Method of manufacturing memory device

A method of manufacturing a memory device, comprising:

• providing a substrate;
• forming a stack layer on the substrate;
• forming a hard mask layer on the stack layer, wherein the hard mask layer has a protrusion, wherein the protrusion extends in a first direction;
• forming a patterned mandrel on the hard mask layer, wherein the patterned mandrel comprises:
• a first mandrel extending in the first direction and disposed adjacent to the protrusion;
• a second mandrel disposed on a top surface of the protrusion;
• and
• a third mandrel extending in a second direction, wherein the first direction intersects the second direction;
• and
• using the patterned mandrel as a mask, sequentially patterning the hard mask layer and the stack layer to form a dummy structure and a word line structure separated from each other on the substrate, wherein a portion of the stack layer corresponding to the first and second mandrels is formed as the dummy structure, and a portion of the stack layer corresponding to the third mandrel is formed as the word line structure.