Controlling Block Copolymer–Substrate Interactions by Homopolymer

Aug 15, 2017 - Control over the orientation of cylindrical and lamellar domains is required for pattern transfer in block copolymer lithography. Previ...
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Controlling Block Copolymer−Substrate Interactions by Homopolymer Brushes/Mats Yuanyuan Pang,†,‡ Lei Wan,§ Guangcheng Huang,†,‡ Xiaosa Zhang,†,‡ Xiaosa Jin,†,‡ Peng Xu,∥ Yadong Liu,† Miaomiao Han,† Guang-Peng Wu,⊥ and Shengxiang Ji*,† †

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Beijing, China § HGST, A Western Digital Company, 5601 Great Oaks Parkway, San Jose, California 95119, United States ∥ College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225009, China ⊥ MOE Laboratory of Macromolecular Synthesis and Functionalization, Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Control over the orientation of cylindrical and lamellar domains is required for pattern transfer in block copolymer lithography. Previous work mainly focuses on the use of random copolymer brushes to control the wetting behaviors of block copolymers (BCPs), but random copolymerization is only limited to a few monomer pairs. Here we demonstrate the use of homopolymer brushes/mats to modify the substrate to form a chemically homogeneous surface. The surface affinity is tuned by changing the monomer substituent, and a variety of wetting behaviors are obtained in BCP films on homopolymer brushes/mats. Three series of hydroxy-terminated or cross-linkable homopolymers, including polymethacrylate, polyacrylate, and polystyrene derivatives, are prepared for controlling the BCP−substrate interaction. Both preferential and nonpreferential wetting behaviors of poly(styrene-b-methyl methacrylate), poly(styrene-b-rac-lactide), and poly(styrene-b-propylene carbonate) films are obtained as the homopolymer structures change in terms of the carbon contents, indicating that this homopolymer approach may be applicable to any BCPs. Moreover, homopolymer brushes can also substitute random copolymer brushes in directed self-assembly of BCP films with density multiplication on the chemical pattern. The use of homopolymer brushes/mats may have advantages over random copolymer brushes or blend brushes in terms of reproducibility and uniformity of brush formation, reduction of defect density, and increase of assembly kinetics.

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INTRODUCTION Block copolymer (BCP) lithography is emerging as one of the leading technologies for the fabrication of sub-10 nm nanostructures.1 One of the major challenges in BCP lithography is to control the orientation of BCP domains.2−5 In BCP thin films, one block usually prefers a particular interface to minimize the interfacial energy, which usually results in either domains lying parallel to the substrate or the formation of nonbulk morphologies.6,7 For pattern transfer applications, the BCP domains need to be oriented perpendicular to the substrate through the film thickness, in which the interactions of both blocks with the interface, e.g., free surface and substrate, are balanced.2,8 Tremendous efforts have been devoted to control the BCP− substrate interfacial interaction by modifying the substrate with self-assembled monolayer,9−11 random copolymer,12−20 and binary homopolymer blend or block cooligomer.21−25 The most common chemical modification approach is to treat the substrate with a random copolymer that contains the same repeating units of the BCP.12−20,26 The random copolymer is © XXXX American Chemical Society

either cross-linked to form a mat or grafted to the substrate surface to form a brush layer. Poly(styrene-ran-methyl methacrylate) (P(S-r-MMA)) is the most widely studied random copolymer and has been demonstrated to be able to sufficiently tune the surface energies of the substrates for the assembly of PS-b-PMMA films. The random copolymer approach has also been successfully extended to poly(styreneran-2-vinylpyridine) (P(S-r-2VP)) brushes for the self-assembly of PS-b-P2VP.20 The advantage of using random copolymers to modify the substrate is that the brush/mat composition can be easily adjusted by simply changing the monomer feed ratio.19 However, given the reactivity difference between the monomer pair, neither P(S-r-MMA) nor P(S-r-2VP) synthesized by radical polymerization is a truly random copolymer, and the random copolymer contains a distribution of short blocks. On the other hand, for most BCPs it is impossible to copolymerize Received: April 9, 2017 Revised: July 6, 2017

