A memory device includes a substrate, a first memory structure including a plurality of first word lines stacked on the substrate in a direction perpendicular to a top surface of the substrate, an inter-metal layer on the first memory structure and including a plurality of intermediate pads connected with separate, respective first word lines of the plurality of first word lines, a second memory structure including a plurality of second word lines stacked on the inter-metal layer in the direction perpendicular to the top surface of the substrate, and an upper metal layer on the second memory structure and including a plurality of upper pads connected with separate, respective second word lines of the plurality of second word lines.