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 h L 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.