Letter pubs.acs.org/NanoLett
Ordered Nanoscale Archimedean Tilings of a Templated 3‑Miktoarm Star Terpolymer Karim Aissou,†,§ Hong Kyoon Choi,†,§ Adam Nunns,‡ Ian Manners,‡ and Caroline A. Ross*,† †
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom S Supporting Information *
ABSTRACT: The directed self-assembly of 3-miktoarm star terpolymer chains (polyisoprene-arm-polystyrene-arm-polyferrocenylethylmethylsilane (3 μ-ISF)) into 2D Archimedean tilings is described. A morphological change from (4.82) to (63) tiling is reported in the 3 μ-ISF thin film blended with PS homopolymer when a greater swelling of PI is achieved during the solvent annealing process. Highly oriented (4.82) tilings were produced by templating the self-assembled three colored structures in blended thin films. The use of (4.82) and (63) tilings as nanolithographic masks to transfer square and triangular hole arrays into the substrate is also demonstrated. KEYWORDS: 3-Miktoarm star terpolymer, thin film, self-assembly, templated substrate, Archimedean tiling, nanolithographic mask
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The (32.4.3.4) tiling, with a high local symmetry (p4gm with 12fold-like Fourier peaks), offers the possibility of constructing novel photonic band gap structures with isotropic properties13−15 from 3 μ-ABC in which the dielectric contrast could be increased by selectively removing one of the microdomains. Another example is the tetragonal symmetry (p4mm), which offers an attractive route to nanolithography for device fabrication compared to the close-packed arrays (p6mm) commonly observed in diblock copolymer systems. Indeed, the square array represents a canonical lithographic structure which is compatible with the standard architecture design used in microelectronic industry.16 Although the bulk morphology of several 3 μ-ABC has been explored, there has been no investigation of their thin film behavior nor their application in nanolithography. In this work, we present the first results showing the self-assembly of 3 μABC thin films. We show that it is possible to obtain a range of 2D Archimedean tiling patterns in a 3 μ-ABC/homopolymer blend including tetragonal (p4mm) by control of the solvent annealing conditions, and that the patterns can be transferred into the substrate by etching processes. Moreover, squaresymmetry arrays with extremely good long-range order are obtained using a graphoepitaxial confinement. These materials offer access to a family of three-color surface patterns that can be incorporated into nanoscale devices. An 3 μ-ABC star terpolymer consisting of polyisoprene (PI, 23 kg·mol−1, ΦPI = 0.39), polystyrene (PS, 22.5 kg·mol−1, ΦPS
he Archimedean tilings described by Johannes Kepler in Harmonices Mundi in 1619 are periodic arrangements of polygons1 which make an infinite 2D tessellation consisting of regular polygons placed edge-to-edge around a vertex. According to Kepler, only 11 tilings can fill the plane without n2 gaps and are denoted (mn1 1 .m2 . ...) where mi refers to the number of sides of each polygon, and the superscript ni denotes the number of adjacent identical polygons around a vertex. For instance, the two-dimensional honeycomb pattern consisting of regular hexagons is denoted (63). Although manmade examples of tilings are abundant, there are few natural systems that exhibit tilings. Thus far, liquid crystals having a specific shape2 and bimodal distributions of colloids3 or inorganic nanoparticles4 have been reported to yield an Archimedean tiling structure. Another example is microphase separation in 3-miktoarm star terpolymers (3 μABC) whose molecules consist of three dissimilar polymer chains, A, B, and C, connected at a junction point. In bulk, several Archimedean tiling structures, denoted (63), (4.82), (4.6.12) and (32.4.3.4), have been reported in microphaseseparated 3 μ-ABC.5−10 In these structures, the junction points of the molecules reside on a line11 that becomes the vertex between polygonal prisms of the three blocks. The ternary phase diagram of 3 μ-ABC is dominated by morphologies consisting of coaxial prisms which offer a large variety of symmetries, including c2mm, p3m1, p4mm, p6mm, and p4gm space groups. These ordered columnar morphologies are highly desirable in photonic and microelectronic applications. For instance, 2D photonic crystals with partial optical band gaps that correspond to visible light have been made from diblock copolymer films.12 © 2013 American Chemical Society
