Negative-Tone Block Copolymer Lithography by In Situ Surface Chemical Modification

Self-Assembly Negative-Tone Block Copolymer Lithography by In Situ Surface Chemical Modification Bong Hoon Kim, Kyeong-Jae Byeon, Ju Young Kim, Jinseung Kim, Hyeong Min Jin, Joong-Yeon Cho, Seong-Jun Jeong, Jonghwa Shin, Heon Lee, * and Sang Ouk Kim * N egative-tone block copolymer (BCP) lithography based on in situ surface chemical modification is introduced as a highly efficient, versatile self-assembled nanopatterning. BCP blends films consisting of end-functionalized low molecular weight poly(styrene- ran -methyl methacrylate) and polystyrene- block -Poly(methyl methacylate) can produce surface vertical BCP nanodomains on various substrates without prior surface chemical treatment. Simple oxygen plasma treatment is employed to activate surface functional group formation at various substrates, where the end-functionalized polymers can be covalently bonded during the thermal annealing of BCP thin films. The covalently bonded brush layer mediates neutral interfacial condition for vertical BCP nanodomain alignment. This straightforward approach for high aspect ratio, vertical self-assembled nanodomain formation facilitates single step, site-specific BCP nanopatterning widely useful for various substrates. Moreover, this approach is compatible with directed self-assembly approaches to produce device oriented laterally ordered nanopatterns. 1. Introduction Block copolymer (BCP) self-assembly has emerged as a nextgeneration nanolithography principle taking advantage of its valuable opportunity to produce ultrafine dense periodic Dr. B. H. Kim, J. Y. Kim, H. M. Jin, Prof. J. Shin, Prof. S. O. Kim Center for Nanomaterials and Chemical Reactions Institute for Basic Science (IBS) Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305 701, Republic of Korea E-mail: sangouk.kim@kaist.ac.kr Dr. K.-J. Byeon, J. Kim, J.-Y. Cho, Prof. H. Lee Department of Materials Science and Engineering Korea University Seongbuk-gu, Anam-ro 145, Seoul 136 701, Republic of Korea E-mail: heonlee@korea.ac.kr Dr. S.-J. Jeong Devices R&D Center SAIT, Samsung Electronics Yongin 446 712, Republic of Korea DOI: 10.1002/smll.201400971 nanopatterns in a scalable manner. [1 5 ] In a BCP lithography, it is crucial to control the orientation of self-assembled nanodomains in BCP thin films along film plane and thickness direction. [6 12 ] BCP thin films with surface perpendicular nanodomains are particularly attractive nanotemplates, as the vertical side wall profiles of such morphologies are greatly advantageous for the robust pattern transfer by selective etching or deposition. [13 23 ] To this end, neutral surface modification has been exploited to balance the interfacial tensions of two chemically different polymer blocks toward a substrate. To date, various surface modification methods employing self-assembled monolayer (SAM), [19 ] layer-bylayer assembly, [24 ] polymer brush and mat, [13,14 ] graphene film [20 ] have been investigated for this purpose. Directed self-assembly (DSA) is the recent direction of BCP self-assembled nanopatterning for device-oriented nanostructures. Chemically or topographically pre-patterned substrates can register the self-assembly of BCP nanodomains such that laterally ordered nanopatterns can be generated over a large area with minimal defects. [19,25 36 ] Meanwhile, precise and selective control of the substrate surface energy is a crucial element for successful DSA process. The preparation of chemically pre-patterned substrate generally requires surface imaging layer formation, whose surface 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 4207

full papers B. H. Kim et al. energy can be precisely modified by a conventional lithographic process. Graphoepitaxy utilizing topographical pattern frequently requires a selective control of surface energy at the bottom surface and trench sidewall of topographic pattern. Unfortunately, such surface modification usually burden additional processing steps and limit the broad utilization of DSA process at various substrates. In this work, we present versatile negative-tone BCP lithography based on in situ surface chemical modification, where surface neutral modification of substrates is simultaneously performed during BCP self-assembly process. After a raw substrate surface is activated by oxygen (O 2 ) plasma treatment, BCP blend films consisting of end-functionalized low molecular weight poly(styrene- ran-methyl methacrylate) P(S- r-mma) [13 ] and a symmetric polystyrene- block - Poly(methyl methacylate) (PS- b -PMMA) self-assemble into surface perpendicular BCP lamellar or cylinder nanotemplates by thermal annealing selectively at the plasma activated region. The O 2 plasma treatment generates reactive hydroxyl groups on the substrate surface, which