Determination of the maximum thickness for directed self-assembly of cylinder-forming PS-b-PMMA films on chemical patterns

Abstract

Previous studies on directed self-assembly (DSA) of cylinder-forming PS-b-PMMA mainly use films with a thickness comparable to the natural period of the block copolymer, L_o. It is unclear how thick the cylinder-forming PS-b-PMMA film can be directed to assemble on chemical patterns. In this work, a feasible approach is developed to determine the maximum thickness for DSA of cylinder-forming PS-b-PMMA films. The chemical pattern with a controlled correlation length (ξ) is fabricated by molecular transfer printing (MTP) by replicating the surface domain pattern of the template blend film and used to direct the self-assembly of the same blend film. The transition from substrate-controlled DSA to surface-controlled self-assembly is located in assembled films on MTP replica patterns with the thickness between 4.9L_o and 5.7L_o by comparing the ξ of the assembled film with the ξ of the original template. DSA of cylinder-forming PS-b-PMMA is achieved in films with thickness up to at least 4.9L_o. When the film is thicker than 5.7L_o, the substrate and surface effects decouple and the free surface controls the film surface morphology.