Orthogonally Aligned Block Copolymer Line Patterns on Minimal

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Orthogonally Aligned Block Copolymer Line Patterns on Minimal Topographic Patterns Jaewon Choi, Yinyong Li, Paul Y. Kim, Feng Liu, Hyeyoung Kim, Duk Man Yu, June Huh, Kenneth R. Carter, and Thomas P. Russell ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17713 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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ACS Applied Materials & Interfaces

Orthogonally Aligned Block Copolymer Line Patterns on Minimal Topographic Patterns Jaewon Choi,† Yinyong Li,† Paul Y. Kim,† Feng Liu,‡§ Hyeyoung Kim,† Duk Man Yu,† June Huh,| Kenneth R. Carter,*† and Thomas P. Russell*†‡┴ †

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, 120

Governors Drive, Amherst, Massachusetts 01003, United States ‡

Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,

Berkeley, California 94720, United States |

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul 02841, Republic of Korea ┴

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Chaoyang District North Third Ring Road 15, Beijing 100029, China KEYWORDS: block copolymers, directed self-assembly, orthogonal alignment, solvent vapor annealing, line pattern, topographic patterns, cylindrical microdomains,

ABSTRACT. We demonstrate the generation of block copolymer (BCP) line patterns oriented orthogonal to a very small (minimal) topographic trench pattern over arbitrarily large areas using 1 ACS Paragon Plus Environment

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solvent vapor annealing (SVA). Increasing the thickness of BCP films induced an orthogonal alignment of the BCP cylindrical microdomains, where full orthogonal alignment of the cylindrical microdomains with respect to the trench direction was obtained at a film thickness of corresponding to 1.70L0. A capillary flow of the solvent across the trenches could be attributed to a critical factor in the alignment of the cylindrical microdomains. Grazing incidence small angle X-ray scattering (GISAXS) was used to determine the orientation function of the microdomains with a value of 0.997 being found reflecting a nearly perfect orientation. This approach to produce orthogonally aligned BCP line patterns could be extended to nanomanufacturing and fabrication of hierarchical nanostructures.

INTRODUCTION

The directed self-assembly (DSA) of block copolymers (BCPs) has gained widespread interest as one of the most promising solutions for overcoming resolution limitations in the current lithographic process in the semiconductor industry, since it provides opportunities to fabricate dense periodic/aperiodic nanostructures with sizes below 10 nm.1-10 In the DSA process, BCP microdomains, such as lamellae, cylinders, or spheres, are typically guided by chemical11-14 or topographic15-18 patterns. Previous studies have shown that one of the key parameters to achieve high-quality BCP patterns on either chemical or topographic patterns is the film thickness. For chemical patterns, Nealey and co-workers demonstrated that, when the period of the striped chemical patterns was commensurate with the bulk cylinder-to-cylinder distance, L1, ( = 2 ⁄√3, where L0 is the domain spacing of hexagonally packed cylindrical microdomains in the bulk), the initial film thickness commensurate with the thickness of L0/2 or L0 was essential for achieving a single layer of defect-free poly(styrene-b-methyl methacrylate) PS-b-PMMA line 2 ACS Paragon Plus Environment

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patterns.19 They found that a deviation of the initial film thickness with only 2 nm from the preferred quantized film thickness resulted in the microdomains with many defects although its reason was not clear. The same group also reported the DSA of a cylinder-forming PS-b-PMMA on the spotted chemical patterns as a function of the film thickness and the commensurability between the pattern dimension and L1.20 In this case, they produced highly ordered hexagonal arrays or hexagonal arrays with numerous defects depending on the commensurate conditions at a given film thickness, which were demonstrated in terms of the minimization of free energy as the PS-b-PMMA equilibrates in the present of the spotted chemical pattern.

