A method includes forming a first mask over a substrate through a double patterning process, wherein the first mask comprises a horizontal portion and a plurality of vertical portions protruding over the horizontal portion, and wherein the vertical portions are spaced apart from each other, applying a first etching process to the first mask until a top surface of a portion of the substrate is exposed, applying a second etching process to the substrate to form intra-device openings and inter-device openings, wherein the inter-device openings are formed at the exposed portion of the substrate, filling the inter-device openings and the intra-device openings to form inter-device insulation regions and intra-device insulation regions and etching back the inter-device insulation regions and the intra-device insulation regions to form a plurality of fins protruding over top surfaces of the inter-device insulation regions and the intra-device insulation regions.