Block copolymer patterns and templates

The rapid development of nanoscience and nanotechnology has

pushed the scale limits of modern functional devices1. The so-

called ‘top-down’ lithography process has been the main driving

force for this miniaturization. The broad scope of top-down

processes, including conventional 193 nm immersion lithography,

extreme ultraviolet (EUV) lithography, X-ray lithography, soft

lithography, and step-and-flash lithography, are in production or

under development for generating or transferring minimized

structures through the reduction of larger mask patterns. To meet

or exceed the predicted pace of lithographic miniaturization

following Moore’s law, unconventional lithography, which could be

described as ‘direct write’ approaches, has also been studied and is

exemplified by electron-beam (e-beam) lithography, three-

dimensional two photon lithography, and multibeam interference

lithography. However, the daunting obstacles to improve

resolution remain the inherent limitations of optical lithography

and the exorbitant cost of lithography production tools.

Block copolymers have become of great interest for high-resolution

patterning because of their simplicity, elegance, and high throughput.

The early investigation of block copolymers as a microlithographic

photoresist can be traced back to late 1980s, largely focused on F- or

Si-containing block copolymers. Because of the surface segregation of

these low surface energy blocks to the top surface, they act as a

hydrophobic overcoat to avoid airborne base contamination and thus

improve the lithographic dimensional stability. The possibility of

micelle formation with these materials facilitates their development in

alternative systems such as supercritical CO2, which has low

environmental impact2. Besides these examples, Si-containing block

copolymers provide high oxygen plasma etch resistance, which makes

them desirable as high-resolution resist materials.

This review describes the chemical and physical aspects of patternableblock copolymers and their use for nanostructure fabrication. Thepatternability of block copolymers results from their ability to self-assemble into microdomains and the manipulation of these patterns by avariety of physical and chemical means. Procedures for achieving long-range lateral order, as well as orientation order of microdomainpatterns, are discussed. The level of control that these strategies affordhas enabled block copolymers to be used as templates for fabricating avariety of nanostructures.

Mingqi Li1 and Christopher K. Ober2,*

1Rohm and Haas Electronic Materials, 455 Forest Street, Marlborough, MA 01752, USA2Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA

*E-mail: [email protected]

ISSN:1369 7021 © Elsevier Ltd 2006

Block copolymerpatterns and templates

SEPTEMBER 2006 | VOLUME 9 | NUMBER 9 30

mailto:[email protected]

An alternative approach to achieve miniaturization goals is the so-

called ‘bottom-up’ approach. This uses the uniform nano- and

supermolecular structures that result from organized macromolecule

packing, the so-called ‘self-assembly’ process. Block copolymers tend to

self-assemble into a variety of well-ordered nanostructures with almost

continuously tunable resolution from several to hundreds of

nanometers because of the chemical immiscibility of the covalently

linked segmental chains3-5. Through various templating processes with

these block copolymer microdomain masks, high-resolution functional

nanostructures can be generated.

This review discusses the chemical and physical aspects of the block

copolymer assembly approach, and how one can use them to generate

controlled high-resolution patterns. Because of the practical

importance of large-domain, oriented, high-resolution patterns, the

means to control the microdomain pattern orientation and lateral

order will be emphasized. In particular, recent progress on the

combination of top-down or direct-write approaches with bottom-up

self-assembly processes to generate multilevel hierarchical

nanostructures will be discussed, since it has been shown to be

effective in generating long-range order and tunable orientations

registered within lithographically defined submicron patterns. Finally,

the use of block copolymer high-resolution patterns as templates to

generate functional nanostructures and nanodevices will be also

addressed.

Block copolymer microdomain patternsThe molecular packing, and thus thermodynamically stable

microdomain patterns, of block copolymers in the bulk state are

governed by the positive mixing enthalpy and low mixing entropy of

component segments. Because of the covalent bonding between the

segments, the system can not macroscopically phase separate, and so

it minimizes the interfacial energy by adopting well-defined

microdomain patterns with constant interfacial curvature and stretched

interfacial chain configurations3,4. For the simplest and most

extensively studied coil-coil diblock copolymers, the molecular weight,

volume fraction of the component, and the degree of segment

incompatibility as expressed by the Flory-Huggins parameter χ are the

three independent parameters used to determine thermostable

morphologies5,6. As shown in Fig. 1, uniform-sized spheres, cylinders,

and lamellae, as well as complex bicontinuous nanostructures, are

theoretically predicted and experimentally observed by varying these

parameters.

To pursue the ideal scenario of facilitating the understanding of

practical systems, such as natural biomacromolecules, extensive

efforts have attempted to study nanostructure pattern formation of

much more complex blocky architectures with different chain segment

rigidity and noncovalent segment interactions. Several excellent

books and reviews have described recent progress in these fields3,7.

However, the entire picture of these complex systems is still not

clear. More experimental as well as theoretical investigations are

needed.

