Chemical Patterns for
Directed Self-Assembly of Lamellae-Forming
Block Copolymers with Density Multiplication of Features
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Abstract
Lamellae-forming polystyrene-<i>block</i>-poly(methyl
methacrylate) (PS-<i>b</i>-PMMA) films, with bulk period <i>L</i><sub>0</sub>, 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><sub>s</sub>. 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><sub>s</sub>/<i>L</i><sub>0</sub>.
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><sub>0</sub>, 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><sub>0</sub> ∼ 0.5–0.6. Directed self-assembly with density multiplication
(<i>n</i> = 4) and <i>W</i>/<i>L</i><sub>0</sub> = 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