Structure, Stability, and Reorganization of 0.5 L₀ Topography in Block Copolymer Thin Films

Abstract

The structure, stability, and reorganization of lamella-forming block copolymer thin film surface topography ("islands" and "holes") were studied under boundary conditions driving the formation of 0.5 L thick structures at short thermal annealing times. Self-consistent field theory predicts that the presence of one perfectly neutral surface renders 0.5 L topography thermodynamically stable relative to 1 L thick features, in agreement with previous experimental observations. The calculated through-film structures match cross-sectional scanning electron micrographs, collectively demonstrating the pinning of edge dislocations at the neutral surface. Remarkably, near-neutral surface compositions exhibit 0.5 L topography metastability upon extended thermal treatment, slowly transitioning to 1 L islands or holes as evidenced by optical and atomic force microscopy. Surface restructuring is rationalized by invoking commensurability effects imposed by slightly preferential surfaces. The results described herein clarify the impact of interfacial interactions on block copolymer self-assembly and solidify an understanding of 0.5 L topography, which is frequently used to determine neutral surface compositions of considerable importance to contemporary technological applications.

Nanoscale Block Copolymer Self‐Assembly and Microscale Polymer Film Dewetting: Progress in Understanding the Role of Interfacial Energies in the Formation of Hierarchical Nanostructures

“…The driving force is the redistribution of polymer chains such that internal minimum energy configurations are achieved. The stable thicknesses correlate with the intrinsic structure periodicity, i.e., the equilibrium polymer domain spacing in microphase‐separated block copolymers, typically referred to as L 0 …”

“…During annealing of a block copolymer film with an initial film thickness not matching this periodicity L 0 the polymer redistributes on the surface forming locally different film thicknesses matching integer n multiples or (0.5+ n ) multiples of L 0 . This process can lead to the generation of holes and is referred to as hole/island formation . The term hole might be misleading because it implies that the bare substrate gets exposed.…”

“…Figure shows an example of the hole/island formation of polystyrene‐ block ‐4‐trimethylsilylstyrene (PS‐ b ‐PTMSS) films with initial film thicknesses incommensurate with the periodicity L 0 , i.e., 1.35 × L 0 (Figure a,b) and 1.65 × L 0 (Figure c,d), respectively. In these investigations by Maher et al, the progression of the hole/island formation during thermal annealing of the BCP films was analyzed at different times. In Figure a, mesas with a thickness of 1.5 × L 0 are formed containing holes of 1 × L 0 after 10 min of annealing.…”

“…Initial thicknesses on (a,b) was 1.35 × L 0 and 1.65 × L 0 in (c,d). Reproduced with permission . Copyright 2016, American Chemical Society.…”

Thermal Approaches to Perpendicular Block Copolymer Microdomains in Thin Films: A Review and Appraisal

“…Fortunately, a much simpler method utilizing hole/island step heights to characterize the neutrality of top coats and substrate‐modifying brushes/mats has been reported by Kim et al . A BCP thin film of incommensurate thickness confined by one preferential and one neutral surface can form “half islands/holes” with step heights of 0.5L 0 in its metastable state (i.e., after short annealing) before transitioning into the more thermodynamically stable L 0 islands/holes . Characterizing the step height of the hole/islands of films confined by a top coat (presuming the top coat can deform to accommodate the formation of holes/islands) provides a facile route to confirm neutrality.…”