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-block-poly(2-vinylpyridine) BCP system (M _w: ∼800 kg/mol) by fast swelling to very high levels of solvent concentration (ϕ_s). Phase separation on timescales of ∼10 min is demonstrated once a thickness-dependent threshold ϕ_s value of ∼0.80-0.86 is achieved, resulting in lamellar feature spacings of over 190 nm. The threshold ϕ_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 ϕ_s, with the kinetically unstable hexagonal perforated lamellar (HPL) phase observed at ϕ_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 ϕ_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 ϕ_s up to a maximum value at which the films transition to a disordered state. Thicker films are shown to possess a higher maximum ϕ_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 ϕ_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 ∼190 nm and heights of up 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.