Film thickness plays an important role in producing uniformly ordered BCP microdomains on topographic patterns. For deep topographic patterns, where the confinement depth is comparable to or larger than L0, Jeong and co-workers showed that the thickness variations of PS-b-PMMA films within the trench patterns resulted in complex morphologies consisting of hexagonal arrays and line patterns of cylindrical microdomains.21 Gopalan and coworkers showed that the film thickness is critical for achieving laterally ordered hexagonal arrays of cylindrical microdomains in the trench patterns with the surfaces weakly preferential to one of the BCP blocks.22 For minimal topographic patterns, where the confinement depth is much less than L0, Russell and co-workers showed that lateral ordering of hexagonal arrays of cylindrical microdomains on the sapphire faceted substrates was lost when the thickness of poly(styrene-b-ethylene oxide) (PS-b-PEO) films was increased from 37 nm (1.25L0) to 46 nm (1.56L0).3 Recently, the same group reported that highly aligned line patterns of cylindrical PS-bPEO microdomains on the minimal trench patterns were lost when the film thickness was increased from 39 nm (1.46L0) to 46 nm (1.72L0), resulting in typical fingerprint patterns.23 It should be noted that the effect of the film thickness on achieving highly ordered BCP 3 ACS Paragon Plus Environment


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microdomains with lateral order is greater in a case for using thermal annealing than that of using solvent vapor annealing (SVA).24 For the DSA of BCP thin films with thermal annealing, the commensurability between the initial film thickness and L0 is a critical parameter to obtain highly ordered BCP patterns.19, 25 In contrast, in the case of using SVA, the BCP thin films typically undergo significant swelling during the SVA so that their final morphology is highly affected by other parameters, such as evaporation rate of solvents and vapor pressure, rather than the film thickness.26-28

Cylinder-forming BCPs can be aligned orthogonal to the underlying ridge direction of a faceted surface as the thickness of BCP films increases.29, 30 For sapphire faceted substrates, Russell and co-workers demonstrated the generation of unidirectionally aligned line patterns of cylindrical microdomains oriented orthogonal to the ridge direction over macroscopic length scales by increasing the BCP film thickness.29 This orthogonal alignment was attributed to the entropic penalties related to the chain packing of the polymer and incommensurability between the pitch of the facets and the cylinder-to-cylinder distance, i.e. 2 ⁄√3. Additionally, the same group reported the effect of the film thickness on the orthogonal alignment of BCP line patterns on the silicon faceted substrates, where the geometry of the silicon faceted substrates is more asymmetric than that of the sapphire faceted substrates.30 In this case, BCP line patterns were oriented parallel to the ridge direction at the film thickness of 35.0 nm (1.25L0), but showed a tendency for being oriented almost orthogonal to the ridge direction when the thickness was increased to 47.6 nm (1.69L0). It should be noted that this orthogonal alignment of BCP line patterns does not occur in the DSA of cylinder-forming BCPs with typical topographic patterns, such as trenches, when the film thickness is increased.23, 31 Recently, based on experiments and simulations, Berggren and co-workers described the orthogonal self-assembly of a cylinder4 ACS Paragon Plus Environment

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forming poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) over the patterned substrates with line, concentric circle, or Y-junction shapes.32 In this case, the height and surface property of topographic patterns were critical for inducing orthogonal self-assembly of PS-b-PDMS line patterns.

Here, we present an orthogonal alignment of PS-b-PEO line patterns of cylindrical microdomains on minimal trench patterns using SVA by increasing the BCP film thickness. Highly aligned line patterns oriented orthogonal to the underlying trench direction were produced over macroscopic areas (~ 1 × 1 cm2) when the film thickness was thicker than 1.70L0 on a minimal trench pattern with a pitch of 139 nm (6.35L0), width of 99 nm (4.52L0), and depth of 15 nm (0.68L0). This orthogonal alignment is attributed a combination of the thicker BCP films on the minimal trench patterns with a solvent field arising from a capillary flow over the trenches. We also produced orthogonally aligned PS-b-PDMS line patterns of cylindrical microdomains on the minimal trench patterns by SVA with relatively low vapor pressure without increasing the film thickness.

