Thin films of copolymers
Researchers using the XSD 8-ID-E beamline at the APS are investigating in-plane and out-of-plane defectivity in thin films of lamellar PS-PMMA copolymers on brushed P(S-r-MMA) substrates.
The team’s research has demonstrated that defectivity in thin films of lamellar copolymers are composed of in-plane topological defects and out-of-plane domain tilt, with little or no correlation between the two types of disorder.
Thin films of symmetric diblock copolymers can spontaneously self-assemble into nanoscale lamellar domains (i.e., nanolines). These materials could improve the resolution of projection lithography by “shrinking” the sizes of patterned features, so the leading semiconductor manufacturers are considering their use in next-generation integrated circuit manufacturing. Lithographic processes require precise control over the placement and orientation of domains with respect to the underlying substrate.
We investigate the ordering of poly(styrene-b-methyl methacrylate) (PS-PMMA) lamellar copolymers (periodicity L0 = 46 nm) confined between a free surface and brushed poly(styrene-r-methyl methacrylate) silicon substrate. The processing temperature was selected to eliminate wetting layers at the top and bottom interfaces, producing approximately neutral boundaries that stabilize perpendicular domain orientations. The PS-PMMA film thickness (t = 0.5L0 − 2.5L0) and brush grafting density (Σ = 0.2–0.6 nm−2) were systematically varied to examine their impacts on in-plane and out-of-plane ordering.
Samples were characterized with a combination of high-resolution microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering. In-plane order at the top of the film (quantified through calculation of orientational correlation lengths) improved with tn, where the exponent n increased from 0.75 to 1 as Σ decreased from 0.6 to 0.2 nm−2. Out-of-plane defects such as tilted domains were detected in all films, and the distribution of domain tilt angles was nearly independent of t and Σ. These studies demonstrate that defectivity in perpendicular lamellar phases is three-dimensional, comprised of in-plane topological defects and out-of-plane domain tilt, with little or no correlation between these two types of disorder. Strong interactions between the block copolymer and underlying substrate may trap both kinds of thermally generated defects.
Nikhila Mahadevapuram, Indranil Mitra, Alona Bozhchenko, Joseph Strzalka and Gila E. Stein, “In-plane and Out-of-plane Defectivity in Thin Films of Lamellar Block Copolymers,” Journal of Polymer Science Part B: Polymer Physics, Full Paper, DOI: 10.1002/polb.23937. Published Online October 29, 2015.