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Optimized Solvent Vapor Annealing for Long-Range Perpendicular Lamellae in PS‑b‑PMMA Films Kyunginn Kim,† Sungmin Park,† Yeongsik Kim,† Joona Bang,§ Cheolmin Park,‡ and Du Yeol Ryu*,† †

Department of Chemical and Biomolecular Engineering and ‡Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea § Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Korea S Supporting Information *

ABSTRACT: We report a simple approach to fabricating highly stable, perpendicularly oriented lamellae through the self-assembly of high-molecular-weight polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA). Using a solvent vapor annealing (SVA) process controlled by modulating the temperature gap between the chamber and bottom plate, followed by a subsequent thermal annealing process, the ordering stability of perpendicularly oriented lamellae was investigated. The temperature gap that regulates the solvent absorption and dewetting times of the swollen block copolymer (BCP) films was adjusted to enable the films to wet a neutral substrate, thereby facilitating ordering and structural development with no ordering failure. An optimized SVA process at a selected temperature gap of 5 °C was found to display the excellent long-term stability, at which highly ordered line arrays composed of perpendicularly oriented lamellae were confined to large-width (3.2 μm) topographic line patterns. This study also provides the effective time window for the SVA process, suggesting a simple and efficient route to fabricating long-range lateral ordering in BCP self-assembled structures.

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the SVA process with toluene/THF mixtures.30,31 Ross and coworkers used toluene/n-heptane mixtures to generate a range of morphologies different from the equilibrium morphology in polystyrene-b-polydimethylsiloxane (PS-b-PDMS) films.32−34 Hillmyer and co-workers also investigated the structural transition of polystyrene-b-polylactide (PS-b-PLA) films using THF as a neutral solvent.35,36 Besides, the SVA of highmolecular-weight BCP on an intermediate nanometer scale (100−200 nm) has recently been highlighted as being useful in optical applications for photonics and polarizers that require the period sizes exceeding 100 nm so as to interact with visible or infrared wavelengths.37,38 Although it formed highly entangled conformations of BCP, the SVA processes that enable the solvent permeation into entangled chains could accelerate the chain mobility by reducing the effective Tg below room temperature, leading to the desired morphologies by regulating parameters such as the chamber volume, temperature, solvent quality, vapor pressure, and annealing time.39−44 Another advantage to applying the SVA processes to highmolecular-weight BCP is that the self-assembly maintains high χN (≫10.5) in an ordered state, which can act as a driving force for ordering during the processes.44,45 Preceding approaches to achieving a desired structure in high-molecular-weight BCPs include the efforts of Thomas and co-workers, who demonstrated a visible BCP photonic crystal

INTRODUCTION Block copolymer (BCP), composed of dissimilar polymers and covalently linked together, self-assembles into periodically ordered structures with tens-of-nanometer feature sizes.1,2 These BCPs have attracted great attention for their utility as nanotemplates and scaffolds. A variety of nanoscopic morphologies, including lamellar, cylindrical, spherical, gyroid, and hexagonally perforated layer (HPL) structures, can be dictated by χN as a function of composition, where χ and N are the Flory−Huggins segmental interaction parameter between the two blocks and the total number of segments, respectively.3,4 Nanoscopic arrays or patterns prepared by BCP selfassembly have been used to meet the increasing demand for many applications such as nanopatterning, nanoporous membranes, drug delivery vehicles, nanoparticles templates, and nanolithography.5−9 Nevertheless, the simple and feasible implementation processes that provide a well-defined regular periodicity remain challenging because the BCP self-assembly naturally generates the defects like dislocations and disclinations (terminal points, junctions, and dots).10−13 Many efforts have been made for controlling and directing the orientations of microphase-separated BCP domains over large areas, using an external field,14−16 solvent evaporation,17−20 graphoepitaxy,21−23 and alignment on lithographically patterned substrates.24−29 For the solvent vapor annealing (SVA), Russell and coworkers reported the controlled cylindrical morphologies in polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) films using © XXXX American Chemical Society

Received: October 5, 2015 Revised: December 28, 2015

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DOI: 10.1021/acs.macromol.5b02188 Macromolecules XXXX, XXX, XXX−XXX

