Block copolymers and conventional lithography

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photoresist in the unexposed regions. The current state-of-the-art uses 193 nm optical lithography to pattern 65 nm half-pitch structures with perfection over full ...
Block copolymers and conventional lithography The lithographic process is arguably the key enabling technology for the digital age. Hundreds of millions of devices can be fabricated on a single chip because patterns with features as small as 50 nm can be written with a remarkable level of perfection, in registration with the underlying substrate, and with complex geometries. As the drive to pattern at ever shrinking length scales continues, however, new imaging materials may be required to meet manufacturing constraints. We highlight some of the recent advances in integrating self-assembling block copolymers into the conventional lithographic process to address issues of resolution and process control. Mark P. Stoykovich* and Paul F. Nealey Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, USA *E-mail: [email protected]

Diblock copolymers are two different types of polymer chains

fabricate using conventional lithographic materials and processes. Block

connected at one end with a covalent bond1,2. Most pairs of

copolymer lithography refers to the use of these materials in the form

polymers are immiscible and blends of polymers tend to phase

of thin films in which the domain structure provides a template for

separate. In the case of diblock copolymers, however, the two

additive or subtractive pattern transfer operations3.

polymers that constitute the material are unable to phase

using a single layer of spherical domains or a thin film of cylindrical

form ordered structures at the molecular scale with domain

domains with the domains oriented perpendicular to the substrate

dimensions of 5-50 nm. The size and shape of the domains in the

(Fig. 1). The latter morphology is advantageous in that high aspect

bulk are dependent on the molecular weight and composition of

ratio templates may be produced with vertical sidewalls and

the copolymer and typically assume morphologies of spheres,

connectivity between the substrate and the free surface of the polymer

cylinders, and lamellae.

film. Unfortunately, substrates are often preferentially wet by one of

The obvious interest in using these materials for patterning is

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Patterns of hexagonal arrays of spots, for example, can be fabricated

separate at macroscopic length scales and instead spontaneously

the blocks of the copolymer and the cylinders tend to orient parallel to

derived from the fact that they self-assemble to form dense arrays of

the substrate. Strategies such as chemical modification4-11 and the

nanostructures with dimensions and spacings that are difficult or

application of external fields12-17 can be used to neutralize or

impossible to create by other means or are prohibitively expensive to

overcome surface and interfacial forces that tend to drive the cylinders

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ISSN:1369 7021 © Elsevier Ltd 2006

Block copolymers and conventional lithography

(a)

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(b)

Fig. 1 Dense arrays of (a) perpendicular cylinders and (b) spherical domains in thin films self-assembled from a PS-b-PMMA copolymer. The PMMA cylinders and spheres (dark) are hexagonally organized at the local scale in the PS matrix (light), but lack long-range order.

to form parallel to the plane of the film. Alternatively, nonequilibrium

Fig. 2 Graphoepitaxial assembly of a cylinder-forming PS-b-PMMA copolymer in topographic grooves. Here 12 rows of PMMA cylinders have been perfectly ordered throughout the entire width and length of the groove. In this topdown scanning electron micrograph, the topographic ridges are the bright regions at the top and bottom of the image. (Adapted and reprinted with permission from47. © 2005 Institute of Physics.)

structures with the desired morphologies can be formed by solvent evaporation18-22 or spin casting23,24. Patterns of dense periodic arrays

for magnetic storage media27,47, nanowire field-effect transistors

of spots fabricated with block copolymer templates have been

(FETs)52, and nanowires.

demonstrated, and in some cases are being commercialized, for the

Recently, we reported the extension of graphoepitaxy to the

fabrication of quantum dots3,25, magnetic storage media26,27, flash

ordering of lamellar-forming block copolymers such that the lamellae

memory devices28, semiconductor capacitors29,30, nanowires13,31,32,

are oriented perpendicular to the substrate and aligned in arrays

photonic crystals33,34, and nanopores35-37. These applications capitalize

parallel to the axis of the grooves53. In contrast to sphere-forming40-46

on self-assembling block copolymer materials to pattern periodic,

and cylinder-forming47,49,50 systems, the grooved substrates must not

uniform-dimension features at the nanoscale at very little expense. The

only be topographically but also chemically patterned to achieve the

use of block copolymer films to pattern dense arrays of features is over

desired configuration of microphase separated domains. The

a decade old and other reviews have covered this topic in detail38,39.

motivation for investigating graphoepitaxy in conjunction with

One strategy to integrate block copolymers with conventional

lamellae-forming polymers is to be able to pattern transfer by reactive

lithography is known as graphoepitaxy and was first demonstrated for

ion etching (RIE) from the block copolymer template to the underlying

sphere-forming block copolymers by Segalman et

al.40.

