Nanostructure formation in spin-cast polystyrene films

The dependence of spin-cast polystyrene film thickness on silicon substrates is examined as a function of concentration (in toluene) and molecular weight. For uniform films ranging in thickness from 100 Å to 1 μm, thickness follows a power law dependence on concentration, with the power varying from 1.25 at low molecular weights, to greater than 2 at the highest molecular weights examined. For very dilute concentrations, atomic force microscopy reveals that the films break up into a uniform distribution of voids or islands with submicron dimensions, as has been observed previously. The apparent film thickness (at molecular weight 690000), is determined by secondary ion mass spectrometry, and is found to vary linearly with concentration for film thicknesses below ∼30 Å, the thickness at which the film begins to break up. At this molecular weight, X-ray reflectivity results indicate that the limiting thickness of the remnant polymer regions is ∼30 Å. Effects of annealing are also investigated.