Substrate-Independent Lamellar Orientation in High-Molecular-Weight

Jun 13, 2014 - Kyunginn Kim , Sungmin Park , Yeongsik Kim , Joona Bang , Cheolmin ... Seyong Kim , Hyun Suk Wang , Youngson Choe , Soo-Hyung Choi , Jo...
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Article pubs.acs.org/Macromolecules

Substrate-Independent Lamellar Orientation in High-MolecularWeight Polystyrene‑b‑poly(methyl methacrylate) Films: Neutral Solvent Vapor and Thermal Annealing Effect Eunyoung Choi,† Sungmin Park,† Hyungju Ahn,† Moongyu Lee,‡ Joona Bang,§ Byeongdu Lee,∥ and Du Yeol Ryu*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea LCD R&D Center, LCD Business, Samsung Display Co., 24 San, Nongseo-dong, Giheung-gu, Yongin 446-711, Korea § Department of Chemical and Biological Engineering, 5 Anam-dong, Seongbuk-gu, Korea University, Seoul 136-701, Korea ∥ X-ray Sciences Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Lamellar microdomain orientation in polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films was controlled by a solvent vapor annealing process, where the high-molecular-weight block copolymer (BCP) was used to self-assemble in a large period of 105 nm. A neutral solvent annealing with tetrahydrofuran vapor screened the difference in the surface energy between the two blocks and the interfacial interactions of the substrate with each block, leading to the substrate-independent perpendicular orientation of lamellar microdomains. Together with thermal annealing of the solvent-annealed BCP film, we demonstrate that highly ordered line arrays of perpendicularly oriented lamellae were well guided in topographic line and disk photoresist patterns composed of the PS-attractive cross-linked copolymer, where the interlamellar d-spacing compliant to the patterns was dependent on the confinement types.

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INTRODUCTION

To address this challenge, a simple blending with homopolymers was used to generate large feature sizes,17−20 while the volume fractions and molecular weights of the added homopolymers were limited by the miscibility issue with the BCP. Thomas and co-workers demonstrated a visible BCP photonic crystal using ternary blends of symmetric polystyreneb-polyisoprene (PS-b-PI)/PS/PI.14,21 They also reported highly oriented microdomain alignment in ultrahigh-molecular-weight (M > 800 kg/mol) BCP films using directional solidification with benzoic acid (BA) as a crystallizable solvent.22 We recently reported that in a high-molecular-weight (256 kg/mol) polystyrene-b-poly(methyl methacrylate) (PS-bPMMA) films on a specific neutral substrate a perpendicular orientation of lamellar microdomains could be achieved by a solvent vapor annealing process, where the saturated solvent vapor permeated and plasticized thin BCP films to elevate chain mobility and develop the periodic nanostructures or ordered arrays.23 Such simple and similar routes have also been proven to be effective in regulating the size and shape of microdomains

Block copolymer (BCP) assembly is of great interest for developing well-defined templates and scaffolds in a typical periodic scale of 10−50 nm owing to microphase separation between the blocks into a variety of nanoscopic morphologies.1−9 Its feature size scale can be dictated by an equation derived from the mean-field theory: L0 ∼ N 2/3χ1/6 for symmetric diblock copolymer in the strong segregation limit (χN ≫10.5),10 where L0, N, and χ are the equilibrium lamellar period, the degree of polymerization (or molecular weight), and the Flory−Huggins segmental interaction parameter between the two blocks, respectively. Recent advances in the BCP assembly have demonstrated nanolithographic application using tailored nanostructures routinely with sub-50 nm feature sizes of template patterns as well as many other applications.11−13 To extend its application to the optical response at the visible or infrared wavelengths that interact with the feature sizes having tenths of micrometers, high-molecular-weight (or great N) BCP can be used to get large periods close to 100 nm,14−16 while the translational ordering of long-chained BCP is seriously precluded by its very low chain mobility due to high conformational entanglement even at elevated temperatures above the glass transition temperature (Tg). © 2014 American Chemical Society

