Flash memory device and method for manufacturing the same

A method for manufacturing a flash memory device, comprising:

• forming a plurality of isolation structures in a substrate, wherein an opening is formed between adjacent two of the isolation structures, wherein a top width of the opening is less than a bottom width of the opening;
• conformally depositing a first silicon seed layer on the substrate and the isolation structures;
• performing a first cycle, wherein the first cycle comprises:
• performing a first deposition process to conformally form a first amorphous silicon layer on the first silicon seed layer, wherein a first recess is defined by the first amorphous silicon layer;
• and
• after the first deposition process, performing a first in-situ chlorine etching process to widen a caliber of the first recess;
• performing a first thermal annealing process to transform the first amorphous silicon layer into a first polysilicon layer;
• and
• performing an amorphous silicon deposition process to form an amorphous silicon layer on the first polysilicon layer and to completely fill the opening.