Epitaxy of Incommensurate Block Copolymer Dot Arrays: Preferential Alignment and Moire Superstructures

In this work we investigated the directed self-assembly of dot forming block copolymers (BCP) deposited in a sequential layer-by-layer fashion. Samples were fabricated by first spin-casting a dot forming BCP (PS-b-PDMS) and converting it to a hexagonal array of silica dots, this non-planar substrate was then used to direct the self-assembly of a second dot forming BCP layer. It was found that the top layer of dots adopted a preferential orientation relative to the bottom dot array, resulting in well-defined Moire superstructures. The observed preferential orientations were shown to be statistically meaningful, as the distributions of the relative rotation angles between the BCP dot arrays were determined using large-area helium-ion microscopy (> 10⁶ dots). Moreover, the preferential rotation angle of these Moire superstructures was found to be uniquely determined by the pitch ratio of the two lattices and relative sizes of the dot features. A systematic investigation into the bottom layer dot features was performed using electron beam lithography, where it was found that three different pattern forming regimes exist depending on the size of the bottom layer features.