Method for forming memory device

A method for forming a memory device, comprising:

• providing a substrate;
• forming a stack comprising a plurality of conductive layers and a plurality of insulating layers being alternatively stacked on the substrate;
• forming a plurality of memory structures on the substrate and the memory structures penetrating the stack;
• forming a plurality of isolation structures on the substrate and the isolation structures penetrating the stack, wherein the isolation structures divide the memory structures into a plurality of first memory structures and a plurality of second memory structures;
• and
• forming a plurality of common source pillars on the substrate and the common source pillars penetrating the stack, wherein the common source pillars directly contact the isolation structures, wherein the stack comprises a plurality of first ridged shape structures and a plurality of second ridged shape structures separated by the isolation structures and the common source pillars, and the first ridged shape structures are interlaced with the second ridged shape structures along a direction parallel to a top surface of the substrate.