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3D-NAND flash memory is currently essential in the semiconductor industry due to the interference issue between memory cells of the conventional planar type. To vertically stack the memory, an oxide-nitride (ON) stack structure is formed using plasma enhanced chemical vapor deposition (PECVD) equipment. Thereafter, a part of the silicon nitride (Si3N4) layer is removed by wet etching using phosphoric acid (H3PO4) to make a space for the memory cell. It is important to selectively wet etch only the Si3N4 film while protecting the silicon oxide (SiO2).