Late mask process for programming read only memories

Details Late mask process for programming read only memories Late mask process for programming read only memories

G - Physics

11

C

352/82.3

G11C 17/00 (2006.01) G11C 11/40 (2006.01) H01L 21/82 (2006.01) H01L 21/8246 (2006.01)

Patent

CA 1204863

LATE MASK PROCESS FOR PROGRAMMING READ ONLY MEMORIES Tarsaim Batra ABSTRACT A late mask programming process is provided for factory programmed ROMs or logic circuitry. MOS transis- tors functioning as ROM cells or in logic circuitry are fabricated by a standard MOS Process. Then, a thin stop layer of silicon nitride is provided over the transistors followed by a layer of silicon dioxide. Programming is accomplished by applying a program mask and etching through the layers overlying the gate regions of selected transis- tors down to the silicon nitride stop layer. The silicon nitride stop layer prevents overetching and shorting of the gates. Then, ions are implanted underneath the gates of the selected MOS transistors to alter their threshold so, for example, as ROM cells they signify a different state than those cells whose transistor gates are not implanted with ions. The silicon nitride layer serves to stop the etch solution but permits the ions to pass through, penetrate the substrate and raise the thresholds of the selected transistors. The silicon dioxide layer stops the ions from being implanted into the nonselected transistors.

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