(402d) Controlling Domain Orientations In Thin Films of Lamellar Copolymers

Thin films of block copolymers are widely investigated for low cost, large area nanopatterning. For applications in semiconductor manufacturing, the copolymer domains must be oriented perpendicular to the substrate through the film thickness.  It is well known that a perpendicular orientation can be sustained in thin films of diblock copolymers if the following criteria are met: (i) The copolymer constituents have similar surface energies; (ii) The substrate is energetically neutral with respect to the copolymer constituents; and (iii) The film thickness is slightly incommensurate with the equilibrium domain periodicity. In this work, we demonstrate that copolymer architecture significantly affects the energetics that control domain orientations in thin films. Two copolymer architectures are compared: An AB diblock of poly(styrene-b-methyl methacrylate) and an ABA triblock of poly(methyl methacrylate-b-styrene-b-methyl methacrylate). Both copolymer systems form lamellar domains with an equilibrium periodicity of 50 nm. All films are cast on ``neutral’’ substrates, and the resulting structures are evaluated with optical microscopy, scanning force microscopy, and grazing-incidence small-angle X-ray scattering. For AB diblock copolymers, domain orientations are very sensitive to film thickness, annealing temperature, and imperfections in the “neutral” substrate coating.  The desired perpendicular orientation is most reliable obtained at high annealing temperatures where PS and PMMA have nearly identical melt surface energies.  For ABA triblock copolymers, the perpendicular domain orientation is stable for all film thicknesses and annealing temperatures that were studied.  These findings are consistent with recent works that consider architectural effects when calculating the copolymer surface tension.[1,2] Significantly, the data demonstrate that triblocks are easier to process for applications in nanopatterning - particularly when high-aspect-ratio nanostructures are required. However, both diblock and triblock films contain a high density of “tilted” or bent domains, and these kinetically-trapped defects should be minimized for most patterning applications.

[1] Khanna et al., Macromolecules 2006, 39, 9346–9356.

[2] Matsen, Macromolecules 2010, 43, 1671–1674.