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Macromolecules Scheme 1. Synthesis of Cross-Linkable or Hydroxy-Terminated Homopolymers

The designed monomers consist of the characteristic “hydrophobic” and “hydrophilic” fractions of BCP units, and the monomer structures can be easily modified to tune the brush affinity through the change of substituents to achieve a wide range of wetting behaviors of BCPs. By adjusting the substituents in monomers, both preferential and nonpreferential (or neutral) homopolymer surfaces for PS-b-PMMA, poly(styrene-b-rac-lactide) (PS-b-PDLLA), and poly(styrene-bpropylene carbonate) (PS-b-PPC) are obtained. The homopolymer brush can also substitute the random copolymer brush in directed self-assembly (DSA) with density multiplication.

the corresponding monomer pairs to form random copolymers, such as poly(styrene-ran-lactide) (P(S-r-LA)), because of the incompatible polymerization mechanism for styrene and lactide. Besides random copolymer brushes/mats, binary homopolymer blend brushes were also developed to control the assembly behaviors of PS-b-PMMA and PS-b-P2VP.22 In this case, the molecular weights of two homopolymers are small enough to prevent the macrophase separation of the binary blend brush with the aid of low molecular weight BCP surfactant. Liu et al.27 and She et al.28 extended the binary blend brush concept and used a two-step grafting strategy to prepare binary homopolymer blend brushes for self-assembly of PS-b-PMMA and PS-b-PLLA films. To prevent the effect of process conditions on the composition of grafted blend brushes, Ji et al. further linked two oligomers to form a block cooligomer which could also sufficiently tune the wetting behavior of high molecular weight PS-b-PMMAs.24 The use of a binary blend brush or a block cooligomer brush solved the difficulty encountered by BCPs with monomer pairs that cannot be randomly copolymerized. These two approaches prove that the brush composition is the key to form a nonpreferential surface and the randomness within a chain is not necessary if there is no macrophase separation in brushes. Here we demonstrate a new, versatile approach for the formation of chemically homogeneous surfaces by using homopolymer brushes/mats for self-assembly of BCP films.

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RESULTS AND DISCUSSION Homopolymers are synthesized through polymerization of (meth)acrylate or styrene derivatives (Scheme 1). Hydroxyterminated homopolymers were synthesized by atom transfer radical polymerization (ATRP) of the corresponding monomers using a hydroxy-containing initiator, named TOH1− 15.29,30 Cross-linkable polymers were prepared by free radical polymerization of monomers with the incorporation of 2 mol % glycidyl methacrylate (GMA), named CL1−5. Cross-linkable CL2 and hydroxy-terminated TOH2 homopolymers were chosen to study the annealing conditions for the brush/mat formation. Both CL2 (∼6 nm thickness) and TOH2 (∼30 nm thickness) homopolymer films were heated at 160 °C under vacuum for different times. When the CL2/TOH2 films were heated for 15 min to 24 h, the thicknesses and water contact B

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Macromolecules Table 1. Estimation of Surface Energy from Contact Angle TOH

CL γs(TOH)a

contact angle (deg) H2O 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a

83.5 82.2 83.2 85.2 83.4 84.1 84.2 89.3 87.7 88.6 75.4 76.5 79.8 84.3 87.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 0.6 0.6 0.9 0.7 0.2 1.3 1.1 0.1 0.2 0.9 0.5 0.9 0.4 0.9

−2

CH2I2

(mJ m )

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

45.2 44.9 43.3 42.8 44.4 45.2 45.9 43.6 42.3 45.9 44.9 45.6 43.8 39.6 38.3

27.7 28.9 32.6 33.6 29.8 27.6 22.7 31.6 34.5 26.0 31.8 29.3 32.6 40.8 42.9

1.3 0.4 0.5 1.0 0.5 1.3 0.4 0.5 1.9 1.3 1.2 1.1 2.0 1.6 1.6

H2O 81.9 84.5 83.3 84.7 83.1

± ± ± ± ±

0.9 0.4 0.8 0.8 0.7

γs (TOH) and γs (CL) were calculated from the Fowkes equation γL(1 + cos θ) = 2[ γLdγSd +

