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
20
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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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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REVIEW FEATURE
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.
24
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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.
REFERENCES
28
1. Bates, F. S., and Fredrickson, G. H., Annu. Rev. Phys. Chem. (1990) 41, 525
3. Park, M., et al., Science (1997) 276, 1401
2. Bates, F. S., and Fredrickson, G. H., Physics Today (1999) 52, 32
4. Mansky, P., et al., J. Mater. Sci. (1995) 30, 1987
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Block copolymers and conventional lithography
5. Mansky, P., et al., Science (1997) 275, 1458 6. Mansky, P., et al., Macromolecules (1997) 30, 6810 7. Heier, J., et al., Macromolecules (1997) 30, 6610 8. Peters, R. D., et al., Langmuir (2000) 16, 4625 9. Kim, T. K., et al., J. Phys. Chem. B (2000) 104, 7403 10. Peters, R. D., et al., Macromolecules (2002) 35, 1822 11. Kim, S. O., et al., Nature (2003) 424, 411 12. Morkved, T. L., et al., Science (1996) 273, 931 13. Thurn-Albrecht, T., et al., Science (2000) 290, 2126 14. DeRouchey, J., et al., Macromolecules (2004) 37, 2538 15. Angelescu, D. E., et al., Adv. Mater. (2004) 16, 1736 16. Park, C., et al., Appl. Phys. Lett. (2001) 79, 848 17. Bodycomb, J., et al., Macromolecules (1999) 32, 2075 18. Kim, G., and Libera, M., Macromolecules (1998) 31, 2569 19. Rosa, C. D., et al., Nature (2000) 405, 433 20. Albalak, R. J., et al., Polymer (1998) 39, 1647 21. Elbs, H., et al., Macromolecules (1999) 32, 1204 22. Kim, S. H., et al., Adv. Mater. (2004) 16, 226 23. Du, P., et al., Adv. Mater. (2004) 16, 953 24. Sundström, L., et al., Appl. Phys. Lett. (2006) 88, 243107 25. Li, R. R., et al., Appl. Phys. Lett. (2000) 76, 1689 26. Cheng, J. Y., et al., Adv. Mater. (2001) 13, 1174 27. Naito, K., et al., IEEE Trans. Magn. (2002) 38, 1949 28. Guarini, K. W., et al., IEEE Tech. Dig. IEDM (2003), 541
REVIEW FEATURE
Montgomery Research Incorporated, San Francisco, (20 January 2005), 18 58. Herr, D. J. C., Update on the extensibility of optical patterning via directed selfassembly. In: Future Fab International, Dustrud, B., (ed.) Montgomery Research Incorporated, San Francisco, (7 January 2006), 20 59. Tanaka, T., et al., Jpn. J. Appl. Phys. (1993) 32, 6059 60. Cao, H. B., et al., J. Vac. Sci. Technol. B (2000) 18, 3303 61. Stoykovich, M. P., et al., Adv. Mater. (2003) 15, 1180 62. Boltau, M., et al., Nature (1998) 391, 877 63. Rockford, L., et al., Phys. Rev. Lett. (1999) 82, 2602 64. Chen, H., and Chakrabarti, A., J. Chem. Phys. (1998) 108, 6897 65. Pereira, G. G., and Williams, D. R. M., Macromolecules (1999) 32, 758 66. Wang, Q., et al., J. Chem. Phys. (2000) 112, 9996 67. Wang, Q., et al., Macromolecules (2000) 33, 4512 68. Tsori, Y., and Andelman, D., J. Chem. Phys. (2001) 115, 1970 69. Tsori, Y., and Andelman, D., Europhys. Lett. (2001) 53, 722 70. Wang, Q., et al., Macromolecules (2003) 36, 1731 71. Yang, X. M., et al., Macromolecules (2000) 33, 9575 72. Rockford, L., et al., Macromolecules (2001) 34, 1487 73. Edwards, E. W., et al., Adv. Mater. (2004) 16, 1315 74. Stoykovich, M. P., et al., Science (2005) 308, 1442 75. Nealey, P. F., et al., IEEE Tech. Dig. IEDM (2005), 356 76. Edwards, E. W., et al., J. Poly. Sci. B: Poly. Phys. (2005) 43, 3444 77. SAMs of silanes are difficult to uniformly and reproducibly deposit on Si substrates, and as such have been replaced by polymer brush imaging layers for most applications.