In this

technique, grooves with typically micron or submicron dimensions are

substrate47,54. In this review, we focus on a strategy to integrate block copolymers

patterned on the substrate by photolithography and etching, and the

into the conventional lithographic process using lithographically

domain structure of block copolymer films deposited in the grooves

defined chemically nanopatterned surfaces. In this technique, surface

nucleates on the walls of the topographic features and propagates

and interfacial forces between the substrate surface and the two blocks

inward so as to be well ordered across the width of the grooves and

of the copolymer are carefully engineered to direct the assembly of

along their axes40-46. The primary purpose of graphoepitaxy is to

nanostructures into thermodynamically stable device-oriented patterns.

enhance the resolution of the conventional lithographic process by

The rationale of this approach is not aimed at improved resolution (or

subdividing the patterned features and to improve the perfection of

sublithographic patterning) with respect to the exposure tool, but

ordering of the dense periodic arrays of nanostructures that are

rather at enhancing process control as a first step and, ultimately,

naturally formed by block copolymers.

greatly enhancing information transfer from the exposure tool to the

Topographic features also allow some control over the placement

imaging material through the use of a self-assembling resist. Before

(registration) of the patterns with respect to the underlying

expanding on these ideas, we first describe the lithographic process as

substrate44,46. If sufficient order can be achieved, addressable arrays for

currently practiced, including its essential attributes for manufacturing

applications such as single-domain magnetic storage media could be

and its limitations.

fabricated. Graphoepitaxy has also been shown to be an effective strategy for ordering cylinder-forming block copolymers with the

Conventional photolithography

domains oriented either perpendicular47,48 (Fig. 2) or parallel49,50 to the

The conceptualization of the integrated circuit, the miniaturization of

substrate, and in geometries more complex than parallel grooves such

such devices to have sub-100 nm features, and the development of

as circles51 and bends49. Application of structures patterned from block

processes for fabricating components at these dimensions are some of

copolymer templates assembled in this way include nanoparticle arrays

the crowning achievements of engineering over the past half century.

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Block copolymers and conventional lithography

Photolithography55, the predominant technique used in the

the 20 nm length scale57. In addition to limitations with CD control

nanofabrication of complex devices, has advanced at a frenetic pace

and LER, standard photoresists are also encountering a problem known

throughout this period. Hallmarks of the modern lithographic process

as pattern collapse, where dense arrays of patterned resist become

used in the semiconductor industry include pattern perfection over

deformed in response to capillary forces present during the

macroscopic areas, dimensional control of features within exacting

development process59-61.

tolerances and margins, and registration and overlay (placement of

Here we present recent advances in the incorporation of self-

features in each layer and with respect to overlying and underlying

assembling materials, specifically block copolymers, into the

layers)55,56.

conventional lithographic process for patterning at the 22 nm

A simplified overview of the lithographic process has four steps: (1) A substrate, typically Si or doped Si, is coated with a film of

technology node and beyond. The directed assembly technique using chemical surface patterns to control and direct the ordering of block

photosensitive polymer-based material known as a photoresist;

copolymer domains is introduced. A principal concept of this work is

(2) The photoresist is exposed to a pattern in the intensity of radiation

that the desired structures represent thermodynamic minima and, as

by the exposure tool and is chemically modified from the

such, facilitate process control in patterned nanoscale features within

photoresist in the unexposed regions. The current state-of-the-art

relevant tolerances and margins. The potential of self-assembling

uses 193 nm optical lithography to pattern 65 nm half-pitch

materials to reproduce the essential attributes of conventional

structures with perfection over full 300 mm diameter Si

photolithography, including pattern perfection, registration, nonregular

substrates56;

device-oriented features, and sufficient process latitude, are discussed

(3) A solvent-based development process selectively removes either the

in detail. Block copolymers also provide the opportunity to overcome

exposed or unexposed photoresist based upon differences in their

the CD control and LER barriers that are currently encountered with

chemistry and solubility rates;

conventional photoresists. Our focus remains on the limits to which

(4) The resulting topographic pattern in the photoresist is transferred to the substrate by selective etching or deposition processes.

