Hierarchical Surface Topography in Block Copolymer Thin Films Induced by Residual Solvent

Abstract

We have investigated the hierarchical surface topography in block copolymer thin films induced by residual solvent using optical microscopy and atomic force microscopy. A polystyrene−poly(2-vinylpyridine) (PS−P2VP) block copolymer forming lamellar morphology was spun-cast from N,N-dimethylformamide (DMF) solution onto a silicon wafer with a native oxide layer. Thin films with various thicknesses between 2.0L0 and 3.5L0, where L0 is the equilibrium lamellar period, were prepared. Typical multilayered lamellae having either islands or holes with quantized thicknesses were observed after annealing at 180 °C due to the strong affinity of the P2VP blocks toward the Si substrate and the preferential wetting of the PS blocks at the free surface. Further annealing, however, resulted in the formation of hierarchical surface topography such as hole-in-hole and/or island-on-island morphology. In addition, holes adjacent to the brush layer with L0/2 thickness tightly bound to the substrate were found to be fractal having a fractal dimension of 1.65. A small amount of residual solvent still remains in the film after spin-casting due to the high boiling temperature of DMF. This induces the lateral strain within the films during the annealing process, which further triggers the formation of the intriguing surface patterns. Autophobic dewetting is also believed to play an important role in the growth of the pattern.