are covalently bonded with end-functionalized P(S- r-mma) random copolymers during the thermally driven self-assembly and mediate neutral interfacial condition. This in situ surface neutralization enables area-selective nanopatterning by site-specific masked plasma treatment, which is similar to the selective pattern formation at the UV radiated surface in a negative-tone photolithography. This strategy exhibits a number of advantages: i) rapid and efficient processing with a minimum number of processing steps, ii) general applicability to various substrates, including titania (TiO 2 ), CoCrPt-alloy, ruthenium (Ru), platinum (Pt), and so on, iii) site-specific surface modification by patterned O 2 plasma treatment, and iv) high compatibility with DSA approaches to generate device oriented nanopatterns. 2. Results and Discussion Figures 1 Aa,b schematically contrast conventional BCP lithography with negative-tone BCP lithography. Conventional BCP lithography employing PS- b-pmma requires neutral surface modification for substrate vertical nanodomain alignment. The neutralized surface is selectively oxidized by light or plasma exposure using E-beam, UV, or oxygen reactive etching (O 2 RIE) to impose surface energy modulation. [19,26,37,38 ] While surface perpendicular lamellar or cylinder nanodomains are assembled on the neutral region, surface parallel nanodomains are assembled at the oxidized region. This process is similar to positive-tone photolithography process where radiative treatment removes patterned structures (Figure 1 Aa). By contrast, in negative-tone BCP lithography, raw substrates induce surface parallel BCP nanodomains, while oxidized region results in perpendicular nanodomains (Figure 1 Ab). Figure 1 B shows the photography and tilted-view SEM image of a typical sample prepared by negative-tone BCP lithography. O 2 plasma-treated region of Au substrate shows surface perpendicular lamellae (region X). By contrast, raw substrate surface show surface parallel lamellar orientation (region Y). This parallel orientation Figure 1. A) Schematic illustrations of a) conventional BCP lithography and b) negative-tone BCP lithography. B) Photography and tiltedview SEM image of typical sample. Surface perpendicular and parallel BCP lamellar nanodomains are induced on the region X and Y, respectively. C) Plane-view SEM images of BCP thin films (thickness: 100 nm), prepared from the blends of lamellar PS- b -PMMA (51 K mol 1 ) and P(S- r -MMA) brush (17 K mol 1 ) (weight ratio: 1:9), on the plasma activated Au substrate as a function of O 2 plasma treatment time (a c). results in the quantization of BCP film thickness such that micrometer scale hole/island terraced morphology is formed. Due to the light scattering from this hole/island morphology, parallel lamellar region shows hazy appearance to naked eye. [39 ] Typical processing steps for negative-tone BCP lithography process is as follows. First, O 2 plasma (50 W, 40 sccm) is treated on a raw substrate to induce surface hydroxyl group. BCP thin film prepared from the blends of polystyrene- block - poly(methyl methacylate) (51 K mol 1, PS-b-PMMA) and hydroxyl-terminated poly(styrene- ran-methyl methacrylate) (17 K mol 1, P(S-r -MMA)) is spin-coated on the activated 4208 www.small-journal.com 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Negative-Tone Block Copolymer Lithography by In Situ Surface Chemical Modification substrate and thermally annealed for self-assembly. We note that during this thermal annealing, two kinds of chemical processes compete each other. One is the covalent bonding between hydroxyl groups of substrate and P(S- r-mma) for neutral wetting and the other is the preferential segregation of PMMA component of PS- b -PMMA at the substrate. The final morphology is dominated by irreversible covalent bonding and shows surface perpendicular BCP nanodomains. Figures 1 Ca c show the SEM images of the BCP blends thin films self-assembled on O 2 plasma treated Au substrates as a function of O 2 plasma treatment time. 60 s plasma treatment was found to be sufficient to induce surface perpendicular BCP nanodomains ( Figure 2A). Our negative tone BCP lithography is effective for various materials, including Au, TiO 2, CoCrPt alloy, Ru, and Pt. Upon these substrates, all tested BCP thin films successfully form surface perpendicular nanodomains given that O 2 plasma (50 W, 40 sccm) is sufficiently treated. Figures 2 B E show the tilted-view SEM images of surface vertical lamellar BCP nanodomains self-assembled on titania (TiO 2, photocatalytic semiconductor), CoCrPt-alloy (magnetic storage material), ruthenium (Ru, highly electric-conductive material), and platinum (Pt, electrocatalystic material) surfaces, respectively. [16 ] As mentioned above, chemical/topographical confinements can enforce the lateral