaporation of thesolvent.[15] Indeed, freshly prepared (non-annealed) thin film thathad been stored in an ambient environment for a maximum of2 days tended to form the patterns much more readily incomparison with films that had been vacuum-annealed at 80 8Cfor 48 h (which showed strong sample to sample variation). Theincreased chain mobility and lowering of the Tg due to someremaining solvent in thin films and the presences of residualstresses after spincoating, as well as the effect on rupturing ordewetting phenomena, has been well documented.[8,15,18]

To study the effect of patterning temperature on the patternsize, we conducted a series of experiments at differenttemperatures. Figure 3 shows the clear increase in pattern sizewith increasing imprinting temperature for both 44 and 60 nmthick films. The line patterns are plotted as increase in widthversus patterning temperature, where the surface area versustemperature is plotted for the hole patterns, as the sharp conicaltips allow the residual stress to be released in two dimensions. Bycontrolling the temperature from 25 to 115 8C, the average widthof trenches in 40 (60) nm-thick PS films can be changed from59nm (71nm) to 341 nm (392 nm). Similarly, the average holeareas changed from 804 nm2 (1017 nm2) to 13463 nm2

(14950 nm2). The feature sizes increased more pronouncedly

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Figure 3. Trench widths (left axis) in 44 nm-(&) and 60 nm-(*) PS films (Mw 50k) and holeareas (right axis) in 40 nm-(&) and 60 nm-(*) PS films versus patterning temperature. Inserts:AFM topography images and cross-section profiles of the patterns corresponding to indicateddata points.

at patterning temperatures above 75 8Cwhich is close to the Tg fora 40–60 nm thin film of PS (70–90 8C as determined byellipsometry measurements).[18] The AFM images in Figure 3show the progression of the feature size increases. From thecross-sectional profile of the AFM, we can observe clearly the rimsaround the imprinted features and rim heights increase with theincrease of the feature sizes. Close agreement between thereorganized rim volume and the volume of the trench or holeimplies that the transformation involves mostly mass transportwithout removal of material. The rims also support a mechanismbased on dewetting phenomena rather than a cavitation of thefilm.[19]

It is known that the size of features formed during dewettingdepends on the film thickness.[7,11] Figure 4 shows the increase in

Figure 4. Plots of line width (&) and hole area (*) as the function of filmthickness. Patterning was conducted with PS films of different thicknessspun cast on HMDS-treated Si at 25 8C.

Adv. Mater. 2009, 21, 1–5 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

feature sizes with increasing film thicknesses.It should be noted that holes and trenches wereonly formed when using sharp tips and films of<60 nm thickness. We found that flat-toppedmolds containing 20 nm lines on a 100 nmpitch did not produce any pattern under theconditions used to produce the patterns shownabove (tested for 12–100 nm thick PS films,room temperature, 5 bar). It was also observedthat the pattern produced by sharp tips did notreach the substrate when 100 nm thick PS filmswere used. In those experiments, we foundtrenches or holes with feature sizes morecommensurate with the tip dimensions(� 12–20 nm) and these patterns almost dis-appeared when the films were annealed at120 8C as a result of viscous flow and relaxationprocess in the mobile polymer film.[20–22]

Finally, a number of other control experi-ments were performed to investigate the limitsof the pressure-induced instability patterningprocedure. In thin-film studies, the surfacewettability and film-substrate interactions areof crucial importance. We found that we couldnot pattern thin PS films on oxygen-plasma-treated Si substrates using room tem-

perature imprinting conditions, reflecting the increased stabilityof PS films on SiO2. We measured contact angles of water and PStoluene solution on oxygen-plasma treated Si wafers (14.58� 48and 3.18� 18, respectively), which are much lower than thecorresponding values for HMDS-Si (110.88� 58 and 45.38� 48,respectively). These results indicate that the pattern size could betuned further by altering the surface chemistry using functionalsilanes.[14] Furthermore, the results discussed above are allobtained using PS with Mw �50k. To investigate the effect ofpolymer Mw, �30 nm thick PS films of Mw of 10.2k, 113.5k, and990.5k were patterned by this approach. We found that thePS-10.2k film gave clear patterns under the same patterningconditions, while PS-990.5k barely showed indentation patternsat room temperature due to its good film stability and highviscosity. These results are in line with other published resultsthat low Mw polymer films are more easily destabilized.[8]