As a result of the distinct character of the component segments,

amphiphilic block copolymers form micelles in selective solvents, with

the solvent-phobic block forming the micelle core and the solvent-

philic block forming the micelle corona8. In concentrated solutions,

block copolymer micelles organize into well-defined nanostructured

patterns, including a lamellar phase, a hexagonal phase of rod-like

micelles, a cubic phase of spherical micelles, as well as a bicontinuous

cubic phase. These high-resolution microstructures have been used in

various block copolymer templating processes.

To mimic the conventional top-down lithographic process that

works on thin-film polymeric resists, block copolymers in thin film

states show much more potential and draw more attention for

nanofabrication applications. In addition to the parameters described in

the bulk state, there are two additional factors that govern block

copolymer thin film patterns: the polymer-air and polymer-substrate

interactions, and the film thickness t relative to the bulk state natural

domain period L0. Even in the simplest compositional symmetrical coil-

coil block copolymer system, although both of the two building blocks

have similar volume ratios and the system thus favors formation of a

lamellar mesophase in the bulk, a very sophisticated picture is revealed

in the thin-film state, as illustrated by Fasolka et al.9. Various aspects

of the thin film block copolymer structures have been presented by

several other reviews10,11.

Fig. 1 Mean-field prediction of the thermodynamic equilibrium phase structures for conformationally symmetric diblock melts. Phases are labeled as: L (lamellar), C

(hexagonal cylinders), G (bicontinuous cubic), S (body-centered cubic spheres). fA is the volume fraction.

Block copolymer patterns and templates REVIEW FEATURE


REVIEW FEATURE Block copolymer patterns and templates


Control of long-range lateral andorientational order of block copolymerpatternsThe intrinsic obstacles for large-scale nanofabrication with block

copolymer patterns lie in two aspects: (1) the inherent small domain

size or, in other words, short-range lateral order; and (2) unfavorable

domain orientation. It generally takes days or weeks for annealing a

block copolymer system to generate micron-sized domains with

minimal energetically costly grain boundaries and defects. In order to

make block copolymer nanopattterns applicable for routine

nanofabrication processes, these obstacles have to be solved

simultaneously in most cases, that is, generating large domain size with

the preferred orientation in short processing periods.

In the bulk state, long-range domain orientation may be generated

by the application of external fields on block copolymer melts or

concentrated solutions. Shear field12, electric fields13-17, elongation or

Alignment category Block copolymer Morphology 1st orientation control 2nd orientation control

Thickness control PS-b-PnAMA21 Multiple Film thickness

PS-b-PBMA22 Lamella Substrate topography

PS-b-PMMA23 Sphere Graphoepitaxy

PS-b-P2VP24 Sphere Graphoepitaxy

PS-b-PFDMS25 Sphere Graphoepitaxy

PFDMS-b-PDMS26 Cylinder Graphoepitaxy

Control interfacial PS-b-PMMA27 Lamella, cylinder Neutral surface

interaction PS-b-PMMA28,29 Lamella Chemically patterned

substrate

PS-b-P2VP30 Lamella Chemically patterned

substrate

PS-b-P2VP31 Cylinder Interfacially segregated

particles

External field PS-b-PMMA16,32 Cylinder Electrical field

PS-b-PEP34 Cylinder Single layer shearing

PS-b-PMMA35 Lamella Eutectic solidification

PS-b-PE36 Cylinder Eutectic solidification

PB-b-PEO37 Lamella Crystallization

PS-b-PB38 Cylinder, lamella Fast solvent evaporation

(flow field)

PS-b-PEO40 Cylinder Fast solvent evaporation

(flow field)

PS-b-P2VP41 Lamella Fast solvent evaporation

(flow field)

PS-b-PB-b-PS42 Cylinder Fast solvent evaporation

(flow field)

PaMS-b-PHS43,44 Cylinder Fast solvent evaporation

(flow field)

Multiple alignment force PS-b-PMMA45 Lamella Electric field Orthogonal electric

field

PS-b-PI47 Cylinder Directional crystallization Graphoepitaxy

PS-b-PMMA48,49 Lamella Chemically patterned Neutral surface

substrate

PS-b-PEP50 Biaxial parallel Preferential interaction Preferential interaction

cylinder with substrate with trough topography

Abbreviations

Polystyrene, PS; poly(n-alkylmethacrylate), PnAMA; poly(n-butylmethacrylate), PBMA; poly(methyl methacrylate), PMMA; poly(2-vinylpyridine), P2VP;

poly(ferrocenyldimethylsilane), PFDMS; poly(dimethylsiloxane), PDMS; polyisoprene, PI; poly(ethylene-alt-propylene), PEP; polyethylene, PE;

polybutadiene, PB; poly(ethyleneoxide), PEO; poly(ferrocenylethylmethylsilane), PFEMS; poly(tert-butylmethacrylate), PtBMA; poly(α-methylstyrene),

PαMS; poly(4-hydroxystyrene), PHS.

