Embodiments of structure and methods for forming a three-dimensional (3D) memory device are provided. In an example, a 3D memory device includes a memory stack, a plurality of channel structures, a slit structure, and a source structure. The memory stack may be over a substrate and may include interleaved a plurality of conductor layers and a plurality of insulating layers extending laterally in the memory stack. The plurality of channel structures may extend vertically through the memory stack into the substrate. The slit structure may extend vertically and laterally in the memory stack and divide the plurality of memory cells into at least one memory block. The slit structure may include a plurality of protruding portions and a plurality of recessed portions arranged vertically along a sidewall of the slit structure.