Correlated roughness, long-range correlations, and dewetting of thin polymer films

The surface morphology of thin polymer films (polystyrene) on top of silicon substrates as prepared by spin-coating and after the dewetting of an initially homogeneous polymer film was examined. We investigated the influence of film thickness, molecular weight, annealing temperature, and used solvent. With diffuse X-ray scattering in the region of total external reflection a depth sensitivity was achieved. The in-plane structure of the untreated samples shows significant differences: Starting from a molecular weight of 28k the samples show conformal roughness over a range of film thickness between 50 and 1160 Å, whereas in an equivalent film thickness range for a low molecular weight sample no correlation between the substrate and the polymer surface was detected. Above the glass transition temperature the roughness correlation was lost and therefore has no influence on the dewetting behavior of the polymer films. A variation of the solvent used for the spin-coating gives rise to different dewetting structures that originate from different in-plane film structures right after the preparation. Thus, diffuse scattering is well suited for probing long-range correlations and mobility.