directed assembly and block copolymers can be pushed for advanced lithography and the emerging directions in this field.

These lithographic steps are iterated with the patterns and materials required to build up multiple layers in the integrated devices. The semiconductor industry is continually striving toward the production of faster and smaller microprocessors and integrated

The concept of using chemical surface patterns to control the longrange organization of phase-separating polymers was first considered

circuits by shrinking the smallest or critical dimension (CD) of the

by Boltau et al.62. An octadecyl mercaptan self-assembled monolayer

fabricated devices. The International Technology Roadmap for the

(SAM) was microcontact printed on a Au substrate to provide a surface

Semiconductor Industry (ITRS) provides detailed timelines and

patterned with regions of varying surface energy. A blend of

requirement lists for the development of new patterning techniques at

polystyrene (PS) and polyvinylpyridine (PVP) homopolymers effectively

future technology nodes (e.g. the 65 nm, 45 nm, 32 nm, 22 nm, and

replicated the SAM surface pattern when the period of the surface

16 nm nodes as defined by the half-pitch of dense features)56.

heterogeneity was commensurate with the coarsened, micron-scale

Guidelines are provided about the projected lithographic techniques for

domains of the phase-separated blend. Rockford et al.63 extended the

each of the desired pattern dimensions, as well as the technological

use of patterned surfaces to nanoscale dimensions and to block

obstacles that remain to be overcome.

copolymer materials that microphase, rather than macrophase,

The current lithographic process, however, is limited by the available

22

Perfection and registration

separate. Striped surfaces of alternating SiO2 and Au with a period of

photoresist materials and cannot be scaled to the future technology

60 nm were generated by grazing angle evaporation on miscut Si

nodes demanded by the semiconductor industry. Already significant

wafers. Only when the period of the lamellar-forming block copolymer

roadblocks are being predicted and encountered in achieving some of

(L0) matched the surface pattern period (LS) were the domains oriented

the requirements set forth in the ITRS. Surprisingly, resolution is not

normal to and directed on the substrate surface. Numerous theoretical

the most pressing challenge. The most critical issues relate to

studies have predicted that dimensional commensurability is required

dimension control and line edge roughness (LER) or, in other words,

for the guided assembly of block copolymers on chemical surface

control in the processing of photoresists56-58. A technology gap exists

patterns64-70. Experimentally, Yang et al.71 have demonstrated an

because significant resources have been allocated to the development

alternative route for fabricating surface chemical patterns. SAMs of

of exposure tools in the past decade, particularly electron-beam and

octadecyltrichlorosilane were deposited on Si substrates and chemically

extreme ultraviolet sources, capable of resolving patterns with

patterned by an interference lithography technique using X-ray light

dimensions of 20 nm or less with the required registration and overlay

and low pressures of oxygen8,9. These chemically striped surfaces were

capabilities, whereas relatively modest investments have been made in

subsequently used to direct the assembly of poly(styrene-block-methyl

the development of imaging materials with the required properties at

methacrylate) (PS-b-PMMA) copolymers.

