Real-Time Atomic Force Microscopy Imaging of Block Copolymer Directed Self Assembly.

Abstract

The kinetics of directed self-assembly of symmetric PS-b-PMMA diblock copolymer on chemically patterned templates were measured during in situ thermal annealing. Although these chemical guide patterns lead to well-aligned, defect-free lamellar patterns at thermodynamic equilibrium, in practice, challenges remain in understanding and optimizing the kinetic evolution for technological applications. High-speed, environmentally controlled atomic force microscopy imaging was used to track pattern evolution on the time scale of individual microdomain connections in real space and time, allowing the direct visualization of defect healing mechanisms. When we apply this highly general technique to films on chemically patterned substrates, we find that pattern alignment is mediated by a metastable nonbulk morphology unique to these samples, referred to as the "stitch" morphology. We observe diverse and anisotropic mechanisms for the conversion from this morphology to equilibrium lamellar stripes. Directed self-assembly on chemical templates is observed to follow exponential kinetics with an apparent energetic barrier of 360 ± 80 kJ/mol from 210-230 °C, a significant enhancement when compared with ordering rates on unpatterned substrates. Ultimately, from local imaging, we find that the presence of a chemical guiding field causes morphological ordering and lamellar alignment to occur irreversibly.