Received: January 1, 2013 Revised: January 18, 2013 Published: January 23, 2013 835
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= 0.35), and amorphous polyferrocenylethylmethylsilane17 (PFS, 20.5 kg·mol−1, ΦPFS = 0.26) was used, where Φ represents the volume fractions. This terpolymer, 3 μ-ISF, with a polydispersity (PDI) of 1.03 was synthesized18 starting from a core molecule designed with three positions of different reactivity following a similar methodology to that of Khanna et al.19 Core-functionalized PS was synthesized by the reaction of living anionic PS with the aldehyde moiety on the core molecule. Then PI and PFS blocks were sequentially attached on the building block using in both cases a copper(I)-catalyzed azide−alkyne cycloaddition “click” reaction strategy. Films of 3 μ-ISF or a blend of 3 μ-ISF with homo-PS (hS, 10 kg·mol−1, PDI = 1.08), designated 3 μ-ISF/hS, were spin coated on oxidized Si substrates from 1 wt % solution of polymer in toluene. The volume fraction of hS (= volume of hS/volume of hS + 3 μ-ISF) was 30% (3 μ-ISF/hS30) or 15% (3 μ-ISF/ hS15). The self-assembly of 3 μ-ISF and 3 μ-ISF/hS thin films with thicknesses up to 60 nm was achieved by exposing samples to a continuous stream of chloroform (CHCl3) vapor produced by bubbling nitrogen gas through the liquid solvent. This continuous flow system20 permits control of the chloroform vapor pressure in the chamber by dilution with a separate N2 stream. To prevent dewetting of the polymer films, a solvent vapor consisting of 7.0−8.5 sccm CHCl3 vapor and 3.0−1.5 sccm N2 (total 10 sccm) was used. The vapor caused swelling of the film during the annealing process then the film was quenched in air by quickly removing the lid of the chamber. Films were imaged by scanning electron microscopy (SEM) after an etch in an oxygen plasma, which removes the PI faster than the PS and leaves regions of oxidized PFS.21,22 Transmission electron microscopy (TEM) samples were prepared by inserting a PEDOT/PSS sacrificial layer between the silicon substrate and the 3 μ-ISF/hS thin film then dissolving the PEDOT/PSS layer and transferring the 3 μ-ISF/ hS film onto the TEM grid.23 Films on PEDOT/PSS had the same surface morphology and period as those on Si wafers. In some cases, films were stained with osmium tetroxide (OsO4), which binds to the PI blocks. Further experimental details are given in the Supporting Information. We first demonstrate the range of morphologies including tilings obtained from 3 μ-ISF films. The bulk morphology of the unblended polymer after thermal annealing for 72 h at 140 °C is shown in Figure 1a, after staining. The OsO4-stained PI (dark) and PS (bright) microdomains form a square symmetry checkerboard with PFS (gray) microdomains present between the PFS and PI, forming a (4.82) Archimedean tiling pattern with period of about 30 nm with the PFS in the square prisms. This is consistent with PFS having the lowest volume fraction. However, the 3 μ-ISF produced a different morphology as a thin film. Supporting Information Figure S1A shows a fingerprint-like thin film morphology in which in-plane cylinders or short vertical lamellae of PFS alternate with cylinders (or spheres) of PI in a PS matrix with a period 30 nm. This was seen in the unblended 3 μ-ISF film over a range of film thicknesses and for thermal as well as solvent annealing. The Flory−Huggins interaction parameter between PI and PFS is higher than that of the two other pairs (χFS ∼ 0.0824 < χIS ∼ 0.1125 ≪ χIF ∼ 0.1726), which suggests that the PFS or PI chains pass through the PS microdomains to form the alternating structure. Blending the 3 μ-ISF with homo-PS led to a variety of morphologies that were sensitive to the thickness and annealing
Figure 1. Gallery of some of the morphologies: (a) TEM image of an unblended bulk 3 μ-ISF film and (b−d) SEM images of 3 μ-ISF/hS15 thin films on untreated Si wafer at (b) 0.7P0, (c) 0.75P0, and (d) 0.85 P0. (e,f) 3 μ-ISF/hS15 thin films on a P2VP-coated surface at (e) 0.75P0 and (f) 0.8P0. Samples were stained with OsO4 to enhance contrast for TEM and SEM imaging. Thin films (b−f) were etched by O2 RIE before SEM observation, so PFS appears bright and regions formerly occupied by PS appear dark. The thickness of the films were (b−e) 34 nm and (f) 42 nm. Scale bars are 200 nm.
