Directed Assembly of Block Copolymers in Thin to Thick Films

Abstract

The extent to which lamellae-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, bulk period L0 = 48 nm) can be directed to assemble on chemically nanopatterned striped surfaces (period LS), with few defects and with domains registered to and extending vertically away from the underlying pattern, was studied as a function of film thickness, commensurability between L0 and LS, and annealing temperature. Two regimes of behavior are identified: thin film (<∼72 nm) and thick film. In the thin film regime, the range of LS/L0 over which the block copolymer could be directed to assemble into ordered linear arrays (with period L = LS) decreased with increasing film thickness. On identical chemical patterns, for example, a film thickness of 30 nm assembled at 0.89 < LS/L0 < 1.04, but a 72 nm film only assembled when LS ∼ L0. In the thin film regime, assembly is cooperative throughout the film thickness, and the behavior can be understood using thermodynamic arguments based on a phenomenological model that includes terms related to the configurational entropy of polymer chains, block–block interfacial energy, and film substrate interfacial energy. In the thick film regime, Directed Assembly occurs only for LS ∼ L0 but can occur in films as thick as 620 nm. In this regime, we present evidence that nucleation and growth of grains occurs independently at the free surface of the film and near the chemically patterned surface. Upon further annealing, the directed orientation and near perfect ordering nucleated on the chemical pattern eventually dominates the structure across the entire film. The thickness through which the directed structure may be realized increases with increasing annealing temperature and time such that under certain conditions (L0 = LS, annealing temperature = 230 °C) PS-b-PMMA films as thick as 620 nm (domain aspect ratio ≈25) exhibited very low levels of defectivity.