Perpendicular orientation control in thin films of POSS-containing block copolymer domains with a top-coat surface treatment

Abstract

The directed self-assembly of block copolymers (BCPs) can extend the resolution of traditional optical lithographic techniques. Recently, Si-containing BCPs have been shown to produce very small features because they have high χ values. Unfortunately, they cannot be aligned perpendicularly to substrates by thermal annealing because Si-containing blocks exhibit lower surface free energies (SFEs) and tend to form top-wetting layers that are parallel to the substrates. The key factor in achieving orientation control of such types of Si-containing BCPs is SFE control. Three types of newly designed polymers for bottom surface layers (BSLs) that exhibit different SFEs were tested with cylinders forming PMMA-block-polyhedral oligomeric silsesquioxane (POSS)-containing poly(methacrylate) (PMMA-b-PMAPOSS) and top-coat (TC) materials to prevent the formation of Si-wetting layers. After annealing at 180 °C for 60 min on a hot plate and ashing with oxygen, the samples annealed with TC, and the optimum BSL showed hexagonally packed cylinders; however, the other BSLs did not have perpendicular orientations. To understand these results, the dispersive and polar components of the BSLs, PMMA homopolymer and PMAPOSS homopolymer were compared. The results demonstrated that the relationships between each of the BSL components and homopolymers were found to be more important indicators than the water contact angles. Three types of bottom surface layers (BSLs-A, B, C) with different surface properties and a top-coat (TC-A) were applied to investigate perpendicular orientation control of PMMA-b-PMAPOSS domains in the thin films with thermal annealing. The combination of BSL-A and TC-A yielded perpendicular cylinders after annealing at 180 °C for 60 min, whereas the other combinations produced parallel cylinders. Water contact angles and surface free energies (SFEs) of the BSLs were evaluated for the perpendicular orientation. It was found that the SFEs were more effectual indicators than the water contact angles.