Direct In Situ Observation of the Early‐Stage Disorder–Order Evolution of Perpendicular Lamellae in Thermally Annealed High‐χ Block Copolymer Thin Films

Abstract The evolution of well‐ordered, sub‐10 nm, perpendicular lamellae from a disordered state is documented directly using in situ atomic force microscopy, cross‐sectional scanning electron microscopy, and grazing‐incidence small angle X‐ray scattering. Since block copolymer (BCP) self‐assembly occurs on extremely fast time scales, directly imaging the growth of perpendicular lamellae remains challenging. The lack of understanding behind early‐stage self‐assembly and how metastable defects can affect the kinetic behavior near thermodynamic equilibrium prevents the realization of defect‐free thin films. By characterizing the evolution of nanostructures using a high‐χ, perpendicular lamella‐forming BCP, poly(polyhedral oligomeric silsesquioxane)‐block‐poly(2,2,2‐trifluoroethyl methacrylate), over a wide temperature range, three stages in the evolution of perpendicular lamellae, characterized mainly by the root‐mean‐squared (rms) roughness, is observed. At temperatures lower than the glass transition temperatures (T_g), the film retains a smooth, disordered surface (rms roughness = 0.352 ± 0.006 nm), characteristic of spin‐coated films. Near the T_g, spinodal decomposition and the appearance of lamellae are detected as the rms roughness increases at 70 °C, before peaking at 82 °C at 0.83 nm, and decaying rapidly with increasing temperatures. Above the T_g, the rms roughness returns to 0.35 ± 0.03 nm, as defects are observed to annihilate sequentially.