angles of CL2 mats were almost constant while the thicknesses and water contact angles of TOH2 brushes increased with the increase of the heating time (Table S1). Solutions of lamellaforming PS-b-PMMA (SM5354) were then spin-coated on these mats/brushes and annealed at 230 °C for 10 min. SEM analysis of these assembled films showed that perpendicular lamellae in the fingerprint-like structures were obtained in all SM5354 films assembled on the eight mats; however, different morphologies were observed in SM5354 films on the eight brushes, and the surface morphologies progressed from hole− island structures (brush thickness ∼ 1.5−2.9 nm) to perpendicular lamellae (brush thickness ∼ 3.2−5.2 nm) (Table S1).31 As the annealing time was longer than 16 h, defect-free perpendicular lamellae were obtained in the SM5354 film on the brush, indicating that the brush was neutral to SM5354. For the rest of the study, all the brushes/ mats were treated at 160 °C for 24 h under vacuum. The surface energy (γs) values of cross-linked mats (CL1−5) and grafted brushes (TOH1−15) were dependent on their molecular structures (Table 1). γs decreased from 45.2 mJ/m2 (TOH1) to 42.8 mJ/m2 (TOH4) with increasing carbon contents but increased to 44.4 mJ/m2 (TOH5). This trend was related to the chemical composition and the surface structure of the brushes. The trend of γs values of CL1−5 was consistent with that of TOH1−5. The γs values of TOH6−10 also have the same trends as TOH1−5. For polystyrene derivatives TOH11−15, γs generally decreased with increasing carbon contents. As anticipated, the surface affinities of homopolymer brushes can be tuned by simply adjusting the homopolymer structures. Self-Assembly of PS-b-PMMA Films on Polymethacrylate Mats. Three PS-b-PMMA BCPs, SM5354 (lamellaeforming), SM4621 (PMMA cylinder-forming), and SM2050 (PS cylinder-forming), were spin-coated on polymethacrylatebased CL1−5 mats, and the films were thermally annealed at elevated temperatures to induce the structure formation. Scanning electron microscopy (SEM) and grazing-incident small-angle X-ray scattering (GISAXS) measurements were performed to determine the film morphologies. Figure 1 shows top-down SEM images of PS-b-PMMA films on the cross-

γs (CL)

contact angle (deg) CH2I2

(mJ m−2)

± ± ± ± ±

45.5 44.4 43.5 43.0 44.2

27.4 29.8 32.2 33.1 30.4

0.8 1.0 1.1 0.7 0.4

γLpγSp ].

Figure 1. SEM images of SM5354, SM4621, and SM2050 films assembled on CL1−5. The film thicknesses are 50 nm for SM5354, 32 nm for SM4621, and 35 nm for SM2050. Scale bars represent 200 nm.

linked mats, with SM5354 and SM4621 films annealed at 230 °C for 10 min and SM2050 films annealed at 250 °C for 3 min. The thicknesses of SM5354, SM4621, and SM2050 films were about 50, 32, and 35 nm, respectively, close to the natural periods (L0) of the corresponding BCPs. Perpendicularly oriented, fingerprint-like structures were achieved in the C

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Figure 2. (A) Array of 2D-GISAXS patterns at an incident angle of 0.14° for films corresponding to Figure 1. (B) Intensity profiles along qy at qz = 0.5 nm−1 from the 2D spectra.

assembled SM5354 films on CL1−4, indicating that these mats were nonpreferential or neutral to SM5354; however, dark spots were observed in the SM5354 film assembled on CL5, indicating that energetics at the modified substrates were not fully balanced and CL5 was slightly preferential to the PMMA block of SM5354. The surface morphologies of SM4621 and SM2050 films were more complicated than those of SM5354 films on CL1−5 mats. A fingerprint-like array of parallel cylinders was observed on the surface of the SM4621 film on CL1. Hexagonal arrays of perpendicular cylinders were obtained in SM4621 films on CL3−4, while a small fraction of parallel cylinders coexisted with perpendicular cylinders in SM4621 films on CL2 and CL5. These results demonstrated that CL3 and CL4 were neutral to SM4621 and CL1 was preferential to the PMMA block, while the structures of CL2 and CL5 were at the border of the neutrality window. A complicated morphology transition was also observed in SM2050 films on CL1−5 mats. Perpendicularly oriented PS cylinders with a fraction of parallel structures were observed on the surface of the SM2050 film on CL1. With the increase of the carbon content, a long-range ordered, hexagonal array of perpendicular PS cylinders was obtained in

