Rapid assembly in block copolymer thin films by sequential solution and thermal annealing with asymmetric reversible processing

Processing pathways significantly impact block copolymer (BCP) self-assembly in thin films. These processing methods may include thermal annealing (TA), direct immersion annealing (DIA), solvent vapor annealing (SVA), etc. Tuning the annealing environment by parallel/sequential processing via different techniques may lead to previously unexplored pathways that rapidly achieve equilibrium (Eq.) morphologies and even new microstructures. With this research, we propose a sequential annealing method coupling TA with DIA to process BCP films. We first demonstrate the series process of DIA followed by TA, where short time DIA rapidly produces metastable lamellar domains of size L_o/2 (L_o = Eq. size) that transform into domains of size L_o on successive TA at an accelerated rate (higher than direct TA). We then studied the reverse transition from TA to DIA microstructure, where we observed notable asymmetry in the reversibility kinetics as the DIA treatment caused instability in TA'ed BCP films for molecular weights beyond entanglement, before producing the metastable state. The chain rearrangement mechanisms between the metastable and Eq. BCP morphologies are analyzed in terms of film swelling, chain diffusion, and in-plane vs. out-of-plane interfacial evolution.