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The most widely studied block copolymer (BCP) for directed self-assembly (DSA) is poly(styrene-block-methyl methacrylate) (PS-b-PMMA). However PS-b-PMMA has a limitation of patterning size in sub-10 nm because the Flory-Huggins interaction parameter (χ) of PS-b-PMMA is approximately 0.038 at room temperature. To obtain sub-10 nm patterning size, many high χ BCPs and systems have been studied. We investigate DSA of PS-b-PMMA with blended an ionic liquid (IL) via thermal annealing with free surface under nitrogen. L0 becomes larger than pure BCP by adding ILs in this system since χ parameter becomes higher. However there is a problem that long time (>30 min) and/or high temperature (>200 °C) annealing conditions causes L 0 shrink than in mild annealing condition (200 °C /1 min). Here, we report IL design to prevent L 0 shrink. The design of IL that we found in this study achieved less than 5 % L 0 shrink at 215 °C for 30 min annealing.