the SM2050 film on CL2, indicating that CL2 was neutral to SM2050. Further increase of carbon contents led to the formation of parallel cylinders in SM2050 films on CL3 and CL4. The surface morphology of the SM2050 film on CL5 contained a small amount of necking domains between perpendicular PS cylinders, exhibiting a “nearly neutral” wetting behavior of SM2050 on CL5 mat. The morphology transition of SM4621 and SM2050 films on these mats clearly suggested that the interfacial interaction between the two blocks and the substrates was considerably different, and cylinder-forming PSb-PMMAs are more sensitive to the chemical structures compared with lamellae-forming PS-b-PMMAs. γs of mats were tunable simply by adjusting the monomer substituents and neutral wetting structures were identified for both lamellaeand cylinder-forming BCPs on these homopolymer mats. Since SEM analysis only captured the features on film surfaces, GISAXS was performed to analyze the morphology and orientation of microdomains within the film.32−34 Figure 2 shows the representative GISAXS patterns of assembled films corresponding to Figure 1. GISAXS patterns of all SM5354 films on CL1−5 exhibited strong first-order scattering peaks at qy = 0.118 nm−1 (d = 53.4 nm) and higher-order peaks with D

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ature; however, films on CL1 and CL5 were still presented with defects in spite of the longer correlation length (Figure S1b,c). When films with different thicknesses, e.g., ∼0.6L0, 0.9L0, and 1.7L0, were thermally annealed at 230 °C, fingerprint-like features were also observed in all films, but with defects presenting in films on CL1 and CL5 for all thicknesses (Figure S2). These results further demonstrated that CL2−4 were “neutral” to SM5354, while CL1 and CL5 were just “nearly neutral” to SM5354. The morphologies of PMMA-cylinderforming SM4621 films assembled on the CL mats were more complicated than those of SM5354 because they were more sensitive to both the annealing temperature and the film thickness. A clear transition from parallel cylinders (CL1) to mixed parallel and perpendicular cylinders (CL2) to hexagonal arrays of perpendicular cylinders (CL3−5) was observed on films that were assembled on CL1−CL5 at 190 °C with L0 thickness (Figure S3a), indicating that CL1 was preferential to the PMMA block and CL3−CL5 mats were neutral to SM4621. When the film thickness was increased to about 2L0, the competition of the surface difference and the substrate effect led to the formation of predominantly perpendicular cylinders with fractions of lying-down cylinders on the film surface (Figure S3b). Ham et al. also reported a similar trend that the domain orientation of cylinder-forming PS-b-PMMA was thickness-dependent on brushes with different affinity to BCPs.33 When the films were annealed at 230 °C, a temperature at which PS and PMMA have nearly equal surface energies, hexagonal arrays of perpendicular cylinders were obtained in films on the substrates modified by CL3 and CL4 with L0 or 2L0 thicknesses (Figure S3c,d). In an analogous study by Ji et al., they showed that the neutral surface could affect the domain orientation in cylinder-forming PS-b-PMMA films up to 6L0 when the surface energy was nearly equal.36 These results demonstrate that a slight change of the brush structure and composition can efficiently modify the interfacial energy between BCP films and the substrates. Self-Assembly of PS-b-PMMA Films on Homopolymer Brushes. The grafted homopolymer brushes were proved to be as efficient as the cross-linked mats for adjusting the wetting behaviors between PS-b-PMMAs and substrates. TOH1−5 brushes were also used to control the wetting behaviors of PSb-PMMA films (Figure S4). Although more defects in SM5354 films on TOH1 and SM4621 films on TOH2 and TOH5 were found than films assembled on their corresponding mats, the trend of assembled morphologies of SM5354, SM4621, and SM2050 films on TOH1−5 was consistent with those on the corresponding CL1−5 mats (Table S2). The homopolymer approach was further extended to polyacrylate and polystyrene derivatives as substrate modification agents. Because of the reactivity difference between GMA and acrylate/styrene, hydroxy-terminated polyacrylate and polystyrene derivatives were used to prevent the effect of the distribution of GMA units in cross-linkable homopolymers on the wetting behaviors of BCPs. SEM images of assembled SM5354, SM4621, and SM2050 films on polyacrylate brushes (TOH6−10) are shown in Figure 3. Perpendicular orientation of lamellar domains was obtained in SM5354 films on TOH6− 10, indicating that TOH6−10 were neutral to SM5354. Nearly neutral brushes were identified for cylinder-forming PS-bPMMAs, e.g. TOH8 and TOH10 for SM4621 and TOH9 for SM2050, and perpendicularly oriented hexagonal cylinders with portions of bridges between cylinders were observed on these films. SEM images of assembled SM5354, SM4621, and

