hierarchical self-assembly of block copolymers for lithography …bonghoonkim.com/1_sci...
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DOI: 10.1002/adma.200702285 NICATIO
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: [email protected]
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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