conditions. The hS enables relaxation of the chains and promotes formation of ordered microdomains, as seen for a linear ISF terpolymer.27 Modeling has shown that the morphology of 3 μ-ABC terpolymers is very sensitive to volume fraction11 so it would be expected that order−order transitions can be driven not only by changes in the hS volume fraction but by changes in the chloroform vapor pressure, which swells the PI preferentially compared to the PS and PFS and lowers the interaction parameters. Figure 1b−d shows the effect of solvent vapor pressure during annealing for 3 μ-ISF/hS15 on a bare silicon wafer. Po represents the room-temperature saturated vapor pressure of chloroform, 200 Torr at 25 °C. At 0.7Po, a mixed morphology consisting of regions of core−shell cylinders, Figure 1b, and regions of a (4.82) tiling pattern was obtained. The close-packed core−shell cylinders had a center− center spacing of 38 nm and consisted of PFS cores within PS shells in a PI matrix, a similar morphology to that seen in a bulk PS−PI−polymethylmethacrylate star terpolymer.8 This structure avoids unfavorable contact between PI and PFS but, as in the unblended 3 μ-ISF, requires chain mixing within the PS microdomains. At a solvent vapor pressure of 0.75Po the (4.82) tiling pattern, Figure 1c, with period 34 nm completely covered the substrate. Unlike the bulk morphology, the PI microdomains (gray) were at the vertices of a checkerboard of PS (dark) and PFS (bright), that is, PS and PFS ideally form octagonal prisms and PI square prisms. The morphological transition from a cylindrical to a tiling pattern is attributed to the decrease in effective χ parameter with increasing vapor pressure that reduces the energy penalty of PI−PFS interfaces and promotes the tiling pattern. A further solvent vapor pressure increase to 0.85Po forms a different “radiation pattern” tiling, Figure 1d with 37 nm period. This can be described as (63) although the prismatic microdomains are not of equal size and do not form regular 836
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Figure 2. (a) TEM image of a 3 μ-ISF/hS30 thin film showing three different phases with (b) its 2D-FFT and FFT profile exhibiting a p4mm phase. The dark PFS, gray PS, and bright PI domains are arranged in a (4.82) tiling pattern called a St. Andrew’s cross. (c) (1.5 × 1.5 μm) and (d) (0.5 × 0.5 μm) 3D AFM topographic images of a 3 μ-ISF/hS30 thin film after etching in oxygen.
instead of the octagonal sites. Decreasing the film thickness recovered the (4.82) tiling with PI at the square positions and PFS at the octagons with approximately the same period. PI is expected to produce a layer at the air interface due to its lower surface energy, and the depletion of PI from the interior of the thinner films may account for the morphological transition. For AFM imaging, Figure 2c,d, an O2 RIE plasma was used to increase the contrast between the PI and the PS microdomains (see also Supporting Information Figure S3C for an etched TEM sample). The shape of the PI microdomains in the TEM images differs from the octagonal shape in the ideal Archimedean (4.82) tiling pattern. The experimental microdomains are not regular squares or octagons and there is evidence of curvature of the boundaries, as seen in ref 10 for a bulk 3 μ-ABC. This microstructure has been called a St. Andrew’s cross28 for the crosses formed by two of the polymer microdomains. The deformation from the ideal Archimedean (4.82) tiling pattern is attributed to the difference in interaction parameters, which favors minimization of the PI−PFS interface area, as well as the nonideal volume fractions of the blocks compared to the ideal 22:39:39 area fraction of (4.82). To enable the use of these microdomain morphologies in nanotechnological applications, it is necessary to control the long-range order of the microdomains by templating and to use the films as masks for pattern transfer. The former was accomplished by the use of topographical gratings in silica fabricated by interference lithography (IL) with a period of 240 nm consisting of 160 nm wide, 50 nm deep trenches separated by 80 nm wide mesas (Figure 3a). The unblended 3 μ-ISF film, which formed in-plane cylinders on a smooth substrate (Supporting Information Figure S1A), formed well-aligned parallel cylinders on the grating (Supporting Information
