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DOI: 10.1002/adma.200702546

Two-Dimensional Patterning of Flexible Designs withHigh Half-Pitch Resolution by Using BlockCopolymer Lithography**

By Toru Yamaguchi* and Hiroshi Yamaguchi

Block copolymer lithography[1] has drawn considerable

attention as a combined top-down/bottom- up nanopatterning

method with molecular-level resolution.[2–5] The key challenge

for its application to nanodevice fabrication lies in achieving

2D patterning by strictly controlling the alignment of the

interfaces of microphase-separated domains of a block

copolymer. In this Communication, we have developed a

flexibly designable 2D patterning method by combining

bottom-up diblock copolymer self-assembly with top-down

electron beam lithography (EBL). An intentionally designed

2D EBL guiding pattern induces unique self-assembly of block

copolymers with extreme accuracy. Both bent lamellae and

concentric cylinders were formed and successfully transferred

to a semiconductor substrate with a 16 nm half-pitch resolu-

tion. Our method increases the applicability of block

copolymer lithography to nanodevice fabrication, as the size

scale is beyond the reach of top-down technology.

Block copolymer lithography involves the use of a mono-

layer of microphase-separated domains of a block copolymer

in a thin film as a lithography template.[6–12] Because only the

chain length of the block copolymers governs the sizes and

periods of these domains, this method will very likely exceed

the resolution limit of state-of-the-art top-down lithography.

The major advantage of this method is the creation of domains

with an approximately consistent size in large quantities and at

a low cost. Many techniques for fabricating self-assembled

domains in a large area have been reported, for example,

graphoepitaxy,[2,3,12–18] electric fields,[7,8] chemical nanopat-

terns,[19–21] shear forces,[22] directional solidification,[23] solvent

annealing,[24] and stabilization by functional block copoly-

mers.[25] In particular, for large-area periodic structures in

spherical domains or vertical cylindrical domains, many

applications have been proposed, for example, quantum

dots,[26] patterned magnetic media,[12,13] flash memory,[27]

capacitors,[28] and nanofilters.[29]

[*] T. Yamaguchi, Dr. H. YamaguchiNTT Basic Research LaboratoriesNTT Corporation3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198 (Japan)E-mail: guchan@nttbrl.jp

[**] This research was partly supported by JSPS KAKENHI (16206003).The authors thank J. Hayashi for the sample preparation, and K.Inokuma (NTT Advanced Technology), and Dr. K. Yamazaki for theEB exposure. Supporting Information is available online from WileyInterScience or from the authors.

� 2008 WILEY-VCH Verlag

Many researches have focused on the realization of perfect

self-assembly without defects over a large area owing to the

wide variety of applications for such a result. On the other

hand, from the viewpoint of nanodevice fabrication it is

important to fabricate nanometer-sized structures at a target

position with pin-point accuracy. With regard to spherical

domains such as a dot template, it has recently been reported

that the 2D registration of a hexagonal array of such domains

can be achieved by using a small protruding spike at the

groove edge[30] or by angled guiding patterns.[31,32] Moreover,

these nanostructures should not be limited to simple periodic

patterns such as dot arrays and straight strip lines, but should

be intentionally designed as 2D patterns that can be applied

to nanodevice circuits such as gates and wiring. With regard

to cylindrical domains as a 2D line template, it has already

been reported that in-plane cylindrical or half-cylindrical

domains can be aligned with the curvature of the guide

patterns.[14,15,17] From the viewpoint of pattern transfer, the

vertical lamellar domain is advantageous as compared to the

spherical or in-plane cylindrical domains owing to its high

aspect ratio. On the other hand, these low-aspect-ratio

domains can also function as an etching template if a block

copolymer with an etch-resistant block is used.[10,33] How-

ever, it is certain that vertical lamellar domains should be

used as the pattern size shrinks into the extreme regime. It

has been successfully demonstrated that vertical lamellar

domains with various 2D shapes and angles can be formed by

epitaxial self-assembly of ternary blends of block copolymers

and homopolymers on chemically nanopatterned surfaces.[20]

