Directed Assembly of Block Copolymers in Thin to Thick Films

The extent to which lamellae-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, bulk period L-0 = 48 nm) can be directed to assemble on chemically nanopatterned striped surfaces (period L-s), 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 L-0 and L-s, and annealing temperature. Two regimes of behavior are identified: thin film (<similar to 72 nm) and thick film. In the thin film regime, the range of L-s/L-0 over which the block copolymer could be directed to assemble into ordered linear arrays (with period L = L-s) decreased with increasing film thickness. On identical chemical patterns, for example, a film thickness of 30 nm assembled at 0.89 < L-s/L-0 < 1.04, but a 72 nm film only assembled when L-s similar to L-0. 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 L-s similar to L-0 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 (L-0 = L-s, annealing temperature = 230 degrees C) PS-b-PMMA films as thick as 620 nm (domain aspect ratio approximate to 25) exhibited very low levels of defectivity.