Memory device and formation method are provided. The memory device includes a substrate; a staircase structure on the substrate; a string driver structure over the staircase structure on a side opposite to the substrate; and a metal routing structure, between the string driver structure and the staircase structure along a vertical direction with respect to a lateral surface of the substrate. The staircase structure includes a plurality of word line tiers. The string driver structure includes a plurality of transistors to individually address the plurality of word line tiers. The string driver structure and the metal routing structure are vertically aligned with the staircase structure based on a lateral central region of the staircase structure.