integer multiples of the first-order peak at qy = 0.232, 0.355 nm−1 (d = 27.1 and 17.7 nm) and even at qy = 0.469 nm−1 (d = 13.4 nm) for the film on CL3 as shown in the corresponding intensity profiles in Figure 2B. GISAXS results in conjunction with the SEM analysis confirmed that lamellar domains oriented perpendicular to the substrate through the films. GISAXS can reveal both the in-plane and out-of-plane orientation of cylinders and is a very sensitive technique to detect even a small fraction of coexisted structures through the film. GISAXS pattern of the cylinder-forming SM4621 film on CL1 has peaks at qy = 0.164, 0.333, and 0.504 nm−1 (d = 38.3, 18.9, and 12.5 nm), corresponding to in-plane line patterns arising from parallel cylinders.35 The d-spacing of 38.3 nm fairly agreed with the center-to-center distance (36.4 nm) between hexagonally packed cylinders in bulk as measured by SAXS. For films assembled on CL3 and CL4, the first-order peaks shifted to qy = 0.197 nm−1 (d = 31.9 nm), which agreed with the rowto-row distance (31.5 nm) of hexagonally packed cylinders in bulk, and the higher-order peaks at qy = 0.340, 0.388, and 0.516 nm−1 (d = 18.5, 16.2, and 12.2 nm), with √3, 2, √7 multiples of the first-order peak, were observed, indicating a typical hexagonally close-packed (HCP) perpendicular cylinders. Moreover, for SM4621 films on CL2 and CL5, peaks at qy = 0.164 nm−1 and qy = 0.197 nm−1 both appeared, which demonstrated that parallel and perpendicular morphologies coexisted in the films, but the HCP perpendicular morphology was dominant in films on these two mats by comparing the intensity of two peaks. GISAXS patterns of SM2050 films also captured a complicated morphology transition on these mats. The peak for the film on CL1 or CL5 had a shoulder in the intensity profile, indicating that a fraction of parallel PS cylinders coexisted with dominant HCP perpendicular PS cylinders. The mixed morphology also destroyed the long-range lateral order of dominant HCP perpendicular cylinders, and high-order peaks were not observed. The pattern on CL2 had a strong first-order peak at 0.191 nm−1 and √3 and 2 multiples of firstorder peaks at 0.327 and 0.384 nm−1, which proved that PS cylinders oriented perpendicular to the substrate with a longrange order. The patterns of films on both CL3 and CL4 exhibited peaks of parallel PS cylinders. The through-film morphology could be constructed by the combination of representative GISAXS and SEM results. The intensity profile of the GISAXS pattern for a given film was similar to the Fourier transform of the corresponding SEM image, which confirmed that the surface analysis was sufficient to determine the wetting behaviors for BCP thin films with L0 thickness on these homopolymer mats. Annealing temperature and film thickness are two important factors for self-assembly behaviors of BCP films. SM5354 and SM4621 films with different thicknesses on CL1−5 were thermally annealed at elevated temperatures to investigate the efficacy of homopolymer mats on balancing the interfacial energy between the PS-b-PMMA film and the mat. SM5354 films with thickness of 50 nm were thermally annealed at 190 °C (12 h), 230 °C (10 min), and 250 °C (5 min). Perpendicular orientation of lamellar domains in fingerprintlike structures was achieved in SM5354 films on the five mats (Figure S1). Although the annealing time was longer at 190 °C, gray spots were observed on all samples (Figure S1a). This type of defect was attributed to the slight difference of free surface energy between PS and PMMA blocks at 190 °C. The defect density decreased with the increase of the annealing temperE

