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Hierarchical Self-Assembly of Block Copolymersfor Lithography-Free Nanopatterning**

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By Bong Hoon Kim, Dong Ok Shin, Seong-Jun Jeong, Chong Min Koo, Sang Chul Jeon,

Wook Jung Hwang, Sumi Lee, Moon Gyu Lee, and Sang Ouk Kim*

Hierarchical self-assembly is the ultimate fabrication process

for complex nanostructures and ensures molecular-level pat-

tern precision and parallel processing.[1–5] Natural building

blocks, such as proteins, DNAs, and phospholipids, undergo

various levels of intra- and intermolecular assembly having

multiple length scales. Their complex functionalities and high

selectivity are largely owing to the resultant hierarchical

structures. Furthermore, the intimately interacting multiscale

orderings of a hierarchical assembly may provide an oppor-

tunity for directing over the diverse length scales simulta-

neously. However, developing a hierarchical self-assembly for

large-scale nanofabrication processes is still in its infancy.[6,7]

Block copolymers are self-assembling materials composed

of covalently linked macromolecular blocks. Owing to their

diverse chemical functionalities and precise tunability over

the shape and size of the self-assembled nanostructures,

block copolymers have been extensively utilized for nanofab-

rication.[8,9] However, the spontaneously assembled morphol-

ogy of block copolymers consists of randomly oriented

nanodomains with a large number of defects. For diverse

[*] Prof. S. O. Kim, B. H. Kim, D. O. Shin, S.-J. JeongDepartment of Materials Science and EngineeringKAIST Institute for the NanocenturyKorea Advanced Institute of Science and Technology (KAIST)Daejeon 305-701 (Korea)E-mail: sangouk.kim@kaist.ac.kr

Dr. C. M. KooHybrid Materials Research CenterKorea Institute of Science and Technology (KIST)Cheongryang Seoul P.O. BOX 131 (Korea)

S. C. Jeon, Dr. W. J. HwangNational Nanofab Center (NNFC)373-1 Guseong-dong Yuseong-guDaejeon 305-701 (Korea)

Dr. S. Lee, M. G. LeeSamsung Advanced Institute of Technology (SAIT)Mt. 14-1 Nongseo-dong Giheung-gu Yongin-siGyunggi-do 446-712 (Korea)

[**] We thank Sung Soon Bai for providing technical support for SEMcharacterization. This work was supported by the SamsungAdvanced Institute of Technology (SAIT), the Korea ResearchFoundation (KRF-2005-003-D00085), the second stage of the BrainKorea 21 Project, the Korea Science & Engineering Foundation(KOSEF) (R01-2005-000-10456-0), the Korean Ministry of Scienceand Technology, and the Fundamental R&D Program for CoreTechnology of Materials funded by the Korean Ministry ofCommerce, Industry and Energy. Supporting Information isavailable online from Wiley InterScience or from the author.

Adv. Mater. 2008, 20, 2303–2307 � 2008 WILEY-VCH Verlag G

advanced applications, such as optical elements,[10] sensor

arrays,[11] or nanofluidics,[12] control over the lateral orienta-

tion and orderings of these nanodomains is required. To date,

various methods such as the application of external fields,[13–15]

templated assembly upon prepatterned surfaces,[16–20] and

directional solidification[21,22] have been developed for the

well-ordered nanoscale morphologies of block copolymers.

However, those approaches require complicated facilities and

multistep processing for applying external fields or lithographic

prepatterning, thereby practically limiting the application to an

arbitrarily large area.

We introduce hierarchical self-assembly of block copoly-

mers as a lithography-free, ultra-large-scale nanopatterning

process. As schematically described in Figure 1, the hierarch-

ical assembly was derived from two levels of spontaneous

orderings. The large-scale ordering was defined by the periodic

thickness modulation of a block-copolymer film, which was

self-organized from the receding contact line of an evaporating

block-copolymer solution upon a substrate. Its length scale was

tunable over a scale of tens of micrometers by adjusting the

processing parameters. The small-scale ordering corresponds

to the self-assembled nanostructure of a block copolymer.

