Ionic Liquids as Additives to Polystyrene-Block-Poly(Methyl Methacrylate) Enabling Directed Self-Assembly of Patterns with Sub-10 nm Features

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) is one of the prototypical block copolymers in directed self-assembly (DSA) research and development, with standardized protocols in place for processing on industrially relevant 300 mm wafers. Scaling of DSA patterns to pitches below 20 nm using PS-b-PMMA, however, is hindered by the relatively low Flory–Huggins interaction parameter, χ. Here, we investigate the approach of adding small amounts of ionic liquids (ILs) into PS-b-PMMA, which selectively segregates into the PMMA domain and effectively increases the χ parameter and thus the pattern resolution. The amount of IL additive is small enough to result in limited changes in PS-b-PMMA’s surface and interfacial properties, thus maintaining industry-friendly processing by thermal annealing with a free surface. Three different ILs are studied comparatively regarding their compositional process window, capability of increasing χ, and thermal stability. By adding ∼3.1 vol % of the champion IL into a low-molecular-weight PS-b-PMMA (Mn = 10.3k-b-9.5k), we demonstrated DSA on chemically patterned substrates of lamellar structures with feature sizes <8.5 nm. Compatibility of the PS-b-PMMMA/IL blends with the standardized processes that have been previously developed suggests that such blend materials could provide a drop-in solution for sub-10 nm lithography with the processing advantages of PS-b-PMMA.