Table 1 Summary of representative approaches for microdomain orientation control in block copolymer thin films.

compression fields18, and temperature gradients19 effectively induce

single-crystal-like oriented microdomain textures that span large

microscopic dimensions.

In the thin-film case, preferential interaction of one block with the

substrate or the low surface energy of another block forces them to

segregate onto the interfaces. As a result of the general

incommensurability of film thickness t relative to the natural period L0,

which is defined as the chain length in the bulk state at equilibrium,

these surface/interfacial effects drive the anisotropic lamellar and

cylinder domains to preferably align parallel to the substrate20, instead

of the normal direction that is more technologically important for

fabricating high-aspect-ratio nanostructures.

Generally, the strategies applied for orienting block copolymer thin

films have focused on three approaches: (1) film thickness t relative to

the natural period L0 (substrate topography21-26); (2) substrate-

polymer interactions (neutral surfaces27,28, chemically patterned

surfaces29,30, and modified interfacial characteristics31); and (3)

external fields (electric32,33, shear34, eutectic solidification35,36,

crystallization37, solvent evaporation38-44, etc.). Recently, there has

been a trend to try and incorporate multiple alignment strategies into

a single orientation process45-50. Synergy effects of these multiple

driving forces have been shown to yield exceptional long-range lateral

order with desirable orientations in much shorter periods of time. Table

1 exemplifies the various strategies used to generate lateral and

orientation control of block copolymers microdomains.

Multilevel control of block copolymerpatterns by combining top-down/direct-write with the bottom-up approachThrough optimization of fabrication procedures with lateral and

orientation control, large-area well-aligned periodic patterns can be

generated during block copolymer self-assembly. However, many

applications require precise registration of the feature patterns onto an

underlying substrate, as well as spatial control of the microdomain

patterns. Meanwhile, it has been shown that periodic thickness or

surface/interface energy profiles can be generated by lithography to

guide the overlaying block copolymer nanopattern orientation

effectively21-26. In light of these aspects, the convergence of top-

down/direct write with bottom-up approaches has generated much

attention for gaining multilevel control of block copolymer patterns.

The epitaxial growth method, originally defined as the growth and

crystallographic registration of an ordered phase on the surface of

another crystal phase, has been used to generate excellent control of

lateral and orientation order in block copolymers, as exemplified by

the work of Park et al.35 and De Rosa et al.36 on the eutectic

solidification of block copolymer/solvent mixtures. Similarly, the

graphoepitaxy method involves the directing and alignment of

copolymer grains along lithographically defined topographical features

on substrates. Through epitaxial growth of a block copolymer bottom-

up structure on a substrate with a periodic thickness profile21, a

micropattern with different lamellar domain orientations can be

achieved that mirrors the substrate topography (Fig. 2). Segalman et

al.24 have studied spherical poly(styrene-b-2-vinylpyridine) (PS-b-

P2VP) block copolymers on patterned mesa substrates. Epitaxial

growth of spherical microdomains was nucleated by the mesa edge.

Long-range lateral order was formed across the entire substrate when

the periodic spacing of the mesa edges was comparable with the grain

size. Sundrani et al.50 used a micromachined Si substrate and were

able to align cylindrical poly(styrene-b-ethylene-alt-propylene) (PS-b-

PEP) domains biaxially to be parallel to both the surface and step

edges. A wide range of block copolymers have been epitaxially grown

on patterned substrates defined by e-beam26 and soft lithography51 to

form functional, hierarchically organized structures with lateral order

control.

Instead of periodic modification of film thickness, chemically

patterned surfaces defined by top-down or direct-write approaches

generate periodic surface/interfacial energy profiles. The groups of

Russell27,28,31, Krausch30, and Nealey29 have done extensive work on

this approach. With the appropriate surface grating and boundary

conditions, lateral control over nanostructures has been shown to

propagate microns away from the surface (deep into the film), thus

providing true three-dimensional control of the self-assembly process



Fig. 2 Graphoepitaxy method. Schematic cross sections of block copolymer thin film morphologies grown on a topographic pattern . A micropattern with different

lamellar domain orientation is shown: (a) surface-parallel lamellae, typical of film thickness t greater than the natural equilibrium period L0; and (b) surface-

perpendicular lamellae, typical of film thickness t < L0. (Adapted from21 and reprinted with permission. © 2000 American Chemical Society.)

(a)

(b)



Fig. 4 Novel patternable block copolymers to achieve spatially controlled nanostructures. (a) An asymmetric PαMS-b-PHS copolymer/photoacid

generator/crosslinker solution is spin-coated onto a Si substrate and forms vertical PαMS cylinders as a result of rapid solvent evaporation. (b) 248 nm stepper

exposure and subsequent development forms micropatterns. (c) Strong ultraviolet irradiation under high vacuum removes PαMS, thus generating patterned

nanochannels.

