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Perpendicular Orientation Control without Interfacial Treatment of RAFT-Synthesized High‑χ Block Copolymer Thin Films with Sub-10 nm Features Prepared via Thermal Annealing Ryuichi Nakatani,† Hiroki Takano,† Alvin Chandra,† Yasunari Yoshimura,† Lei Wang,† Yoshinori Suzuki,† Yuki Tanaka,† Rina Maeda,† Naoko Kihara,‡ Shinya Minegishi,‡ Ken Miyagi,‡ Yuusuke Kasahara,‡ Hironobu Sato,‡ Yuriko Seino,‡ Tsukasa Azuma,‡ Hideaki Yokoyama,§ Christopher K. Ober,∥ and Teruaki Hayakawa*,† †
Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-S8-36 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ DSA Research Department, EUVL Infrastructure Development Center, Inc., 16-1 Onogawa, Tsukuba-shi, Ibaraki 305-8569, Japan § Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ∥ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, United States S Supporting Information *
ABSTRACT: In this study, a series of perpendicular lamellaeforming poly(polyhedral oligomeric silsesquioxane methacrylate-block-2,2,2-trifluoroethyl methacrylate)s (PMAPOSS-bPTFEMAs) was developed based on the bottom-up concept of creating a simple yet effective material by tailoring the chemical properties and molecular composition of the material. The use of silicon (Si)-containing hybrid high-χ block copolymers (BCPs) provides easy access to sub-10 nm feature sizes. However, as the surface free energies (SFEs) of Si-containing polymers are typically vastly lower than organic polymers, this tends to result in the selective segregation of the inorganic block onto the air interface and increased difficulty in controlling the BCP orientation in thin films. Therefore, by balancing the SFEs between the organic and inorganic blocks through the use of poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) on the organic block, a polymer with an SFE similar to Si-containing polymers, orientation control of the BCP domains in thin films becomes much simpler. Herein, perpendicularly oriented BCP thin films with a χeff value of 0.45 were fabricated using simple spin-coating and thermal annealing processes under ambient conditions. The thin films displayed a minimum domain size of L0 = 11 nm, as observed via atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Furthermore, directed self-assembly (DSA) of the BCP on a topographically prepatterned substrate using the grapho-epitaxy method was used to successfully obtain perpendicularly oriented lamellae with a half pitch size of ca. 8 nm. KEYWORDS: block copolymer lithography, perpendicular lamellae, sub-10 nm, directed self-assembly, orientation control, interfacial control, hybrid block copolymer, nonpreferential interface
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INTRODUCTION As the demand for miniaturized devices and increased on-chip density is constantly increasing in the microelectronics industry, the target critical dimensions for semiconductor devices have shifted to successively smaller and smaller sizes.1,2 In the last several decades, a production method that has received growing interest for producing higher levels of pitch subdivision is block copolymer (BCP) lithography in conjunction with directed selfassembly (DSA).3−8 BCPs are of great interest for highresolution patterning because of their affordability and high throughput potential. In addition, as the processing of BCPs is compatible with current patterning and exposure technologies, © XXXX American Chemical Society
it is possible to combine the most advanced exposure tools and the perpendicular lamellar or parallel cylindrical morphologies of self-assembled BCP materials to expand current manufacturing technologies to fabricate line and space (L/S) structures with domain dimensions of 10 nm and less.9−12 In particular, perpendicularly oriented lamellae are preferred for such Special Issue: Block Copolymers for Nanotechnology Applications Received: December 16, 2016 Accepted: March 1, 2017
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DOI: 10.1021/acsami.6b16129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
hexagonal dot patterns with a feature size of less than 10 nm and a dot density of 5.9 Tb/inch2 after solvent annealing or perpendicularly oriented cylinders after thermal annealing with a top-coat.35−37 However, perpendicularly oriented lamellae have yet to be observed in thin films containing these BCPs. This could be attributed to the large SFE gap between the blocks (PMMA, 41.1 mJ/m2; PMAPOSS, 28.7 mJ/m2). Therefore, to develop a polymer that does not require any postsynthesis functionalization and is able to orient into perpendicular lamellae with just thermal annealing and without any interfacial modifications, we refined the polymer design further to obtain a novel BCP with the interfacial affinity control unit already included in the repeating units of one of the blocks, poly(polyhedral oligomeric silsesquioxane methacrylate-block-2,2,2-trifluoroethyl methacrylate) (PMAPOSS-bPTFEMA). In contrast to methyl methacrylate (MMA) which has a methyl group bonded to the methacrylate moiety, TFEMA is functionalized with a hydrophobic 2,2,2-trifluoroethyl group. Fluorine-containing polymers such as PTFEMA not only display increased segregation strengths when compared to analogous nonfluorinated polymers, but also increased etching contrasts.38−41 In addition, the SFEs of PTFEMA and PMAPOSS are highly similar (PTFEMA, 25.1 mJ/m2; PMAPOSS, 28.7 mJ/m2),42 allowing for the easy control of the lamellar orientation and high resolution patterning with a high aspect ratio on thin films.43 In a previous study,42 PMAPOSS-b-PTFEMA was synthesized via living anionic polymerization and self-assembled on thin films to obtain perpendicular lamellae with L0 = 20.7 nm via thermal annealing without any modifications to the top (air/ free surface) and bottom (substrate) interfaces. However, precise control of the primary structure of PMAPOSS-bPTFEMA via living anionic polymerization was not possible. This is likely a result of various side reactions that terminated the propagating chains. We speculated that termination occurred via the intramolecular backbiting of the propagating chain. In view of possible practical applications, a synthetic method for obtaining well-controlled polymers without critical factors such as homopolymer contaminants, metal contaminants, or undesired byproducts is crucial. In this paper, reversible addition−fragmentation chain transfer (RAFT) polymerization was employed to fulfill such requirements.44−46 RAFT is a powerful technique for the preparation of BCPs with low dispersities (
