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Weak segregation of polystyrene-block-poly(methyl methacrylate) block copolymers (PS-b-PMMA BCPs) limits their utility for sub-10 nm lithography. Such limits could be overcome by including ionic liquids (ILs) as additives, which conveniently enhance the demixing between PS and PMMA blocks. Herein, we studied the distribution and interaction evolution of the ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate (HMHF) in PS-b-PMMA BCPs. The segmental dynamics of the rigid, intermediate, and mobile molecular components in PS-b-PMMA/HMHF was characterized by solid state nuclear magnetic resonance spectroscopy, and the results suggested a selective interaction between PMMA and HMHF. The surface energies and interfacial behaviors of the polymer/HMHF mixtures indicated that HMHF was distributed underneath the PMMA surface during heat treatment. Because of this distribution, disordered PS-b-PMMA with HMHF added formed perpendicular sub-10 nm features through thermal annealing. In addition, the effective interaction parameter (χeff) between PS and PMMA/HMHF was calculated from the domain periods of the BCPs. A parameter, ξIL, defined as the ratio of the Coulomb and Lennard-Jones interactions, was introduced to obtain self-consistent χeff values. The fitted ξIL values explained the dependence of χeff on molecular weight due to the dramatically enhanced charge effects for BCPs with low degrees of polymerization. The PS-b-PMMA BCPs still exhibited great compatibility with the later BCP lithography fabrication when using ionic liquid additives.
Chen et al. (Mon,) studied this question.