Hierarchical Directed Self-Assembly of Diblock Copolymers for Modified Pattern Symmetry

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com Application area of BCP self-assembled thin films is highly restricted by the limited degree of freedom in pattern symmetry, typically including hexagonal close-packed cylinders/spheres and bilateral symmetric lamellae/cylinder arrays. Sparse density modulated nanopatterns and modified pattern symmetries are demanded for many different applications, such as electrical separation of active/non-active area, optical guiding application, and ligand patterning for cell migratory.[24–28] To this end, previous approaches have introduced BCP blend systems and elaborately designed ABC triblock copolymers or starshape BCPs. Han et al. reported the symmetry modified lamellar patterns by using binary blends of polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) and polystyrene-block-poly(4-hydroxystyrene) (PS-b-PHS). The strong hydrogen bonding between P2VP and PHS blocks led to asymmetric lamellar morphologies.[29] Tang et al. exploited hydrogen bonding mediated A-B/B-C diblock copolymer blend system for square symmetry patterns.[30] Square symmetry patterns were also reported by Chuang et al. by means of ABC triblock copolymer systems.[31] Polyisoprene-block-polystyrene-blockpoly ferrocenylsilane (PI-b-PS-b-PFS) thin film exhibit square patterns consisting of alternating PI and PFS cylinders oriented perpendicular to the substrate within a PS matrix. Besides, Choi et al. reported square symmetry knitting pattern by using polyisoprene-arm-polystyrene-arm-polyferrocenylethylmethysilane (PI-PS-PFS) miktoarm star terpolymer.[32] Although those approaches successfully demonstrated symmetry modification of self-assembled morphologies, narrow choice of block copolymer components and their complicated synthetic procedure remains formidable challenge. Meanwhile, pattern symmetry control based on well-established diblock copolymer selfassembly system has been rarely demonstrated.[33–35] In this work, we report hierarchical self-assembly in the thin films of widely used poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock BCPs for the nanoscale patterning with modified pattern symmetry. In our approach, various combinations of cylinder and lamellar PS-b-PMMA BCP layers are sequentially assembled into hierarchical nanopatterns. The secondary BCP layers are monolithically integrated into the primary BCP layers, allowing vertical nano domain orientation and conformability to pre-existing chemical and physical guide pattern. Grazing-incidence small-angle X-ray (GISAXS) measurements exactly characterize the internal Hierarchical Directed Self-Assembly of Diblock Copolymers for Modified Pattern Symmetry