29. Black, C. T., et al., Appl. Phys. Lett. (2001) 79, 409
78. Ryu, D. Y., et al., Science (2005) 308, 236
30. Black, C. T., et al., IEEE Electron Device Lett. (2004) 25, 622
79. Pallandre, A., et al., Nano Lett. (2004) 4, 365
31. Lopes, W. A., and Jaeger, H. M., Nature (2001) 414, 735
80. Lercel, M. J., et al., Appl. Phys. Lett. (1996) 68, 1504
32. Kim, H. C., et al., Adv. Mater. (2001) 13, 795
81. Solak, H. H., et al., Microelectron. Eng. (2003) 67, 56
33. Urbas, A., et al., Adv. Mater. (2000) 12, 812
82. Solak, H. H., et al., J. Phys. D: Appl. Phys. (2006) 39, R171
34. Urbas, A. M., et al., Adv. Mater. (2002) 14, 1850
83. Pellerin, K., et al., unpublished results
35. Chan, V. Z.-H., et al., Science (1999) 286, 1716
84. Wilmes, G. M., et al., Macromolecules (2006) 39, 2435
36. Xu, T., et al., Polymer (2001) 42, 9091
85. Edwards, E. W., et al., J. Vac. Sci. Technol. B (2006) 24, 340
37. Jeong, U. Y., et al., Adv. Mater. (2002) 14, 274
86. Stoykovich, M. P., et al., (2006), unpublished results
38. Hawker, C. J., and Russell, T. P., MRS Bull. (2005) 30, 952
87. Mayes, A. M., et al., Macromolecules (1994) 27, 7447
39. Segalman, R. A., Mater. Sci. Eng. R (2005) 48, 191
88. Koneripalli, N., et al., Macromolecules (1998) 31, 3498
40. Segalman, R. A., et al., Adv. Mater. (2001) 13, 1152
89. Bates, F. S., et al., Phys. Rev. Lett. (1997) 79, 849
41. Cheng, J. Y., et al., Appl. Phys. Lett. (2002) 81, 3657
90. Corvazier, L., et al., J. Mater. Chem. (2001) 11, 2864
42. Segalman, R. A., et al., Phys. Rev. Lett. (2003) 91, 196101
91. Torikai, N., et al., Macromolecules (1997) 30, 5698
43. Segalman, R. A., et al., Macromolecules (2003) 36, 6831
92. Muller, M., and Smith, G. D., J. Poly. Sci. B: Poly. Phys. (2005) 43, 934
44. Cheng, J. Y., et al., Adv. Mater. (2003) 15, 1599
93. Daoulas, K. Ch., et al., Soft Matter (2006) 2, 573
45. Cheng, J. Y., et al., Nat. Mater. (2004) 3, 823
94. Gido, S. P., and Thomas, E. L., Macromolecules (1994) 27, 6137
46. Cheng, J. Y., et al., Adv. Mater. (2006) 18, 597
95. Burgaz, E., and Gido, S. P., Macromolecules (2000) 33, 8739
47. Xiao, S. G., et al., Nanotechnology (2005) 16, S324
96. Duque, D., et al., J. Chem. Phys. (2002) 117, 10315
48. Li, H. W., and Huck, W. T. S., Nano Lett. (2004) 4, 1633
97. Tanaka, Y., et al., J. Vac. Sci. Technol. B (1998) 16, 3509
49. Sundrani, D, et al., Nano Lett. (2004) 4, 273
98. Edwards, E. W., et al., (2006), unpublished results
50. Sundrani, D, et al., Langmuir (2004) 20, 5091
99. Wilmes, G. M., et al., Investigating the limits of surface-directed self-assembly of pure block copolymers. Presented at MRS Fall Meeting, Boston, MA (2005)
51. Black, C. T., and Bezencenet, O., IEEE Trans. Nanotech. (2004) 3, 412 52. Black, C. T., Appl. Phys. Lett. (2005) 87, 163116
100. Semenov, A. N., Macromolecules (1993) 26, 6617
53. Park, S. M., et al., (2006), unpublished results
101. Fukukawa, K., et al., Macromolecules (2005) 38, 263
54. Shin, K., et al., Nano Lett. (2002) 2, 933
102. Semiconductor Research Corporation/National Nanotechnology Initiative Directed Self Assembly of Materials for Patterning Workshop, Madison, WI (16 June 2005)
55. Thompson, L. F., et al., (eds.) Introduction to Microlithography, 2nd edition, American Chemical Society, Washington, D.C., (1994) 56. International Technology Roadmap for Semiconductors (ITRS) 2005 Edition, Semiconductor Industry Association, San Jose, CA, 2005 57. Herr, D. J. C., The extensibility of optical patterning via directed self-assembly of nano-engineered imaging materials. In: Future Fab International, Dustrud, B., (ed.)
103. Schellenberg, F. M., and Torres, J. A. R., Proc. SPIE (2006) 6151, 61513L 104. Daoulas, K. Ch., et al., Phys. Rev. Lett. (2006) 96, 036104 105. Daoulas, K. Ch., et al., J. Poly. Sci. B: Poly. Phys. (2006), in press 106. La, Y. H., et al., Nano Lett. (2005) 5, 1379
SEPTEMBER 2006 | VOLUME 9 | NUMBER 9
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