RESULTS AND DISCUSSION Figure 1 shows the schematic illustration of the DSA process to generate orthogonally aligned BCP line patterns. We first fabricated minimal trench patterns with the pitch of 139 nm (6.35L0), width of 99 nm (4.52L0), and depth of 15 nm (0.68L0), where the surface is hydrophilic, using nanoimprint lithography (NIL) (Figure 1a). The details of the procedure for the fabrication of minimal trench patterns can be found in our previous work.18 After fabrication of the minimal

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trench patterns, thin films of a cylinder-forming PS-b-PEO (L0 = 21.9 nm in the bulk, see Supporting Information, Figure S1) with different thicknesses from 22.6 nm (1.03L0) to 41.0 nm (1.87L0), as measured on a flat silicon substrate before SVA, were prepared on the patterned substrates using spin-coating (Figure 1b). Then, the PS-b-PEO films were solvent-vapor annealed using tetrahydrofuran (THF) and water, where THF is a good solvent for both blocks and water is a selective solvent for the PEO block, to achieve well-defined line patterns of cylindrical microdomains oriented orthogonal to the underlying trench direction (Figure 1c). Figure 2 shows scanning force microscopy (SFM) phase images of solvent-vapor annealed PS-b-PEO thin films on the minimal trench patterns at different film thicknesses. As shown in Figure 2a, when the film thickness was 22.6 nm (1.03L0), the line patterns consisting of cylindrical microdomains oriented parallel to the film surface were obtained without breakage in the film. When the film thickness was increased to 24.7 nm (1.13L0) (Figure 2b), two different orientations coexisted, where the cylindrical microdomains were oriented nearly (1) parallel or (2) orthogonal to the trench direction. As seen in Figure 2c–e, further increasing the film thickness resulted in an increase in the fraction of microdomains oriented orthogonal to the trench direction with no trace of the underlying minimal trench pattern, where many defects, such as dislocations and disclinations, exist in the films. However, the annihilation of these defects was found by further increasing the thickness, so that highly aligned line patterns of cylindrical microdomains oriented orthogonal to the underlying trench direction were achieved over the entire patterned substrate (~1 × 1 cm2) at the film thickness of 37.3 nm (1.70L0) (Figure 2f) and maintained even for films with a thickness of 41.0 nm (1.87L0) (Figure 2g). We observed identical SFM images to those in Figure 2f and g when many different areas (>30 positions) were randomly scanned using


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SFM on the patterned surface of ~1 × 1 cm2. This will be discussed in more detail later using grazing incidence small angle X-ray scattering (GISAXS) experiments. As shown in Figure 3, the orientations of the line patterns in Figure 2 were quantitatively analyzed using image processing, as described by Murphy et al.,33 where the distribution of the orientation angle is plotted in histograms with Gaussian curve fitting. Figure 3a shows the schematic illustration of the orientation angles of line patterns of the cylindrical microdomains with respect to the underlying trench direction, where the orientation angle of 0 corresponds to the microdomains being oriented parallel to the underlying trench direction. From Figure 3b–f, we see that the PS-b-PEO line patterns showed the tendency for orienting orthogonal as the film thickness was increased. Specifically, in the thickness range of 24.7 nm (1.13L0) to 31.8 nm (1.45L0) (Figure 3b–d), bimodal distributions representing coexisting orientations are observed: line patterns oriented nearly (1) parallel or (2) orthogonal to the trench direction. The average parallel orientations were −1.4° ± 9.6°, −2.0° ± 8.4°, and −1.2° ± 8.0° for the 24.7 nm (1.13L0), 26.9 nm (1.23L0), and 31.8 nm (1.45L0) thicknesses, respectively, showing a decrease in probability of parallel orientation as the film thickness was increased. In contrast, the average orthogonal orientations were 93.4° ± 23.0°, 79.0° ± 14.0°, and 90.3° ± 17.1° for the 24.7 nm (1.13L0), 26.9 nm (1.23L0), and 31.8 nm (1.45L0) thicknesses, respectively, which gradually increased in probability of orthogonal orientation with increasing the film thickness. At the film thickness of 35.0 nm (1.60L0) (Figure 3e), the orthogonal alignment with an average orientation of 90.6° ± 12.5° was predominant, whereas the parallel orientation was rarely seen (inset of Figure 3e). When the thickness was increased to 37.3 nm (1.70L0) (Figure 3f), highly aligned line patterns oriented orthogonal to the underlying trench direction with an average orientation of 90.0° ± 7.3° were observed. Recently, Berggren and co-workers demonstrated the fabrication of 7 ACS Paragon Plus Environment