Received: April 6, 2014 Revised: May 28, 2014 Published: June 13, 2014 3969

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Figure 1. Water contact angle (WCA) (the first row) of a variety of substrates from hydrophilic Si (31.4°) to hydrophobic PS-grafted (92.2°) substrates, the SEM images (the second row) of the solvent-annealed PS-b-PMMA films on the substrates, and the SEM images (the third row) by thermal annealing of the solvent-annealed PS-b-PMMA films. A tilt-angle SEM image in the inset displayed the lamellar microdomains oriented normal to the film surface.

of lamellar microdomains was achieved by a solvent annealing process that was sequentially combined with thermal annealing process for equilibrium. We also demonstrate the wall guidance effect on perpendicularly oriented lamellae in topographic line and disk photoresist patterns as well as the interlamellar dspacing compliant to the patterns.

as well as their orientation by varying solvent affinity (or selectivity) to the blocks.24−33 The experimental and simulation studies on the cylinder-forming BCP films confined between two surfaces showed that the phase diagram of the morphologies and orientation depended on the strength of the interfacial interactions and commensurability between the equilibrium period and film thickness.34−36 It should be noted that the solvent vapor annealing process is sensitive to environmental conditions including humidity, evaporation rate of vapor, and temperature that determine the vapor pressure and quality of solvents.24,27,37−39 On the other hands, spontaneous microphase separation of the BCP in the thin film geometry resulted in preferential orientation of microdomain arrays or random orientation.40−42 A parallel orientation of lamellar and cylindrical microdomains would be favorable with preferential interactions of the substrate with one block and/or the large difference in surface energies of the two blocks. The perpendicular orientation of microdomains, however, was achievable by controlling the interfacial and surface interactions as an external field.43−47 A successful case study of the latter may be attributed to polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) system, not only because the difference in surface energies of the two blocks was little but also the substrates were readily tuned with the balanced (or neutral) interfacial interactions toward each block.43−48 The ordering and directing orientation of microdomains are nevertheless an everlasting concern to accommodate the increasing needs for a variety of long-range ordered BCP assemblies, since there are many different types of BCPs. In this paper, we used a high-molecular-weight (331 kg/mol) PS-b-PMMA to order and direct the orientation of lamellar microdomains in BCP films on a variety of substrates using a solvent vapor annealing process with tetrahydrofuran (THF) that is a neutral solvent for the two blocks of PS-b-PMMA, followed by quickly evaporating the solvent to preserve the film structure. The substrate-independent perpendicular orientation

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EXPERIMENTAL SECTION

A symmetric PS-b-PMMA copolymer was synthesized by sequential anionic polymerization of styrene and methyl methacrylate in tetrahydrofuran (THF) solvent, which was performed at −78 °C in the presence of LiCl (high purity, Aldrich) under purified argon. A secbutyllithium was used as an initiator. The number-averaged molecular weight (Mn), as characterized by size-exclusion chromatography (SEC), was 331 kg/mol with a narrow dispersity (less than 1.05). The volume fraction of PS (ϕPS) in the BCP was determined to be 0.500 by 1H nuclear magnetic resonance (1H NMR), based on the mass densities of the two components (1.05 and 1.184 g/cm3 for PS and PMMA, respectively). The equilibrium period (or interlamellar spacings, L0) of the BCP was evaluated to be 105 nm by the GISAXS pattern for a thermally annealed film. The PS-b-PMMA films were prepared by spin-coating at typically 3000−5500 rpm for 60 s; 2−3 wt % BCP solutions in toluene were applied to the substrates to control the film thickness of 2L0 (210 nm). The film thicknesses were measured by spectroscopic ellipsometry (SE MG-1000, Nano-view Co.) at an incidence angle of 70.5°. The topographic line and disk patterns were fabricated on an underlying oxide (2 nm) layer of Si substrate using an I-line photolithography process with a negative photoresist (SU-8; Microchem). Tetrahydrofuran (THF; high purity, Aldrich) was used for solvent vapor annealing of PS-b-PMMA films. A cylindrical brass chamber was devised to set the volume (V = 706.5 cm3) and surface area (S = 78.5 cm2) of the solvent, where the solvent uptake in the films was precisely regulated by the annealing time under the condition of S/V = 0.111 cm−1. The chamber was completely sealed by Teflon cap with Chemraz (Greene Tweed Co.) O-ring, and the temperature in a closed chamber was set at constant 10 °C for time control experiment. The thermal annealing condition for the BCP films (only after solvent annealing process with THF vapor) was optimized at 230 °C for 3 h 3970

