Reversible polystyrene-block-poly(methyl methacrylate) copolymer films with perpendicular orientation by ultra-thin polystyrene substrates

•The bottom views of block copolymer films were observed directly by SEM to understand the orientation behaviors of block copolymers at the interface.•Ultra-thin polymer films above strongly polar substrates were neutral for obtaining perpendicularly orientation of PS-b-PMMA BCP.•Perpendicular orientation in the block copolymer was through the film up to down by comparing the bottom views with the top. Detecting the inner structures in the perpendicularly orientated block copolymer (BCP) films is beneficial to understand the in-depth arrangement of nano-domains. This study presents a method to reverse the perpendicularly orientated polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP films for observing their bottom morphologies. Only PS brush or mat was used as neutral substrates, because strongly polar groups inside the PS chains exposed to balance the interfacial energies of PS-OH chemical substrates. The perpendicular window for fingerprint morphology of PS-b-PMMA BCP was among 1.16∼1.44 nm of PS brush on silicon wafer with silanol group. For crosslinked PS mat on water soluble substrate of sodium polyacrylate, however, perpendicular window was shortened to be around 1.37 nm, which was confirmed by both interfacial energy and images of scanning electron microscope (SEM). The bottom views of PS-b-PMMA block copolymer films with perpendicular and mixed orientation were observed freshly. There was no obvious difference between the fingerprints of top and bottom, indicating that perpendicular orientation in the block copolymer was through the film up to down. Therefore, the challenges of degrees of perfection from directed assembly of various morphologies are deduced to be originated from match of the morphology with the e-beamed prepattern or defects in the prepattern.