Method of manufacturing flash memory device

A method of manufacturing a flash memory device, including the steps of sequentially forming a tunnel oxide film, a first polysilicon layer, a nitride film, and a hard mask film on a semiconductor substrate, etching the hard mask film, the nitride film, the first polysilicon layer, and a predetermined region of the tunnel oxide films and then etching the exposed semiconductor substrate by a predetermined depth, thereby forming trenches, forming an oxide film on the entire structure so that the trenches are buried, polishing the oxide film and stripping the hard mask film, forming isolation films, etching the isolation films by a predetermined thickness, wherein when the nitride film is etched, the first polysilicon layer is also etched to form a trapezoid profile, stripping the nitride film remaining after the nitride film is etched, and forming a dielectric layer, and burying the entire top surface with a second polysilicon layer and a gate conductive film.