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Block copolymers and conventional lithography

The aforementioned nanoscale techniques, while providing small

REVIEW FEATURE

beam lithography55 is first used to pattern dense lines and spaces with

areas of guided self-assembly riddled with defects, did not achieve the

a period LS in a photoresist film that has been deposited on the

defect-free, registered ordering that is necessary for widespread

imaging layer. Next the topographic pattern in the photoresist layer is

lithographic application63,71,72. Even the most basic requirements

transferred to a chemical pattern in the imaging layer by means of an

demanded by the ITRS were far from being satisfied. Only statistical

oxygen plasma treatment step. The remaining photoresist material is

measures such as a two-dimensional order parameter could be used to

then removed by solvent treatment to reveal the chemical surface

characterize the quality of the directed patterns that were achieved63.

pattern that has a period of LS. Regions of the imaging layer exposed to

The lack of defect-free ordering and registration most likely resulted

the oxygen plasma become chemically modified to contain oxygen-

from a high density of imperfections in the surface pattern being

containing moieties and have a high surface energy83. In comparison,

translated into the self-assembled morphologies.

the regions of the imaging layer covered by the photoresist are

Directed assembly of block copolymers using chemical surface patterns, however, has recently been improved to the point where it is now a promising approach for

nanolithography11,73-76.

A functional

protected from the oxygen plasma and remain chemically unmodified with a lower surface energy. A thin film (typically L0, respectively, to accommodate the periodicities imposed by the surface pattern.

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Block copolymers and conventional lithography

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(a)

(b)

Fig. 6 Blends of block copolymers and homopolymers for directed assembly. (a) Lamellar-forming blends of 51 kg mol-1 and 104 kg mol-1 PS-b-PMMA can achieve periodicities between those of the pure block copolymers L0 = 32 nm and 48 nm, respectively. The 1:3 and 1:1 blends produce lamellar periodicities of 45 nm and 41 nm, respectively, and can be directed to assemble on chemically nanopatterned PS brushes. (Adapted and reprinted with permission from85. © 2006 American Vacuum Society.) (b) Symmetric blends of the 104 kg mol-1 PS-b-PMMA block copolymer and 40 kg mol-1 homopolymers can be precisely tuned to achieve dimensions ranging from L0 = 48 nm (pure block copolymer) up to LB = 95 nm (4:3:3 PS-b-PMMA:PS:PMMA).

energy contrast provides less of a driving force for chain stretching or

lamellar periodicities of LB = 45 nm and 41 nm, respectively85. These

compression and, consequently, defect-free assembly can be achieved

materials were also used for directed assembly on chemical surface

over a narrow range of LS (only LS = L0). The greatest process latitude

patterns and were observed to behave identically to pure block

for directed assembly can thus be achieved when using chemical

copolymers85. For example, defect-free assembly on a patterned PS

surface patterns with the largest interfacial energy

contrast73.

Recently, Wilmes et al.84 have shown that arcs with radii of

brush was achieved when LS ≈ LB ± 10% LB. Ternary blends of a block copolymer and its corresponding

curvature as small as L0/4 can be fabricated with perfection using pure

homopolymers (e.g. PS-b-PMMA plus PS and PMMA homopolymers)

block copolymers. The arc geometry is nearly identical to dense lines

can access an even wider range of self-assembled dimensions. Simply

with an additional free energy penalty for bending the lamellar

by tuning the composition of the blend, through parameters such as

interfaces. For perfect directed assembly with LS = L0, this interfacial

the volume fraction of the total homopolymer and the homopolymer

bending energy must therefore be balanced by the interfacial

molecular weight, it is possible to swell the blend domains to greater

interactions between the surface pattern and the block copolymer

than twice the size of the pure block copolymer domains86,89-91. Fig. 6b

domains.

demonstrates how lamellar ternary blend dimensions scale with the

Block copolymer blends may also be used to expand the range of dimensions accessible to directed assembly85,86. The ability to control

total homopolymer volume fraction, φH. For example, a pure symmetric block copolymer (φH = 0) with L0 = 48 nm can be adjusted by the

the self-assembled dimension is of significant importance in

addition of 40 vol.% total homopolymer (φH = 0.4) to create a lamellar

nanofabrication where multiple length scales are often required in close

phase with a periodicity of LB = 70 nm. These lamellar-forming blend

proximity. Fig. 6 details how block copolymer-block copolymer blends85

materials have also been used for directed assembly and have

and block copolymer-homopolymer blends86 can precisely tune the

comparable process latitude to pure block copolymers and binary block

natural lamellar periodicity of the blend system, LB. Binary blends of a

copolymer blends86.