(a)

(b)

(c)

Fig. 3 Schematic of controlling block copolymer thin film orientations by using a heterogeneous chemically patterned surface defined by top-down lithography.

(Fig. 3). Recently, Stoykovich et al.49 demonstrated that small amounts

of homopolymer can segregate to the high curvature substrate features

to relieve associated strains, thus providing high fidelity and

registration of block copolymer domains with arbitrarily shaped

chemically patterned substrate features.

The convergence of top-down/direct-write approaches with

bottom-up fabrication may be achieved through design of the block

copolymer architecture. By incorporating multiple functionalities into

the building blocks, the merits of various types of patterning can be

achieved in the same block systems. Bal et al.52 and Spatz et al.53 have

used direct write e-beam lithography to generate spatial control of

poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and PS-b-P2VP

microdomains, respectively. Through the incorporation of high-

resolution poly(4-hydroxystyrene) (PHS) photoresist and poly(α-

methylstyrene) (PαMS) in the same diblock architecture, researchers

have successfully achieved microdomain spatial control through

high-resolution top-down deep ultraviolet lithographic

processes43,44. Large-area, uniform, nanometer-sized cylinders in

submicron-sized patterns were generated through simple fabrication

processes (Fig. 4).



Templating process Block copolymer Morphology Application

Nanolithographic patterning PS-b-PB Cylinder Si3N4, Ge nanodots56

PS-b-PI Sphere Si3N4, metal semiconductor nanodots56-58

PS-b-PMMA Sphere Co74Pt26 and Co74Cr6Pt20 magnetic media23

PS-b-PMMA Cylinder Semiconductor capacitors59

PS-b-P2VP Spherical micelles GaAs quantum dots, diamond nanocolumns60

PS-b-P2VP Spherical micelles Nanoporous Au films61

PS-b-PFDMS Sphere Silica nanopillars25

PI-b-PFDMS Cylinder Co magnetic media62

Patterning of nanoparticles or PS-b-PMMA Lamella Au islands63

nanoclusters PS-b-PMMA Lamella Co nanoclusters71

PS-b-PtBA Sphere Pt, Ag, or PbS nanoclusters64

PS-b-P2VP Sphere Au islands63

PS-b-P2VP Lamella Fe, Fe-Co, Co-Ni nanoparticles66

PS-b-P2VP Spherical micelles Au nanoparticles60,61

PS-b-PB-b-PS Cylinder Nanopattering of BaTi65

PS-b-PI-b-PS Lamella Au nanoparticles67

PS-b-PFEMS Cylinder Carbon nanotubes68

PS-b-PFEMS Cylinder Magnetic α-Fe2O3 nanoparticles39

PMTCDD-b-P2NB) Sphere and wormlike PbS, CdS nanoclusters69

Lysine-b-cysteine diblock copolypeptide Sphere Silica, Au nanoparticles70

Mesoporous and nanoporous PI-b-PEO Multiple Mesoporous aluminosilicate72

materials PEO-b-PPO-b-PEO Hexagonal micelles Mesoporous silica and oxides73

PEO-b-PPO-b-PEO Spherical micelles Nanopores for low-k dielectric applications74

PEO-b-PPO-b-PEO Hexagonal micelles Mesoporous metal oxides75

PB-b-PVP Spherical micelles Mesoporous silica76

PS-b-PB Cylinder Nanoporous polymer56

PS-b-PI Sphere Nanoporous polymer57,58

PS-b-PMMA Cylinder Nanoporous polymer77

PtBA-b-PCEMA Cylinder Nanoporous polymer78

PS-b-PLA Cylinder Nanoporous polymer79

PCHE-b-PLA Cylinder Nanoporous polymer80

P MS-b-PHS Cylinder Nanoporous polymer43,44

PPDPS-b-P4VP Cylinder Nanoporous polymer81

PVPDMPS-b-PI-b-PVPDMPS Lamella, sphere/ Nanoporous polymer82

cylinder

PI-b-PMDSS-b-PI Double gyroid Nanoporous polymer83

PS-b-P2VP-b-PtBMA Perforated lamella Nanoporous polymer84

PPQ-b-PS Spherical micelle Microporous materials85

PDOPPV-b-PS Spherical micelle Microporous materials86

PS-b-PPP Spherical micelle Microporous materials87

Nanoreplication PS-b-PMMA Cylinder Ferromagnetic nanowires88

PS-b-PMMA Cylinder SiO2 nanoposts89

PS-b-PMMA Cylinder Metal nanodots, nanoporous metal films90

PS-b-PMMA Cylinder Nanoelectrode arrays91

PS-b-PMMA Cylinder Surface patterned PDMS92

PS-b-PMMA Cylinder CdSe nanorods93

PS-b-PI Sphere Metal nanodots57,58

PS-b-P2VP Spherical micelles Nanoporous Au films61

PDOPPV-b-PS Spherical micelles Hexagonal-packed Al cup arrays86

Abbreviations

Polystyrene, PS; polybutadiene, PB; polyisoprene, PI; poly(methyl methacrylate), PMMA; poly(2-vinylpyridine), P2VP; poly(ferrocenyldimethylsilane),

