Morphological Reconstruction and Ordering in Films of Sphere-Forming Block Copolymers on Striped Chemically Patterned Surfaces

The morphology of thin films of sphere-forming block copolymers on striped chemically patterned surfaces was investigated as a function of thickness and commensurability of the pattern period ( L s ) and the lattice spacing of the spherical structure ( L o ), using scanning electron microscopy and atomic force microscopy. In all cases, a linear wetting layer forms on the chemical pattern. As the film thickness increases from 10 to 25 nm, protuberances in the wetting layer appear. In 57 nm thick films, when L s and L o are sufficiently commensurate, the protuberances in the knurled, linear wetting domain and the spheres in the overlying layer of P(S- b -MMA) coordinate during equilibration, resulting in an ordered hexagonal array of partial spherical domains. The range of commensurability over which such an ordered hexagonal structure is achieved decreases when the film thickness is increased to 70 nm. In general, the process leads to a pattern transformation from a series of parallel lines at the substrate surface to an ordered, hexagonal array of spots at the free surface of the film as the block copolymer starts to reconstruct its bulk morphology away from the chemically patterned surface.