Additionally, the spincoating solvent also has effect on thedewetting patterning process, as high boiling point solvents dryslower, allowing more strain relaxation, resulting in finerdewetting features (we did not investigate an alternativeexplanation: the presence of higher levels of remaining solvents).The patterns in the film were stable at room temperature for atleast 2 months, without signifant increases in size. Whenpatterned films were annealed films at the polymer Tg for 5min,AFM measurements and SEM images showed that the patternsizes increased considerably (several micrometers), and the rimheights decreased while rim widths increased. These results arein line with ‘standard’ dewetting results.[7–11]

Since instabilities are a universal phenomenon of polymer thinfilms, our technique is applicable to many polymers. Here wedemonstrate the line and hole patterns in a conjugated polymer,poly(3-hexylthiophene) (P3HT, Mw 87k), and poly(methyl metha-crylate) (PMMA, Mw 120k) (Fig. 5). Very uniform, 98 nm wide

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Figure 5. AFM images of a) 98 nm wide lines in 38 nm thick P3HT filmproduced at 100 8C, 5 bar 50 s, and b) 28 nm wide (in diameter) hole arrayin 52 nm thick PMMA film produced at 25 8C, 5 bar, 50 s. (c) and (b) are the3-dimentional morphologies of the patterns in (a) and (b), respectively.

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trenches were formed in 38 nm thick P3HT films at 100 8C at apressure of 5 bar for 50 s. Likewise, regular 28 nm wide holes(measured at the hole bottom) were produced in 60 nm thickPMMA films at 25 8C at a pressure of 5 bar for 50 s.

In summary, we have demonstrated how the residual stressesin spincoated films, which so easily leads to undesirablerupturing of thin polymer films at elevated temperatures, canbe exploited to produce highly controlled nanoscale patterns viapressure-induced local rupturing. Our technique not only allows asystematic study of the properties of non-annealed films, but alsoserves as an important lithographic tool in that no residue is left inthe patterned areas. The sizes of the holes and trenches formedhere can be tuned from the nano- to the micrometer-scale bycontrolling the film thickness, substrate wettability, patterningtemperature and post annealing time (the latter not shown above,but annealing of the films above the Tg leads to a further retractionof the films and coarsening of the holes).

The rupturing induced by pressure through a sharp tip array isin contrast to ‘classical’ dewetting or cavitation studies, which takeplace at temperatures above the Tg. Likewise, NIL is usuallycarried out at high temperatures and high pressure (10–40 bar).The very sharp tips used here lead to very rapid pattern formation(seconds rather than minutes) in films at room temperature (orslightly above) and at low pressures (3–5 bar) (although the localpressure under the sharp tips will be much higher). In furthercontrast to standard NIL is the absence of a residual ‘skim’ layer,which obviates the need for an additional dry-etching step beforemetallization or other evaporation processes that are part ofdevice fabrication procedures. Apart from the patterning results,the experiments should also be of relevance for polymerelectronics studies, where spincoating is used as a standardfabrication tool, but where the residual stressesmight well impactfilm morphology and hence device performance.

Experimental

Si substrates were cleaned in DI water, acetone, iso-propanol sonicationbath for 15min each before use. Following a 30-second oxygen plasma

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(power 100Watt), Si substrates were coated with a HMDS by vapor phasedeposition at 175 8C for 15min. Contact angles were measure using ahome built contact angle rig fitted with a microscope and camera. Theroughness of HMDS layers and of the PS films, as determined by atomicforce microscopy (AFM) was below 1 nm.

Polystyrene (PS, Mw 50k, PolyScience Inc.) films of certain thicknesswere spun cast from toluene solution of various concentration onto freshHMDS-treated, oxygen-plasma-treated and untreated Si substrates withdifferent spin speeds. Cleaning and coating were performed in a class100 clean room. Patterning was performed on freshly prepared PS filmsstored in ambient atmosphere and, for control experiment, dried PS filmsby vacuum annealing at 80 8C for 48 h. Thickness was checked by Alpha-SESpectroscopic Ellipsometer and Dektak Profilometer. 12, 20, 44, 60, and100 nm thick PS films (Mw 50k), and also 20–30 nm thick film made frompolystyrene of Mw of 10.2k, 113.5k, and 990.5k were studied in this work.Polymethyl methacrylate (PMMA, Mw¼ 120k, Aldrich) films and poly-3-hexythiophene (P3HT, Mw¼ 87k, Aldrich) films were prepared andpatterned in the same way as PS films. The glass transition temperaturesof the polymer thin films that were used in our study were measured usingan Alpha-SE Spectroscopic Ellipsometer.

