The present invention relates to a gate structure of a flash memory cell and method of forming the same, and method of forming a dielectric film. The method of forming the dielectric film in the flash memory cell comprises the steps of preparing a wafer including a tunnel oxide film formed in a given region of a semiconductor substrate, a polysilicon film formed on the tunnel oxide film, and an oxide film and a silicon nitride film formed on the polysilicon film; preparing a work cell in which a voltage is applied to the rear side of the semiconductor substrate used as a work electrode in which the silicon nitride film is formed, a relative electrode and a reference electrode are kept by a given distance so that they can be immersed in electrolyte, and in which an ultraviolet rays source is formed on an upper side to illuminate an ultraviolet rays to a work electrode; and performing electro-chemical etch using silicon dissociation reaction for the wafer mounted on the work cell to form porosities in the silicon nitride film.