hexagons and has been seen in bulk PI−PS−polydimethylsiloxane/homopolymer blends.28 The morphological change from (4.82) to (63) is promoted by the greater swelling of PI in chloroform at high vapor pressure29 that increases the effective volume fraction of PI. Additional morphologies, such as inplane core shell cylinders (Figure 1e) and a previously unreported thin film morphology of PFS spheres in PS separated by PI (Figure 1f), were produced when the substrate was coated with a P2VP brush which is preferential to PI (see Supporting Information Figure S2 for further details). Increasing the hS content improved the ordering of the microdomains to give correlation lengths of ∼1 μm in the films. This phenomenon is because the unstable hexagonal (63) phase is not observed for solvent pressure of 0.8P0 upon the addition of extra hS, showing that the hS favors (4.82) over (63). Figure 2 and Supporting Information Figure S3 show atomic force microscopy (AFM) and TEM images of unstained 60 nm thick films of 3 μ-ISF/hS30. In the TEM image, Figure 2a and Supporting Information Figure S3A, the darkest regions correspond to PFS iron-rich domains, while the brighter regions are PI and PS with PS slightly darker due to a higher density (dPS = 1.05 g·cm−3 and dPI = 0.91 g·cm−3). The 2DFFT from a 1 μm area of the TEM image reveals a tetragonal symmetry with an interdomain spacing, p, of 38.5 nm extracted from the first-order peak, q* (q* = 0.163 nm−1), of the FFT intensity profile (Figure 2b). The presence of higher-order peaks with positions in the ratio 1:21/2:41/2:51/2:81/2:101/2 and 161/2 is in accordance with a p4mm phase where the first five peaks are associated with the (10), (11), (20), (21), and (22) diffraction planes, respectively, of a square lattice. This blend forms a (4.82) tiling but unlike that reported in Figure 1c for 3 μ-ISF/hP15, the PFS now occupies the square 837
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symmetry. This result is emphasized by the corresponding 2DFFT that shows a common orientation of the spots from the grating and the microdomains (Figure 3c). The grating pitch, Λ, was 240 nm (Λ = 2π/q* where q*= 0.026 nm−1) whereas the 3 μ-ISF/hS thin film exhibited a period of 40 nm (p = 2π/ q** where q** = 0.157 nm−1) perpendicular to the grooves, and 37 nm parallel to the grooves, which can be compared with that on a smooth substrate, 38.5 nm. This indicates an orthorhombic distortion to symmetry p2mm driven by the incommensurability of the template, resulting in two unit cells of the star terpolymer on the mesas and four in the trenches. Supporting Information Figure S4 presents a 3D-AFM topographic image obtained from the best ordering condition for 3 μ-ISF/hS30 of average thickness 60 nm on the grating template, which corresponded to a continuous film of thickness ∼39 nm above the mesa and ∼81 nm in the trenches. The polymer film was nearly planar with peak-valley amplitude of 4 nm. Finally, pattern transfer from the 3 μ-ISF/hS films was demonstrated. Figure 4a,b shows the pattern transfer process from (4.82) (with PI and PFS forming octagonal prisms as in Figure 1c) and (63) (as in Figure 1d) tilings of stained 3 μ-ISF/ 15hS films into an oxidized silicon substrate. The PS microdomains were removed by oxygen RIE, then the substrate was etched with CF4 and a further oxygen RIE removed the residual polymer film, leaving only oxidized PFS. Unlike circular pits formed from square symmetry patterns from AB diblock copolymer or ABC linear triblock terpolymer films, the pits here have a polygonal shape, respectively, square and triangular. Figure 4c shows pattern transfer from a (4.82) unstained 3 μISF/30hS film with PI and PS forming octagonal prisms (as in Figure 2). The PI was removed by an oxygen plasma, followed by pattern transfer into the silica and complete removal of the terpolymer. In summary, highly ordered Archimedean tiling patterns with ∼40 nm period have been realized from the self-assembly of 3miktoarm star terpolymer thin films blended with homopolymer chains. Excellent long-range order has been achieved by templating the self-assembly by a topographical substrate. It is likely that other tiling patterns, including even quasi-periodic structures31 can be achieved by adjustments of polymer composition, homopolymer blend and annealing conditions. For example, 3 μ-ISF 40:37:23 formed a (4.6.12) tiling in bulk and a (3.4.6.4) tiling as a thin film blended with 15% hS, while a neat PI-PS-PEO (40:55:19) miktoarm star terpolymer film formed a (63) tiling with ∼30 nm period. These new geometries, which cannot be obtained from linear terpolymers, provide a new toolset for pattern generation by block
Figure 3. (a) Schematic illustration of the 240 nm template used in this study. (b) SEM images of a solvent annealed unstained 3 μ-ISF/ hS30 thin film deposited on the template and etched in O2 to remove the (dark) PI phase. (c) The 2D-FFT of the SEM image and its FFT profile show that the vertically oriented columns are organized into a p4mm phase.
