Enhanced vertical ordering of block copolymer films by tuning molecular mass

Abstract

We demonstrate that an increase in the molecular mass (chain length N) of a cylinder-forming PS–PMMA block copolymer (BCP), and thus the Flory–Huggins interaction strength χN, allows us to form well-organized surface patterns having the technologically interesting perpendicular cylinder BCP orientation with respect to the substrate. Tuning the polymer mass also allows for a precise control of the in-plane BCP cylinder pattern wavelength λ and gives rise to a local height variation of the BCP film in which the average film roughness varies in direct proportion to λ. At a fixed ordering temperature (T = 182 °C), we observe an orientation transition with increasing BCP molecular mass from a parallel to a perpendicular orientation. Based on the findings of the present work, and accumulated results from our former studies of BCP ordering, we propose as a general principle that increasing the BCP segregation strength by either lowering temperature or increasing the BCP mass, enhances the extent of vertical ordering in BCP thin films. We suggest that this effect arises because the segregation strength χN controls the shear rigidity of self-assembled BCP structures.