The 1D V-shaped tip array, periodicity 3mm, tip height 1.5mm, and 2Dtip array, periodicity 2.12mm, tip height 0.3–0.5mm, tip radius< 10 nm forboth arrays, are commercially available (NT-MDT). Patterning wasconducted using a nanoimprinter (Obducat) at room temperature(constant at 25 8C in clean room) or various elevated temperatures withapplied force of 3–5 bar for 10–50 s. The sample plate of the nanoimprinterwas pre-heated to the temperature that would be used for imprinting andthe instability patterning took place right after the sample (Si tip mastercovered PS film on Si substrate) was placed onto pre-heated plate. Thepattern morphologies were checked by SEM (Leo Variable pressure SEM)and AFM (Dimension 3100 AFM). All the pattern widths and diameterswere measured at the trench and hole bottoms and average values weretaken from at least ten measurements.

Acknowledgements

This work was supported by Kodak European Research, Cambridge and theCambridge Gates Trust (XH).

Received: December 1, 2008

Revised: December 19, 2008

Published online:

[1] S. Y. Chou, P. R. Krauss, P. J. Renstrom, Science 1996, 272, 85.

[2] L. J. Guo, Adv. Mater. 2007, 19, 495.

[3] N. E. Voicu, S. Ludwigs, E. J. W. Crossland, P. Andrew, U. Steiner, Adv.

Mater. 2007, 19, 757.

[4] J. P. Rolland, B. W. Maynor, L. E. Euliss, A. E. Exner, G. M. Denison, J. M.

DeSimone, J. Am. Chem. Soc. 2005, 127, 10096.

[5] B. D. Gates, Q. B. Xu, J. C. Love, D. B. Wolfe, G. M. Whitesides, Annu. Rev.

Mater. Res. 2004, 34, 339.

[6] A. G. Jones, C. Balocco, R. King, A. M. Song, Appl. Phys. Lett. 2006, 89,

013119.

[7] A. Sharma, G. Reiter, J. Colloid Interface Sci. 1996, 178, 383.

[8] G. Reiter, M. Hamieh, P. Damman, S. Sclavons, S. Gabriele, T. Vilmin, E.

Raphael, Nat. Mater. 2005, 4, 754.

[9] G. Reiter, Phys. Rev. Lett. 1992, 68, 75.

[10] R. Seemann, S. Herminghaus, K. Jacobs, Phys. Rev. Lett. 2001, 86, 5534.

[11] G. Reiter, Langmuir 1993, 9, 1344.

[12] B. Du, F. Xie, Y. Wang, Z. Yang, O. K. C. Tsui, Langmuir 2002, 18, 8510.

[13] J. Mahnke, H. J. Schulze, K. W. Stockelhuber, B. Radoev, Colloids Surf. A

1999, 157, 1.

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[14] G. G. Baralia, C. Filiatre, B. Nysten, A. M. Jonas, Adv. Mater. 2007, 19,

4453.

[15] G. Reiter, P. G. de Gennes, Eur. Phys. J. E 2001, 6, 25.

[16] C. Bolline, S. Cuenot, B. Nysten, A. M. Jonas, Eur. Phys. J. E 2003, 12, 389.

[17] N. Stutzmann, R. H. Friend, H. Sirringhaus, Science 2003, 299, 5614.

[18] L. Singh, P. J. Ludovice, C. L. Henderson, Thin Solid Films 2004, 449, 231.

Adv. Mater. 2009, 21, 1–5 � 2009 WILEY-VCH Verlag Gmb

[19] A. Faldi, R. J. Composto, K. I. Winey, Langmuir 1995, 11, 4855.

[20] Y. Ding, H. W. Ro, K. J. Alvine, B. C. Okerberg, J. Zhou, J. F. Douglas, A.

Karim, C. L. Soles, Adv. Funct. Mater. 2008, 18, 1854.

[21] Z. Fakhraai, J. A. Forrest, Science 2008, 319, 600.

[22] Y. Ding, H. W. Ro, T. A. Germer, J. F. Douglas, B. C. Okerberg, A. Karim, C.

L. Soles, ACS Nano 2008, 1, 84.

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