Embodiments of staircase and contact structures of a three-dimensional (3D) memory device and fabrication method thereof are disclosed. The 3D memory device includes a semiconductor substrate and a plurality of through-substrate-trenches penetrating the semiconductor substrate. The 3D memory device also includes a film stack disposed on a first surface of the semiconductor substrate extending through the through-substrate-trenches to a second surface of the semiconductor substrate, wherein the film stack includes alternating conductive and dielectric layers. The 3D memory device also includes a staircase structure formed at an edge of the film stack.