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A neutral substrate was prepared using hydroxyl end-functionalized poly(styrene-r-methyl methacrylate) (HO-P(S-r-MMA)) with a styrene mole fraction (XS) ∼ 0.55 and Mn = ∼15 kg/mol with a dispersity (Đ) of 1.30. The PS composition of 55 mol % was turned out to balance the interfacial interactions to orient the lamellar microdomains normal to the substrate.48 The surface modifications to enable the grafting reactions onto the substrates were accomplished by thermally annealing the thin films of HO-P(S-r-MMA) on standard Si wafers at 170 °C for 3 days under vacuum, leading to the anchoring of random copolymer chains (∼5.5 nm) onto the substrate after thoroughly rinsing with toluene. Tetrahydrofuran (THF; high purity, Aldrich) was used for the SVA process applied to 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 solvent, where the solvent absorption of BCP films was precisely monitored over the annealing time under conditions of S/V = 0.111 cm−1. The chamber was completely sealed with a Teflon cap using a Chemraz (Greene Tweed Co.) O-ring, and the reference temperature in the closed chamber was set to 20 °C for the timecontrolled experiments. The bottom plate of the chamber was preheated to set temperatures at 23, 25, 28, and 30 °C prior to placing the BCP samples onto the sample stage. The SVA experiments were then performed at various temperature gaps between the reference temperature (20 °C) and bottom temperatures. The thermal annealing condition for BCP films (applied after the SVA process under THF vapor) was 230 °C for 6 h under vacuum to reduce the possible thermal degradation at higher temperatures because no further changes in the d-spacing were observed over periods longer than 6 h. Grazing-incidence small-angle X-ray scattering (GISAXS) and SAXS experiments were performed at the 9A and 4C beamlines, respectively, at the Pohang Accelerator Laboratory (PAL), Korea. The operating conditions involved a wavelength of 1.11 Å and a sample-to-detector distance of 6.462 m. To probe internal film structures, an incidence angle (αi) was set to 0.140° above the critical angle (0.114°) of PS-bPMMA films. 2D GISAXS patterns were recorded using a 2D detector (SX 165, Rayonix) positioned at the end of a vacuum guide tube with an exposure time of 10 s. SEM images of the dry-etched PS-b-PMMA film were measured using field emission scanning electron microscopy (FE-SEM; JSM-7001F, JEOL) under an accelerating voltage of 15.0 kV and a semi-in-lens detector. The phase contrast between the PS and PMMA blocks was enhanced by applying asymmetric dry (or plasma) etching (VITA, Femto Sci.) that rapidly etched away the PMMA block, operated with an O2/Ar mixture (in a 5/1 volume ratio) under an RF power of 100 W at 150 mTorr and 18 sccm. The topographic line patterns were fabricated on a neutral substrate using an I-line photolithography process with a negative photoresist (SU-8; Microchem).

using the ternary blends of polystyrene-b-polyisoprene (PS-bPI) and the respective PS/PI homopolymers.38 Thomas et al. also reported a tunable photonic crystal behavior using polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films, in which the periodic lamellar microdomains oriented parallel to the substrate were prepared using a SVA process with a neutral solvent.46 Symmetric polystyrene-b-polyisoprene (PS-b-PI) was examined by Okamoto and co-workers to characterize the solvent quality effects from a neutral to selective to one block, as an approach to tuning photonic crystal properties.47 Recently, we used the SVA process with a neutral solvent to high- and ultrahigh-molecular-weight polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films coated on a neutral substrate in order to direct the orientation of large-period lamellar microdomains (exceeding 100 nm) in BCP films.44 The perpendicularly oriented lamellae displayed a long-range ordered structure during the SVA process applied over large areas and a thermal annealing step. However, the desired morphology, of high ordering quality, was achieved over a narrow annealing period (5−10 min) under solvent vapor, since the SVA process need to cease immediately before the saturated BCP films begin to fluctuate and dewet the substrate. This narrow processing period impeded practical applications to continuous industrial processes, particularly during the fabrication of in-cell polarizers and liquid crystal displays (LCDs). To address this issue on the time-controlled SVA processes applied to BCP self-assembly, it is crucial to ensure the long-term stability of a long-range ordered structure simply by suppressing the early stage film dewetting. Herein, we introduce an optimized SVA process controlled by modulating the temperature gap that regulates the solvent absorption and dewetting times of the swollen BCP films. We attempted to achieve a long annealing time that produces longrange lateral ordering in BCP self-assembled structures. Together with the thickness-independent perpendicular orientation of lamellar microdomains using a combination process (SVA + thermal annealing), we defined the ordering stability of PS-b-PMMA films in large-width topographic line patterns, which was evaluated with respect to the annealing time under solvent vapor, thereby leading to a plot of the effective time window for producing highly ordered line arrays of perpendicularly oriented lamellae as a function of temperature gap. These results provide informative guidelines for longterm stable, long-range ordered perpendicular lamellae using the self-assembly of high-molecular-weight PS-b-PMMA.

