Patent ID: 6872624

Claim:
A method for fabricating a nonvolatile semiconductor memory device, comprising: a first step of forming a gate structure on a semiconductor substrate, the gate structure comprising a tunnel insulation film being in contact with the semiconductor substrate, a floating gate electrode being in contact with the tunnel insulation film, a control gate electrode facing the floating gate with an intervening capacitive insulation film; a second step of forming ion injection adjustment films comprising an insulation film being in contact with the floating gate electrode at least on side surfaces of the floating gate electrode; a third step of injecting impurity ions into an active region beside the gate structure in the semiconductor substrate by using the gate structure and each ion injection adjustment film as masks, such that the impurity ions do not reach the lower portion of the floating gate electrode in the active region; and a fourth step of thermally diffusing the injected impurity ions by performing heat treatment on the active region, such that the impurity ions reach the lower portion of the floating gate electrode in the active region; wherein, in the second step, the film thickness of each ion injection adjustment film is chosen so as to prevent the impurity ions from being injected into the tunnel insulation film and to allow the impurity ions to reach a portion below a side end portion of the floating gate electrode in the active region as a result of diffusive scattering and the thermal diffusion of the impurity ions into the semiconductor substrate.