ordering of BCP nanodomains by DSA principle. [19,25 36 ] In Figures 3, 4, we demonstrate that typical DSA principles of epitaxial self-assembly and graphoepitaxy can be synergistically integrated with negative-tone BCP lithography for highly ordered BCP nanopatterns on various substrates. UV imprint was employed to prepare chemically/topographically prepatterned structures to direct BCP assembly (Supporting Information 1, Figure S1). [40,41 ] Figure 3 A shows that two kinds of BCP films with different molecular weights are self-assembled on the similar chemical patterns. The Au line pattern with the thickness of 50 nm was formed by conventional photoresist lift-off process for UV nanoimprinted pattern (Figure 3 Aa). Two different BCP films prepared from the blends of PS- b-pmma (thickness 200 and 140 nm for 211 and 51 K mol 1, respectively) and P(S- r -MMA) brush (solute weight ratio 9 : 1) were spin-coated and thermally annealed upon plasma activated substrates (50 W, 40 sccm, 60 s). Considering the thin thickness (50 nm) of Au layer, the resultant pattern can be regarded as chemical pattern consisting of alternate Au and Si stripes. When a high molecular weight lamellar BCP film (211 K mol 1 ) was self-assembled on the chemically patterned surface, randomly oriented lamellae were induced over the entire area, ignoring the chemical pattern (Figure 3 Ab). By contrast, when low molecular weight lamellar BCP film (51 K mol 1 ) is deposited on the chemical pattern, hierarchically ordered morphology is formed, as shown in Figure 3 Ac. Upon Si stripes, surface parallel lamellae were formed regardless of O 2 plasma treatment time. Si wafer is a well-known high surface energy substrate with native oxide layer. Preferential wetting of PMMA component is predominant at the Si stripes (Supporting Information 2). [42 ] Surface vertical lamellae are formed on Au stripes to constitute alternating surface parallel and perpendicular lamellar morphology. It is Figure 2. A) Schematic illustration for surface vertical lamellar BCP nanodomain formation procedure on various raw substrates. Tiltedview SEM images of substrate vertical BCP nanodomains prepared on B) titania, C) CoCrPt-alloy, D) ruthenium, and E) platinum, respectively. noteworthy that vertical lamellae are highly aligned along the perpendicular direction of Au stripe to minimize the energy penalty at the boundary between surface parallel and vertical lamellar region by forming twist grain boundary morphology (Figure 3A(c)). [43,44 ] Figure 3 B shows that the self-assembled morphology of BCP blends film deposited on a topographical pre-patterned structure. UV imprint pattern is fabricated on a Si wafer, above which E-beam evaporated 10-nm-thick Au layer was 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 4209

full papers B. H. Kim et al. the O 2 plasma treatment, the polymer residue layer remaining after UV imprint could be completely washed away at the bottom surface (supporting information Figure S3). By contrast, the side walls of photoresist pattern were not influenced by vertically oriented plasma treatment such that the photoresist side walls maintain preferential wettability for PS component. BCP blends films including P(S- r-mma) brush was deposited and thermally annealed (Figure 4 Aa) on the activated surface. Figure 4 Ab shows the plane- and tilted-view SEM images of BCP lamellae highly aligned along the UV imprint trench patterns. Figure 4 B presents that BCP nanotemplates assembled on Au substrate can be transferred onto nonplanar, threedimensional substrates. [45 47 ] Transferrable patterning procedure is schematically described in Figure 4 Ba. First, the UV imprint pattern is fabricated on the Au/ SiO 2 substrate. O 2 plasma is treated to activate Au surface and the BCP thin film is spin-coated and thermally annealed to form highly aligned BCP nanotemplate. The sacrificial SiO 2 layer is easily wetetched by HF solution. The BCP nanotemplate supported by Au substrate is floated at the surface of aqueous media and recovered at the desired location of nonplanar or flexible substrates, such as glass pipette surface (Figure 4Bb). Figure 3. A) a) Schematic illustration of highly-aligned lamellar nanodomain on the chemical prepattern consisting of Si and Au lines. Plane- and tilted-view SEM images of b) high and c) low molecular weight BCPs (211 and 51 K mol 1, respectively) self-assembled on the same chemical pattern. B) a) Schematic illustration and b) plane-view SEM image of the BCP lamellae highly aligned across the trench width of topographical pre-pattern. deposited by 45 tilt deposition (Figure 3 Ba). Since surface vertical lamellar BCP nanodomains are induced on the O 2 plasma activated Au surface, lamellar BCP nanodomains were oriented perpendicular to both the trench sidewall and bottom substrate to form lamellar