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RESULTS AND DISCUSSION

Lamella-forming PS-b-PMMA films were spin-coated onto a neutral substrate that balances the interfacial interactions toward the two blocks, where an end-functionalized random copolymer was used to generate the neutral brushes anchored to the surface. The BCP films were subjected to a SVA process with THF as a neutral solvent.44 During the SVA process, the solvent vapor in a closed chamber permeated BCP films and significantly enhanced the microphase ordering by elevating the chain mobility of the swollen BCP films.49 A notable drawback of this approach is that the BCP films saturated with solvent vapor fluctuated and dewetted during the early periods of the SVA process; this ordering failure seriously impeded practical applications of BCP self-assembled structures. Typically in time-controlled SVA processes, nevertheless, the high-quality ordered BCP films were obtained by trial and error, once the processes had been optimized to identify a narrow and limited period immediately prior to film dewetting. The difficulties associated with the early appearance of dewetting could be

EXPERIMENTAL SECTION

A symmetric PS-b-PMMA copolymer was synthesized by the sequential anionic polymerization of styrene and methyl methacrylate in tetrahydrofuran (THF) solvent; this reaction was performed at −78 °C in the presence of LiCl (high purity, Aldrich) under purified argon using sec-butyllithium as an initiator. The number-averaged molecular weight (Mn), as characterized by size-exclusion chromatography (SEC), was 232 kg/mol with a narrow dispersity (Đ = Mw/Mn) less than 1.05. The volume fraction of PS (ΦPS) in 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 period (or interlamellar spacings, L0) of the BCP was measured to be 88.4 nm based on the GISAXS patterns obtained from the thermally annealed films. The PS-b-PMMA films were prepared by spin-coating typically at 2000−4500 rpm for 60 s using 1−10 wt % BCP solutions in toluene. The film thicknesses were measured by spectroscopic ellipsometry (SE MG-1000, Nanoview Co.) at an incidence angle of 70.0°. B