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Figure 4. SEM images of SM5354, SM4621, and SM2050 films selfassembled on hydroxy-terminated substituted polystyrene brushes. The film thicknesses are 50, 45, and 35 nm for SM5354, SM4621, and SM2050, respectively. The annealing temperatures are 230 °C for SM5354, 190 °C for SM4621, and 250 °C for SM2050. Scale bars represent 200 nm.

Figure 3. SEM images of SM5354, SM4621, and SM2050 films selfassembled on hydroxy-terminated polyacrylate brushes. The film thicknesses are 50, 45, and 35 nm for SM5354, SM4621, and SM2050, respectively. The annealing temperatures are 230 °C for SM5354, 190 °C for SM4621, and 250 °C for SM2050. Scale bars represent 200 nm.

SM2050 films on substituted polystyrene brushes (TOH11− 15) are shown in Figure 4. Neutral brushes were located for three PS-b-PMMAs. Perpendicular fingerprint-like lamellar structures were obtained in SM5354 films on TOH14−15, while perpendicular HCP cylinders were obtained in SM4621 films on TOH14 and SM2050 films on TOH13. Despite that some monomers of the polymethacrylate, polyacrylate, and substituted polystyrene brushes have similar elemental compositions, e.g., C10H10O2 for TOH1, TOH7, and TOH11, the assembly behaviors of same PS-b-PMMAs on these three brushes are different (Table S2). This observation revealed that the surface properties of homopolymer brushes/ mats were determined by not only their elemental compositions but also their functional groups. Self-Assembly of PS-b-PDLLA and PS-b-PPC on Homopolymer Mats/Brushes. The use of homopolymer mats or brushes to control the BCP−substrate interaction is also applicable to BCPs other than PS-b-PMMA. The versatility of this homopolymer approach was demonstrated on the assembly of PS-b-PPC and PS-b-PDLLA films. Both PS-b-PPC and PS-b-PDLLA have nearly equal surface energies between blocks, and perpendicular orientation of microdomains can be expected if the substrate interface is neutral to these BCPs.37,38 Films of lamellae-forming PS-b-PPC (Mn = 8K−8K), lamellaeforming PS-b-PDLLA (Mn = 20K−23K), and cylinder-forming PS-b-PDLLA (Mn = 20K−10K) were assembled on homopolymer mats or brushes. The interactions between the modified substrates and the overlying BCP films varied with the

homopolymer structures, and different film morphologies were observed on these brushes or mats (Figure S5). Neutral surfaces were identified for these three BCPs, and perpendicularly oriented fingerprint-like structures were achieved on CL2 and TOH7 for lamellae-forming PS-b-PPC and PS-b-PDLLA, respectively, while a hexagonal array of perpendicularly oriented cylinders was obtained in the cylinder-forming PS-b-PDLLA film on TOH11 (Figure 5). These results indicate that rational design of monomers may lead to a neutral homopolymer brush for any BCP. Directed Self-Assembly (DSA) of BCP Films on Sparse Chemical Patterns Backfilled with Homopolymer Brushes. DSA of BCP films with density multiplication has

Figure 5. SEM images of BCP films self-assembled on homopolymer mats or brushes. (A) PS-b-PPC (8K−8K) on CL2 was thermally annealed at 130 °C for 30 min, (B) PS-b-PDLLA (20K−10K) on TOH11, and (C) PS-b-PDLLA (20K−23K) on TOH7 were thermally annealed at 180 °C for 12 h. F

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Figure 6. DSA of SM2222 films with density multiplication. (A) Schematic of the DSA process. (B) SEM images of 3× and 4× density multiplication on chemical pattern backfilled hydroxy-terminated homopolymer brushes TOH1−5. The pattern periods Ls are 76 and 102 nm, respectively. Scale bars represent 200 nm.

