A method for forming self-assembled patterns on a substrate surface is provided. First, a block copolymer layer, which comprises a block copolymer having two or more immiscible polymeric block components, is applied onto a substrate that comprises a substrate surface with a trench therein. The trench specifically includes at least one narrow region flanked by two wide regions, and wherein the trench has a width variation of more than 50%. Annealing is subsequently carried out to effectuate phase separation between the two or more immiscible polymeric block components in the block copolymer layer, thereby forming periodic patterns that are defined by repeating structural units. Specifically, the periodic patterns at the narrow region of the trench are aligned in a predetermined direction and are essentially free of defects. Block copolymer films formed by the above-described method as well as semiconductor structures comprising such block copolymer films are also described.