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independence in thin BCP films also indicated that the structural development and microdomain orientation were propagated from top surface at air/polymer interface in contact with solvent vapor rather than polymer/substrate interface. However, some topological features like wrinkles or ridges in the solvent-annealed PS-b-PMMA films were generated by the reduction of film thickness due to the fast evaporation of solvent. At the second stages of thermal annealing process at 230 °C for 3 h, the surface ordering structures of BCP films were preserved and topologically flattened, as shown in the third row of Figure 1 (will be discussed later). Solvent absorption for PS-b-PMMA film was evaluated at constant 10 °C using in situ spectroscopic ellipsometry, as shown in in Figure 2 as a function of annealing time. The

under vacuum to reduce the possible thermal degradation at higher temperatures because no further change in d-spacing was observed for longer than 3 h. To enhance the phase contrast between the two blocks in PS-b-PMMA films, asymmetric dry (or plasma) etching (VITA, Femto Sci.) mode was operated with an O2/Ar (5/1 in volume ratio) mixture by a RF power of 100 W at 150 mTorr and 18 sccm. Grazing-incidence small-angle X-ray scattering (GISAXS) experiments were performed at the 12-ID-B beamline of the Advanced Photon Source (APS) at Argonne National Laboratory and the 9A beamline at Pohang Accelerator Laboratory (PAL), Korea. The operating conditions were chosen as a wavelength of 1.34 Å and a sample-to-detector distance of 3.68 m. To probe internal film structures, incident angle (αi) was set at 0.170° above the critical angle (0.140°) for PS-b-PMMA films. 2D GISAXS patterns were recorded by using a 2D detector (Pilatus 2M, DECTRIS Ltd.) positioned at the end of a vacuum guide tube with an exposure time of 10 s. To examine the surface morphology of PS-b-PMMA films, scanning force microscopy (SFM; Dimension 3100, Digital Instrument Co.) was operated in the tapping mode. The standard silicon nitride probes were used at 3% offset below their resonance frequencies ranging 250−350 kHz. Height and phase images were taken at a scanning speed of 7 μm/s. The SEM images of dry-etched PS-bPMMA film were obtained with field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL) under an accelerating voltage of 5.0 kV using a semi-in-lens detector.

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RESULTS AND DISCUSSION A desired control of nanoscopic structures in thin BCP films has required tuning the surface or interfacial properties on the substrates that pertains to wetting and dewetting the polymer films. For this reason, many efforts have been dedicated to attempts to characterize the interfacial interactions of the substrates, although a notable drawback to these approaches was impeded by the different surface-specific chemistries of a wide range of substrates.43−49 We first used a variety of substrates with varying surface properties from hydrophilic to hydrophobic substrates, as displayed in Figure 1 by water contact angle (WCA). On a variety of substrates, the surface ordering and microdomain orientation in PS-b-PMMA (331 kg/mol) films with the film thickness of 2L0 (210 nm) were studied when the films on the substrates were subjected to a solvent vapor annealing process with THF. During a solvent annealing process, the saturated solvent vapor in a closed chamber permeates into thin BCP films and elevates the chain mobility for an optimized time, then quickly evaporating the solvent to preserve the film structure. Figure 1 demonstrates the substrate-independent perpendicular orientation of lamellar microdomains in BCP films on the substrates that vary from hydrophilic Si (31.4°) to hydrophobic PS-grafted (92.2°) substrates based on WCA. For the BCP films on underlying layers from the PMMA-selective oxide of Si substrate to PS-grafted substrate, the in-plane scanning electron microscopy (SEM) images showed the long stripe patterns, and a representative tilt-angle SEM image in inset displayed the lamellar microdomains oriented normal to the film surface. Here, the brighter and darker phases correspond to the PS and PMMA blocks, respectively, due to the etch contrast between the two blocks after asymmetric dry (or plasma) etching with an O2/Ar (5/1 in volume ratio) mixture. It is worthwhile to point out that this neutral solvent annealing process enabled the lamellar microdomains of high-molecular-weight BCP to be oriented normal to the film surface regardless of the substrate types, which was caused by screening the difference in the surface energy between the two blocks and the interfacial interactions of the substrates with each block. The substrate