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thickness (t/t0) at various temperature gaps, as measured by in situ spectroscopic ellipsometry, corresponded to the extent of solvent absorption of PS-b-PMMA films as a function of annealing time under solvent vapor. The swelling ratio of PS-bPMMA film at the reference temperature (20 °C) rapidly increased to t/t0 ≈ 3.4 during the early stage annealing over 30 min and then it gradually slowed at longer annealing times up to 50 min prior to film dewetting. As the bottom temperature increased, the swelling ratios of PS-b-PMMA films similarly increased for 10 min, but each plateau at the late stages diminished discretely as the temperature gap increased. Particularly at the late stages, an increase in the temperature gap led to a decrease in the solvent absorption of BCP films due to vapor pressure difference, making the balance between the solvent evaporation and absorption processes in the BCP films. Therefore, the plateau thickness (t/t0) of quasi-equilibrium swelling ratio decreased with increasing temperature gap, as shown in Figure 2b. A noteworthy feature in this system is that the film dewetting was remarkably suppressed in PS-b-PMMA films at temperature gaps of 3, 5, 8, and 10 °C, whereas the films at the reference temperature of 20 °C dewetted the substrate after 50 min. Based on the dilution approximation with the volume fraction (Φ) of polymer in the solution, by Φ = t0/t, the effective interaction parameter (χeff) during the SVA process was calculated by χeff = χΦ due to the elevated screening effects of the solvent molecules incorporated into the interfaces between the two blocks.51 Figure 2c presents χeffN at various temperature gaps as a function of annealing time under solvent vapor, where χ = 0.0425 + 4.046/T was applied for a symmetric PS-b-PMMA.52 As the χeff was inversely proportional to the swelling ratio of BCP films, the values of χeffN decreased remarkably from 98.8 to less than 50 during the early stage SVA process (within 10 min). Intriguingly, the values of χeffN obtained at longer annealing times remained still above 10.5 for an order-to-disorder transition, indicating that during the SVA process the PS-b-PMMA films annealed at the temperature gaps tested here experienced a strong driving force to form an ordered lamellar morphology in the strong segregation limit (SSL; where χeffN ≫ 10.5 in an ordered state). The surface ordering for PS-b-PMMA films was investigated during the SVA process conducted at various temperature gaps. Figure 3 shows scanning electron microscope (SEM) images of 190 nm thick PS-b-PMMA films, where the solvent-annealed films at the reference (20 °C) and elevated temperature of 25 °C were thermally annealed at 230 °C for 6 h under vacuum. Although the structural development of BCP films occurred during the SVA process, the morphology or ordered structure was set immediately after rapid solvent evaporation. The surface topology was flattened and stabilized during thermal annealing of the solvent-annealed films due to the enhanced short-range segregation at the interfaces between the two blocks. In contrast, only thermal annealing process even at higher temperatures (∼240 °C) produced a less-ordered structure because the molecular weight was too high to overcome the local free energy barriers associated with the ordering and defect annihilation, indicating the necessity for the SVA process in high-molecular-weight BCP.44 The as-cast film (Figure 3a) displayed a poorly ordered structure, but the timedependent ordered structures (Figure 3b−d) of the swollen BCP films at the reference temperature of 20 °C indicated rapid structural development from short to long striped patterns of the lamellar microdomains oriented normal to the substrate (as

mitigated by lengthening the annealing time without dewetting, which ensured the long-term stability of long-range ordered self-assembled structures. For this purpose, we controlled the vapor pressure difference of a neutral solvent in the closed chamber using a temperature gap system that regulates the solvent absorption and dewetting times of the swollen BCP films. It should be pointed out that Elbs and Krausch demonstrated the controlled solvent absorption of polymer films using the temperature gap system to determine χ between the polymers and solvent.50 Figure 1 depicts the chamber device used for the SVA process, in which the BCP films were placed onto the sample

Figure 1. Schematic illustration of a chamber device for the SVA process, where the temperatures on the bottom plate were set to 23, 25, 28, and 30 °C under a chamber temperature (T1) of 20 °C (the reference temperature).

stage close to the bottom plate. The temperatures on the bottom plate were set to 23, 25, 28, and 30 °C under a chamber temperature (T1) of 20 °C (the reference temperature); these temperatures were assumed to be identical with the sample temperature. On the basis of Fick’s law, the vapor diffusion flux (J) can be given by p(P2 − P1) (1) L due to vapor pressure difference (ΔP = P2 − P1), where p is the permeability coefficient and L is the diffusion length that is assumed to be a distance between the top and bottom plate. This equation is further modified using Clausius−Clapeyron equation by J=−

⎫ ⎧ ⎛ ΔH ⎛ 1 ⎪ ⎪ 1 ⎞⎞ vap exp⎜⎜ − J ∼ −⎨ ⎜ − ⎟⎟⎟ − 1⎬ ⎪ ⎪ R ⎝ T2 T1 ⎠⎠ ⎭ ⎩ ⎝

(2)

where ΔHvap and R are the evaporation enthalpy (33 000 J/mol at T1 = 293 K) of THF and the ideal gas constant, respectively. Since an increase in bottom temperatures (T2, hereafter referred to as the temperature gap) by 3, 5, 8, and 10 °C was applied to the system, the vapor pressure difference (ΔP) between the top (P1) and bottom plate (P2) made the flux increased in upward (− sign) direction from the bottom plate, leading to a decrease in the solvent absorption of polymer film that is close to the bottom plate. Figure 2a shows the swelling ratio of a 190 nm thick PS-bPMMA film annealed with THF in the closed chamber with bottom temperatures (T2) by 3, 5, 8, and 10 °C under the reference temperature (T1) of 20 °C. The normalized film C

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Figure 2. (a) Normalized thickness (t/t0) of a 190 nm thick PS-b-PMMA film annealed with THF in the closed chamber with bottom temperatures (T2) by 3, 5, 8, and 10 °C under the reference temperature (T1) of 20 °C. The solvent absorption of BCP films was measured by in situ spectroscopic ellipsometry. (b) Plateau thickness (t/t0) of quasi-equilibrium swelling as a function temperature gap. The data were taken at 120 min during the SVA process at various temperature gaps. (c) The calculated χeffN as a function of annealing time under solvent vapor.

