A method for manufacturing NAND memory cells includes providing a substrate having a first doped region formed therein; forming a first dielectric layer, a storage layer and a patterned hard mask on the substrate; forming a STI in the substrate through the patterned hard mask and removing the patterned hard mask to define a plurality of recesses; forming a second dielectric layer and a first conductive layer filling the recesses on the substrate; and performing a planarization process to remove a portion of the first conductive layer and the second dielectric layer to form a plurality of self-aligned islanding gate structures.