low molecular weight (e.g. 51 kg

mol-1)

and a high molecular weight

(e.g. 104 kg mol-1) symmetric PS-b-PMMA copolymer result in lamellar

Nonregular device-oriented structures

phases with periodicities intermediate to that of the pure

The directed assembly of block copolymers must demonstrate the

copolymers87,88.

ability to fabricate nonregular device-oriented structures prior to being

The 1:3 and 1:1 by volume blends in Fig. 6a have

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Block copolymers and conventional lithography

(a)

(b)

corners in order to accommodate the dimensional differences between the linear period and the corner-to-corner period in this geometry. The remaining geometries in Fig. 7 were also fabricated with block copolymer blends and the success of the assembly process, particularly for the T-junction geometry, was undoubtedly enhanced by the redistribution of homopolymer in the thin films94-96. The arc

(d)

(c)

geometries should not be confused with the bend geometries in which the sharp corners have a much smaller radius of curvature and induce localized dimensional variations in the pattern74. It is expected that, while pure block copolymers may be suitable for many of the essential integrated circuit geometries, block copolymer blends will provide the necessary flexibility to achieve the entire spectrum of structures, dimensions, and feature densities.

Fig. 7 Block copolymers have been directed to assemble into, clockwise from top-left: (a) 90° bends, (b) arcs, (c) T-junctions, and (d) lines that end in a fixed position.

CD control and LER Self-assembling materials such as block copolymers are particularly

considered a viable nanofabrication and next-generation lithography

attractive for nanolithography because they offer potential improvements

technique57,58. Integrated circuits and memory arrays contain elements

in the CD control and LER of the fabricated structures. Conventional

that are more complex geometrically than the simple periodic arrays of

lithography with chemically amplified photoresists is a diffusion-limited

lines or spots available from pure block copolymers. The gate layer of

process. Small deviations in the exposure dose (Figs. 8a and 8b) or

most integrated circuits, for example, requires the patterning of dense

post-exposure bake temperature can lead to large variations in the final

lines, bent lines with sharp corners, lines that end at specific positions,

structure dimension55,97. Furthermore, the dimensional control and LER

T-junctions, jogs, and arrays of spots. Recently, many of these

issues present in conventional lithography will become increasingly

geometries have been successfully patterned by the directed assembly

troublesome with each successive technology node56.

of block copolymers (Fig. 7).

Thermodynamically controlled processes such as self-assembly, on

The periodic lines with corners geometry (Fig. 7a, top left) was

the other hand, proceed until the resulting morphology achieves a

considered both with experiments and molecular-level simulations74.

minimized free energy at equilibrium. As discussed earlier, the

Experimentally 45º, 90º, and 135º bends with a surface periodicity of

thermodynamics of the block copolymer system determines the overall

65 nm < LS < 80 nm were patterned. A block copolymer-homopolymer

shape and size of the domains1,2. The block copolymer assembly

blend was found to assemble with perfection in both the linear and

process is therefore able to correct or self-heal for irregularities, e.g.

sharp corner sections, providing the first evidence that directed

dimensional variations or defects, in the chemical surface

assembly could fabricate nonregular geometries74. Single chain in mean

pattern75,98,99. The influence of the surface pattern duty cycle, defined

field (SCMF)

simulations92,93

performed on this system indicate that

as the ratio of the width of the chemically unmodified stripes to the

the bend corners have a higher homopolymer concentration, by

overall periodicity, on directed assembly has been considered with both

6-7 vol.%, than the linear sections of the lamellae. The localized

experiments and SCMF simulations75,98. The symmetric block

redistribution of homopolymer in the film swelled the domains in the

copolymer considered had a duty cycle of 0.5, while surface patterns

(a)

(b)

(c)

Fig. 8 Enhanced dimensional control provided by self-assembling materials. Traditional photoresist materials display poor dimensional control in response to processing parameters such as (a) low and (b) high exposure doses that create wide and narrow lines, respectively. (c) SCMF simulations (shown here) and experiments have demonstrated that block copolymers on chemical surface patterns with a duty cycle of 0.60 are able to self-correct and produce 1:1 lines and spaces. The PS and PMMA domains are shown in red and blue, respectively.

