A method of forming a semiconductor device includes forming a mold structure on a substrate, forming a first mask layer having a deposition thickness on the mold structure and patterning the first mask layer to form first mask openings which expose the mold structure. The mold structure is etched to form holes that penetrate the mold structure. The first mask layer is thinned to form mask portions having thickness smaller than the deposition thickness. Conductive patterns are formed to fill the holes and the first mask openings. The first mask layer including the mask portions is etched to expose the mold structure. The conductive patterns include protrusions. A chemical mechanical polishing process is performed to remove the protrusions of the conductive patterns.