plication in our study were also more hydrophilic than the optimal neutral brushes for self-assembly of lamellae-forming PS-b-PMMA.

been successfully demonstrated for efficiently multiplying the feature density.39−43 In this scenario, the composition of backfilling random copolymer brushes is critical for defect-free assembly and the vertical profile of domains.42 Instead of using random copolymer brushes, TOH1−5 brushes were backfilled into the interspace between cross-linked PS (XPS) stripes to form patterns with alternated XPS guiding lines and TOH brush background (Figure 6A). The pattern periods (Ls) were 70−110 nm with a step of 2 nm and spanned 3 and 4 multiples of L0 of lamellae-forming SM2222 (L0 = 25 nm). SM2222 films with a thickness of 30 nm were spin-coated onto the modified patterns and thermally annealed to reach the equilibrium state. XPS guiding stripes were preferentially wetted by the PS block, while PMMA and PS domains interpolated on the TOH brush filled regions. Figure 6B shows the SEM images of the assembled films on chemical pattern backfilled with TOH1−5. The bright lines represented the location of the guiding XPS stripes in the pattern. Defect-free assembly of perpendicular lamellae with 3× and 4× density multiplication was obtained in SM2222 films on chemical patterns backfilled with TOH1, TOH2, and TOH5, while the background area was almost featureless on patterns backfilled with TOH3 and TOH4. Uniform lamellae with low defect densities were obtained at a narrow range of Ls = 74−78 nm for 3× density multiplication and a relatively wide range of Ls = 98−106 nm for 4× density multiplication on both chemical patterns backfilled TOH1 and TOH2 (Figure S6). The reason for this phenomenon is that the interfacial energy between BCP and chemical pattern is minimized by both the XPS guide lines and the backfilled homopolymer brush, not only the homopolymer brush itself. The interaction between the background brush and the overlying BCP film varied with the homopolymer brush structures, which dictated the assembled film morphologies. Liu et al. studied the effect of the composition of backfilled brushes on DSA with density multiplication.42 In their analysis, defect-free DSA with density multiplication was realized on patterns backfilled with brushes that were more hydrophilic than the neutral brushes to minimize the free energy since the guiding XPS lines were hydrophobic. Analogous to Liu’s work, the optimal backfilled brush for DSA with density multi-

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CONCLUSIONS In summary, a homopolymer approach has been developed for controlling the interfacial interaction between the overlying BCP and the substrate. The ability to create a surface with homopolymers guarantees the formation of chemically homogeneous surfaces and overcomes the composition inhomogeneity in random copolymer brushes and blend brushes. This homopolymer approach allows the tenability over a wide range of surface affinities, including nonpreferential and preferential wetting behaviors, by simply changing the monomer structure. This method has been demonstrated on three different types of BCPs, and perpendicular oriented microdomains in lamellae- and cylinder-forming BCP thin films have been achieved by adjusting the structure of homopolymer brushes and mats. The fundamental and technological impact is that neutral brushes can now be easily deposited for conceivably any BCPs. Furthermore, the homopolymer brushes have the potential application in DSA of BCP with density multiplication, which might reduce the defect density in the assembled films and increase the assembly kinetics.

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METHODS

Materials Preparation. All reagents were purchased from SigmaAldrich. Silicon wafers were purchased from Silicon Inc. and cleaned with a piranha solution (30:70 v/v of H2O2/H2SO4) at 140 °C for 30 min prior to use. PS-b-PMMA block copolymers, referred to as SM5354 (Mn = 53K−54K, L0 = 53 nm), SM2222 (Mn = 22K−22K, L0 = 25 nm), SM4621 (Mn = 46K−21K, lattice spacing L0 = 36 nm), and SM2050 (Mn = 20K−50K, L0 = 40 nm) according to their molecular weights of each block, were purchased from Polymer Source Inc. PS-bPPC (Mn = 8K−8K, L0 = 17 nm, PDI = 1.01) and PS-b-PDLLA (Mn = 20K−23K, L0 = 31 nm, PDI = 1.17, and Mn = 20K−10K, L0 = 27 nm, PDI = 1.15) were synthesized according to the literature.38,44 Monomers 1−15 were synthesized through the esterification of phenols or alcohols in anhydrous tetrahydrofuran with excessive methacryloyl/acryloyl chloride in the presence of triethylamine or through the esterification of 4-vinylbenzyl chloride with sodium carboxylates in anhydrous N,N′-dimethylformamide. The crossG