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Block copolymers and conventional lithography

REVIEW FEATURE

with duty cycles ranging from 0.25 to 0.70 were fabricated. Perfect

able to achieve the sub-1 nm control required for lithography at the

ordering of the block copolymer was achieved for pattern duty cycles

22 nm node75,98, the block copolymer components may need to be

between 0.35 and 0.65. For the largest duty cycles (Fig. 8c), the

chosen specifically to address these issues. The block copolymer

domain interfaces were tilted away from the substrate normal.

chemistry may also be optimized for etch selectivity for removing one

However, molecular-level simulations indicate that the interface tilting

of the two polymer blocks and for etching into the substrate.

is less than ~10° and less than what would be predicted on the basis of

Potentially, the etch resistance of one of the blocks could be improved

simple volumetric arguments (>20°). In addition, the widths of the

by incorporating inorganic components directly in the polymer, e.g. as

lamellar domains at half the film thickness were exactly equal and

an Fe-containing26,41 or Si-containing101 polymer. Further materials

corresponded to the block copolymer duty cycle of 0.5. The resulting

improvements will be required throughout the directed assembly

polymer structures were self-corrected for the dimensional variation in

process including in the areas of directly patternable imaging layers,

the underlying surface pattern and are more suitable as templates for

etch resistant polymers, degradable or crosslinking polymers, and

pattern transfer processes to the substrate. Furthermore, lateral

possibly functional copolymer components (e.g. by incorporating a

roughness and

gaps99

in the chemically modified regions of the surface

pattern stripes can be overcome by the block copolymer such that defect-free structures are formed with long-range order.

conducting polymer domain). The challenges facing directed self-assembly for insertion in the ITRS were addressed at a recent workshop cosponsored by the

The LER of the self-assembled structures is also a thermodynamic

Semiconductor Industry and National Nanotechnology Initiative102. In

property of the system and is theoretically predicted to be sub-1 nm in

addition to the materials issues addressed above, specific lithographic

dimension. The interfacial width or roughness between the block

questions remain about the possibility of assembling full integrated

copolymer domains is in theory proportional to χ-0.5, where χ is the

circuit layouts using block copolymers. The feasibility of fabricating the

interaction parameter between the blocks of the copolymer100. Simply

essential and desired set of lithographic structures used in integrated

by choosing polymer components that are more incompatible it may

circuit layouts will need to be explored, including periodic and isolated

be possible to reduce the LER of the block copolymer structures further.

lines and contact openings. Once the individual lithographic elements

At present it has not been demonstrated, however, that one can

have been demonstrated, it will be necessary to integrate these

achieve the sub-2 nm LER requirement in the ITRS after removing one

components into a complete circuit layout. The ability to fabricate

block or after pattern transfer

operations56,58.

features with multiple sizes and pitches in a single layer of the chip would also be desirable, but may be difficult to achieve because the

Emerging directions and conclusions

structure dimensions are typically limited by the choice of self-

The insertion of block copolymers into the conventional lithographic

assembling material (e.g. LS ≈ L0 ± 10% L0). Multiple-layer devices have

process has progressed appreciably over the past decade, from the first

yet to be attempted by directed assembly and it will be necessary to

demonstrations of directed assembly on chemical surface

demonstrate that this approach can satisfy the projected ITRS

patterns62,63,71 to the fabrication of complex device-oriented

alignment and registration requirements. Achieving these goals for full

geometries11,74. Directed assembly has been refined to the point where

integrated circuits by directed assembly may demand the redesign of

it can reproduce many of the essential attributes of photolithography

the layouts to be more amenable for self-assembling materials. This

including pattern perfection, registration, and arbitrarily shaped

concept of design for manufacturing should take advantage of the

geometries and, as such, is being considered as a legitimate next-

inherent periodicities, length scales, and geometries naturally formed

generation lithography technique for insertion at the sub-22 nm

by the block copolymers and is believed to be a viable option at future

technology nodes. Unlike conventional photoresists, block copolymer

technology nodes103. Furthermore, theoretical frameworks and

materials have the advantage of being scalable to future technology

molecular-level simulations will need to be developed for predicting

nodes including those sub-10 nm in dimension simply through the

whether the desired pattern geometries can be directed to assemble

rational choice of molecular parameters such as the degree of

without defects. Alternatively, these computer tools could be used to

polymerization and the interaction parameter. Significant work

determine the optimal block copolymer material for self-assembling a

remains, however, prior to the widespread adoption of self-assembling

specific geometry or set of components.