This method is excellent, but it requires the pitch of the

chemical prepatterns created by top-down lithography to be

similar to the repeating period of lamellar domains. From the

viewpoint of resolution, a graphoepitaxial technique[2,3] that

can align self-assembled domains with a smaller pitch than

that aligned by the prepatterns should also be used. A 2D

graphoepitaxy of lamellar domains in very confined spaces

such as nanopores[34–36] and electrospinning fibers[37] has

already been reported; this suggests that such self-assembled

domains, which are never obtained in a bulk state or in a thin

film, can be formed. However, it is impossible to position

these domains accurately at any target area on a substrate of

our interest. To the best of our knowledge, there has been no

previous report of a 2D alignment method of vertical

lamellae domains using graphoepitaxy that successfully

satisfies requirements such as high resolution, pattern

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transferability, and intentional 2D positioning in the extreme

regime.

In this study, we report a flexible 2D patterning method by

combining bottom-up diblock copolymer self-assembly with

top-down electron beam lithography (EBL) to satisfy the

above-mentioned requirements for nanodevice fabrication.

We have already devised a method for the graphoepitaxy of the

lamellar domains of symmetric diblock copolymers using resist

patterns as alignment guides.[38] By applying this method to a

2D self-assembly, we have demonstrated the formation of bent

vertical lamellar domains and concentric vertical cylindrical

domains with a 16 nm half-pitch. This method may have

practical advantages in fabricating 2D nanopatterns in the

16 nm regime and beyond, such as high resolution, high etching

durability, accurate positioning, and wide flexibility in a

pattern layout. For the first time, block copolymer self-

assembly in combination with high-precision EBL has enabled

the realization of flexible 2D self-assembly of diblock

copolymers.

First, we briefly explain the alignment method used in this

study. To form a dense line pattern by using lamellar domains,

the lamellar interface must be aligned in two directions:

perpendicular to the substrate and parallel to the sidewall of

the guide pattern. To achieve this, it is essential to create a

difference between the surface affinity of the substrate and that

of the sidewall by using different materials. The use of a resist

pattern as an alignment guide easily meets this requirement

because the surface modification of the substrate and the

formation of guide patterns with a different affinity can be

carried out separately.

We formed a neutralization layer on the substrate (Fig. 1a),

which is a very effective procedure for the perpendicular

alignment of the domain interface.[39–41] It prevents the either

block domain from preferentially wetting the substrate

because the surface affinity of the neutralization layer is

approximately half that of either of the domains of the block

copolymer. This enables the easy alignment of the lamellae

perpendicular to the substrate. Second, we formed 2D

hydrophilic guiding patterns on the neutralization layer using

Figure 1. Schematic diagram of the alignment approach. a) The neutral-ization layer formed on the substrate. b) 2D hydrogen silsesquioxaneguiding patterns formed on the neutralization layer. c) Block copolymerthat was coated, and d) annealed to induce confined self-assembly.

Adv. Mater. 2008, 20, 1684–1689 � 2008 WILEY-VCH Verla

a negative-tone hydrogen silsesquioxane (HSQ) resist,[42,43]

