Combinatorial mapping of substrate step edge effects on diblock copolymer thin film morphology and orientation.

We have used a combinatorial gradient technique to map precisely how the terrace structure and microdomain lattice alignment in a thin film of a sphere-forming diblock copolymer are affected by both the thickness of the copolymer film and the height of a series of parallel step edges fabricated on the substrate. We find that for film thicknesses slightly incommensurate with integer numbers of sphere layers, the step edges act as nucleation sites for regions with one more or one fewer layers of spheres. We also find that for our system, the hexagonal lattice formed by a single layer of spheres on the low side of a step edge is aligned along the direction of the step edge only where the film on the high side is sufficiently thin to support only a wetting layer of copolymer material. This work will guide the tuning of film thickness and step height in future studies and applications of graphoepitaxy in block copolymer films.