A method of fabricating a semiconductor device is provided. The method may include forming an interlayer insulating layer on a structure with a cell region and a peripheral circuit region, forming a first mask layer on the interlayer insulating layer, forming trenches in the first mask layer exposing the interlayer insulating layer by patterning the first mask layer on the peripheral circuit region, and forming key mask patterns in the trenches. An etch selectivity of the first mask patterns with respect to the interlayer insulating layer may be greater than that of the key mask patterns with respect to the interlayer insulating layer.