LETTER pubs.acs.org/NanoLett
Highly Ordered Square Arrays from a Templated ABC Triblock Terpolymer Jeong Gon Son,† Jessica Gwyther,§ Jae-Byum Chang,† Karl K. Berggren,‡ Ian Manners,§ and Caroline A. Ross*,† †
Department of Materials Science and Engineering and ‡Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § School of Chemistry, University of Bristol, Bristol BS8 ITS U.K.
bS Supporting Information ABSTRACT: Square-symmetry patterns are of interest in nanolithography but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate highly ordered 44 nm period square patterns formed in a thin film of polyisoprene-blockpolystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer by controlling the film thickness, solvent anneal conditions, the surface chemistry and topography of the substrates. The square patterns consist of PFS pillars that remained after removal of the PI and PS with an oxygen plasma. On an unpatterned smooth substrate, the average grain size of the square pattern was increased dramatically to several micrometers by the use of brush layers and specific solvent anneal conditions. Templated self-assembly of well-ordered square patterns was demonstrated on substrates containing nanoscale topographical sidewalls and posts, written by electron beam lithography, in which the sidewalls and base of the substrate were independently chemically functionalized. KEYWORDS: Block copolymer, triblock terpolymer, self-assembly, templated self-assembly, square pattern, nanolithography
elf-assembly of block copolymer thin films enables the formation of highly regular sub-10 nm microdomain patterns at low cost in a simple process and is currently of great interest for nanolithography and device fabrication.15 Many groups have studied the self-assembly of diblock copolymer films, but these materials are limited to certain geometries such as arrays of parallel lines or close-packed dots. To extend the available variety of microdomain geometries, triblock terpolymer self-assembly has been explored,68 which can generate geometries such as ring-shape patterns9 or square-symmetry patterns.10 In particular, the square-symmetry array is a key geometry that is considered essential for future device fabrication and which may be useful in making high density magnetic patterned media11 and via arrays in integrated circuits. Several strategies have been developed for making square-symmetry arrays from block copolymer self-assembly, including the templated self-assembly of a diblock copolymer on a square chemically patterned substrate with the same periodicity as the copolymer,12 body-centered cubic packing of spherical microdomains in triblock terpolymer films of specific thickness,13 square-symmetry cylindrical microdomains from a blend of two hydrogen-bonded diblock copolymers,14,15 diblock copolymer blend films in small square wells,16 and the self-assembly of thin films of a triblock terpolymer with a bulk square symmetry.10 Despite these successes, the square-symmetry patterns do not have good long-range order in the absence of templating because
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microphase separation leads to the formation of regions of microdomains with short-range order but random in-plane orientation. The extent of short-range order, that is, the average “grain size” of the pattern, is determined by the annealing process and by the surface properties of the substrate.17 Long-range order has been imposed on diblock copolymer thin films by using chemical or topographical substrate patterns,3,4,1824 for example the use of sparse arrays of topographical posts to pattern dense arrays of spherical25 or cylindrical26 microdomains, driven by the strong affinity of the template to one domain of the diblock copolymer. However, there has been very little work on the templating of triblock terpolymer films, at least in part because it is less straightforward to select appropriate topographical or chemical substrate patterns to direct the assembly of a triblock terpolymer. As an example, we showed previously that the orientation of the microdomains in a square-pattern triblock terpolymer film with respect to a shallow substrate trench could be controlled by using a brush layer.10 In this paper, we demonstrate first the formation of highly ordered square patterns from thin films of a triblock terpolymer by use of a substrate brush layer, and second, registration of the Received: April 14, 2011 Revised: May 23, 2011 Published: June 16, 2011 2849
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Figure 1. (a) Schematics of the perpendicular orientation of alternating cylinders with square symmetry in an ABC triblock terpolymer film using solvent annealing. (bd) SEM images of oxidized PFS microdomains from a 32 nm thick film of PI-b-PS-b-PFS triblock terpolymer blended with 15 wt % homopolymer PS after solvent annealing in chloroform vapor, after (b) 128%, (c) 152%, and (d) 178% swelling during solvent annealing. Oxygen plasma etching removes the PI and PS domains revealing oxidized PFS cylinders. Only the 152% swelled film formed highly ordered square arrays with period 44 nm. (e) Thicker films, such as 37 nm, showed in-plane cylinders when the films were swelled by 152%.