Figure S1B). The in-plane cylinders were aligned orthogonal to the grating direction with a spacing of 30 nm extracted from the 2D-FFT. Orthogonal in-plane orientations of cylindrical microdomains with respect to topographical gratings have been observed in diblock copolymer systems30 and attributed to incommensurability between the template dimensions and the microdomain period. The 3 μ-ISF/hS30 film formed an extremely well ordered (4.82) tiling pattern on the topographical template (Figure 3b) with the same microdomain arrangement as in Figure 2. The sample comprised a single grain orientation with tetragonal
Figure 4. Pattern transfer from 3 μ-ISF/hS into oxidized silicon substrates. (a) From a (4.82) tiling and (b) from a (63) tiling of 3 μ-ISF/15hS, both stained with OsO4. The dark features are the pits corresponding to the locations of the PS microdomains. White spots are residual oxidized PFS, making a 3-level structure. (c) From unstained 3 μ-ISF/hS30 into 20 nm thick silica. The SEM image shows a square array of dark pits corresponding to the locations of the PI microdomains. The inset is a 750 nm × 750 nm 3D AFM topographic image of the sample. 838
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(21) Korczagin, I.; Lammertink, R. G. H.; Hempenius, M. A.; Golze, S.; Vancso, G. J. Adv. Polym. Sci. 2006, 200, 91−117. (22) Nunns, A.; Gwyther, J.; Manners, I. Polymer 2012, http://dx.doi. org/10.1016/j.polymer.2012.11.057. (23) Aissou, K.; Fleury, G.; Pécastaings, G.; Alnasser, T.; Mornet, S.; Goglio, G.; Hadziioannou, G. Langmuir 2011, 27, 14481−14488. (24) Eitouni, H. B.; Balsara, N. P.; Hahn, H.; Pople, J. A.; Hempenius, M. A. Macromolecules 2002, 35, 7765−7772. (25) Lecommandoux, S.; Borsali, R.; Schappacher, M.; Deffieux, A.; Narayanan, T.; Rochas, C. Macromolecules 2004, 37, 1843−1848. (26) χIF was estimated from a strong segregation theory (SST) calculation described previously: (a) Aissou, K.; Baron, T.; Kogelschatz, M.; Pascale, A. Macromolecules 2007, 40, 5054−5059. using Lammertink’s experimental results: (b) Lammertink, R. G. H.; Hempenius, M. A.; van den Enk, J. E.; Chan, V. Z. H.; Thomas, E. L.; Vancso, G. J. Adv. Mater. 2000, 12, 98−103. (27) Chuang, V. P.; Gwyther, J.; Mickiewicz, R. A.; Manners, I.; Ross, C. A. Nano Lett. 2009, 9, 4364−4369. (28) Yamauchi, K.; Akasaka, S.; Hasegawa, H.; Iatrou, H.; Hadjichristidis, N. Macromolecules 2005, 38, 8022−8027. (29) Choi, H. K.; Gwyther, J.; Manners, I.; Ross, C. A. ACS Nano 2012, 25, 8342−8348. (30) Hong, S. W.; Huh, J.; Gu, X.; Lee, D. H.; Jo, W. H.; Park, S.; Xu, T.; Russell, T. P. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 1402−1406. (31) Hayashida, K.; Dotera, T.; Takano, A.; Matsushita, Y. Phys. Rev. Lett. 2007, 98, 195502−195506.
copolymer lithography, and additionally could form a variety of “three-colored” surfaces whose chemical heterogeneity could be used to guide, for example, magnetically or optically functional nanoparticles with different ligands into complex arrangements.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.R. gratefully acknowledges financial support from the National Science Foundation, the Semiconductor Research Corporation and TSMC. I.M. acknowledges the support of the EPSRC. We thank Nicolas Aimon for his help with TEM.
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REFERENCES
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