which has crucial advantages as an alignment guide, such as

high heat resistance and low line-edge roughness (Fig. 1b). This

material has already been used for the directed self-assembly

of spherical domains of diblock copolymers.[31] In this study,

we pay attention to the hydrophilicity of this material after

patterning (cross linking). The use of hydrophilic guiding

patterns leads to the lateral alignment of the lamellar domains

Figure 2. a) AFM phase image of lateral lamellar domains inside sixgrooves separated by HSQ guiding patterns with a pitch of 170 nm. Phaseimaging is a standard technique for mapping the spatial variations in thesurface elasticity. The lamellar structures can be observed owing to thedifference in the elasticity between the poly(styrene) (PS) and poly(methylmethacrylate) (PMMA) domains. The dark domain corresponds to the PSdomain that is harder than the PMMA domain (bright domain). b)Cross-sectional phase profiles obtained by averaging the phase image of(a) along the groove. c) Cross-sectional topographic profile obtained byaveraging the topographic image (not shown here) corresponding to (a)along the groove. The PMMA domains are observed even in the topo-graphic image because they swell up to a height of 1 nm or less on themeniscus surface of the block copolymer film. The difference in heightbetween the top surface of the guide pattern and the film surface isapproximately 10 nm, which prevents the PMMA domains directly wettingthe sidewall surface from being observed in AFM images. In all figures, thedots represent the positions of the guide patterns.

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Figure 3. a) SEM image of self-assembled domains in spaces confinedbetween rectangular guiding patterns. Three dark lines in a confinementseparated by HSQ guiding patterns (bright white lines) correspond to thePMMA domains. b) SEM image of retained PS domains after the removalof the PMMA domains. It is noteworthy that the widths of these four linesare approximately the same. This fact strongly supports the model shownin Figure 1 in which the PMMA domain preferentially wets the sidewall ofthe HSQ guide patterns. c,d) SEM images of etched patterns after theentire pattern transfer process is conducted, including the dry developmentof the PMMA domains, SiO2 etching, and Si substrate etching. Unfortu-nately, the Si substrate was not etched in this study because the opening ofthe PS template could be filled up with an etching protection layer duringetching process. The pitches of the guiding patterns are 140 nm (4Laconfinement) for (a–c) and 170 nm (5La confinement) for (d). Scale barsare 200 nm.

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because the ether block domain, which has a surface affinity

close to that of the hydrophilic sidewall of the guiding pattern,

preferentially wets the sidewall. Third, we spin-coated a thin

film of a block copolymer over the guide patterns (Fig. 1c).

Finally, we annealed the film to induce lateral alignment as well

as microphase separation (Fig. 1d).

Before describing the 2D self-assembly of block copolymers,

we first refer to the 1D periodic, lateral alignment of lamellar

domains induced by graphoepitaxy using straight guiding

patterns. This enables the lateral alignment of lamellar

domains with at least five periods in the spaces confined

between the guide patterns. Figure 2a shows the atomic force

microscopy (AFM) phase image of lateral lamellar domains

inside six grooves separated by HSQ guiding patterns with a

pitch of 170 nm. In all the grooves, four dark domains

corresponding to the poly(methyl methacrylate) (PMMA)

domains are distinctly separated from each other, and they are

successively aligned over a length of 1mm. Figure 2b and c

shows the averaged cross-sectional profiles of the AFM phase

and topographic images, respectively, of the sample shown in

Figure 2a. In Figure 2b, four weak downward peaks

corresponding to the PMMA domains are clearly observed

between the strong downward peaks corresponding to the

guide patterns. The average repeating period of the laterally

aligned lamellar domains (La) is approximately 32 nm. This

period is approximately 12% greater than that of the lamellar

period (L0¼ 28 nm) obtained from the step height of the

parallel lamellar domains formed on a flat substrate.[38] This

slight difference is partly due to the fact that the lamellar

period and the width of the spaces confined between the guide

patterns are not commensurate. This fact causes a variation in

the pitch of the self-assembled domains inside a groove.[15]

Considering that the HSQ guide patterns are approximately

15 nm wide, the confined space must be approximately 155 nm

wide, which corresponds to approximately 5La. This result

confirms that the HSQ guiding patterns actually function as a

guide for lateral alignment and also that lamellae with a width

of 5La are laterally aligned between the HSQ guiding patterns.

Next, we describe the 2D self-assembly of symmetric diblock

copolymers. Figure 3a shows an scanning electron microscopy

(SEM) image of self-assembled domains in spaces confined

between rectangular guides with a pitch of 140 nm (4La

confinement). It is clear that the three dark lines corresponding

to the PMMA domains are almost perfectly aligned with the

guide patterns in the straight regions, both in the vertical and

horizontal directions. Of particular interest is the fact that

three PMMA domains are forced to bend along the guiding

pattern around the corner.