Figure 3. SEM images of 190 nm thick PS-b-PMMA films, where the solvent-annealed films at the reference (20 °C) and elevated temperature of 25 °C were thermally annealed at 230 °C for 6 h under vacuum. The time-dependent ordered structures of the swollen BCP films were observed at (a) the as-cast, (b−h) the reference temperature of 20 °C, and (i−o) an elevated temperature of 25 °C (a temperature gap of 5 °C). The insets show the corresponding OM images.

perpendicularly oriented lamellae. However, the BCP films annealed for 60 min and longer annealing times (Figure 3g,h) displayed the serious loss in the ordering and orientation as a result of ordering failure. Unlike the local surface SEM images shown in Figure 3g,h, the macroscopic optical microscopy (OM) images revealed distinct dewetting structures, as shown in the insets.

will be discussed further in Figures 4 and 5). It could be indicative of the accelerated translational ordering among perpendicularly oriented lamellae during the early stage annealing over 30 min, which was in good agreement with the early stage solvent absorption behavior observed in PS-bPMMA films, as shown in Figure 2a. For annealing time up to 50 min, the lamellar grain sizes slightly increased as the defects slowly annihilated, further enhancing the lateral ordering of D

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Figure 4. (a) GISAXS patterns of 190 nm thick PS-b-PMMA films prepared by the SVA process for 2 h at a temperature gap of 5 °C (left), followed by thermal annealing at 230 °C for 6 h under vacuum (right). (b) The line intensity scans along 2θf at αf = 0.240° from the GISAXS patterns obtained from the as-cast, solvent-annealed (using 5 °C gap), and subsequently thermally annealed (at 230 °C for 6 h under vacuum) films. (c) The d-spacings of the solvent-annealed films at various temperature gaps and subsequent thermally annealed films as a function of annealing time under solvent vapor. The open circles with an arrow denote the recovery into the thermal equilibrium d-spacing of 88.4 nm. (d) The d-spacing plateau as a function of volume fraction of polymer (Φ) in swollen BCP films.

A temperature gap of 5 °C readily regulated the solvent permeation into BCP films and suppressed dewetting of the swollen BCP films, most likely because the swelling-controlled films at an elevated temperature were viscous enough to cover the substrate over longer annealing times under solvent vapor. In addition to promoting BCP self-assembly, the long-term stability over longer annealing times facilitated defect annihilation and improved the long-range lateral ordering among perpendicularly oriented lamellae. These effects are beneficial to continuous processes for further steps. Grazing-incidence small-angle X-ray scattering (GISAXS) measurements were performed to probe the internal structures of PS-b-PMMA films, as shown in Figure 4. The grazing incidence scattering geometry was defined by the αf and 2θf, which are the exit angles of the X-ray beam along the out-ofplane scattering direction normal to the sample surface and along the in-plane scattering direction parallel to the sample surface, respectively. Here, q = (4π/λ) sin θ is the in-plane component of the scattering vector, where λ and θ are the wavelength and scattering angle, respectively. The incidence angle (αi) was set at 0.140° above the critical angle (0.114°) of PS-b-PMMA films to ensure that structural information was collected across the entire film thickness. Figure 4a shows the GISAXS patterns of 190 nm thick PS-b-PMMA films prepared

Considering a small and negligible difference in the surface tension (γ) between the neutral substrate (γN ∼ 30.0 erg/cm2) and BCP film (γB ∼ 30.0 erg/cm2),53 the increasing instability of BCP films during the SVA process could be expressed by the negative spreading coefficient (SP) in terms of the polar components of the surface tensions according to S P = γSN − (γSB + γBN) ≈ −γBN

