Chemical Patterns for Directed Self-Assembly of Lamellae-Forming Block Copolymers with Density Multiplication of Features

Lamellae-forming polystyrene-<i>block</i>-poly­(methyl methacrylate) (PS-<i>b</i>-PMMA) films, with bulk period <i>L</i>₀, were directed to assemble on lithographically nanopatterned surfaces. The chemical pattern was comprised of “guiding” stripes of cross-linked polystyrene (X-PS) or poly­(methyl methacrylate) (X-PMMA) mats, with width <i>W</i>, and interspatial “background” regions of a random copolymer brush of styrene and methyl methacrylate (P­(S-<i>r</i>-MMA)). The fraction of styrene (<i>f</i>) in the brush was varied to control the chemistry of the background regions. The period of the pattern was <i>L</i>_s. After assembly, the density of the features (domains) in the block copolymer film was an integer multiple (<i>n</i>) of the density of features of the chemical pattern, where <i>n</i> = <i>L</i>_s/<i>L</i>₀. The quality of the assembled PS-<i>b</i>-PMMA films into patterns of dense lines as a function of <i>n</i>, <i>W</i>/<i>L</i>₀, and <i>f</i> was analyzed with top-down scanning electron microscopy. The most effective background chemistry for directed assembly with density multiplication corresponded to a brush chemistry (<i>f</i>) that minimized the interfacial energy between the background regions and the composition of the film overlying the background regions. The three-dimensional structure of the domains within the film was investigated using cross-sectional SEM and Monte Carlo simulations of a coarse-grained model and was found most closely to resemble perpendicularly oriented lamellae when <i>W</i>/<i>L</i>₀ ∼ 0.5–0.6. Directed self-assembly with density multiplication (<i>n</i> = 4) and <i>W</i>/<i>L</i>₀ = 1 or 1.5 yields pattern of high quality, parallel linear structures on the top surface of the assembled films, but complex, three-dimensional structures within the film