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the orthogonally aligned PS-b-PDMS line patterns with respect to the direction of the underlying hydrogen silsesquioxane (HSQ) line templates.32 Based on self-consistent field theory (SCFT) modeling, they argued that the key parameters to induce the orthogonal alignment of PS-bPDMS line patterns were the height and chemical preference of the guiding templates. However, in this case, the thickness of BCP films was fixed at L0, whereas the orthogonal orientation in our study was achieved at thicker films (> 1.70L0). The packing frustration of BCP chains on the triangular groove topography of a faceted sapphire substrate, the major reason for the orthogonal orientation of BCP microdomains,29 is less severe in the rectangular groove topography of the minimal trench patterns used in this study. Therefore, we speculate that a more plausible factor for the orientation of the microdomains is a solvent field arising from a capillary flow into the trenches as the solvent evaporates for the thicker films (Figure 4). Such a solvent field can give rise to a gradient in annealing temperature across the trenches, which can lead to an orthogonal orientation of the cylindrical microdomains with respect to the trench direction. A detailed investigation on the origin of the orthogonal alignment for the PS-b-PEO system is currently underway. Since SFM is limited to examining the local ordering of BCP microdomains over an area of several square micrometers, GISAXS was used to elucidate the lateral ordering and orientation of line patterns of cylindrical microdomains over macroscopic distances.34, 35 Figure 5a shows the schematic illustration of GISAXS experiments, where the sample stage is rotated about the surface normal during the measurements. We defined the rotation angle, Ψ, as the angle between the direction of the incident X-ray beam and the direction of PS-b-PEO line patterns on the minimal trench pattern. Since the PS-b-PEO line patterns were oriented orthogonal to the trench direction, the X-ray beam was first aligned parallel to the trench 8 ACS Paragon Plus Environment

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direction, which is defined as Ψ = 90°. The sample stage was then rotated to Ψ = 0°. Subsequently, the sample stage was rotated 2° to 10°. The incident angle was set at 0.18°, which is above the critical angle of the polymer film (0.16°), allowing us to characterize the BCP morphology throughout the film. Figure 5b–h show the 2-D GISAXS patterns of the PS-b-PEO thin film on the minimal trench pattern at different Ψ, where qy is the in-plane scattering vector and qz is the out-of-plane scattering vector. As shown in Figure 5b, when the direction of the Xray beam was parallel to the trench direction (Ψ = 90°), strong streaks in the vertical direction arising from the minimal trench pattern were observed. In addition, these streaks were accompanied by a semicircle-like scattering pattern because of the intersection of the Ewald sphere with the grating truncation rods, which is characteristic of GISAXS patterns when the direction of the X-ray beam is parallel to the direction of the grating.18, 36, 37 It should be noted that the semicircle-like scattering pattern is very sensitive to the alignment of the trench direction with respect to the direction of the incoming X-ray beam.38, 39 As seen in Figure 5c, when the direction of the X-ray beam was parallel to the direction of the PS-b-PEO line patterns (Ψ = 0°), Bragg rods corresponding to the PS-b-PEO microdomains were observed. As the sample stage was rotated (Figure 5d–h), that is, increasing Ψ, a change in the intensity of Bragg rods was observed. The corresponding in-plane profiles from the 2-D GISAXS patterns are shown in Figure 5i. At Ψ = 90°, the strong reflections arising from the underlying minimal trench pattern were observed. However, when Ψ = 0°, three reflections coming from the PS-b-PEO line patterns were seen with scattering vector ratios of 1:2:3 relative to the first-order reflection. In this case, strong reflections from the underlying minimal trench pattern were not observed, since the direction of the trench pattern and the direction of the X-ray beam were not the same. The domain spacing of the PS-b-PEO line patterns of cylindrical microdomains was calculated to be



