A memory device includes a periphery wafer, a memory array chip stack, and a plurality of first conductive contacts. The periphery wafer has a functional surface. The memory array chip stack is disposed on the periphery wafer and has a functional surface, in which the functional surface of the periphery wafer faces toward the functional surface of the memory array chip stack, and a first side of the memory array chip stack is in a staircase configuration. The first conductive contacts are on the first side of the memory array chip stack, and between and interconnecting the functional surface of the periphery wafer and the functional surface of the memory array chip stack.