Methods and structures of a three-dimensional memory device are disclosed. In an example, the disclosed method comprises forming a plurality of dielectric stacks stacked on one another over a substrate to create a multiple-stack staircase structure. Each one of the plurality of dielectric stacks comprises a plurality of dielectric pairs arranged along a direction perpendicular to a top surface of the substrate. The method further comprises forming a filling structure that surrounds the multiple-stack staircase structure, forming a semiconductor channel extending through the multiple-staircase structure, wherein the semiconductor channel comprises unaligned sidewall surfaces, and forming a supporting pillar extending through at least one of the multiple-staircase structure and the filling structure, wherein the supporting pillar comprises aligned sidewall surfaces.