To clarify not only the surface domain morphology but

also the domain structure of laterally aligned lamellae inside a

film, the PMMA domain was selectively removed by deep

ultraviolet (DUV) light exposure and subsequent development

with acetic acid.[8] Figure 3b shows an SEM image of the

self-assembled domains after the removal of the PMMA

domains. Four poly(styrene) (PS) domains are retained in the

confined space, although they collapsed and stuck together

www.advmat.de � 2008 WILEY-VCH Verlag GmbH

owing to the surface tension of water during the rinsing step.

It is clear that these PS domains are continuously bent at right

angles, although the width of confinement is locally modulated

at the corner. The maximum width of confinement at the

corner is approximately 5.5La (ca. 177 nm), which corresponds

to the square root of two times the width of confinement in the

straight region, 4La (ca. 125 nm). In the straight confinement

with a width of 5.5La, it is expected that more than 5 PS

domains should be retained; however, the number of repeating

periods does not change and remains 4 at the corner. This

indicates that the block copolymers rearrange themselves to

form bent lamellae in the corner area of 4La� 4La, while

maintaining the lamellar interface perpendicular to the

surface; this rearrangement is induced by both topographical

confinement and the varying surface affinities of the contacting

surfaces such as the bottom neutral surface, two adjacent

hydrophilic sidewalls, and vertical stripes of PS and PMMA

domains at the boundary interface between the corner and the

straight regions. It is noteworthy that these domain structures

are formed in a pure block copolymer system, which can be

achieved with high flexibility in the domain shape and period in

graphoepitaxy of lamellar domains of diblock copolymers.

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Figure 4. AFM phase images of the self-assembled domains in confinedspaces between the hexagonal guiding patterns with pitches of a) 140 nm(4La confinement), and b) 170 nm (5La confinement) between oppositesides. The bright and dark regions correspond to the PMMA and PSdomains, respectively. Height profiles along line XX0 are also shown aboveeach image. SEM images of etched patterns in c) 4La and d) 5La confine-ments after the entire pattern transfer process is conducted, including thedry development of the PMMA domains, SiO2 etching, and Si substrateetching. e–h) SEM images of retained PS domains after the removal of thePMMA domain by DUV exposure and development with acetic acid.Pitches of guiding patterns between opposite sides are e) 140 nm (4La),f) 170 nm (5La), g) 110 nm (3La), and h) 210 nm (6La). SEM images ofretained PS domains in linear confinements are superimposed on eachfigure for reference. Scale bars are 200 nm.

We attempted to transfer patterns from these bent lamellae

onto a substrate. The PMMA domains were selectively

removed by dry development with oxygen plasma because

the wet process using DUV exposure and development can no

longer be used for the pattern transfer, as shown in Figure 3b.

After the removal of the PMMA domains, a 4 nm thick SiO2

layer just below the neutralization layer was etched with the

remaining PS template using CF4-based gas chemistry. Figure

3c and d show the SEM images of the etched patterns. It is clear

that bent lamellae with a half-pitch of 16 nm were successfully

transferred to the SiO2 layer for 4La and 5La confinements.

This result undoubtedly indicates that the graphoepitaxy of

lamellar domains is definitely advantageous for pattern

transfer in 16 nm regimes and beyond.

Furthermore, we have succeeded in not only bending line

patterns but also constructing new nanopatterns, which has

never been performed in an unguided film, by confining the

phase-separated diblock copolymers two-dimensionally. The

AFM phase images of the self-assembled domains inside

the hexagonal cells are shown in Figure 4a and b. The pitches of

the guide patterns between opposite sides are 140 nm (�4La)

