Controlling Domain Spacing and Grain Size in Cylindrical Block Copolymer Thin Films by Means of Thermal and Solvent Vapor Annealing

Abstract

Real-time grazing-incidence small-angle X-ray scattering (GISAXS) experiments were used to study the self-assembly of cylinder-forming block copolymers (BCPs) in thin films during thermal annealing and solvent vapor annealing. BCP thin films were annealed in near-neutral solvent vapor for solvent vapor annealing and on a hot plate under an inert gas atmosphere for thermal annealing. The initially ordered films were heated or swollen to induce an order–disorder transition (ODT) and then cooled or the solvent was removed, respectively. The domain spacings of BCPs as determined from in situ GISAXS measurements during solvent removal and cooling were analyzed with respect to the polymer concentration and the reciprocal temperature. Close to the ODT the domain spacing was found to be nearly identical for thermal and solvent vapor annealing. At lower solvent concentrations ϕ and lower temperatures T, the domain spacing was found to increase for both thermal and solvent vapor annealing until structural reorganization in the film was limited by the slow kinetics at solvent concentrations and temperatures close to the glass transition. In this regime, the domain spacing in solvent annealed films was found to be higher than that in thermally annealed films, which is likely due to a significantly smaller diffusion coefficient in the case of thermal annealing. On the basis of an ex situ scanning electron microscopy characterization of annealed block copolymer thin films, we show that the grain size of the cylindrical microdomains can be strongly increased by annealing films close to the ODT. Well below ϕODT and TODT the formation of large grains is kinetically limited. In thermally annealed films the grain size was found to be smaller than that for the solvent annealed films, which was attributed to a smaller diffusion coefficient in the absence of solvent.