JPS55128862A - Junction breakdown write-in type semiconductor memory device and method of fabricating the same - Google Patents

Abstract

PURPOSE:To protect a peripheral circuit of a semiconductor memory device when writing in the device by forming the depth of an emitter and base junction in a ROM cell shallower than the peripheral circuit to lower the writing-in level. CONSTITUTION:An n<->-type epitaxial layer is formed through an n<+>-type buried layer on a p<->-type silicon substrate, and isolated by an oxide film. Openings are selectively perforated at the surface oxide film 8, B is diffused in one n<->-type layer to form a p-type base 6, and P is diffused to the buried layer in the other layer to form an n<+>-type collector pickup layer 7. An opening is then perforated at the oxide film, a thin oxide film 9 is formed therethrough, and superimposed with a polysilicon film 10 thereon. The film 10 is retained selectively only at the ROM side I, P ion is implanted thereon, expanded and diffused to form n<+>-type emitters 11, 12 thereon. The film 10 is then removed to form electrodes B, E and C thereon. Since the ion is implanted through the thick film on the ROM side in this configuration, the emitter and base junction is formed shallower. Since the shallower junction is more readily broken in general, the writing level of the ROM may be suitably lowered by selecting the thickness of the thin oxide film 9 so as to protect the peripheral circuit when writing in the ROM.