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28.3 nm (= 2π/qy) from the first-order reflection. This value is larger than those observed in the bulk (L0 = 21.9 nm). This could be attributed to the combination of SVA process with the incommensurability between the pattern dimensions (pitch and width) and L0. In the presence of solvent molecules in BCP films during SVA, the microdomains can be stretched or compressed to accommodate given topographic constraints.3, 18, 30 With increasing Ψ from 0° to 10°, two distinct changes in the scattering profiles were found: (1) the first-order reflection becomes weaker and broader and (2) the higher-order reflections vanish. Since the scattering from the microdomains is dependent on Ψ, we can quantify the degree of orientation of BCP line patterns using the orientation parameter, f, for the 2-D object in the film, which is calculated using the following function,29, 40

= 2〈cos 〉 − 1

〈cos 〉 =

∑ ∑

(1)

(2)

where, Ψ is the rotation angle and I(Ψ) is the intensity of the first-order reflection at each Ψ from the GISAXS patterns. Figure 6 shows the normalized scattering intensity of the first-order reflection as a function of Ψ. Based on the integration of the intensity, f was calculated to be 0.997, indicating the extraordinarily high degree of orientation of the line pattern over macroscopic length scales (~1 × 1 cm2). It is noted that f = 1.0 for perfect orientation and f = 0 for random orientation. We have explored the orthogonal alignment of different BCPs on the minimal trench patterns. For this purpose, thin films of a cylinder-forming PS-b-PDMS (L0 = 38.2 nm) with a thickness of 38.8 nm (1.02L0), as measured on a flat silicon substrate prior to SVA, were


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prepared on the minimal trench patterns using spin-coating. Then, SVA was performed using acetone, THF, or toluene vapor for the same annealing time. It is noted that all solvents are more selective for the PS blocks because of the differences in the solubility parameters between the polymers and solvents (|δpolymer − δsolvent|) (Table 1).41 Interestingly, the alignment of PS-b-PDMS line patterns of cylindrical microdomains on the minimal trench pattern was affected by the nature of the solvent vapor without increasing the film thickness, as shown in the SFM images in Figure 7. We note that SFM imaging of PS-b-PDMS microdomains was affected by the thin surface layer of PDMS at the top surface in the films. As shown in Figure 7a, when the PS-bPDMS thin film was annealed using acetone vapor, a typical fingerprint pattern of cylindrical microdomains was produced on the minimal trench pattern. This is confirmed by the 2-D fast Fourier transform (FFT) (inset of Figure 7a), which shows a halo, indicating randomly oriented microdomains. It should be noted that a chain of spots in the middle of the FFT arises from the underlying minimal trench pattern. Similarly, in the case of SVA with THF vapor (Figure 7b), the fingerprint patterns of cylindrical microdomains with shorter correlation lengths were generated on the minimal trench pattern. However, when the film was annealed with toluene vapor (Figure 7c), we found that the line patterns of cylindrical microdomains were oriented orthogonal to the underlying trench direction. The FFT (inset of Figure 7c) also showed diffuse spots parallel to the axis of the chain of spots in the middle of the FFT, confirming orthogonally aligned PS-b-PDMS line patterns with respect to the underlying trench direction. Previously, Ross and Jung reported the orthogonal alignment of PS-b-PDMS line patterns within the trench patterns modified with a PDMS brush layer.42 In this case, the origin of the orthogonal alignment was attributed to the wide mesas of the trench pattern and SVA with relatively low vapor pressure. However, in our study, orthogonally aligned PS-b-PDMS line patterns exist over the



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trench patterns. Moreover, in our system, both PS and PDMS blocks are not attracted to the surface of the minimal trench pattern due to its hydrophilicity. Thus, we argue that the relatively low vapor pressure of toluene is a factor in driving the orthogonal alignment of PS-b-PDMS line patterns of cylindrical microdomains on the minimal trench pattern. As shown in Table 1, the vapor pressure of toluene is noticeably smaller than that of the other solvents,43 suggesting that low volatility of the solvent can provide more time, even for highly incompatible PS-b-PDMS microdomains, to be orthogonally aligned to the trench direction by directing field, possibly by the solvent field across the minimal trench pattern. However, more investigation is needed to clarify the relationship between the vapor pressure of the solvents and the orthogonal alignment of PS-b-PDMS line patterns of cylindrical microdomains on the minimal trench patterns.