PFDMS; poly(tert-butylacrylate), PtBA; poly(ferrocenylethylmethylsilane), PFEMS; poly(methyltetracylododecene), PMTCDD; poly(substituted-2-

norbornene), P2NB; poly(ethyleneoxide), PEO; poly(propyleneoxide), PPO; poly(butadiene-b-vinylpyridinium), PVP; poly(4-vinylphenyldimethyl-2-

propoxysilane), PVPDMPS; poly(cinnamoyl-ethylmethacrylate), PCEMA; poly(pentamethyldisilylstyrene), PPMDS; polylactide, PLA;

poly(cyclohexylethylene), PCHE; poly(α-methylstyrene), PαMS; poly(4-hydroxystyrene), PHS; pentadecyl phenol modified Polystyrene, PDPPS; poly(tert-

butylmethacrylate), PtBMA; poly(tert-butylacrylate), PtBA; poly(paraphenylene), PPP.

Table 2 Summary of representative templating processes using high-resolution block copolymer nanopatterns.

Templating with high-resolution blockcopolymer patternsThe well-defined microdomains of block copolymer systems can

themselves serve as device materials, for example as photonic

crystals54 or stimuli-sensitive materials55. However, most applications

for block copolymer nanopatterns use them as templates for

generating functional nanostructures. Patterned materials with a

greater range of properties than polymeric matrices, such as inorganic

substrates, nanoparticles, nanopores, and metals, may be fabricated for

various nanotechnology applications. Table 2 includes representative

templating processes and corresponding applications that have used

high-resolution, well-controlled block copolymer nanopatterns.

One of the most actively studied areas in the block copolymer

templating process is the use of block copolymers as nanolithographic

resists23,25,56-62. The general fabrication process begins with casting

composition-asymmetric diblock copolymers onto various substrates to

form thin films with two-dimensional, large-area, periodic close-packed

sphere patterns or vertical cylinder arrays. Through a subsequent ion-

etching process, high-resolution periodic nanostructures are transferred

to the substrate with high fidelity, thanks to the different etching

sensitivities of the two building blocks. The regular nanopatterns on the

substrates may be used for a variety of applications, ranging from

quantum dot arrays56-58 and on-chip decoupling capacitors59 to high-

density magnetic recording media23,62.

The first work on template nanolithographic processes was carried

out by Park et al.56 with spherical poly(styrene-b-butadiene) (PS-b-PB)

or poly(styrene-b-isoprene) (PS-b-PI) thin films. As illustrated in Fig. 5,

well-defined compositional asymmetric microdomains, with PB or PI as

minor phases, were formed on a substrate. Ozone-removal or selective

staining of the minor phase transferred the block copolymer thin-film

patterns to either positive or negative resists for a subsequent phase-

selective etching process. An extremely high-density periodic array

(~1011 holes/cm2) of holes and dots could be generated on the

substrates. Since this groundbreaking work, there have been consistent

efforts to improve the aspect ratio of fabricated patterns for real-time

nanofabrication. For example, the Russell group and several others have

improved the etching process by using vertical cylindrical patterns of

PS-b-PMMA as the etching mask59. Cheng et al.25,62 improved the

etching sensitivity of the block copolymers by incorporating

organometallic poly(ferrocenyl dimethylsilane) (PFDMS) into the block



Fig. 5 Schematic of a block copolymer nanolithography process. (a) Cross-sectional view of a nanolithography template consisting of a uniform monolayer of

polybutadiene (PB) spherical microdomains on Si3N4. PB wets the air and substrate interfaces. (b) Process flow when an ozonated copolymer film is used as a

positive resist, which produces holes in Si3N4. (c) Processing flow when an Os-stained copolymer film is used as a negative resist, which produces dots in Si3N4.

(Reprinted with permission from56. © 1997 American Association for the Advancement of Science.)

(a)

(b) (c)

copolymer architecture, while Spatz et al.60 and Haupt et al.61 achieved

the same goal by loading Au into the P2VP core of PS-b-P2VP micelles.

