Morphology and surface relief structures of asymmetric poly(styrene-block-ferrocenylsilane) thin films

Thin films of an asymmetric styrene-block-ferrocenyldimethylsilane copolymer were studied on solid silicon substrates as well as free-standing. In the bulk the diblock forms poly(ferrocenylsilane) cylinders in a poly(styrene) matrix. The thin films were analyzed by a variety of techniques including atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray reflectivity, secondary ion mass spectrometry (SIMS), and optical microscopy. From X-ray reflectivity measurements it was concluded that both blocks were present at the substrate, whereas poly(styrene) wetted the free surface. The formation of islands and holes was observed by optical microscopy and AFM, as a result of a mismatch between the film thickness and the equilibrium spacing between layers of cylinders. Upon longer annealing of the films, the islands disappeared due to a change of the internal morphology of the thin film. After selective removal of the organic matrix by means of oxygen reactive ion etching, the phase-separated structure could be visualized by AFM. The morphology of the films altered upon annealing from an inplane cylindrical to a hexagonal morphology. The structure in free-standing films could be analyzed by TEM. After annealing, both types of orientations of cylinders (parallel and perpendicular with respect to the surface) were found to be stable.