materials for patterning applications in the semiconductor industry57,58. One of the most pressing needs is a conclusive demonstration of

Using block copolymers and self-assembling materials for the direct fabrication of complex three-dimensional structures in thin films

the enhanced long-range dimensional control and low-frequency LER

promises to be an emerging area for directed assembly and

characteristics provided by the block copolymer materials. These

nanolithography. In top-down lithography and all of the work described

measurements must be performed both on the self-assembled

thus far, the structures formed are two-dimensional in nature. Control

structures and the final features etched in the substrate. Although the

over the structure geometries, long-range ordering, and positioning has

CD studies detailed earlier suggest that PS-b-PMMA copolymers will be

been achieved within the plane of the film. However, there has been no

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REVIEW FEATURE

Block copolymers and conventional lithography

Fig. 9 Thin films of a lamellar-forming PS-b-PMMA copolymer on two-dimensional surfaces chemically patterned with a square array of spots form threedimensional bicontinuous morphologies. Top-down scanning electron micrograph (top right) shows that a series of spots arise on the free surface. Mean field simulation results (bottom right) indicate that both of the copolymer domains (the blue domains were removed from the image for clarity) of the self-assembled morphology are continuous and connect the substrate to the free surface. The light blue spots specify the position of the underlying surface pattern and the green represents the interface between the blue and the red domains. The PS and PMMA domains are shown in red and blue, respectively.

variation in the structures in the direction normal to the substrate. It

(a)

may be possible to encode additional information into the system through the choice of self-assembling material such that threedimensional structures are formed in a single processing step. In the case of the semiconductor industry, the formation of three-dimensional structures can be conceptualized as a route for simultaneously

(b)

patterning multiple layers in an integrated device, e.g. as a combined gate and contact feature. Already block copolymers on chemical surface patterns with mismatched geometries have been used to induce the formation of bicontinuous, network morphologies (Fig. 9)104,105. These types of bicontinuous structures show more immediate potential as membranes, but through careful design of the chemical surface pattern it may be possible to fabricate structures relevant for integrated circuits. An alternative approach to threedimensional structures could involve the directed assembly of a block

Fig. 10 Hierarchical phase transition from cylinders to spheres on chemically patterned substrates. (a) A cylindrical PS-b-PtBA has been organized on surface patterns with LS = 57.5 nm and thermochemically reacted to (b) a spherical PS-b-PAA copolymer that remained perfectly ordered. (Adapted and reprinted with permission from106. © 2005 American Chemical Society.)

copolymer using a chemically striped surface, followed by conversion of the structures to a more functional geometry106. Fig. 10a demonstrates

In conclusion, the directed assembly of block copolymers on

the directed assembly of a cylindrical poly(styrene-block-tert-butyl

chemically heterogeneous surfaces is a promising route for patterning

acrylate) (PS-b-PtBA) copolymer in a thin film. The aligned PtBA

at the nanoscale. The directed assembly technique is able to replicate

cylinder domains are subsequently converted through a

the perfection, registration, and geometries attained by

thermochemical reaction at 160°C to a linear, hierarchical arrangement

photolithography, while achieving the advantageous atomic-level

of spherical poly(acrylic anhydride) (PAA) nanodots in three dimensions

control inherent to self-assembling materials. Improvements to the

(Fig.

10b)106.

These dot arrays, with row-to-row spacings controlled by

materials and the assembly process will be critical in determining

the surface pattern dimension LS = 57.5 nm and regular separations of

whether block copolymers or other smart materials are eventually

~49 nm between nanodots within a row, could potentially be used in

considered the best option for fabrication at sub-22 nm length

the fabrication of magnetic storage media.

scales.

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28

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