A method includes forming a patterned hard mask layer, with a trench formed in the patterned hard mask layer, dispensing a Bulk Co-Polymer (BCP) coating in the trench, wherein the BCP coating comprises a mix of a first material and a second material different from the first material. The method further includes treating the BCP coating with a chemical to form a first plurality of strips of the first material and a second plurality of strips of the second material, with the first plurality of strips and the second plurality of strips allocated in an alternating layout. The second plurality of strips is selectively etched, and the first plurality of strips is left in the trench.