A method for forming a 3D memory device is disclosed. The method includes: forming an first insulating layer on a substrate in a peripheral region, the first insulating layer having a slope near a boundary between the peripheral region and a core region of the substrate; forming an alternating conductive/dielectric stack on the substrate and the slope of the first insulating layer, a lateral portion of the alternating conductive/dielectric stack extending along a top surface of the substrate in the core region, and an inclined portion of the alternating conductive/dielectric stack extending along the slope of the first insulating layer; and forming a plurality of contacts to electrically contact a plurality of conductive layers in the inclined portion of the alternating conductive/dielectric stack.