CONCLUSION

In summary, we achieved orthogonally aligned BCP line patterns on minimal trench patterns with the pitch of 139 nm (6.35L0), width of 99 nm (4.52L0), and depth of 15 nm (0.68L0) with SVA. For PS-b-PEO microdomains, increasing the film thickness induced the orthogonal alignment, resulting in highly aligned line patterns of cylindrical microdomains oriented orthogonal to the trench direction, with an orientation parameter of 0.997, over macroscopic distances (~1 × 1 cm2). This orthogonal alignment appears to be affected by the combination of thicker films with the solvent field arising from a capillary flow across the trenches. For PS-bPDMS microdomains, the orthogonal alignment of line patterns on the minimal trench patterns was attributed to SVA with the relatively low vapor pressure of toluene. Compared with the previous methods to produce orthogonally aligned BCP line patterns over large areas with 12 ACS Paragon Plus Environment

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respect to underlying topographic features, such as shearing with UV-crosslinking44 or nanotransfer printing,45 the approach presented here is easier to use and could open a new avenue for building up hierarchical nanostructures and be useful in nanomanufacturing.

EXPERIMENTAL METHODS Materials. Poly(styrene-b-ethylene oxide) (PS-b-PEO) (Mn = 20.5 kg mol−1 and Mn = 7.0 kg mol−1 for PS and PEO blocks, respectively, PDI = 1.05) and poly(styrene-b-dimethylsiloxane) PS-b-PDMS (Mn = 31.0 kg mol−1 and Mn = 14.5 kg mol−1 for PS and PDMS blocks, respectively, PDI = 1.15) were purchased from Polymer Source, Inc. and used as received. Benzene (anhydrous, 99.8%), toluene (anhydrous, 99.8%), and tetrahydrofuran (THF) (anhydrous, ≥ 99.9%, inhibitor-free) were purchased from Sigma-Aldrich. Acetone (≥ 99.5%) was purchased from Fisher Chemical. Each material was used without further purification. Epoxy permanent resist (version 142, samples A-Z) was supplied from Microchem Corp. for the fabrication of silsesquioxane (SSQ)-based films. Fabrication of Minimal Trench Patterns. Nanoimprint molds composed of “hardened”, crosslinked PDMS elastomer (h-PDMS) were replicated from a silicon master mold having the pitch of 140 nm, line width of 70 nm, and height of 50 nm according to the previously described method.18 SSQ-based films were spin-coated at 2000 rpm for 60 s from propylene glycol monomethyl ether acetate (20 wt%) onto a silicon substrate with a native oxide layer (orientation of (100), University Wafer, Inc.). The thickness of the as-spun SSQ films was 274 ± 4 nm. Then, the SSQ films were nanoimprinted and UV cured with the h-PDMS mold with grating lines using the nanoimprinter (NX-2000, Nanonex) operating at 80 psi for 5 min. To decrease the 13 ACS Paragon Plus Environment


