Perpendicular Orientation of Domains in Cylinder-Forming Block Copolymer Thick Films by Controlled Interfacial Interactions

Abstract

We report the induction of perpendicularly oriented cylindrical domains in PS-b-PMMA block copolymer (BCP) films thicker than 100 nm by thermally annealing on a substrate modified with a random copolymer. The effects of annealing temperature, composition of the substrate-modifying random copolymer, and BCP film thickness on the morphology of PMMA cylinder forming PS-b-PMMA were studied. For BCP films thicker than 100 nm, the fabrication of perpendicular PMMA cylinders is highly dependent on both the substrate-modifying random copolymer and the annealing temperature as these two parameters control the interactions of the BCP with the substrate and the free surface, respectively. We found the best perpendicular structures to be created by using a random copolymer brush with a styrene fraction (F St) near 0.70 and an annealing temperature near 230 °C. Perpendicular cylinder structures were achieved in ∼300 nm thick films using these conditions. When the BCP film was thicker than 300 nm, nucleation and growth of the microdomains proceeded independently from each interface. We present scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM) images of these perpendicular structures and explain the results on the basis of previous simulation reports.

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“…[97] Block copolymers in thin film states are used to create surfaces with densely packed periodic nanopatterns whose characteristic length scales are typically tunable within 5-50 nm. [98] In addition to the parameters described in the bulk state, the nanopatterns and nanodomain spacing in the film state can be additionally controlled through film thickness, [99] polymer-air and polymer-substrate interactions, [100] or prepatterned substrates. [101] To gain deeper knowledge on the fundamental science of phase separation of block copolymers, and how to direct the self-assembly process, the reader is referred to other review articles.…”

Can ionic liquid additives be used to extend the scope of poly(styrene)-block-poly(methyl methacrylate) for directed self-assembly?

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