Concentric half-domain spacing morphologies and anomalous domain stretching in microwave annealed block copolymer thin films†

Abstract

Block copolymer (BCP) films hold significant promise for a wide array of technological applications, including nanopatterning, nanophotonics, polymer electrolytes, and optical waveguides. However, the practical realization of these applications is often hindered by the slow kinetics of the ordering of block copolymers, attributed to the inherently glassy dynamics of polymeric soft materials under standard processing conditions. The diverse range of BCP morphologies further highlights the unique self-assembly characteristics of polymeric materials. In this study, we employ a microwave annealing method that generates a high substrate heating rate (18 °C s⁻¹) to rapidly order lamellar BCP thin films on a high-resistivity boron-doped silicon substrate. This substrate efficiently absorbs microwave energy, creating a rapid and substantial z-temperature gradient in the BCP film. The high-temperature annealing facilitated by microwave heating generates 1L₀ surface terraces composed of unconventional rim-like morphologies with a 0.5L₀ (half domain spacing) height, forming half-domain height island-on-island and hole-in-hole topographies. We hypothesize that these topographies are related to the highly dynamic through-film thickness temperature gradient. Notably, reducing the substrate heating rate to 13.5 °C s⁻¹ only produces interesting 0.5L₀ top surface structures. Additionally, the elevated high temperatures of microwave annealing significantly increase the vertical lamellar domain size, L₀, of the BCP film surface topography, which we believe corresponds to an “intermediate segregation” regime of chain stretching. This domain size enhancement is due to the synergy of the reduced interaction parameter between blocks and improved interlayer diffusional dynamics resulting from the sharp temperature spike and rapid vitrification. These unique morphological effects, exclusive to microwave annealing, are not seen in conventional thermal or solvent annealing and open new avenues for microwave substrate-directed self-assembly (MS-DSA) to create unique surface and internal BCP morphologies for specialized applications.