Pathway-Dependent Grain Coarsening of Block Copolymer Patterns under Controlled Solvent Evaporation

Abstract

Solvent evaporation annealing (SEA) is a straightforward, single-step casting and annealing method of block copolymers (BCP) processing yielding large-grained morphologies in a very short time. Here, we present a quantitative analysis of BCP graincoarsening in thin films under controlled evaporation of the solvent. Our study is aimed at understanding time and BCP concentration influence on the rate of the lateral growth of BCP grains. By systematically investigating the coarsening kinetics at various BCP concentrations, we observed a steeply decreasing exponential dependence of the kinetics power-law time exponent on polymer concentration. We used this dependence to formulate a mathematical model of BCP ordering under nonstationary conditions and a 2D, time-and concentrationdependent coarsening rate diagram, which can be used as an aid in engineering the BCP processing pathway in SEA and also in other directed self-assembly methods that utilize BCP−solvent interactions such as solvent vapor annealing.

Solvent-assisted self-assembly of block copolymer thin films

“…Further investigations of the SEA mechanism revealed that a relatively narrow concentration window, just above the φ ODT (0.33 < φ < 0.40) facilitates rapid coarsening of BCP grains with the later drying stage inconsequential for the formation of large grains. 148 This result indicates the importance of the early stages of BCP ordering and underscores the utility of BCP processing strategies in which the ODT transition is crossed and grains are nucleated from a disordered state, an approach exploited in other direct casting methods and some field-assisted DSA methods. 149,150…”

“…Further investigations of the SEA mechanism revealed that a relatively narrow concentration window, just above the j ODT (0.33 o j o 0.40) facilitates rapid coarsening of BCP grains with the later drying stage inconsequential for the formation of large grains. 148 This result indicates 146 Copyright 2015 American Chemical Society, (c-e) adapted with permission from Macromolecules, 52, 5026. 147 Copyright 2019 American Chemical Society, (f-h) adapted with permission from Macromolecules, 53, 11178.…”

“…Recently, a semi-empirical kinetic model of grain coarsening in solvent assisted ordering under non-constant BCP concentration was proposed. 148 The kinetic exponent a has been found to exponentially decrease with the BCP concentration (j):…”

“…285 Solvent-assisted grain coarsening pathways can be easily computed once the grain-coarsening rate map, i.e., dx/dt = f (j, t) is determined from constant BCP concentration annealing experiments. 148 This approach, valid under a general assumption that instantaneous coarsening rate is only dependent on the current grain size and BCP concentration, is illustrated by Fig. 13a which shows two examples of grain-coarsening trajectories corresponding to SVA (green curve) and SEA (red curve) experiments mapped onto the j-t space.…”

“…Adapted with permission from ACS Macro Lett., 11, 121. 148 Copyright 2022 American Chemical Society.…”