Its length scale was on a scale of tens of nanometers, as

determined by the molecular weight of the block copolymer.

Unlike ordinary hierarchical assembly, where a small-scale

structure determines the further organization into a larger

structure, the large-scale structure directs the small-scale

ordering in our approach. This offers a unique opportunity to

attain control over nanoscale morphology by manipulating a

microscale structure.

A symmetric polystyrene-block-poly(methyl methacrylate)

(PS-b-PMMA, molecular weight: 104 kg mol�1, lamellar spa-

cing: 48 nm) diblock copolymer was dissolved in toluene and

used for the hierarchical assembly. A micropatterned film of

the block copolymer was prepared from the polymer solution

confined between a glass plate and silicon substrate, as

described in Figure 1a. Upon the slow movement of the

upper glass plate, the polymer film, having a periodic thickness

modulation, was self-organized from the receding edge of the

polymer solution.[23,24] Because of the evaporation of volatile

toluene at the solution front, polymer residue was deposited at

the edge of the polymer solution and temporarily retarded the

movement of the solution edge such that the receding velocity

of solution edge was periodically varied. The resultant polymer

film left on the silicon substrate exhibited thickness variation

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Figure 1. Experimental procedure for the hierarchical nanopatterning process of a block copo-lymer. a) The micropatterned block-copolymer film was spontaneously organized from theevaporating block-copolymer solution. b,c) The subsequent annealing at high temperature ledto the spontaneous alignment of nanoscale lamellar morphology along the thickness gradient ofthe patterned film.

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reflecting the periodic velocity variation of solution edge. Note

that the surface of the underlying substrate was neutrally

modified for the PS and PMMA blocks to induce lamellae of a

PS-b-PMMA block copolymer that are perpendicularly

oriented to the surface.[25] Figure 2a presents the height

profile of a micropatterned film along the direction that the

depositing solution had propagated. It shows a periodic

thickness variation having a sinusoidal shape. The character-

istic length scales of the periodic pattern were tunable by

varying the polymer concentration in the solution or the

receding velocity of the contact line.[24] In a typical sample,

the pattern period was tunable over tens of micrometers and

the film thickness at the highest region ranged from a

few hundred nanometers to a few micrometers. After the

self-organized micropatterning process, the block-copolymer

film was substantially annealed at a high temperature.

When a micropatterned film was examined by field-emission

scanning electron microscopy (FE-SEM) after annealing, it

demonstrated a marvelous hierarchical morphology of micro-

scale stripes consisting of alternately well-aligned and

randomly aligned lamellae (Fig. 2b and c). The well-aligned

lamellae appeared at the thick part of the film, and their

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

orientation was orthogonal to that of the

microscale pattern (Supporting Information,

Fig. S1). Figure 2b shows a hierarchically

ordered morphology extending over a broad

field of 0.4 mm� 0.05 mm. A close examina-tion in a magnified image (Fig. 2c) demon-

strates a remarkably high degree of ordering

of nanoscale morphology in the well-aligned

region. This clearly contrasts with the

morphology at the randomly aligned region

with a distinctive boundary between the two

areas.

The morphology inside a hierarchically

ordered film was investigated by cross-

sectional SEM imaging, as presented in

Figure 3. The width of the hill-shaped

thickness modulation was about 75mm and

well-aligned lamellae extended over 65mm.

The corresponding aspect ratio of polymeric

nanowires exceeded 2600 (Supporting Infor-

mation, Fig. S2). Because the underlying

silicon surface was neutrally treated for

PS-b-PMMA, the lamellae were oriented

perpendicularly to the surface throughout

the film thickness.[25] The film thickness at

the highest region was about 1.4mm, where

the aspect ratio of the lamellar morphology

in the surface-normal direction was over 55.