(3)

where γSN and γSB are the surface tensions at solvent-vapor/ neutral substrate and solvent-vapor/BCP interfaces, respectively, and γBN is the interfacial tension between BCP and neutral substrate. Because of the similar chemical compositions of PS-b-PMMA and the neutral brush composed of P(S-rMMA), the γSN is approximately equal to γSB, resulting in SP ≈ −γBN. Therefore, the magnitude of the negative spreading coefficient increased as the solvent absorption of BCP films increased; this effect destabilized the film surface and dewetted the substrate. Furthermore, a decrease in the viscosity during the SVA process kinetically promoted film dewetting. Surprisingly, the solvent-annealed films at an elevated bottom temperature of 25 °C (Figure 3i−o and insets) displayed no ordering failure over longer annealing times (∼7 h) of the SVA process, while the SEM images showed slightly slower structural development during the early stages (first 30 min). E

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Figure 5. SEM image of PS-b-PMMA films prepared with various thicknesses from 0.6L0 (50 nm) to 10.3L0 (912 nm), where the films that had been solvent-annealed for 2 h at a select temperature gap of 5 °C were subsequently thermally annealed at 230 °C for 6 h under vacuum. The film thicknesses of relevance were normalized by an equilibrium lamellar period (L0 = 88.4 nm). The perpendicular orientation of lamellar microdomains was demonstrated in the tilt-view SEM images of the 50 and 912 nm thick films, irrespective of film thickness.

by the SVA process for 2 h at a temperature gap of 5 °C (left), followed by thermal annealing at 230 °C for 6 h under vacuum (right). Bragg rods for the primary peaks observed at 2θf = 0.083° (q = 0.082 nm−1) along the out-of-plane scattering direction indicated that the lamellar microdomains were oriented normal to the film surface. In the thermally annealed film, these features were intensified along with higher-order peaks in the in-plane scattering direction along the horizon of αf = 0.240° due to the enhanced short-range segregation at the interfaces between the two blocks. Figure 4b presents the line intensity scans along 2θf at αf = 0.240° from the GISAXS patterns obtained from the as-cast, solvent-annealed (using 5 °C gap), and subsequently thermally annealed (at 230 °C for 6 h under vacuum) films. Compared with the weak and broad primary peak observed in the as-cast film, the SVA process for 2 h at a temperature gap of 5 °C intensified the primary peak with the scattering vector ratios of q/q* = 1:2:3 that arisen from slightly asymmetric lamellae due to a slightly selective property of THF to the PS block. The primary peak further developed upon thermal annealing with distinct odd-integer-number scattering vector ratios of q/q* = 1:3:5 due to the volumetric symmetry between the two blocks, thereby confirming the development of perpendicularly oriented lamellae using a combination process (SVA + thermal annealing). The shift of q* toward lower q values during the two processes corresponded to an increase in the interlamellar d-spacings by d = 2π/q*. Figure 4c shows the d-spacings of the solvent-annealed films at various temperature gaps and subsequent thermally annealed films as a function of annealing time under solvent vapor. The small d-spacing (66.9 nm) in the as-cast film was attributed to nonequilibrium (kinetically trapped) conditions after spincoating BCP solution onto the substrate. The SVA process increased the d-spacings to different degrees during solvent absorption of BCP films depending on the temperature gap. Unlike the early appearance of film dewetting at the reference temperature of 20 °C, the d-spacing plateaus at each

temperature gap characterized the quasi-equilibrium d-spacing in semidiluted BCPs in the presence of a neutral solvent. This state persisted for longer annealing times under solvent vapor. As the temperature gap increased, the d-spacing plateau increased since the solvent absorption of BCP films decreased or the volume fraction (Φ) of polymer increased, as shown in Figure 4d. The mean-field theory predicts that the d-spacing in semidiluted BCP with a neutral solvent can be given by the power law d ‐spacing ∼ Φ β

