Ratchana Limary

Block copolymer thin films: pattern formation and phase behavior

Abstract During the last decade, research on thin, sub-micron thick, block copolymer films was devoted toward understanding and controlling microstructural and topographical features at temperatures T T ODT , where T ODT is the order–disorder transition temperature below which thermodynamic interactions favor the formation of ordered (phase separated) microstructures. Symmetric diblock copolymers, the subject of this review, undergo an isotropic to lamellar transition when T T ODT . Topographical features, ‘islands’ or ‘holes,’ of these films typically reflect the underlying phase separation; and the dimension of these features, normal to the substrate is equal to the interlamellar spacing, L . Two aspects of block copolymer thin films that have not received much attention are discussed in this paper: (1) pattern formation in symmetric block copolymers under conditions of T > T ODT ; and (2) phase behavior of thin film symmetric diblock copolymer/homopolymer mixtures, when T T ODT .

Block Copolymer Thin Films Above and Below the Order-Disorder Transition Temperature

Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm.