A semiconductor device include; a substrate including a cell array region and a key region, a stack structure on the cell array region including vertically stacked electrodes, a dummy structure on the key region, a vertical channel structure penetrating the stack structure to connect the substrate, a dummy pillar penetrating the first dummy structure, an interlayer dielectric layer on the stack structure and the dummy structure, wherein an upper portion of the interlayer dielectric layer on the dummy structure includes a key pattern that vertically overlaps the dummy pillar, and a capping layer on the key region and covering the key pattern.