and 170 nm (�5La). For the 4La confinement (Fig. 4a), it is

clearly observed that a concentric semicylindrical domain

comprising PMMA (dark) and PS (bright) domains with a

PMMA core formed with a high-yield (>90%). However,

in several cells, the PMMA core diminishes. As clearly

observed in the height profile in Figure 4a, the film thickness in

these cells is thicker than that in other cells by only around

2 nm. This implies that PS domains whose surface tensions

(�39.4 mN m�1 at 20 8C, number-average molecular mass

Mn¼ 9300 g mol�1) are slightly lower than that of the PMMA

domains (�41.1 mN m�1 at 20 8C, M¼ 3000 g mol�1) partially

segregate at the film surface and distort the domain morphology

at the surface.[44] For the 5La confinement (Fig. 4b), concentric

semicylindrical domains with the PS core are actually formed in

some cells; however, the domain morphology at the surface is

distorted in almost all the cells.

In order to clarify the domain structure inside a film,

these self-assembled domains were transferred to the SiO2

layer via a dry etching process, as previously described.

Figure 4c and d show SEM images of the etched SiO2 surface

in the 4La and 5La confinements, respectively. It is clearly

observed that dotlike and circular scratches are formed in

many cells for the 4La confinement. The cells in which

the PMMA core is not transferred are thought to correspond

to cells in which the domain morphology is distorted, as

shown in Figure 4a. A slight difference in thickness leads to

the failure of pattern transfer. Conversely, this implies that the

success of pattern transfer constitutes indirect evidence of

the domain interface being perpendicular to the surface inside

a film. The surface domain morphology does not necessarily

reflect the internal domain structures.

Finally, we investigated the dependence of the domain

morphology on the width of a hexagonal confinement. To

observe the domain structure more clearly, the PMMA domain

was removed by DUV exposure and wet development, as

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previously described. Figure 4e–h shows SEM images of the

remaining PS domains after the removal of the PMMA domains.

It is evident that PS rings are formed in all the cells, although they

are deformed and partially removed; this confirms that the con-

centric cylindrical domains with vertical interfaces are formed in

a hexagonal confinement. As expected from the results of

self-assembly between the straight guides, (n�1)/2 layers of

concentric PS rings with a PS core are formed in an nLa

confinement when n is an odd number; on the other hand, n/2

layers of PS rings with a PMMA core are formed when n is an

even number. These results are consistent with those of previou-

sly reported experimental[34,35,37] and theoretical studies[45,46] on

the self-assembly of symmetric diblock copolymers in cylindrical

confinements and in which the width of confinement is

commensurate with the lamellar period and one of the segments

shows a strong attractive interaction with the sidewall of the

hexagonal guides. The most important result is that we have

experimentally demonstrated that these concentric cylindrical

rings can be artificially formed inside lithographically created

guides; in other words, they can be formed at any target position

on a substrate with pinpoint accuracy.

In summary, we have investigated 2D self-assembly of the

symmetric diblock copolymer PS-b-PMMA in an intentionally

designed confinement. The combination of the neutralization

of the bottom surface and the use of HSQ resist patterns as an

alignment guide enables the graphoepitaxy of lateral lamellar

domains in confined spaces. For rectangular confinement,

vertical lamellar domains with a thickness of 5La and with a

half-pitch of 16 nm can be forced to bend using right-angled

guiding patterns. For hexagonal confinement, we have

successfully demonstrated that concentric cylindrical domains

are formed; these domains are characterized by high controll-

ability of the number of layers of the PS or PMMA rings, which

is achieved by varying the width of confinement between

opposite sides. This precise control of the alignment of lamellar

domains can be achieved by first taking full advantage of the

bottom-up self-assembly of block copolymers and the top-down

fabrication of the alignment guide by using high-precision

electron-beam lithography. This method has the significant

advantage of maintaining strict control on the alignment of

self-assembled nanodomains that can never be formed in a

bulk state or a thin film, thereby enabling the flexible

fabrication of 2D self-assembled domains. We are convinced

that this method has the potential to be a powerful technique

employing a combined top-down/bottom-up approach for 2D

nanopatterning in nanodevice fabrication on a size scale

beyond the reach of the state-of-art top-down lithography.

