Three-dimensional memory and fabrication method thereof

A three-dimensional memory device, comprising:

• a stack structure including alternately stacked dielectric layers and gate layers;
• and
• a gate line slit structure extending in a first horizontal direction in the stack structure, comprising:
• a main portion penetrating through the stack structure in a vertical direction, and
• a plurality of protruding portions protruding outward from the main portion along a second horizontal direction perpendicular to the first horizontal direction, wherein each protruding portion has an end in contact with a corresponding gate layer, and an average thickness of at least one protruding portion in the vertical direction is greater than a thickness of at least one corresponding gate layer.