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, chi. 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 chi 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 Its are studied comparatively regarding their compositional process window, capability of increasing x, and thermal stability. By adding similar to 3.1 vol % of the champion IL into a low-molecular-weight PS-b-PMMA (M-n = 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.