A method of forming a semiconductor device comprises forming a mask pattern over an etch target layer, forming an ion implantation region in the mask pattern through an ion implantation process, and forming an ion non-implantation region within the mask pattern, removing the ion implantation region on a top surface of the ion non-implantation region, removing the ion non-implantation region, and patterning the etch target layer by using spacers that comprise the ion implantation region as an etch mask.