DOI: 10.1021/acs.macromol.7b00743 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules linkable polymers were synthesized via classical free-radical polymerization using AIBN as the initiator and 2% mole fraction of GMA as the cross-linking agent. The hydroxy-terminated homopolymers were synthesized via ATRP using 2-hydroxyethyl α-bromoisobutyrate as the initiator. Substrate Modification and BCP Film Preparation. The homopolymer mat/brush formation was analogous to that of random copolymer mat/brush.19,45 Films of cross-linkable homopolymers (CL1−5, ∼6 nm thickness) and hydroxy-terminated homopolymers (TOH1−15, ∼30 nm thickness) were spin-coated on silicon substrates and annealed at 160 °C for 24 h under vacuum. After annealing, the brushes/mats were sonicated in toluene for 5 times, and the thicknesses of resulting brushes or mats were ∼4−6 nm. BCP solutions were spin-coated on modified wafers to produce films with different thicknesses. These films were thermally annealed at various temperatures under vacuum for different times to develop the equilibrium morphology. Chemical Pattern Preparation and DSA of SM2222 with Density Multiplication. The resist pattern was prepared as described in the literature.46 The resist patterns were then trimmed to the target width and transferred to the XPS layer by a timed 10W O2 RIE for 70−200 s to afford a pattern with Ls = 70−110 nm with a step of 2 nm. After the removal of the resist with chlorobenzene, the films of different hydroxy-terminated polymethacrylates were spin-coated onto the patterned substrates. The substrates were then annealed at 160 °C for 24 h to graft the homopolymer brushes into the regions between the XPS stripes. Excessive hydroxy-terminated polymethacrylates were removed by sonication in toluene to yield chemical patterns of alternating XPS stripes and polymethacrylate stripes. SM2222 film was spin-coated from a 1.0 wt % solution in toluene at 4000 rpm and then annealed for 1 h at 250 °C in a glovebox to develop the equilibrium morphology. Characterization. Mn and polydispersity index (PDI) were determined by gel permeation chromatography (GPC) relative to a PMMA or a PS standard with THF as the eluent. Mn and PDI ranged from 20K−30K, 1.56−2.00 for the cross-linkable polymers and 10K− 20K, 1.06−1.30 for the hydroxy-terminated homopolymers, respectively. Film thickness was measured with a spectroscopic ellipsometer (Sentech SE800). The static contact angles of deionized water and diiodomethane were measured at ambient temperature using a KRÜ SS model DSA 30 video goniometer. The corresponding surface energies were calculated using a geometric mean combining rule proposed by Owens and Wendt.47 A Hitachi S-4800 field-emission scanning electron microscope (SEM) was used to image the block copolymer films using a 1 kV acceleration voltage. The SEM analysis was performed on the assembled BCP films without any treatment, e.g., etching or staining. GISAXS measurements were performed at the Beamline BL16B1 at the Shanghai Synchrotron Radiation Facility (SSRF), China, with a wavelength of 1.24 Å. GISAXS patterns were taken at an incidence angle of 0.14°, which was above the critical angle for the block copolymer samples (RC ≈ 0.135°) and below the silicon critical angle. For each sample the exposure time was 300 s with a sample-to-detector distance of 2210 mm.

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Shengxiang Ji: 0000-0003-0336-0530 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the financial support from the National Natural Science Foundation of China (Nos. 51173181 and 51373166), “The Hundred Talents Program” from the Chinese Academy of Sciences, and Department of Science and Technology of Jilin Province (Nos. 20150204027GX and 20160414032GH), China. The authors also thank the SSRF for providing the beamline (BL16B1).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00743. Additional tables and SEM images (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.J.). ORCID

Lei Wan: 0000-0001-6805-2155 Guang-Peng Wu: 0000-0001-8935-964X H

DOI: 10.1021/acs.macromol.7b00743 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b00743 Macromolecules XXXX, XXX, XXX−XXX