Figure 2. Normalized thickness (t/t0) or swelling ratio of PS-b-PMMA film and the calculated χeffN as a function of annealing time with a neutral solvent vapor. Solvent absorption was measured at constant 10 °C using in situ spectroscopic ellipsometry.

normalized thickness (t/t0) dictates the swelling ratio of BCP films with a neutral solvent vapor. For a given neutral solvent, the effective interaction parameter (χeff) can be diminished by

χeff = χ Φ due to the screening effect of the solvent molecules incorporated at the interfaces between the two blocks,50 where χ = 0.0425 + 4.046/T for PS-b-PMMA,51 and the average volume fraction (Φ) of polymer in the solution was evaluated by Φ = t0/t. A rapid swelling kinetics for the first ∼25 min indicated that the solvent vapor uptake occurred efficiently at early stage to facilitate the ordering of long-chained BCP along with a significant decrease in χeffN for 25 min from 184.5 to 63.1. Over 25 min of solvent annealing process, the swelling kinetics was gradually retarded through a state of t/t0 ≈ 3.5 that corresponds to χeffN = 53.3. However, the diminished χeffN during solvent annealing process was still far from a disordered phase at χeffN ∼ 10.495, indicating that the swollen BCP film was in an ordered lamellar morphology and the chain mobility was significantly accelerated without losing the strong driving force to segregate between the two blocks. Therefore, this approach to high-molecular-weight BCP films should be distinguished from an ordering process by the concentrationgradient field of solvent with low-molecular-weight BCP films.25,27 3971

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Figure 3. SFM phase images for (a−d) the solvent-annealed PS-b-PMMA films for each time and (e) thermally annealed film at 230 °C for 3 h with (d) sample; (f−j) the corresponding GISAXS images. The S and TS denote to the solvent annealing and thermal annealing of the solvent-annealed PS-b-PMMA film.

Figure 4. (a) Line intensity scanned along 2θf at αf = 0.147° from the corresponding GISAXS patterns for PS-b-PMMA films. (b) The d-spacing (d = 2π/q*) for PS-b-PMMA films as a function of solvent annealing time. The d-spacing by thermal annealing at 230 °C for 3 h was indicated by closed dots with arrows.

b-PMMA films, as shown in Figure 3f−j. In the grazing incidence scattering geometry, αf and 2θf are the exit angles of X-ray beam along the out-of-plane scattering normal to the sample surface, and along the in-plane scattering parallel to the sample surface, respectively. Here, q = (4π/λ) sin θf is the inplane component of the scattering vector, where λ is the wavelength and θ is the scattering angle. The incident angle (αi) was set at 0.170° above the critical angle (0.140°) for PS-bPMMA films to ensure the structural information across the entire film thickness. With increasing solvent annealing time, the in-plane scattering for the BCP films as well as the Bragg rods at 2θf = 0.084° (q = 0.069 nm−1) along the out-of-plane scattering were significantly enhanced, and their weak higherorder peaks became discernible along the horizon of αf = 0.140°. It was also plotted in detail in Figure 4a by the line scan intensities from the corresponding GISAXS patterns. These results indicated that the solvent annealing process with neutral solvent led to the development of the perpendicular orientation

Figure 3a−d presents scanning force microscopy (SFM) phase images for the solvent-annealed PS-b-PMMA films, where the large-area (2 × 2 cm2) films on a flat Si substrate were set to a thickness of 2L0 (210 nm). An as-cast film revealed a poorly or less ordered structure, but the timedependent morphology of the swollen BCP films exhibited the structural development from short to long stripe patterns of the lamellar microdomains oriented normal to the film surface. At late stage of solvent annealing time for 40 min, the translational ordering of perpendicularly oriented lamellae could be attributed to high χeffN ≈ 55.6 ≫ 10.5 still in the strong segregation limit (SSL). When the solvent-annealed PS-bPMMA film (for 40 min) was thermally annealed (at 230 °C for 3 h), the phase morphology was preserved with the topology being flattened, while the interlamellar spacing was increased, as shown in Figure 3e. We performed grazing-incidence small-angle X-ray scattering (GISAXS) measurement to probe the internal structure of PS3972

