A method of forming a semiconductor device is disclosed. The method includes providing a device having a substrate and a hard mask layer over the substrate; forming a mandrel over the hard mask layer; depositing a material layer on sidewalls of the mandrel; implanting a dopant into the material layer; performing an etching process on the hard mask layer using the mandrel and the material layer as an etching mask, thereby forming a patterned hard mask layer, wherein the etching process concurrently produces a dielectric layer deposited on sidewalls of the patterned hard mask layer, the dielectric layer containing the dopant; and forming a fin by etching the substrate using the patterned hard mask layer and the dielectric layer collectively as an etching mask.