TIME EVOLUTION OF THE FREE-SURFACE OF ULTRATHIN COPOLYMER FILMS

Optical microscopy and X-ray reflectivity have been used to investigate the behavior of ultrathin films of a symmetric diblock poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) copolymer deposited on a silicon substrate. Thick films of such a copolymer exhibit, at equilibrium, a multilayer structure where the lamellae, of thickness L, are oriented parallel to the substrate and the PS and PMMA blocks are located respectively at the air/polymer and substrate/polymer interfaces. The influence of the thickness constraints on the morphology of the free surface has been studied. For copolymer whose molecular weight M(w) is 91 500, spin-coated films with thicknesses less than the lamellar period have been annealed under vacuum at 170-degrees-C. Different morphologies are encountered depending on the annealing time and the thickness. For thin films whose thickness t(ac) is such that L/2 < t(ac) < L, either a stretched monolayer or the formation of islands is observed. When the film thickness is equal to L/2, a stable monolayer of low surface roughness is found, while, for thicknesses less than L/2, the monolayer is not stable.