Morphology of Thin Films of Diblock Copolymers on Surfaces Micropatterned with Regions of Different Interfacial Energy

Abstract

Self-assembled (SA) films of octadecyltrichlorosilane were patterned by exposure to X-rays in air through a mask to produce chemically patterned surfaces. The irradiated regions of the SA films underwent surface chemical modification from nonpolar, hydrophobic surfaces to polar, hydrophilic surfaces. The degree of chemical modification increased with increasing exposure dose. The behavior of thin films of symmetric poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) was investigated on the chemically patterned surfaces exposed to X-rays at doses ranging from 800 to 2000 mJ/cm2. P(S-b-MMA) films exhibited symmetric, neutral, and asymmetric wetting on these surfaces with increasing dose. The area and height of islands and holes that formed on the free surface of the films depended on the interfacial energy between the film and the surface. If the difference in interfacial energies (Δγ) between the surface and both PS and PMMA was small, islands or holes formed that had a smaller average area and smaller step height than islands or holes that formed on surfaces where Δγ was large. Additionally, perpendicular lamellae were detected at the edge of islands when Δγ was small but were not detected at the edge of islands when Δγ was large. The island/hole morphologies observed over regions where Δγ was small were concluded to be the result of slow rates of formation and growth of the topography. Films of initial thickness from 1.75 to 2.75Lo, where Lo is the bulk lamellar period, were studied on chemically patterned surfaces where both exposed and unexposed regions had large values of Δγ. On these surfaces, regions of the film were observed to remain flat and featureless even when the film thickness did not equal a quantized value of the thickness. These flat regions were concluded to be due to either mass transport of excess material from island-forming regions across the pattern boundaries to regions with deficits (hole-forming regions) or deformation of lamellae in the outer layer of the block copolymer film. On surfaces with large values of Δγ, ordering was fast and allowed large-scale cooperation between adjacent regions of the polymer film to set up the film morphology during the very early stages of annealing.