stack along the topographic trenches (Figure 3 Bb). Figure 4 A shows that the lamellar alignment along the trench side walls can also be induced by negative-tone BCP lithography. Firstly, UV imprint photoresist pattern was fabricated on a bare Au substrate. O 2 plasma was treated to activate the bottom Au surface (50 W, 40 sccm, 180 s). During 3. Conclusion In a DSA strategy employing BCP thin films, how to control nanoscale local surface energy is a challenging technological issue. We have demonstrated negative-tone BCP lithography, which can directly generate self-assembled BCP nanotemplates on various substrates with minimum number of processing steps. As BCP thin film initially contains surface energy neutralizer, substrate can be neutrally modified in situ during thermal annealing for vertical self-assembled nanostructures. O 2 plasma is used to activate the substrate surface for reactive functional groups, which also enables area-selective surface activation for hierarchically patterned structure formation. In addition, negative tone BCP lithography can be judiciously integrated with DSA principle for device oriented laterally ordered nanopatterns. Our strategy offers a valuable route to broaden the practical applicability [ 48,49 ] of BCP lithography for diver substrate materials with precise area-selectivity. 4210 www.small-journal.com 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Negative-Tone Block Copolymer Lithography by In Situ Surface Chemical Modification BCP Blend Thin Film : End-functionalized (hydroxyl group) P(S- r-mma) random copolymer was synthesized using nitroxide-mediated living radical polymerization, as described elsewhere. [13,50] PS-b -PMMA diblock copolymers (51 Kg mol 1, 94 Kg mol 1, and 211 Kg mol 1 ) were purchased from Polymer Source, Inc. and used as received. O 2 plasma (50 W, 40 sccm, 60 s) was treated on a raw substrate to induce surface reactive functional group. BCP thin films prepared from the blends of PSb -PMMA and P(S- r -MMA) (9:1 vol. ratio) were spin-coated (2 wt% in toluene, 3000 rpm) on the activated substrates and thermally annealed (200 C for 24 h) for self-assembly. UV Nanoimprinting Lithography (NIL): A Si master template, containing nanoscale line patterns (line width: 300 nm and pitch: 600 nm) were fabricated by e-beam lithography and reactive ion etching (RIE) processes. Polymer mold was replicated from Si master template by hot embossing method. In this step, polyvinyl chloride (PVC) film was used as transparent polymer replica mold. After fabricating the PVC replica mold, imprint resin was coated on the bare substrate by drop-let method. UV curable imprinting resin was used, which consisted of base monomer (benzylmetacrylate, C 11 H 12 O 2 ) mixed with various additives such as viscosity modifier, anti-sticking agent and UV photo-initiator. UV NIL process was performed at 20 atm pressure while exposing the stack of the PVC/ resin/substrate to UV for 10 min. [40] Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements B.H.K., J.Y.K., H.M.J., and S.O.K. were supported by Institute for Basic Science (IBS) [CA1301 02]. K.-J.B., J.K., J.-Y.C., and H.L. were supported by the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012M3A7B4035323). Figure 4. A) a) Schematic illustration and b) plane- and tilted-view SEM images of the BCP lamellar nanodomains aligned along the trench side walls of topographical pattern. B) a) Schematic illustration and b) photography and plane-view SEM images of highly-aligned BCP nanopatterns transferred onto the nonplanar surface of glass pipette. 4. Experimental Section Substrate Preparation : Si wafers were used as base substrates for self-assembled nanopattering. Thin layers of Au (100 nm), Ru (60 nm), and TiO 2 films (60 nm) were deposited on the surface of Si water by e-beam evaporation, PEALD, and ALD, respectively. Pt (40 nm) and CoCrPt (30 nm) films were deposited by sputtering. [1] J. Bang, U. Jeong, D. Y. Ryu, T. P. Russell, C. J. Hawker, Adv. Mater. 2009, 21, 4769. [2] M. P. Stoykovich, P. F. Nealey, Mater. Today 2006, 9, 20. [3] C. T. Black, R. Ruiz, G. Breyta, J. Y. Cheng, M. E. Colburn, K. W. Guarini, H.-C. Kim, Y. Zhang, IBM J. Res. Dev. 2007, 51, 605. [4] B. H. Kim, J. Y. Kim, S. O. Kim, Soft Matter 2013, 9, 2780. [5] S.-J. Jeong, J. Y. Kim, B. H. Kim, H.-S. Moon, S. O. Kim, Mater. Today 2013, 16, 468. [6] I. P. Campbell, G. J. Lau, J. L. Feaver, M. P. Stoykovich, Macromolecules 2012, 45, 1587. [7] I. P. Campbell, S. Hirokawa, M. P. Stoykovich, Macromolecules 2013, 46, 9599. [8] I. P. Campbell, C. He, M. P. Stoykovich, ACS Macro Lett. 2013, 2, 918. [9] K. W. Gotrik, A. F. Hannon, J. G. Son, B. Keller, A. Alexander-Katz, C. A. Ross, ACS Nano 2012, 6, 8052. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 4211

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