Method of manufacturing flash memory device

1. A method of manufacturing a flash memory, comprising the steps of:

performing an ion implantation process for adjusting a threshold voltage on a semiconductor substrate;

sequentially forming a tunnel oxide film, a first polysilicon film and a pad nitride film on the semiconductor substrate;

etching the pad oxide film, the first polysilicon film, the tunnel oxide film and the semiconductor substrate to form a trench defining an active region and a device isolation region;

forming a nitride film on an entire surface of the trench by means of an annealing process to prevent the implanted ions for adjusting the threshold voltage from diffusing to the device isolation region;

forming a side wall oxide film on a surface of the nitride film and a sidewall of the polysilicon film; and

forming a device isolation film by filling up inside the trench.

2. A method of claim 1 , wherein the side wall oxide film is formed by a dry oxidation method at a temperature in the range of 800 to 950 ° C.

3. A method of claim 1 , wherein the annealing process is performed under N 2 O atmosphere at a temperature in the range of 800 to 900 ° C.

4. A method of claim 1 , after the step of forming a device isolation film, further comprising the steps of:

eliminating the pad nitride film;

forming a second polysilicon film for a floating gate on the structure where the pad nitride film is eliminated;

forming a dielectric film on the structure where the second polysilicon film is formed; and

forming a third polysilicon film for a control gate on the dielectric film.