Understanding Film Thickness‐Dependent Block Copolymer Self‐Assembly by Controlled Polymer Dewetting on Prepatterned Surfaces

Abstract

Block copolymer (BCP) lithography is a versatile bottom‐up approach for the creation of regular nanoscale patterns on large surface areas. The pattern morphology evolving during the microphase separation of a BCP is strongly dependent on the polymer film thickness. Thus, surface wetting as well as interfacial energies between polymer and substrate determine the polymer behavior, however, the complex interplay of those effects is not yet fully understood. In this work, a model describing the film thickness dependence of BCP self‐assembly on prepatterned surfaces is proposed. Polymer dewetting on nanohole‐patterned surfaces is controlled using different prepattern dimensions, polymer amounts, and microphase‐separation temperatures. This is found to allow for a precise local film thickness modulation and thus allows to guide BCP self‐assembly into arrays of tailored hierarchical nanoarchitectures. Analytical calculations of the total surface free energies of the microphase‐separated polymer of different film thicknesses confined inside nanoholes confirm the model. The insights contribute to the understanding of fundamental processes in polymer dewetting and BCP self‐assembly and allow for the controlled creation of advanced hierarchical nanostructures on large areas for applications in optics, plasmonics, and biomedical devices.

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

“…Again, energetically favorable configurations correlate with the film thickness in units of L 0 . This effect was, for instance, investigated by Perego and co-workers [81] on random copolymer (RCP) functionalized silica and by Brassat et al [14] on preferentially wetted silica as well as on platinum and gold surfaces. Thus, during hole/island formation or dewetting of BCP films, which both lead to local changes of the polymer thickness, different polymer orientations can form locally minimizing the total surface free energy F by optimization of the interfacial areas.…”

“…The scanning electron microscopy (SEM) image in Figure 4c shows a film thickness gradient of a cylinderforming PS-b-PMMA block copolymer on a nonfunctionalized silica surface after thermal annealing in vacuum. [14] The film thickness increases from left to right. Along this gradient, the orientation of PMMA cylinders in a PS matrix changes from parallel (stripe patterns from top view) through a mixed region to perpendicular cylinders (dots from top view).…”

“…Thus, the minimum energy orientation is film thickness dependent (Figure 4b,e). [14,19,67,68,[77][78][79][80] Figure 4c,d gives examples of the changing polymer domain orientation with increasing polymer film thickness. The scanning electron microscopy (SEM) image in Figure 4c shows a film thickness gradient of a cylinderforming PS-b-PMMA block copolymer on a nonfunctionalized silica surface after thermal annealing in vacuum.…”

“…b,c) Reproduced with permission. [14] Copyright 2019, Wiley VCH. d) TM-SFM image of a microphase-separated SBS triblock copolymer with increasing film thickness from left to right.…”

“…[11] One crucial issue in advancing BCP lithography to its next technological level, i.e., rendering the technique applicable for versatile device integration, is the fundamental understanding of the self-assembly process enabling control of the pattern formation and its guiding into desired surface architectures. In particular, interactions between the polymer and solid surfaces determine the morphology of nanostructures: polymer wetting behavior, [12][13][14] nanopattern orientation, [15][16][17][18][19] nanopattern uniformity through a thin film, [20][21][22] and defect density [22][23][24][25] in BCP thin films are fundamentally investigated by research groups contributing with insights from chemistry, physics, and material science.…”