microdomains using topographical posts and square wells. This work extends the ability to precisely register and control microdomain locations to triblock terpolymers, facilitating their use in nanolithography and augmenting the range of available pattern geometries. The films were made from a polyisoprene-block-polystyreneblock-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer in which the bulk morphology consists of PI and PFS cylinders in an alternating square-symmetry arrangement within a PS matrix. We previously showed that thin films of this blend could produce square-symmetry patterns with limited long-range order.10 We show here that highly ordered square patterns with average grain size on the scale of micrometers can be achieved by controlling the surface chemistry of the substrate using a brush layer that differs chemically from all three of the blocks in the triblock terpolymer. Furthermore, topographical templates in which the sidewalls and substrate are treated independently with different brush layers were used to impose long-range order and registration of the square patterns. An A-b-B-b-C triblock terpolymer can form alternating square symmetry cylinders of A and C in a matrix of B (Figure 1a) when B is the majority block and the FloryHuggins interaction
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parameter between A and C, χAC is higher than χAB and χBC.6,7 PI-b-PS-b-PFS is a suitable material according to the solubility parameters δ, which are 17.0 (MPa)1/2 for PI, 18.5 (MPa)1/2 for PS and 18.7 (MPa)1/2 for PFS.27,28 The interaction parameter χAB is proportional to the square of the difference in solubility parameters between A and B.29 We used an 82 kg/mol PI-b-PS-b-PFS triblock terpolymer with volume fractions of 25, 65, and 10%, respectively, blended with 15 wt % of 27 kg/mol PS homopolymer that increases the range of film thicknesses over which perpendicular cylinders form.10 Additionally, because the PFS moiety contains iron and silicon, it etches slowly compared to the PI and PS blocks when treated with an oxygen plasma, which simplifies pattern transfer to other materials. Thin films of the triblock/homopolymer blend were formed on brush-coated Si wafers by spin-coating a toluene solution of the blend. The polymer film was solvent annealed at room temperature in chloroform vapor to obtain square-symmetry microdomain patterns. The solvent vapor pressure in the chamber was controlled using a leak valve, Figure 1a, which was adjusted to vary the degree of swelling of the films, the mobility of the block segments30 and the interaction parameters,31 and to promote self-assembly of the microdomains.32 Solvent annealing can change the volume fractions of the domains as a result of selective uptake of solvent molecules within each block, which can drive orderorder transitions,24,30 and the effective χ parameter (χeff) decreases linearly with the fraction of solvent molecules incorporated.29 The film thickness change caused by swelling of the films was monitored using in situ film thickness measurement by spectral reflectometry.30 Chloroform vapor was used to swell the films from 128 to 178% thickness change. Figure 1bd shows SEM images after etching of 32 nm (the reference unswelled film thickness) thick PI-b-PS-b-PFS triblock terpolymer/15% PS homopolymer blended films swelled to (b) 128, (c) 152, and (d) 178% of the unswelled film thickness. In the case of the 128% swelled film, the final PFS morphology, observed after removing the PI and PS using an oxygen plasma, was poorly ordered indicating that this degree of swelling was not enough to provide an adequate mobility. The 152% swelled film formed highly ordered square array patterns showing an optimum degree of mobility without lowering the interaction parameter excessively, but the 178% swelled films again formed poorly ordered microdomain patterns. In this case the poor order may be a result of a change in volume fraction of the domains. Figure S1 of the Supporting Information shows that homopolymers of the three blocks swell similarly to the triblock blend of chloroform at low swelling ratios, but at high swelling ratios the PFS swells less than the other blocks, which may promote a morphological transition. In addition, the high swelling ratio is expected to lower the interaction parameters (χeffPS/PI, χeffPS/PFS, χeffPI/PFS), which lowers the driving force for microphase separation. The thickness of the films also affects the morphology and orientation of the microdomains. At 32 nm thickness, square symmetry perpendicular cylinder arrays were formed, as shown in Figure 1c. However, when 37 nm thickness films were swelled by 152%, the films formed discrete terraced structures containing both in-plane and perpendicular cylinders (Figure 1e). Thicker films, such as 60 nm, showed predominantly in-plane cylinders. Such an orientation change with film thickness is well established for diblock copolymer thin films, both theoretically33 and experimentally,34 and is related to the commensurability between the microdomain period and the film thickness. 2850
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Figure 2. SEM images and grain sizes of square-array patterns from a PI-b-PS-b-PFS blended with 15% PS thin film on (a) Si wafer, (b) PS brush, (c) PFS brush, (d) PB brush, (e) P2VP brush, (f) PEO brush-coated substrates. (g) Average grain sizes of the square patterns were measured based on lowmagnification SEM images. The grain size of the square pattern is increased dramatically to several micrometers by the use of P2VP or PEO brush layers.
