Abstract: A method for manufacturing a flash memory device is provided. The method includes forming a plurality of isolation structures in a substrate, an opening is formed between two adjacent isolation structures, and conformally depositing a first silicon seed layer on the substrate and the isolation structures and performing a first cycle. The first cycle includes performing a first deposition process to conformally form a first amorphous silicon layer on the first silicon seed layer. A first recess is defined by the first amorphous silicon layer. A first in-situ chlorine etching process is performed to widen the caliber of the first recess. The method includes performing a first thermal annealing process to transform the first amorphous silicon layer into a first polysilicon layer. The method includes performing an amorphous silicon deposition process to form an amorphous silicon layer on the first polysilicon layer and completely fill the opening.