Since neither an external field nor a

lithographic prepatterning process was

applied to the block-copolymer film, the

observed lamellar alignment was hardly

anticipated. In order to attain a detailed

understanding of this unique behavior, the

morphological evolution was characterized

while varying the microscale morphology of a film and thermal

annealing conditions. As expected, a block-copolymer film

having a uniform thickness did not show any preferential

alignment of lamellae (Supporting Information, Fig. S3). The

lamellae became macroscopically aligned provided that a

thickness modulation was introduced by micropatterning. In

the early stage of thermal annealing, the morphology of a

thickness-modulated film consisted of randomly oriented

lamellae throughout the whole film plane, signifying that no

preferential lamellar alignment occurred during the preceding

self-organized micropatterning process (Supporting Informa-

tion, Fig. S4). As annealing proceeded, well-aligned lamellar

domains were nucleated from the thick part of the film (Fig.

4a), gradually propagated following the thickness gradient

(Fig. 4b), and finally overwhelmed most of the patterned film

(Fig. 4c). Such a directional domain growth could be caused by

the influence of film thickness upon the defect annihilation

rate. Since lamellae were oriented perpendicularly to the

substrate surface, the defect morphology at the film surface

penetrated throughout the film thickness. Therefore, the defect

energy throughout the film thickness, which is given by the core

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Figure 2. Hierarchically organized PS-b-PMMA diblock-copolymer film, havingboth microscale and nanoscale orderings. The micropatterned block-copolymerfilm was spontaneously organized from the receding contact line of evaporatingpolymer solution with a substrate surface [24]. The depositing solution propagatedalong the x-direction and the resultant microscale stripes were extended in they-direction. Subsequent annealing at a high temperature led to the self-alignmentof lamellae in the x-direction. a) Height profile for a micropatterned block-copolymer film. The thickness modulation had a sinusoidal shape, having aperiod of about 60mm. b) Top-down scanning electron microscopy (SEM) imageof a hierarchically organized block-copolymer film over a large area of0.4mm� 0.05mm. The periodically appearing dark regions correspond to thevalley of the sinusoidal thickness modulation, where nanoscale morphologyconsists of randomly oriented lamellae. The periodically appearing bright regionscorrespond to the thick part of film where lamellae were well-aligned in thex-direction. c) High-resolution SEM images demonstrating the nanoscale order-ing along a half pitch of microscale pattern. The highly ordered lamellarmorphology in the well-aligned region clearly contrasts to the morphology inthe randomly aligned region.

energy and the energy penalty for the morphology distortion

around the core, was proportional to the film thickness.[26] The

influence of film thickness upon the defect energy could impose

the gradient in the defect annihilation rate along the thickness

gradient (Supporting Information, Fig. S3).[27] As a conse-

quence, the well-aligned lamellar domains were nucleated at

the thick part of the film and propagated following the

thickness gradient upon an isothermal annealing process.

The lamellae became oriented along the thickness gradient

upon the nucleation of well-ordered domains (Fig. 4a). The

spontaneous lamellar alignment should involve the anisotropic

mobility of defects. Since defects were annihilated through the

Adv. Mater. 2008, 20, 2303–2307 � 2008 WILEY-VCH Verl

interaction with oppositely signed defects,[26–28] the nucleation

and growth of well-ordered domains was accompanied by the

propagation of defects along the thickness gradient (Fig. 4a and

b). Note that, because of the topological constraint of lame-

llar morphology, the defect mobility is higher in the lamellar

parallel direction (climb) than in the lamellar perpendicular

direction (glide).[28,29] The movement in the lamellar parallel

direction requires the short-range diffusion of polymer

molecules whereas movement in the lamellar perpendicular

direction accompanies the sequential breakage and recombi-

nation of adjacent lamellae, which cause a high energy penalty.