Block copolymer microdomains have also been investigated as

nanocompartments for fabricating spatially defined functional

nanoparticles or nanoclusters. The two general approaches for these

nanoreactor applications differ in the sequence of the reactant loading

process. In the first, phase-selective aggregation of nanoparticles or

precursors onto well-defined block copolymer patterns leaves

nanoparticles or nanoclusters registered with the bottom-up template

patterns in the subsequent reaction63-67. The second approach involves

the incorporation of functional precursors into block copolymer

structures through pre-attachment to one of the block copolymer

building components, or homogeneously mixed with the block

copolymer. The subsequent self-assembly process gives functional

precursors within one of the block copolymer microdomains, and thus

patterned nanoparticles or nanoclusters are generated after final

pyrolysis or sequestration. This in situ nanoreactor generation approach

has been best exemplified with the work on poly(ferrocenes)39,68. By

making homogeneous solutions of organometallic complexes, metal or

metal alloy nanoparticles have also been generated within the P2VP

domains of PS-b-2VP block copolymers60,61,69 and other systems70,71.

Porosity control of inorganic and organic nanoporous materials is

increasingly critical for high-technology applications, such as filtration

membranes, pattern templates, and photonic band gap materials.

Because of their high-resolution nanopattern formation, block

copolymers are a focus of study as templates or matrices for

fabricating nanoporous and mesoporous materials with defined

porosity. Depending on the different block copolymer systems used,

three general nanofabrication processes have been applied, as

illustrated in Fig. 6:

• Organic block copolymer micelle solutions are used as structure-

directing reagents in a sol-gel mixture with inorganic precursors.

The subsequent removal of the template organic materials affords

mesoporous inorganic materials with uniform pore size and high

degrees of orientation order72-76. The most studied micelle system

is poly(alkylene oxide) containing diblock72 or triblock73-75

copolymer systems, which have been used to fabricate mesoporous

silica or mixed oxides;

• Block copolymer thin films or bulk matrices are first formed with

good lateral organization and microstructure orientation. The

subsequent fixing of the major phase and selective removal of

functional minor phase of microdomains, through either chemical or

physical degradation, generates a defined porosity in crosslinked

organic polymer bulk matrices or thin films. Various diblock

copolymer systems, including a siloxane-functionalized PS-b-PI

system, PS-b-PB56, PS-b-PI56, PS-b-PMMA77, poly(tert-

butylacrylate-b-2-cinnamoylethylmethacrylate) and related

systems78, poly(styrene-b-lactide) and related systems79,80,

pentadecyl phenol modified poly(styrene-b-4-vinylpyridine)81,

poly(α-methylstyrene-b-isoprene)43, poly(α-methylstyrene-b-4-



Fig. 6 Schematic of three approaches to generate nanoporous and mesoporous materials with block copolymers. (a) Block copolymer micelle templating for

mesoporous inorganic materials. Block copolymer micelles form a hexagonal array, silicate species (in blue) then occupy the spaces between the cylinders. The

removal of the micelle template leaves hollow cylinders. (b) Block copolymer matrix for nanoporous materials. Block copolymers form a hexagonal cylinder phase

in a bulk or thin film state. Subsequent crosslinking fixes the matrix. Hollow channels are generated by removing the minor phase. (c) Rod-coil block copolymer for

microporous materials. Solution-cast micellar films consist of multilayers of hexagonally ordered arrays of spherical holes. (Adapted from85 and reprinted with

permission. © 1999 American Association for the Advancement of Science.)

(a)

(b)

(c)

hydroxystyrene)43,44, as well as some triblock systems82-84 have

been reported to create ordered monolithic polymers in bulk and

thin film states;

• Casting films of several rod-coil block copolymer micelles from a

selective solvent generates microporous films on the substrates85-87,

possibly caused by the vitrification of the pattern created by the

condensation of a water droplet during the fast evaporation of

organic solvents.

One of the most important applications of nanoporous membranes

is as nanoscaffolds to replicate the structural features of the

nanopores, or patterns, into various materials. The most effective,

well-studied system is the PS-b-PMMA system that the Russell group

and others have investigated88-93. By using nanoporous films

generated from prealigned, perpendicular PS-b-PMMA cylinder

nanopatterns as templates, the Russell group88 has fabricated high-

density vertical arrays of ferromagnetic Co nanowires through

subsequent direct current electrodeposition processes (Fig. 7). Using

similar processes, they have also demonstrated the replication of

nanoscale features into a variety of materials, including the formation

of SiO2 posts89, metal nanodots90, nanoporous metal films90,

nanoelectrode arrays91, surface-patterned poly(dimethyl siloxane)

elastomers92, and CdSe nanorods93.

Outlook Block copolymer research has progressed rapidly during the past

decade. The continuous drive to understand the self-assembly behavior,

control the lateral packing and orientation of their bottom-up patterns,

and make use of their versatility to serve as templates to fabricate

various nanodevices, has taken researchers to the horizon of imminent

large-scale, wide-range production. However, there are still some areas

that need to be better explored to implement their full potential.

Many strategies to achieve lateral and orientational control of

three-dimensional block copolymer nanostructures have been reviewed

here. However, most of the processes are not trivial. It still remains a

puzzle as to how best incorporate them into nanofabrication processes

in a time-efficient manner and with good reproducibility and a large

processing window. The combination of biaxial or multiple driving

forces has shown promise and could be an essential means for paving

the path to large-scale, real-time production.