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trench dimensions, the patterned SSQ films, i.e. nanoimprinted trench patterns, were annealed at 500 °C in air for 2 h, finally producing minimal trench patterns with the pitch of 139 nm, width of 99 nm, and depth of 15 nm. DSA of PS-b-PEO Thin Films. The minimal trench patterns were cleaned with carbon dioxide snow jet, followed by UV-Ozone cleaning (UVO cleaner model 342, Jelight Company Inc.). PSb-PEO thin films were prepared onto the minimal trench patterns by spin-coating from PS-bPEO solution in benzene. Before solvent vapor annealing (SVA), the thicknesses of the films were measured on a flat silicon substrate using the ellipsometer (Model LSE, Gaertner Scientific Corp.), where the film thickness was varied by controlling the concentration of the PS-b-PEO solution (0.6% − 1.0% (wt/v)) and the speed (1700 rpm − 3000 rpm) of spin-coating. SVA was performed in a sealed glass jar (volume of the jar = 46.5 cm3, surface area of THF = 12.6 cm2, and surface area of water = 1.3 cm2) at room temperature. To avoid dewetting, PS-b-PEO thin films were pre-swollen with water for 10 min, followed by SVA with THF and water for 60 min. The solvent vapor-annealed films were dried in air at room temperature for 6 hrs. DSA of PS-b-PDMS Thin Films. PS-b-PDMS thin films were spin-coated onto the minimal trench patterns from 1.6% (wt/v) PS-b-PDMS solution in toluene. Prior to SVA, the thicknesses of the films were 38.8 nm on a flat silicon substrate, as measured by the ellipsometer (Model LSE, Gaertner Scientific Corp.). PS-b-PDMS films were solvent-vapor annealed with acetone, THF, or toluene vapor in three individually sealed glass jars at room temperature for 60 min: (1) SVA with acetone vapor (volume of the jar = 46.5 cm3 and surface area of acetone = 12.6 cm2), (2) SVA with THF vapor (volume of the jar = 46.5 cm3 and surface area of THF = 12.6 cm2), and (3) SVA with toluene vapor (volume of the jar = 46.5 cm3 and surface area of toluene = 12.6 cm2). The solvent vapor-annealed films were dried in air at room temperature for 6 hrs. 14 ACS Paragon Plus Environment

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Characterization. Scanning Force Microscopy (SFM). Solvent-vapor-annealed BCP thin films on the minimal trench patterns were investigated using a SFM (Dimension 3100, Digital Instruments) operated in the tapping mode, where many different areas (>30 positions) were randomly scanned at each film thickness on the patterned surface of (~1 × 1 cm2). Small Angle X-ray Scattering (SAXS). SAXS measurements were performed at the University of Massachusetts, Amherst using an in-house setup from Molecular Metrology Inc. It uses a 30 W microsource (Bede) with a 30 µm by 30 µm spot size matched to a Maxflux optical system (Osmic), leading to a low-divergence beam of monochromatic Cu Kα radiation (wavelength λ = 0.1542 nm). Grazing Incidence Small Angle X-ray Scattering (GISAXS). GISAXS measurements were performed at Beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, at an X-ray energy of 10 keV.46 The sample-to-detector distance was calibrated using a silver behenate standard. Scattering images were collected on a Pilatus 2M 2-D pixel detector (Dectris). During GISAXS measurements on the PS-b-PEO microdomains on the minimal trench patterns, the sample stage was rotated about the surface normal, where the rotation angle of Ψ = 90° is defined when the X-ray beam is parallel to the trench direction. Then, the sample stage was rotated 90° to find PS-b-PEO line patterns (oriented orthogonal to the trench direction), which is defined as Ψ = 0°. The sample stage was rotated from Ψ = 0° to Ψ = 10°. The incidence angle (αi) was fixed at 0.18°, which is above the critical angle of the polymer film (0.16°). For a beam height of 125 µm, the footprint of the X-ray beam varies between ∼72 mm (αi = 0.10°) and ∼36 mm (αi = 0.20°), allowing X-rays to probe the BCP microdomains over the entire length of the patterned substrate (∼1 × 1 cm2 in size)



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Image Processing of Orientations of BCP Line Patterns. SFM images were processed using MATLAB with the procedure developed by Murphy et al..33 Image processing began with enhancing the contrast between two BCP domains by multiplying pixel values with themselves. Each pixel was then replaced with median value of adjacent eight pixels to reduce noise (i.e. median filter), and the filtered frame was transformed into a binary image by thresholding; black (0) and white (1) represent PS and PEO respectively. After being skeletonized into single-pixel width, each domain was trimmed to remove unwanted short branches formed by domain thickness undulation. To smooth out pixelated skeletons, the coordinates of each pixel were repeatedly adjusted to an average location of adjacent two pixels, but within ± 0.5 pixel from the initial coordinates. Finally, the orientations of each domain were calculated by dot product of two unit vectors: (1) unit vector between every two connected pixels in skeletons and (2) unit vector parallel to the trench line pattern.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SAXS data of the PS-b-PEO, time evolution of SFM images of PS-b-PEO thin films during SVA, SFM image of PS-b-PEO thin films on minimal trench patterns, and swelling ratio of BCP films (PDF) AUTHOR INFORMATION Corresponding Author 16 ACS Paragon Plus Environment

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*

E-mail: [email protected]

*

E-mail: [email protected]

Present Addresses §

Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, P. R.

China Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) supported Center for Hierarchical Manufacturing (CMMI-1025020) at the University of Massachusetts, Amherst and by the Air Force Office of Science Research under contract 16RT1602. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract No. DE-AC02-05CH11231. J.C. acknowledges Samsung Scholarship from the Samsung Foundation for financial support.

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Figure 1. Schematic illustration of the DSA process. (a) Minimal trench patterns were fabricated using nanoimprint lithography (NIL). (b) PS-b-PEO films with different thicknesses were prepared by spin-coating, and then solvent-vapor annealed with THF and water. (c) Highly aligned PS-b-PEO line patterns oriented orthogonal to the underlying trench direction were achieved for thicker films.


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Figure 2. SFM phase images of solvent-vapor annealed PS-b-PEO thin films on minimal trench patterns at different film thicknesses: (a) 22.6 nm (1.03L0), (b) 24.7 nm (1.13L0), (c) 26.9 nm (1.23L0), (d) 31.8 nm (1.45L0), (e) 35.0 nm (1.60L0), (f) 37.3 nm (1.70L0), and (g) 41.0 nm (1.87L0). The arrow of each image indicates the direction of the underlying trench pattern. All scale bars are 200 nm.



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Figure 3. Orientation angle distribution from image analysis of PS-b-PEO line patterns on minimal trench patterns. (a) Schematic illustration of the defined orientation angles of line patterns with respect to the underlying trench direction. (b–f) Orientation angle distribution at different film thicknesses: (b) 24.7 nm (1.13L0), (c) 26.9 nm (1.23L0), (d) 31.8 nm (1.45L0), (e) 35.0 nm (1.60L0), and (f) 37.3 nm (1.70L0). The inset in (e) shows the orientation probability between −30° and 30°.


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Figure 4. Schematic illustration of the proposed mechanism for a solvent field arising from a capillary flow into the trenches.



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Figure 5. GISAXS measurements of solvent-vapor annealed PS-b-PEO thin films on minimal trench patterns. (a) Schematic illustration of GISAXS experiments, where Ψ is the rotation angle. We define Ψ = 0° when the direction of the X-ray beam is parallel to the direction of PS-b-PEO line pattern. (b–h) 2-D GISAXS patterns taken at different Ψ: (b) Ψ = 90°, (c) Ψ = 0°, (d) Ψ = 2°, (e) Ψ = 4°, (f) Ψ = 6°, (g) Ψ = 8°, and (h) Ψ = 10°. (i) In-plane scattering profiles corresponding to a horizontal cut of (b–h) at qz = 0.320 nm–1. At Ψ = 0°, three peaks with scattering vector ratio of 1 : 2 : 3 relative to the first-order peak were observed.


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Figure 6. Normalized scattering intensity of the first-order reflection from GISAXS patterns as a function of Ψ. Using this data, f was calculated to be 0.997, which is characteristic of exceptionally aligned PS-b-PEO line patterns oriented orthogonal to the trench direction.



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Figure 7. SFM phase images of PS-b-PDMS microdomains on minimal trench patterns annealed with different solvent vapors: (a) acetone, (b) THF, and (c) toluene. In (a–c), the film thickness was fixed at 38.8 nm (1.02L0). The inset of each image shows the corresponding 2-D FFT. The white arrow of each image exhibits the underlying trench direction. The yellow arrow in (c) indicates the direction of PS-b-PDMS line patterns. All scale bars are 200 nm.


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Table 1. Solubility parameters of the polymers and solvents and vapor pressures of the solvents used in the PS-b-PDMS experiments.41, 43

Polymer or Solvent

Solubility parameter (MPa)1/2

Vapor pressure at 25 °C (kPa)

PS

18.6

–

PDMS

15.4

–

Acetone

20.1

30.8

THF

19.4

21.6

Toluene

18.2

3.79



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Orthogonally Aligned Block Copolymer Line Patterns on Minimal

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