(4)

where the value of β is 0.22 in the strong segregation limit (SSL).54 The data were fit to the power law equation to yield a value of β = 0.237 for the films that were solvent-annealed at various temperature gaps and the reference temperature of 20 °C. This value is within the experimental error range of the measured values. Figure 5 shows SEM images of PS-b-PMMA films prepared with various thicknesses of 50−912 nm, where the films that had been solvent-annealed for 2 h at a selected temperature gap of 5 °C were subsequently thermally annealed at 230 °C for 6 h under vacuum. Note that the annealing time under solvent vapor was not limited to 2 h but was extendable to 7 h without film dewetting. The film thicknesses of relevance were normalized by an equilibrium lamellar period (L0 = 88.4 nm), and the increase in film thickness is overlaid to guide the eyes. The long striped patterns in all surface images corresponded to the lamellar microdomains oriented normal to the substrate, irrespective of the film thickness, as demonstrated in the tiltview SEM images of the 50 and 912 nm thick films. Particularly in 10.3L0 thick film, the aspect ratio of perpendicularly oriented lamellae was approximately 20 with no ordering failure present on the film surface. This thickness independence of perpendicular orientation (or incommensurability) was predominantly attributed to the controlled SVA process that accelerated the chain mobility and screened the difference in F

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Figure 6. (a) SEM image of PS-b-PMMA film confined to a 3.2 μm wide topographic confinement. The films subjected to the SVA process for 7 h at a temperature gap of 5 °C were subsequently thermally annealed at 230 °C for 6 h under vacuum. For clarity, the phase contrast in the SEM image was enhanced by asymmetric plasma etching with O2/Ar (in a 5/1 volume ratio) that rapidly etched away the PMMA block. (b) Average defect density (ρ) of BCP films in a trench pattern as a function of annealing time under solvent vapor.

solvent vapor. During the early stage SVA process (i.e., the first 30 min), the defect density decreased significantly to rapidly annihilate defects by coarsening the short line patterns of perpendicularly oriented lamellae, while the films prepared at the reference temperature of 20 °C yielded defect densities up to 40 min prior to film dewetting. As the temperature gap increased, the defect annihilation rate slowed over 2 h, but the aligned line arrays of perpendicularly oriented lamellae were highly stable with no ordering failure. It should be also noted that the defect densities measured after subsequent thermal annealing steps were consistent with those obtained from the films that had been solvent-annealed at various temperature gaps, since most defect annihilation occurred during the SVA process and the morphologies were fixed immediately after rapid evaporation of solvent. More importantly, the ordering stability of PS-b-PMMA films confined to trench patterns was quantified during the SVA process with respect to the annealing time under solvent vapor to evaluate the effective time window at various temperature gaps. We defined the time window starting from the moment that the defect density dropped below 1 μm−2 and ending at the upper time limit prior to film dewetting; this condition was far stricter than that demanded for standard optical polarizers. Figure 7 summarizes the time window evaluated by line arrays of perpendicularly oriented lamellae in the trench patterns as a function of temperature gap, where the lower and upper time limits are indicated in red color. The films that were solvent-annealed at the reference temperature of 20 °C displayed a narrow time window from 30 to 40 min to produce highly ordered line arrays of perpendicularly oriented lamellae. As the temperature gap increased (or the bottom temperature increased) to 5 °C during the SVA process, the ordering stability that began at 50 min was significantly enhanced to 7 h, yielding the widest time window obtained at a temperature gap of 5 °C. We presume that it is a consequence of a trade-off between two competing effects, as follows. The solvent absorption of BCP films decreased linearly as the temperature gap increased during the SVA process due to vapor pressure difference (ΔP) in the closed chamber. By contrast, a higher bottom temperature close to sample temperature promoted fluctuations in the saturated BCP films. The swelling-controlled