The wealth of microstructures available and the tunability of

structural dimensions of block copolymers make them suitable for a

variety of applications. However, current theoretical and experimental

research has generally focused on a few, well-studied diblock systems.

New functional systems, such as metal-containing block copolymers,

have shown great value in terms of simplifying and improving the

nanofabrication process. The payback/investment ratio could be very

high, thus more investigation of these complicated and functionalized

block copolymer systems are necessary.

The convergence of top-down/direct-write processes with bottom-

up nanopattern formation has generated great interest in directing the

self-assembly process and gaining spatial control of hierarchical

structures. Through the integration of block copolymer structure

control into well-developed lithographic technologies, structures with

dimensions far beyond the expectation of Moore’s law could be

generated with precise substrate registration and spatial control.

Further challenges remain, such as the optimization of the synergy

effect between different processes and integration of block copolymer

bottom-up self-assembly with unconventional three-dimensional

lithographic processes.

Block copolymer high-resolution nanopatterns have been applied as

templates for fabricating various nanomaterials and nanodevices with

high fidelity. However, we can conclude that we have seen only the tip

of the iceberg, and we expect that the ongoing rapid development of

nanoscience and nanotechnology will ultimately realize the full

potential of these fascinating materials.



Fig. 7 Schematic of the nanoreplication of high-density nanowires from a

nanoporous block copolymer thin film. An asymmetric PS-b-PMMA diblock

copolymer was aligned to form vertical PMMA cylinders under an electric field.

After removal of the PMMA minor component, a nanoporous film is formed. By

electrodeposition, an array of nanowires can be replicated in the porous

template (Adapted from88 and reprinted with permission. © 2000 American

Association for the Advancement of Science.)