Because of the anisotropic mobility, the propagation of defects

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Figure 3. Tilted SEM images showing both the top surface and cross section of a PS-b-PMMA film that demonstrates thickness modulation. Ablock-copolymer film on a silicon substrate was cryofractured to reveal the cross section. The sample was tilted 458 during SEM observation to reveal boththe top surface and cross section. The white scale bar corresponds to 5mm. a) Tilted SEM image for a hill-shaped thickness modulation (thehigh-resolution image is presented in the Supporting Information, Fig. S2) b,c) SEM images for well-aligned lamellae that appeared where the PS-b-PMMAfilm had a sufficient thickness gradient. The lamellae were well-aligned along the thickness gradient and were observed on the top side of the film. In thecross section, lamellae were oriented perpendicularly to surface throughout the film thickness. d) The lamellar arrangement around the boundary betweena well-aligned region and a randomly aligned region. The boundary is indicated with the white dotted line. Randomly aligned lamellae appeared where thePS-b-PMMA film had a modest thickness gradient. In the cross section, lamellae were oriented perpendicularly to the surface regardless of the filmthickness.

Figure 4. Top-down SEM images of a PS-b-PMMA film observed at the various stages of well-aligned domain growth. a) The nucleation of aligned domainsat the thick part of the PS-b-PMMA film. The preferential movement of the defects along the thickness gradient led to the spontaneous alignment oflamellae following the thickness gradient. b) Lamellar arrangement at the boundary between a propagating well-aligned domain and a randomly orienteddomain observed at the intermediate stage of domain growth. The domain boundary indicated by the white dotted line was propagating along the thicknessgradient. The major defect structure in the well-aligned domain was edge dislocation, oriented normally to the domain boundary. Disclinations as well asdislocations were present in the randomly aligned region. c) The tilted SEM image showing the cross-sectional morphology of a patterned film having asteep thickness gradient at its edge. Lamellae became well-aligned even in the vicinity of the film edge, provided that the film had a sufficient thicknessgradient. The bare wafer surface was exposed on the right side of the image.

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along a thickness gradient could raise the spontaneous

alignment of lamellae in the same direction (Supporting

Information, Fig. S5). Figure 4c shows the cross-sectional

morphology of a patterned film at the edge of the patterned

region. The bare wafer surface was exposed on the right side of

the image. The well-aligned lamellae almost reached the edge

of the patterned film. Lamellae became aligned even in the

very thin part of the film, provided that the film had a sufficient

thickness gradient.

In summary, we have demonstrated a hierarchical nano-

patterning process using self-assembling block copolymers.

The microscale architecture in a block-copolymer film directed

the spontaneous alignment of the nanoscale morphology

through the directional growth of well-ordered domains. This

facile and robust nanopatterning process employing the two

levels of spontaneous orderings represents a versatile pathway

to control nanoscale morphology by manipulating microscale

architecture. Because of the simplicity of the microscale

patterning process, our approach is readily applicable to an

arbitrarily large area. Furthermore, the shape and character-

istic length scales of a microscale pattern could be finely tuned

in conjunction with other noninvasive patterning techniques

such as imprinting or contact printing.[30] This could potentially

allow for diversifying the shape and length scales of the

hierarchical structures through all-parallel processing.

Experimental

A symmetric diblock copolymer, polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA, number-average molecular weight, Mn,of PS block: 52 kg mol�1, Mn of PMMA block: 52 kg mol

�1, PolymerSource, Inc.), was dissolved in toluene with a concentration ofapproximately 1–4 wt%. The prepared solution was confined in the gapbetween a slide glass and a silicon wafer yielding a straight contact lineof the polymer solution along the edge of the slide glass. The surface ofthe underlying substrate was neutrally treated to have identicalinterfacial tensions for PS and PMMA blocks. The glass plate locatedon top was mechanically drawn at a constant velocity (2–40mm s�1).Since the polymer residue deposited at the evaporating front of thepolymer solution temporarily pinned the receding contact line,the contact line underwent successive pinning and depinning to thesubstrate surface. As a consequence, the block-copolymer film,spontaneously organized from the receding contact line, had periodicthickness variation. The prepared micropatterned film was completelydried at room temperature and was sufficiently annealed at 190 8C. Theresultant hierarchically nanopatterned film was characterized by usinga 3D confocal measurement system (surf, Nano Focus) and FE-SEM(S4800, Hitachi).

Received: September 8, 2007Revised: November 2, 2007

Published online: May 26, 2008

Adv. Mater. 2008, 20, 2303–2307 � 2008 WILEY-VCH Verl

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