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Figure 5. In-plane SEM images for PS-b-PMMA films confined in 1.5 μm wide topographic line trench patterns. Each solvent-annealed film was thermally annealed at 230 °C for 3 h, as displayed up and down. The phase contrast between the two blocks was enhanced with dry (or plasma) O2/ Ar (5/1 in volume ratio) etching.

with a movement in q* toward lower q region from 0.078 to 0.069 nm−1 due to the accelerated chain mobility from the spincast frozen state of BCP films. For further solvent annealing through 25 min, a constant d-spacing of 92.0 nm was maintained, which is a quasi-equilibrium d-spacing of semidiluted BCP with a neutral solvent. This value was still smaller than the equilibrium d-spacing (105 nm) due to the screening effect of the solvent molecules incorporated at the interfaces between the two blocks.54 On thermal annealing of the solventannealed films, however, an entire recovery to the equilibrium d-spacing was seen in the 35 and 40 min solvent-annealed films, while a less increased d-spacing (∼98.5 nm) was observed in the early stage (less than 25 min) solvent-annealed films due to the insufficient translational ordering during solvent annealing process, as indicated by the closed dots in Figure 4b. This thermal annealing effect indicated from this experiment that the ordering process of high-molecular-weight PS-b-PMMA films was dominated by the solvent annealing process with neutral solvent, and only short-range segregation between the two blocks was derived by thermal annealing. The above combinational process was applied to PS-bPMMA films in topographic confinements in order to guide the lateral and vertical alignment of lamellar microdomains. A negative type SU-8 photoresist (Microchem) was employed to fabricate topographic patterns on an underlying oxide (2 nm) layer of Si substrate using an I-line photolithography process, where the trench walls of cross-linked photoresist are PSattractive presumably due to the molecular similarity in aromatic structures between the PS block and SU-8. Figure 5 displays in-plane SEM images for PS-b-PMMA films confined in 1.5 μm wide topographic line trench patterns, where the phase contrast between the two blocks was enhanced with dry (or plasma) O2/Ar (5/1 in volume ratio) etching. Here, the ratio C/L0 corresponds to ≈14.2, where the confinement width C is the distance across trench walls and L0 = 105 nm. As shown in Figure 5a−f, an as-cast film in a line trench pattern indicated a poorly or less ordered structure, which was very similar to that on a flat Si substrate (Figure 3a). The swollen films with increasing solvent annealing time exhibited the

of lamellar microdomains. It was also relatively consistent with the GISAXS patterns measured at αi = 0.120° (Figure S1) targeting near the film surface, indicating the structural development of overall film thickness during solvent annealing, rather than the concentration-gradient of solvent. However, a couple of tiny arc-shaped bendings (corresponding to partial Debye−Scherrer ring) indicated a little distribution of slightly tilted lamellar microdomains deviated from perpendicularly oriented lamellae in the interior film.52 For the GISAXS pattern of thermally annealed (at 230 °C for 3 h) PS-b-PMMA film through solvent annealing process for 40 min, this imperfection was more discernible, as shown in Figure 3j. It could be attributed to the fact that thermal energy is not favorable with the perpendicular orientation of lamellar microdomains on an underlying PMMA-selective oxide layer. Nevertheless, the preserved microdomain orientation and morphology were a consequence of only enhanced shortrange segregation by thermal annealing in close proximity to interfaces between the two blocks. Figure 4a shows the line intensity scanned along 2θf at αf = 0.147° from the corresponding GISAXS patterns. With increasing solvent annealing time, the intensities of the primary peak (at q*) increased with a distinction of the higher-order peaks (at q/q* = 1:2:3). When the solvent-annealed PS-bPMMA film (for 40 min) was thermally annealed, the q* shifted toward lower q region, indicating an increase in interlamellar d-spacing. The disappearance of the secondorder peak in a thermally annealed film (TS 40 min) was caused by the volumetric symmetry between the two blocks in the thermally equilibrated BCP. The d-spacing (d = 2π/q*) for PS-b-PMMA films during solvent annealing and subsequent thermal annealing process is plotted in Figure 4b as a function of annealing time. In the PS-b-PMMA films prepared from BCP solutions in toluene, a smallest d-spacing (80.4 nm) of as-cast film was presumably caused by the kinetic nonequilibrium by fast evaporation of solvent, where the BCP chains were frozen or kinetically trapped.53 Upon solvent annealing for 25 min, the dspacing gradually increased from 80.4 to 91.9 nm in accordance 3973