We now describe the effect of surface chemistry on the average grain size of the square-array pattern. Ordering of diblock copolymer domain patterns is known to be significantly affected by the surface properties of the substrate,17 and we previously found large differences in the correlation length of cylindrical polystyrene-b-polydimethylsiloxane (PS-b-PDMS) block copolymer thin films deposited on pristine Si wafers, on PS brushcoated and on PDMS brush-coated substrates.35 Most studies of brush effects use a brush consisting of one of the blocks of the copolymer, but it is also possible to use a different brush chemistry. In this work, we select brushes of PEO (polyethylene oxide), PB (polybutadiene), and P2VP (poly(2-vinylpyridine)) in addition to PS and PFS brushes. Brushes were formed by grafting hydroxyl-terminated short-chain homopolymers onto oxidized Si wafers, then rinsing away the ungrafted chains. Figure 2 shows SEM images of PI-b-PS-b-PFS/PS blend films on various substrates after oxygen plasma treatment, and the average grain size obtained from the images. Because the optimum film thicknesses to form perpendicular cylinder square arrays can differ according to the wetting behavior of the film on the substrate, we selected the optimum thickness as just below the thickness at which in-plane cylinders start to form. This was 32 nm, except for the PFS brush sample where the optimum film thickness was 39 nm. On an untreated prime Si wafer (see Figure 2a), a relatively small grain size was observed because the hydroxyl-terminated polar native oxide surface strongly interacted with the polymer chains and lowered the mobility. In the case of both PS and PFS brushes, the films also showed very small grain size (Figure 2b,c). However, on brush substrates that differ from the three blocks in the triblock terpolymer, such as PB, P2VP, and PEO, (Figure 2df), the films formed surprisingly highly ordered PFS square arrays. Figure 2g shows the average grain sizes on each substrate. While the average grain size on the Si wafer, PS brush and PFS brush sample were 319, 214, and 165 nm, respectively, the PB brush, P2VP brush, and PEO brush substrate, showed much larger grain sizes, 848 nm, 1.22, and 1.75 μm, respectively. To investigate the differences between the self-assembly of the films on different brush substrates, the top and bottom morphologies of the films were examined using atomic force microscopy
Figure 3. (a) AFM top surface image of thin films of PI-b-PS-b-PFS blended with 15% PS. A featureless surface was observed at the top surface of the films. (bd) AFM bottom interface images of the films on (b) PS brush, (c) PFS brush, and (d) PEO brush-coated substrates. Highly ordered square hole patterns were observed at the bottom interface of PEO and P2VP brush samples indicating the PFS cylinders contact the lower interface, while no features were observed at the bottom interface of films on PS brush or PFS brush substrates.
