Directed Self-Assembly of Block Copolymer Thin Films Using Minimal Topographic Patterns

We demonstrate that a minimal topographic pattern with a confinement depth (D) much less than the domain spacing of block copolymers (L₀) can be used to achieve highly ordered hexagonal arrays or unidirectionally aligned line patterns over large areas. Cylinder-forming poly(styrene-b-ethylene oxide) (PS-b-PEO) thin films were prepared on a series of minimal single trench patterns with different widths (W) and D. Upon thermal annealing, hexagonal arrays of cylindrical microdomains propagated away from the edges of a single trench, providing insight into the minimum pitch (P) of the trench necessary to fully order hexagonal arrays. The confinement trench D of 0.30L₀, the W in the range of 1.26L₀ to 2.16L₀, and the P as long as 18.84L₀ were found to be effective for the generation of laterally ordered hexagonal arrays with the density amplification up by a factor of 17, within the minimally patterned trench surfaces of 100 μm by 100 μm. Furthermore, we produced line patterns of cylindrical microdomains by using solvent vapor annealing on the minimally patterned trench surfaces. However, highly aligned line patterns could be achieved only on the patterned surface with P = 5.75L₀, W = 1.26L₀, and D = 0.30L₀ because the influence of the minimally patterned trench surface on the lateral ordering decreased as the P and W increase at the fixed D, resulting in poor ordering. These findings suggest that the minimal topographic pattern is more effective in guiding hexagonal arrays than in guiding line patterns.