Patent ID: 7691721

Claim:
A method for manufacturing a flash memory device, the method comprising: (a) providing a semiconductor comprising a screen oxide film is formed; (b) performing an ion implantation process for a well and a threshold voltage; (c) forming a pad nitride film on the screen oxide film; (d) patterning the pad nitride film and the screen oxide film to partially expose the semiconductor substrate; (e) forming spacers on sidewalls of the patterned pad nitride and screen oxide films before forming a trench; (f) oxidizing the exposed semiconductor substrate between the spacers to form a surface oxide film having two sides on the exposed top surface of the semiconductor substrate and under the spacers; (g) etching the surface oxide film and the semiconductor substrate exposed between the spacers to form a trench in the semiconductor substrate, wherein the surface oxide film remains under the spacer after etching the surface oxide film; (h) forming an insulation film for field isolation to fill up the trench; (i) removing the pad nitride film; (j) partially etching the insulation film for field isolation while removing the spacers and the screen oxide film to form a field isolation film; and (k) oxidizing the overall structure including the field isolation film, thereby forming a tunnel oxide film, wherein the surface oxide film is formed after forming the spacers.