(AFM) without any etching. In each case, the top surface of the films was featureless (Figure 3a) suggesting that preferential wetting of one component of the triblock terpolymer occurred at the top surface of the films regardless of substrate. This preferential wetting originates from surface energy minimization of the films. The surface energies of PI and PS are 32 and 40 mN/m, respectively, and the surface energy of PFS is known to be slightly higher than that of PS,36 so we expect the PI domains to be selectively exposed at the top surface of the films. However, at the bottom interface the morphology was highly dependent on the brush layer. To observe the bottom surface, 50 nm SiO2 layer deposited Si wafer was used as the substrate. The films were immersed in 5 wt % HF aqueous solution to dissolve the SiO2 2851
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Nano Letters sacrificial layer and release the polymer film, which was picked up on another substrate to expose the bottom interface of the film. The films on PS or PFS brush-coated substrates did not show ordered patterns at the bottom interface (Figure 3b,c), which was attributed to selective wetting of the brush-coated substrate by the PS and PFS blocks, respectively. However, on the PEO or P2VP brush-coated substrate, well-ordered square arrays of hole patterns were observed (Figure 3d). We believe that the Sicontaining PFS domains selectively reacted with the HF solution, enabling their locations to be observed.37 The square symmetry hole arrays revealed that PFS domains directly contact the substrate. Schematics of the film cross sections are shown in the insets of Figure 3bd. The films on PS or PFS coated brush substrate were assumed to form a PI layer at the top surface and fully wetted PS or PFS layers at the bottom interface, respectively. These surface layers transformed the alternating cylinder morphology into a spherical morphology at this thickness condition, which degraded the tendency for square packing that exists between the parallel cylinders in bulk. However, when the triblock terpolymer films were annealed on the PEO or P2VP brushed substrate, the PFS and PI cylinders contacted the bottom surface and oriented perpendicularly to the film to form the square symmetry pattern. The PEO (δ ∼ 20.2 (MPa)1/2) and P2VP (δ ∼ 20.4 (MPa)1/2)28 brushes are relatively hydrophilic compared to all three of the triblock terpolymer blocks. However, solvent annealing affects the magnitude of the interfacial energy between the brushes and the three blocks of the triblock terpolymer. Interfacial energy γ is proportional to the square root of the FloryHuggins interaction parameter χ and therefore to the difference in solubility parameters.29 Solvent annealing lowers the effective χ parameter (χeff) linearly as more molecules are incorporated at the interface.29 Brushes made from one of the components (PI, PS, or PFS) promote preferential wetting with zero interfacial energy regardless of the presence of solvent molecules, but for the PEO or P2VP brushes, incorporation of more solvent molecules lowers χeff between any of the domains and the brush, which makes the brush closer to being a neutral surface (i.e., γeff PI-PEO, γeff PS-PEO, and γeff PFS-PEO become more similar). This promotes a perpendicular orientation of the cylindrical triblock terpolymer domains on a PEO or P2VP brush substrate with good long-range order. Several groups have also reported that solvent annealing promotes perpendicular orientation of block copolymer microdomains in thin films even on energetically non-neutral substrates.32 Thus, long-range order of the triblock terpolymer films was successfully accomplished by optimization of solvent annealing, film thickness and surface chemistry of the substrate. We now show how these well-ordered square patterns can be templated using topographically guiding wall and post patterns to enable pattern registration. Topographical ridge patterns with a rectangular layout were prepared by electron-beam patterning of an negative-tone inorganic hydrogen silsesquioxane (HSQ) resist (Figure 4a). The wall height was 32 nm, similar to the film thickness, and the wall width was 20 nm. Preferential wetting of the sidewalls by one block is desirable in order to promote ordering,3,35 while nonpreferential wetting is desirable on the horizontal surface of the substrate to improve the ordering as discussed above. These considerations suggest that a heterogeneous surface is required38 with different brush layers on the vertical and horizontal surfaces. To accomplish this, a PFS brush
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Figure 4. Highly ordered square arrays formed from a triblock terpolymer film in an HSQ template. (a) Schematic of a square array of microdomain triblock terpolymer films within a topographical template with preferential PFS brush coated sidewalls and nonpreferential PEO brush coated horizontal surfaces. (b,c) SEM images of highly aligned square arrays of oxidized PFS microdomains in (b) 1.5 and 2 μm 3 μm and (c) 0.5 μm, 1 μm, and 1.5 μm 4 μm templates.
