Optimization and Control of Large Block Copolymer Self-Assembly via Precision Solvent Vapor Annealing

The self-assembly of ultra-high molecular weight (UHMW) block copolymers (BCPs) remains a complex and time-consuming endeavor owing to the high kinetic penalties associated with long polymer chain entanglement. In this work, we report a unique strategy of overcoming these kinetic barriers through precision solvent annealing of an UHMW polystyrene-blockpoly(2-vinylpyridine) BCP system (M-w: similar to 800 kg/mol) by fast swelling to very high levels of solvent concentration (phi(s)). Phase separation on timescales of similar to 10 min is demonstrated once a thickness-dependent threshold phi(s) value of similar to 0.80-0.86 is achieved, resulting in lamellar feature spacings of over 190 nm. The threshold phi(s) value was found to be greater for films with higher dry thickness (D-0) values. Tunability of the domain morphology is achieved through controlled variation of both D-0 and phi(s) with the kinetically unstable hexagonal perforated lamellar (HPL) phase observed at phi(s) values of -0.67 and D-0 values of 59-110 nm. This HPL phase can be controllably induced into an order-order transition to a lamellar morphology upon further increase of phi(s) to 0.80 or above. As confirmed by grazing-incidence small-angle X-ray scattering, the lateral ordering of the lamellar domains is shown to improve with increasing phi(s) up to a maximum value at which the films transition to a disordered state. Thicker films are shown to possess a higher maximum phi(s) value before transitioning to a disordered state. The swelling rate is shown to moderately influence the lateral ordering of the phase-separated structures, while the amount of hold time at a particular value of phi(s) does not notably enhance the phase separation process. These large period self-assembled lamellar domains are then employed to facilitate pattern transfer using a liquid-phase infiltration method, followed by plasma etching, generating ordered, high aspect ratio Si nanowall structures with spacings of similar to 190 nm and heights of up to similar to 500 nm. This work underpins the feasibility of a room-temperature, solvent-based annealing approach for the reliable and scalable fabrication of sub-wavelength nanostructures via BCP lithography.