In one embodiment, the semiconductor device includes a stack of alternating interlayer insulating layers and conductive layers on a substrate. Each of the conductive layers extends in a first direction less than a previous one of the conductive layers to define a landing portion of the previous one of the conductive layers. An insulating plug is in one of the conductive layers under one of the landing portions, and a contact plug extends from an upper surface of the one of the landing portions.