the surface energy of the two blocks at the solvent-vapor/BCP interface. It is in contrast to the microdomain orientations in the thermally annealed PS-b-PMMA films that were commensurable with the film thickness.55 It should be noted that this process is very favorable and compatible with nanolithographic applications that require a high aspect ratio in perpendicularly oriented lamellae to achieve etching contrast. The 190 nm thick PS-b-PMMA films were applied to a wide (3.2 μm) trench region that was topographically confined to direct the lateral alignment of perpendicularly oriented lamellae. An I-line photolithography process was used to fabricate line-trench patterns using a negative-type SU-8 photoresist (Microchem) on a neutral substrate. The trench walls (cross-linked photoresist) were not permeable to the solvent vapor, and they were PS-selective presumably due to the molecular similarities between the aromatic structures of the PS block and SU-8. Figure 6a shows a SEM image of PS-bPMMA film confined to trench patterns. The film subjected to the SVA process for 7 h at a temperature gap of 5 °C was subsequently thermally annealed at 230 °C for 6 h under vacuum. The line arrays of perpendicularly oriented lamellae, corresponding to the PS block, were laterally straightened along both trench walls due to the guiding effects, leading to highly ordered line arrays of perpendicularly oriented lamellae in wide trench patterns. The ratio C/L0 corresponded to 36, where C is the confinement width (3.2 μm) across the trench walls, such that the 36−37 lines of perpendicularly oriented lamellae were present, indicating that the lamellar microdomains conformed to the limited space of the trench patterns. However, the compressed end lines of the PS block near both trench walls were attributed to limited stretching during the thermal annealing process since the film thickness close to the walls slightly increased; this effect is tolerable for practical applications once the dual I-line photolithography process is employed over large areas. The defects including dislocations and disclinations (terminal points, junctions, and dots) were analyzed through automatic triangulation, providing neighboring defect distances in the areas of 3.2 × 6 μm2 for the surface morphologies of BCP films. Figure 6b shows the average defect density (ρ) of PS-b-PMMA films in a trench pattern as a function of annealing time under G

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Article

Macromolecules

dewetting the substrate. For the swelling-controlled films annealed at various temperature gaps, longer annealing times facilitated defect annihilation to produce long-range lateral ordering among perpendicularly oriented lamellae in the trench patterns. The widest time window was obtained at an optimal temperature gap of 5 °C, enabling us to apply the system to wider (5 μm) trench patterns (Figure S1). The ordering stability of nontrenched PS-b-PMMA films (Figure S2), as evaluated on a flat neutral substrate, was comparable to that of trenched films. This study suggests a simple and feasible methodology to achieve long-term stable, long-range lateral ordering for further continuous processes with BCP selfassembly using a temperature gap system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02188. SEM image of PS-b-PMMA film confined to a 5 μm wide topographic confinement (Figure S1); the ordering stability (or time window) of PS-b-PMMA films evaluated on a flat neutral substrate (Figure S2) (PDF)

Figure 7. Ordering stability (or time window) of PS-b-PMMA films as a function of temperature gap, which was evaluated by line-arrays of perpendicularly oriented lamellae in a large-width (3.2 μm) topographic confinement. The lower and upper time limits are indicated in red color.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

films annealed at a temperature gap of 5 °C displayed long delays to film dewetting while preserving the chain mobility, which allowed overcoming the local free energy barriers associated with defect annihilation. Further increasing temperature gap by 10 °C, however, delayed defect annihilation due to an increase in the viscosity of the swollen BCP films. Under these conditions, the chain mobility was too low to overcome the local free energy barriers. In addition, the film dewetting occurred earlier than that of the films that were solventannealed at a temperature gap of 5 °C due to the fluctuation effects pronounced at higher temperatures, making the time window narrower again. Consequently, the ordering stability of PS-b-PMMA films was significantly improved by modulating the temperature gap that regulates the solvent absorption (or swelling) and dewetting times of the swollen BCP films during the SVA process. These effects were exploited to achieve highly ordered line arrays of perpendicularly oriented lamellae, resulting in the widest time window at an optimal temperature gap of 5 °C.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Samsung Research Funding Center of Samsung Electronics under the Project Number SRFC-MA1301-03.

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REFERENCES

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CONCLUSIONS A temperature gap system was used to regulate the solvent absorption and dewetting times of the swollen PS-b-PMMA films. Highly stable, perpendicularly oriented lamellar structures were achieved in the SVA processed films that were subsequently thermally annealed to stabilize the surface topology. The temperature gap was adjusted to wet the films onto a neutral substrate over long annealing times during the SVA processes, thereby facilitating structural development of long-range lateral ordering in high-molecular-weight BCP selfassembled structures. The ordering stability of highly ordered line arrays of perpendicularly oriented lamellae in wide (3.2 μm) trench patterns was evaluated for various annealing times under solvent vapor, where the ordering time window was defined as the earliest time at which the defect density dropped below 1 μm−2 and the latest time at which the film began H

DOI: 10.1021/acs.macromol.5b02188 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02188 Macromolecules XXXX, XXX, XXX−XXX