Step Edges in Thin Films of Lamellar-Forming Diblock Copolymer

Abstract

Self-consistent field theory (SCFT) is used to study the step edges that occur in thin films of lamellar-forming diblock copolymer, when the surfaces each have an affinity for one of the polymer components. We examine film morphologies consisting of a stack of ν continuous monolayers and one semi-infinite bilayer, the edge of which creates the step. The line tension of each step morphology is evaluated and phase diagrams are constructed showing the conditions under which the various morphologies are stable. The predicted behavior is then compared to experiment. Interestingly, our atomic force microscopy (AFM) images of terraced films reveal a distinct change in the character of the steps with increasing ν, which is qualitatively consistent with our SCFT phase diagrams. Direct quantitative comparisons are not possible because the SCFT is not yet able to probe the large polymer/air surface tensions characteristic of experiment. ■ INTRODUCTIONThe behavior of block copolymer melts on solid substrates has received considerable attention over the last two decades motivated by a wide range of applications. 1−3 Not surprisingly, the earliest work 4−8 involved the simplest conceivable system, thin films of symmetric AB diblock copolymer on flat substrates. In the bulk, symmetric diblock copolymer forms a lamellar phase of alternating A-and B-rich domains, where each period, L 0 , consists of two monolayers. The main effect of confining the diblock copolymer to a thin film is to orient the domains. When the substrate and air surfaces of the film each have an affinity for one of the polymer components, the lamellae orient in the plane of the film, thereby forming a parallel lamellar (L ν ∥ ) phase consisting of ν diblock monolayers. Because of the strong tendency to maintain the preferred domain size of the bulk, the film height is quantized, H ≈ L 0 ν/ 2. Even values of ν are selected when the two surfaces have an affinity for the same component, and odd values occur when the two surfaces have opposite affinities. Usually the amount of material cast on the substrate is incommensurate with the preferred domain size, which leads to macrophase separation into thin and thick regions (i.e., terraces) separated by steps of height L 0 . The topography of the air surface changes from islands to a bicontinuous network to holes as the area of the thicker region increases. 4 The behavior becomes particularly interesting with the formation of elaborate terraced structures, when droplets 9−11 or rings 12 of diblock copolymer are deposited on the substrate.The presence of step edges separating different film thicknesses is ubiquitous in block copolymer films, and yet they have received relatively little theoretical attention. A step edge between the L ν ∥ and L ν+2 ∥ phases will typically involve ν continuous monolayers plus one semi-infinite bilayer as depicted schematically in Figure 1. Ausserréet al. 5 initially argued that the bilayer should exist one monolayer below the air surface, rather than on top as in Figure 1. Fu...

Boundary-directed epitaxy of block copolymers

“…5d, e). Similar slanted lamellae have been observed at step edges in di-BCP films 53 . The slanted lamellae act as a template to convert the lamellar domains on the stripes from horizontal to vertical.…”

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

“…The one shortcoming of the SCFT is that we cannot solve the field equations for realistic values of Ah and Bh, and consequentially the step edges will be somewhat sharper than in experiment. 12 Although this will underestimate the line tensions, 0.5 and 1, of the 0.5 L0 and 1 L0 step edges, respectively, SCFT should still provide a reasonable prediction of their ratio, 0.5/1.…”

“…33,34 SCFT has recently been used to provide what is undoubtedly the most accurate theoretical treatment of 1 L0 step edges in block copolymer films. 12 Here, we use the same approach for the 0.5 L0 step edges. and Bh, respectively.…”

Rapid ordering of block copolymer thin films

“…Thicknesses slightly above commensurability have an excess of islands on their surface; those slightly below commensurability exhibit holes; intervening thicknesses can display bicontinuous surface features [260]. These surface features nucleate as small, smooth bumps/divots, which then grow in size and exhibit relatively sharp walls [261,262]. The appearance of these features is preceded by an incubation phase [258,263,264]: a meta-stable state where the film is nominally flat.…”