Experimental

Neutralization Layer: A crosslinked film composed of an alternatingcopolymer of poly(a-methyl styrene) and poly(methyl methacrylate)(Mw: 114 kg mol�1, Mw/Mn: 1.96, Polymer Source Inc.) was used as theneutralization layer [38]. The polymer solution was prepared by addinga cross-linker and a thermal acid generator (TAG) in ratios of 30 and10 wt% to the alternating copolymer dissolved in 2-methoxyethylacetate. We used 1,3,5-trimethyl-2,4,6-(triacetoxymethyl) benzene as a

www.advmat.de � 2008 WILEY-VCH Verlag GmbH

crosslinker and cyclohexylmethyl (2-oxocyclohexyl) sulfonium tri-fluoromethane sulfonate (CMS-105, Midori Kagaku) as a TAG. Thesolution was spin coated onto a 4 nm thick SiO2/Si substrate to athickness of approximately 10 nm. The film was baked on a hot plate at200 8C for 2 min. The baking process resulted in the crosslinking ofthe alternating copolymer by means of the crosslinker through anacid-catalyzed reaction, and the film then becomes insoluble in thethinner solvent for polymer solutions used in the following process.The unreacted substances were removed by dipping the crosslinkedfilm in toluene for 90 s. The reaction mechanism has been describedelsewhere [47].

Guiding Patterns: Hydrogen silsesquioxane (HSQ) (Fox-16, DowCorning), which is a negative-tone electron-beam resist [42, 43], wasused as a guide pattern for alignment. The solution diluted withmethylisobutylketone was spin-coated onto the neutralization layer toa thickness of 40 nm. The film was prebaked on a hot plate at 110 8C for1 min. It was then exposed to an electron beam with an accelerationvoltage of 100 kV (VB6UHR, Vistec Lithography) [48]. The film wasdeveloped in a 2.38% tetramethyl ammonium hydroxide (TMAH)solution for 60 s.

Block Copolymer: Symmetric poly(styrene)-b-poly(methyl metha-crylate) (Mn: 36 kg mol�1, Mw/Mn: 1.07, lamellar period Lo: 28 nm,Polymer Source Inc.) was used as the block copolymer for the lamellarstructure. The toluene solution of this block copolymer was spin-coatedover the HSQ pattern on the neutralization layer to a thickness of30 nm. The film was then annealed in an oven at 195 8C in a N2

atmosphere for 16 h to induce lateral alignment as well as microphaseseparation.

Patern Transfer: The dry development of the PMMA domains wasconducted by exposing the sample to 200 W O2 electron cyclotronresonance (ECR) (ECR-6002, Canon ANELVA) plasma at a pressureof 2� 10�4 Torr (1 Torr¼ 1.333� 102 Pa) for 17 s. The etch rate for aPMMA film was �140 nm min�1 and the etch rate ratio of PMMA/PSwas �2.2. The SiO2 layer was etched by reactive ion etching (RIE)(Tokuda) at 60 W in CF4/CHF3/Ar (24 vol% CF4, 6 vol% CHF3,70 vol% Ar) at a pressure of 6 Pa. The etch rate of an SiO2 film layerwas �33 nm min�1 and the etch rate ratio of SiO2/PS was �3.0. The Sisubstrate was etched by ECR etching at 370 W in Cl2/O2/SF6 (84 vol%Cl2, 8 vol% O2, 8 vol% SF6). The etch rate for the Si substrate was�120 nm min�1.

Characterization: The microphase-separated domains wereobserved by scanning electron microscopy (SEM) (S-7800, Hitachi)and atomic force microscopy (AFM) (SPI3800/SPA500, SII nano-technology) in the dynamic force mode. Before SEM observation, thePMMA domains were removed by DUV exposure (dose: 1 J cm�2,lamp: UXM-501MD (Ushio)) and development with acetic acid for60 s. All intensities are assumed to be emitted at 254 nm. Etchedpatterns were also observed by SEM.

Received: October 10, 2007Revised: December 11, 2007

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