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Figure 6. (a) Large-area SEM image for PS-b-PMMA film confined in 1.5 μm wide topographic line trench patterns by the combinational process. (b) Averaged defect density (ρ) of PS-b-PMMA films on a flat Si substrate and in a 1.5 μm wide topographic line trench pattern as a function of solvent annealing time, where the defect density with combinational process was indicated by the closed symbols.

of perpendicularly oriented lamellae in a line trench pattern, which is consistent with C/L0 ≈ 14.2. It is worthwhile to point out that a large-area directed self-assembly of high-molecularweight PS-b-PMMA could be achieved by the combinational process, although the underlying oxide layer on the bottom was non-neutral and PMMA-selective, as demonstrated in Figure 6a by a SEM image. The defects including dislocation and disclination (terminal point, junction, and dot) were analyzed with the automatic triangulation program by the neighboring defect distances in an area of 2.5 × 2.5 μm2 for the in-plane morphologies of BCP films. Figure 6b shows the averaged defect density (ρ) of PS-bPMMA films on a flat Si substrate and in a 1.5 μm wide topographic line trench pattern as a function of solvent annealing time, where the defect density with combinational process was indicated by the closed symbols. As the swelling kinetics was accelerated by solvent absorption into BCP films, the defect density of BCP films decreased rapidly during solvent annealing to 35 min due to the defect annihilation by coarsening of the short stripe lines of perpendicularly oriented lamellae. With further solvent vapor annealing to 50 min, the defect density diminished to less than 5 μm−2. The defects in a trench pattern remarkably decreased more than that on a flat Si substrate because of the PS-attractive wall guidance or confinement effect that enabled the line arrays to be laterally aligned parallel to trench walls. It should be also pointed out that the defect densities upon further thermal annealing were consistent with those on solvent vapor annealing, confirming

structural development with longer stripe patterns of lamellar microdomains. Furthermore, the line arrays of perpendicularly oriented lamellae on neutral substrate-free condition were laterally aligned along both trench walls due to the guiding effect. It was speculated that the cross-linked copolymer of PSattractive photoresist was not permeable to the saturated solvent vapor, while the difference in the surface energy between the two blocks and the interfacial interactions of the substrates with each block were efficiently mediated by the neutral solvent vapor. The BCP films were then thermally annealed at 230 °C for 3 h immediately after taking them during solvent annealing process, as shown in Figure 5g−l. The surface ordering structures at the solvent-annealed states were preserved at the second stages of thermal annealing process, indicating that the molecular weight of BCP was too high to overcome the local free energy barriers associated with the translational ordering. The effective lateral alignment of perpendicularly oriented lamellae by the guiding effect was developed during the first solvent annealing step. However, through thermal annealing process, their morphologies in a line trench pattern were remarkably improved by flattening, straightening, and contrasting between the two blocks. This result also indicated that the second thermal annealing step was a complementary process to relieve the residual strains in BCP films for topological stabilization arising from the enhanced short-range segregation between the two blocks. As shown in Figure 5l, an optimal combinational (solvent + thermal annealing) process with the BCP film led to the distinct 14 (or 15 in minor) PS line arrays 3974

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Figure 7. (a) In-plane SEM images for PS-b-PMMA films confined in 3.0 μm diameter topographic disk pattern. (b) Interlamellar spacing (or dspacing) as a function of confinement distance that denotes to the center-to-wall and wall-to-wall distances in the disk and line patterns, respectively.

