Hierarchical pattern formation in thin film diblock copolymers above the order-disorder transition temperature

Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at temperatures higher than the bulk order-disorder transition temperature T-ODT are shown to dewet silicon substrates, forming topographical features that depend on the initial film thickness h. Films of thickness h < 3.5 nm dewet the substrate, forming bicontinuous spinodal-like patterns. When 3.5 nm < h < h(L) = 7 nm, discrete holes are observed randomly throughout the film. For films of thickness in the range h(L) < h < h(H) = 35 nm, the copolymer exhibited autophobic behavior, whereby the top layer of thickness (h - h(L)) dewets a dense "brush" of ordered copolymer of height hL anchored to the silicon substrate. The morphologies, which include a bicontinuous spinodal pattern for films of thickness in the range h(L) < h < 19 nm, and discrete holes, for films of thickness in the range 19 nm < h < h(H), eventually evolve into droplets. Films of h > 35 nm remained stable, with smooth surfaces. The time-dependent evolution of the "spinodal" structures that evolve in the autophobic regime is discussed. In addition, the existence of surface-induced ordering of the copolymer at temperatures above the bulk T-ODT is also discussed.