Symmetric Diblock Copolymer Thin Films on Rough Substrates. Kinetics and Structure Formation in Pure Block Copolymer Thin Films

The effect of substrate roughness on the orientation of lamellar microdomains of symmetric poly(styrene)-block-poly(methyl methacrylate) [PS-b-PMMA] was investigated. Thin films of three molecular weights of PS-b-PMMA were prepared on organic polyimide and inorganic indium tin oxide substrates whose surfaces were characterized for roughness and surface energy. It was shown, through cross-section transmission electron microscopy (TEM) and dynamic secondary ion mass spectroscopy (dSIMS), that above a critical substrate roughness all three molecular weights of PS-b-PMMA produced a perpendicular lamellar orientation. Using atomic force microscopy (AFM) and PS-b-PMMA thin films on an array of polyimide substrates of varied substrate roughness, a critical substrate roughness was identified, below which a parallel orientation was observed. This behavior was modeled simply and showed that the critical roughness determined by AFM represents an underestimate of the true critical roughness of the substrate. Finally, a series of TEM cross sections of thin films on rough and smooth substrates, annealed to different stages of reaching equilibrium, are shown and discussed in terms of the dynamics of ordering in block copolymer thin films.