(molecular weight ∼5 kg/mol) was first grafted over the entire substrate, then treated with a short (∼2 s) oxygen reactive ion etch (RIE) that anisotropically etches the PFS brush on the horizontal surfaces. Then the substrate was treated with a hydroxyl-terminated PEO brush that coated the horizontal surfaces without displacing the PFS brush on the sidewalls. The triblock terpolymer/PS homopolymer blend films formed highly ordered and aligned square symmetry dot arrays in micrometer-size rectangular cages, shown in Figure 4b,c. Rectangular cages of 500 nm 500 nm, 2 μm 3 μm and 1.5 μm 4 μm each formed a single-grain pattern. The orientation of the base vector of the PFS microdomain array parallel to the wall and the spacing between the PFS domains and the wall are consistent with a brush layer of PFS being present at the wall, so the first row of 2852
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Figure 5. (a) Schematic and (b,c) SEM images of highly ordered square arrays formed on a sparse 2D lattice of HSQ (b) 88 nm 132 nm single posts and (c) 44 nm spacing double posts (brighter dots). The substrate was functionalized with a PEO brush layer and the post sidewalls were coated with PFS brush layer.
cylindrical microdomains that forms adjacent to the wall consist of PI.10 The height of the HSQ sidewalls also affected the ordering of the square patterns, as can be seen in Figure S2 in the Supporting Information. When the height exceeded the polymer film thickness, for example 50 nm, the film formed a meniscus near the sidewalls and in-plane cylinders were observed near the sidewalls. Sidewalls that were thinner than the film thickness, for example, 25 nm, led to distortions in the square-symmetry arrays. Sparse arrays of posts were also effective in templating the square arrays, as illustrated in Figure 5a. Arrays of electron-beam patterned HSQ posts were made with 88 nm 132 nm spacing, commensurate with the triblock terpolymer pattern period, L0 = 44 nm. Arrays were also made containing paired posts with 44 nm spacing in which the double-post motif was repeated on a 176 nm 132 nm period lattice. The height of the posts was 25 nm, slightly below the film thickness, and the diameter was approximately 10 nm. The post sidewalls were coated with a PFS
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brush and the horizontal surface of the substrate with a PEO brush. The PFS-coated posts acted as surrogates for perpendicular cylindrical PFS microdomains. Figure 5b shows square dot arrays formed on a commensurate post lattice with period ratio Lpost,x/L0 = 2.0 and Lpost,y/L0 = 3.0. The basis vectors of the post lattice and that of triblock terpolymer microdomain lattice are accurately aligned. Even sparser double-post arrays with Lpost,x/ L0 = 3.0 and Lpost,y/L0 = 4.0 were also effective at templating the array. We found earlier for post-templating of close-packed sphere arrays from a diblock copolymer that, as Lpost increases, the energy difference between different orientations of the microdomain lattice is reduced and multiple microdomain lattice orientations form on different regions of the post lattice. In the present case of the triblock terpolymer, however, this tendency is suppressed by the use of the double-post motif, which promotes a parallel alignment of the post lattice and the microdomain lattice. This square pattern post templating is the first demonstration of the use of a topographical pattern to precisely register the location and orientation of the microdomains of a triblock terpolymer film. In summary, highly ordered square patterns with 44 nm period and average grain size on the micrometer scale have been demonstrated in films of a PI-b-PS-b-PFS triblock terpolymer blended with 15% PS homopolymer by the use of a substrate brush chemistry that differs from the three blocks of the triblock terpolymer, combined with solvent annealing to produce a specific swelling ratio of the film. Moreover, topographical templates consisting of rectangular walls grafted with a PFS brush could produce “single grain” square symmetry patterns inside the templates, while post arrays were able to accurately align and register the arrays. These highly ordered square arrays can be useful as an etch mask for other functional materials, facilitating the fabrication of devices based on a Cartesian layout, such as integrated