Interestingly, the line width in a 3.0 μm disk pattern was found to be varied with increasing radius, which is similar to a zone-plate-like concentric nanopattern. Figure 7b shows the interlamellar spacing (or d-spacing) as a function of confinement distance that denotes to the center-to-wall and wall-towall distances in the disk and line patterns, respectively. The dspacing in a disk pattern gradually decreased from the equilibrium d-spacing (105 nm) to 83 nm at 1.3 μm radius and then remarkably diminished to 72.3 nm near the end wall. On thermal annealing of the solvent-annealed BCP films in a confinement pattern, the chain stretching of lamellar microdomains toward equilibrium d-spacing (105 nm) by the shortrange segregation between the two blocks was hampered in the limited space normal to the lamellar plane (or parallel to the wall), and it was minimized near trench walls of the disk and line patterns. Thus, the significant decrease in d-spacing near trench walls was attributed to this frustration effect on the expansion of lamellar d-spacing during thermal annealing process. In a topographic line trench pattern, a relatively consistent d-spacing ∼105 nm in the middle (∼90%) area of the line arrays indicated that the residual strains in the BCP were mostly relieved particularly in a direction parallel to the lamellar plane. However, the frustration effect in the disk pattern was placed in all radial directions normal to the lamellar plane (or parallel to the wall), leading to an asymmetrically contracted d-spacing of perpendicularly oriented lamellae.

that most of defect annihilation (or lateral ordering) occurred during the solvent vapor annealing step with a neutral solvent. For further consideration, we examined the ordering fidelity of lamellar microdomains in a specific curvature condition. A negative disk pattern in 3.0 μm diameter was fabricated with the SU-8 on an underlying oxide layer of Si substrate. The effective solvent annealing time for the lateral ordering was extended to 90 min, which would be caused by higher free energy barrier associated with the translational ordering in a curvature geometry.55 The solvent-annealed PS-b-PMMA films in this topographic disk pattern were thermally annealed at 230 °C for 3 h, leading to the concentric patterns of perpendicularly oriented lamellae, as shown in Figure 7a by in-plane SEM images. The 18 and 17 PMMA-block (darker) rings were seen in the PMMA-block and PS-block centered images, respectively, as similarly observed in the literatures,56,57 although the ratio C/L0 was 14.2. The outermost brighter phase in the SEM images corresponds to the PS block along the end walls of SU-8 due to the attractive interactions with the PS block, indicating that during solvent annealing process the lateral alignment of perpendicularly oriented lamellae initiated from the PSattractive wall guidance in the patterns and propagated into the center that locates either of the two blocks. It was also suggested that the favorable wall interactions were greater than the free energy penalty for bending lamellae in a disk pattern.55 3975

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CONCLUSIONS Using a solvent vapor annealing process with THF that is a neutral solvent for the two blocks, the ordering process and microdomain orientation in high-molecular-weight PS-bPMMA films on a variety of substrates were studied. This solvent annealing process screened the difference in the surface energy between the two blocks and the interfacial interactions of the substrates with each block, efficiently enabling the lamellar microdomains of BCP to be oriented normal to the film surface, irrespective of the substrate types. We examined the structural development of lamellar morphology during solvent annealing that was sequentially combined with thermal annealing process for equilibrium, demonstrating that the second thermal annealing step was a complementary process to relieve the residual strains in BCP films for topological stabilization arising from the enhanced short-range segregation between the two blocks. During a solvent annealing process, the highly ordered line arrays of perpendicularly oriented lamellae guided in topographic line and disk patterns on non-neutral substrate were presumably attributed to the PS-attractive property of crosslinked photoresist and the impermeability to the saturated solvent vapor. The defect-free line arrays of perpendicularly oriented lamellae could be achieved by the combinational process in a line trench pattern as a feasible route to a large-area directed self-assembly of high-molecular-weight PS-b-PMMA. In contrast, an asymmetrically contracted and concentric nanopattern of perpendicularly oriented lamellae was attributed to the frustration effect on the radial expansion of lamellar dspacing during thermal annealing process in a disk pattern with curvature geometry. This strategy developed in the present study suggests a simple and efficient route for directing orientation of microdomain arrays in the thin films of BCP selfassembly with large feature sizes having tenths of micrometers and for further applications to nanolithographic pattern transfer to target materials.

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ASSOCIATED CONTENT

S Supporting Information *

GISAXS patterns measured at αi = 0.100° and 0.170°, the line scan intensities along 2θf at αf = 0.147° from the corresponding GISAXS patterns, and the d-spacing (d = 2π/q*) comparison (Figure S1). 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] (D.Y.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nuclear R&D Programs, APCPI ERC (2007-0056091), and Converging Research Center (2010K001430) funded by the Ministry of Science, ICT & Future Planning (MSIP), Korea. We also acknowledge the partial support from Samsung Display Co., Korea.

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