Method of late programming a read only memory

Abstract

Transistors (10) having lateral gaps between their source and drain and the gate are interconnected in a ROM to receive program code. In one embodiment of the invention (FIG. 3), the gaps of selected transistors (42) are subjected to a phosphorous implant (44) to create lightly doped n.sup.- regions (26,28) connecting the source (18) and drain (20) to the gate (14), and function normally. The other transistors (40) do not receive the phosphorous implant, and thus have a higher threshold voltage. In another embodiment of the invention (FIG. 2) all of the transistors receive the phosphorous implant to create the n.sup.- regions (26,28) connecting the source and drain to the gate, and the n.sup.- regions of selected transistors (32) are counter-doped with a boron implant (34) so as to raise their threshold voltages, while the other transistors (30) are not counter-doped and function normally. In both embodiments, programming can occur late in the processing of the ROM.

Dependencies are inferred from the claim text.