Manufacturing method of semiconductor memory device

A method of manufacturing a semiconductor memory device, the method comprising:

• forming a preliminary memory cell array on a support structure, the preliminary memory cell array with a stack structure and cell pillars, wherein the stack structure includes interlayer insulating layers and conductive layers that are alternately stacked on the support structure, and wherein each of the cell pillars has a first part that penetrates the stack structure and a second part that extends into the support structure from the first part;
• removing the support structure to expose the second part of each of the cell pillars;
• forming a protective layer that covers the second part of each of the cell pillars, wherein the protective layer has a first inclined surface and a second inclined surface that face each other, wherein a first opening is interposed between the first inclined surface and the second inclined surface, wherein the first inclined surface and the second inclined surface are disposed between the cell pillars, and wherein a width of the first opening narrows as the first opening moves farther from the second part;
• forming a mask pattern that exposes the first opening on the protective layer;
• and
• etching at least one conductive layer among the conductive layers that is adjacent to the first opening, thereby isolating the at least one conductive layer into select lines.