Aspects of the disclosure provide a semiconductor device. The semiconductor device includes gate layers and insulating layers that are stacked alternatingly along a direction perpendicular to a substrate of the semiconductor device in an array region upon the substrate. Further, the semiconductor device includes an array of channel structures that is formed in the array region. The gate layers and the insulating layers are stacked in a staircase form with stair steps having non-uniform stair depths in a connection region upon the substrate. Further, the semiconductor device includes contact structures to the gate layers. The contact structures are formed on the stair steps that have the non-uniform stair depths.