In certain aspects, a three-dimensional (3D) memory device includes a stack structure, and a slit structure extending. The stack structure includes interleaved conductive layers and dielectric layers. Edges of the interleaved conductive layers and dielectric layers define a staircase structure. Each one of the conductive layers has a thickened portion in the staircase structure. The thickened portion extends along a first direction. The slit structure extends through the stack structure and along a second direction perpendicular to the first direction, such that the slit structure cuts off at least one, but not all, of the thickened portions of the conductive layers.