REFERENCES

1. Whitesides, G. M., et al., Science (1991) 254, 1312

2. Ober, C. K., et al., Adv. Mater. (1997) 9, 1039

3. Hamley, I. W., et al., The Physics of Block Copolymers, Oxford Science

Publications, USA, (1998), 26

4. Leibler, L., Macromolecules (1980) 13, 1602

5. Bates, F. S., and Fredrickson, G. H., Annu. Rev. Phys. Chem. (1990) 41, 525

6. Matsen, M. W., and Bates, F. S., Macromolecules (1996) 29, 1091

7. Fredrickson, G. H., and Bates, F. S., Annu. Rev. Mater. Sci. (1996) 26, 501

8. Tuzar, Z., and Kratochvil, P., Micelles of Block and Graft Copolymers in Solutions.

In: Surface and Colloid Science, Matijevic, E., (ed.), Plenum Press, New York,

(1993) 15, 1

9. Fasolka, M. J., and Mayes, A. M., Annu. Rev. Mater. Res. (2001) 31, 323

10. Li, M., et al., Adv. Polym. Sci. (2005) 190, 183

11. Matsen, M. W., J. Chem. Phys. (1997) 106, 7781

12. Chen, Z.-R., et al., Science (1997) 277, 1248

13. Chao, C.-Y., et al., Adv. Funct. Mater. (2004) 14, 364

14. Amundson, K., et al., Macromolecules (1993) 26, 2698

15. Amundson, K., et al., Macromolecules (1994) 27, 6559

16. Mansky, P., et al., Macromolecules (1998) 31, 4399

17. Böker, A., et al., Macromolecules (2002) 35, 1319

18. Arsenault, A. C., et al., J. Am. Chem. Soc. (2005) 127, 9954

19. Bodycomb, J., et al., Macromolecules (1999) 32, 2075

20. Russell, T. P., et al., Macromolecules (1989) 22, 4600

21. Fasolka, M. J., et al., Macromolecules (2000) 33, 5702

22. Fasolka, M. J., et al., Phys. Rev. Lett. (1997) 79, 3018

23. Asakawa, K., et al., J. Photopolym. Sci. Technol. (2002) 15, 465

24. Segalman, R. A., et al., Adv. Mater. (2001) 13, 1152

25. Cheng, J. Y., et al., Appl. Phys. Lett. (2002) 81, 3657

26. Massey, J. A., et al., J. Am. Chem. Soc. (2001) 123, 3147

27. Huang, E., et al., Nature (1998) 395, 757

28. Rockford, L., et al., Phys. Rev. Lett. (1999) 82, 2602

29. Kim, S. O., et al., Nature (2003) 424, 411

30. Heier, J., et al., Macromolecules (1997) 30, 6610

31. Lin, Y., et al., Nature (2005) 434, 55

32. Morkved, T. L., et al., Science (1996) 273, 931

33. Elhadj, S., et al., Appl. Phys. Lett. (2003) 82, 871

34. Angelescu, D. E., et al., Adv. Mater. (2004) 16, 1736

35. Park, C., et al., Macromolecules (2001) 34, 2602

36. De Rosa, C., et al., Nature (2000) 405, 433

37. Reiter, G., et al., Phys. Rev. Lett. (1999) 83, 3844

38. Turturro, A., et al., Polymer (1995) 36, 3987

39. Temple, K., et al., Adv. Mater. (2003) 15, 297

40. Lin, Z. Q., et al., Adv. Mater. (2002) 14, 1373

41. Fukunaga, K., et al., Macromolecules (2000) 33, 947

42. Kim, G., and Libera, M., Macromolecules (1998) 31, 2569

43. Du, P., et al., Adv. Mater. (2004) 16, 953

44. Li, M., et al., Chem. Mater. (2004) 16, 3800

45. Xu, T., et al., Macromolecules (2003) 36, 7296

46. Kimura, M., et al., Langmuir (2003) 19, 9910

47. Park, C., et al., Appl. Phys. Lett. (2001) 79, 848

48. Edwards, E. W., et al., Adv. Mater. (2004) 16, 1315

49. Stoykovich, M. P., et al., Science (2005) 308, 1442

50. Sundrani, D., et al, Langmuir (2004) 20, 5091

51. Deng, T., et al., Langmuir (2002) 18, 6719

52. Bal, M., et al., Appl. Phys. Lett. (2002) 81, 3479

53. Spatz, J. P., et al., Adv. Mater. (2002) 14, 1827

54. Urbas, A., et al., Macromolecules (1999) 32, 4748

55. Ruokolainen, J., et al., Science (1998) 280, 557

56. Park, M., et al., Science (1997) 276, 1401

57. Park, M., et al., Appl. Phys. Lett. (2001) 79, 257

58. Li, R. R., et al., Appl. Phys. Lett. (2000) 76, 1689

59. Black, C. T., et al., Appl. Phys. Lett. (2001) 79, 409

60. Spatz, J. P., et al., Adv. Mater. (1999) 11, 149

61. Haupt, M., et al., Adv. Mater. (2003) 15, 829

62. Cheng, J. Y., et al., Adv. Mater. (2001) 13, 1174

63. Morkved, T. L., et al., Appl. Phys. Lett. (1994) 64, 422

64. Brown, G. D., and Watkins, J. J., Mater. Res. Soc. Symp. Proc. (2000) 584, 169

65. Lee, T., et al., Langmuir (1997) 13, 3866

66. Kane, R. S., et al., Chem. Mater. (1996) 8, 1919

67. Ansari, I. A., and Hamley, I. W., J. Mater. Chem. (2003) 13, 2412

68. Lu, J. Q., et al., Chem. Mater. (2005) 17, 2227

69. Abes, J. I., et al., Chem. Mater. (2003) 15, 1125

70. Wong, M. S., et al., Nano Lett. (2002) 2, 583

71. Tadd, E. H., et al., Langmuir (2002) 18, 2378

72. Templin, M., et al., Science (1997) 278, 1795

73. Zhao, D., et al., Science (1998) 279, 548

74. Yang, S., et al., Chem. Mater. (2002) 14, 369

75. Yang, P., et al., Nature (1998) 396, 152

76. Krämer, E., et al., Langmuir (1998) 14, 2027

77. Jeong, U., et al., Adv. Mater. (2002) 14, 274

78. Liu, G., et al., Chem. Mater. (1999) 11, 2233

79. Zalusky, A. S, et al., J. Am. Chem. Soc. (2002) 124, 12761

80. Wolf, J.H., and Hillmyer, M. A., Langmuir (2003) 19, 6553

81. Mäki-Ontto, R., et al., Adv. Mater. (2001) 13, 117

82. Lee, J. S., et al., Macromolecules (1988) 21, 274

83. Chan, V. Z.-H., et al., Science (1999) 286, 1716

84. Ludwigs, S., et al., Nat. Mater. (2003) 2, 744

85. Jenekhe, S. A., and Chen, X. L., Science (1999) 283, 372

86. de Boer, B., et al., Adv. Mater. (2000) 12, 1581

87. Widawski, G., et al., Nature (1994) 369, 387

88. Thurn-Albrecht, T., et al., Science (2000) 290, 2126

89. Kim, H.-C., et al., Adv. Mater. (2001) 13, 795

90. Shin, K., et al., Nano Lett. (2002) 2, 933

91. Jeoung, E., et al., Langmuir (2001) 17, 6396

92. Kim, D. H., et al., Adv. Mater. (2003) 15, 811

93. Zhang, Q., et al., J. Am. Chem. Soc. (2006) 128, 3898



Acknowledgments The research reported here was supported by the National Science Foundation,

as well as the Cornell Center for Materials Research and the Cornell

Nanobiotechnology Center. We also thank Cornell Nanofabrication Center for

use of their facilities.

Documents

Block copolymer patterns and templates