The present disclosure describes method and structure of a three-dimensional memory device. The memory device includes a substrate and a plurality of wordlines extending along a first direction over the substrate. The first direction is along the x direction. The plurality of wordlines form a staircase structure in a first region. A plurality of channels are formed in a second region and through the plurality of wordlines. The second region abuts the first region at a region boundary. The memory device also includes an insulating slit formed in the first and second regions and along the first direction. A first width of the insulating slit in the first region measured in a second direction is greater than a second width of the insulating slit in the second region measured in the second direction.