circuits. Methods. Substrate and Template Preparation. To graft the brushes onto the oxidized Si substrates, hydroxyl-terminated homopolymers, including PS-OH (Mw ∼ 3 kg/mol, Polymer Source), PB-OH (Mw ∼ 5 kg/mol, Polymer Source), PFS-OH (Mw ∼ 5 kg/mol, synthesized) solutions in toluene, P2VP-OH (Mw ∼ 6 kg/mol, Polymer Source) solution in DMF, PEO-OH (Mw ∼ 5 kg/mol, Polymer Source) solution in chloroform were spin-coated on the substrate and annealed at 170 °C overnight in vacuum. The substrates were then rinsed using the same spincoating solvents to remove physically adsorbed polymer molecules. The templates were fabricated using electron beam patterning of hydrogen silsesquioxane (HSQ), a negative-tone electron beam resist. HSQ films (FOx 2% solids from Dow Corning) were spin-coated on silicon substrates. Single-pixel dots and line patterns were exposed in a Raith 150 electron-beam lithography tool at 30 kV acceleration voltage. The samples were developed in a 0.25 M NaOH/0.7 M NaCl in distilled water to remove unexposed resist and to reveal the topographical nanostructures. To obtain preferential wetting on the sidewalls and nonpreferential wetting on the horizontal surfaces of substrate, the patterned substrates were spin-coated with hydroxyl-terminated PFS brush and thermally treated at 170 °C overnight in vacuum. A short (∼2 s) oxygen reactive ion etching (RIE) treatment was performed to anisotropically etch the brush layer. Then the hydroxyl-terminated PEO brush was spin-coated and thermally treated at 170 °C in vacuum. Self-Assembly of Triblock Terpolymer. We synthesized polyisoprene-b-polystyrene-b-polyferrocenylsilane (PI-b-PS-b-PFS) 2853
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Nano Letters with volume fractions of 25, 65, and 10%, respectively. The total molecular weight of this triblock terpolymer was approximately 82 kg/mol. The details of PI-b-PS-b-PFS triblock terpolymer synthesis are given in a previous article.10 The 1 wt % of the triblock terpolymer was blended with 15 wt % of PS homopolymer (Mn ∼ 27 kg/mol, PDI ∼ 1.05, Polymer Source Inc.) solution in toluene. This blended solution was spin-coated on various brush coated Si substrates with thickness of approximately 32 nm. Solvent annealing with controlled vapor pressure was executed using 1 mL of chloroform in a glass chamber with a leak valve until the solvent was fully evaporated. For in situ film thickness measurement, a Filmetrics F20-UV (Filmetrics Inc.) instrument was used. Oxygen reactive ion etching was carried out to remove the PI and PS microdomains leaving square-symmetry arrays of oxidized PFS microdomains on the substrate. Characterization. To observe the morphology of the films at the filmsubstrate interface, a Si wafer with 50 nm thick SiO2 layer was used as the substrate. The films were partially immersed in 5 wt % HF aqueous solution to dissolve the SiO2 sacrificial layer and release the polymer film, which was picked up on another substrate to expose the bottom interface of the film. The morphology of the patterns was observed by field emissionscanning electron microscope (FE-SEM, Zeiss/Leo Gemini 982) operated at 5 kV and AFM (Digital Instrument, Nanoscope IIIA) in tapping mode. The samples for FE-SEM were coated with a thin AuPd alloy film in order to avoid charging effects. Average grain sizes of square patterns were calculated as the square root of the average grain area, calculated by image analysis from sets of 5 SEM images (4 μm 3 μm).
’ ASSOCIATED CONTENT
bS
Supporting Information. Degree of swelling of homopolymers compared to triblock blend films during solvent annealing with controlled vapor pressure and additional SEM images showing effects of template heights on the long-range order of square arrays in self-assembled triblock terpolymer films. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Science Foundation, the Semiconductor Research Corporation, the UCLA FENA Center, the Office of Naval Research, and the EPSRC. The Research Laboratory of Electronics Scanning-Electron-Beam Lithography Facility provided facilities for this work. ’ REFERENCES (1) Lazzari, M.; Lopez-Quintela, M. A. Adv. Mater. 2003, 15 (19), 1583–1594. (2) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276 (5317), 1401–1404. (3) Segalman, R. A.; Yokoyama, H.; Kramer, E. J. Adv. Mater. 2001, 13 (15), 1152–þ.
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