Programmable and erasable mos memory device

Abstract

An electrically programmable and electrically erasable MOS memory device having a floating gate which is separated from the semiconductor substrate by a thin oxide layer, the memory device also having an impurity implant in the substrate which extends under an edge of the floating gate beneath the thin oxide layer. In one embodiment the thin oxide layer underlies the entire floating gate while in another embodiment only a portion of a small thin side window extends under the floating gate's edge. Also disclosed is a fabrication process in which the one embodiment is formed by first forming the floating gate over the thin oxide layer and then implanting the impurity near an edge of the floating gate. Later steps with heating cause the implanted impurity to diffuse under the floating gate edge. An alternative process first forms a window in the gate oxide layer and implants the impurity through the window. The window is filled with a thin oxide layer and the floating gate is formed so that its edge lies over a portion of the window. Control gates, sources and drains are formed last.