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metal mask rom is very different from the conventional mask rom concept.
It is formed by the selectable vias,not the VT imp to the mos transistors in conventinal mask rom
I've an ambiguity about MMR. Some IC book says that MMR is programmed using contact mask layer only. Some other IC book says that MMR is programmed using metal-1 layer only. Which programming methodology is true to MMR ?
For serial rom ( implant code ), all the non-selected gates are active ( on ) and the selected gate is off, once the gate is coded, it will be on, no matter the gate voltage.
In same condition but replace the ion-implantation by a metal which short the source and Drain of selected gate can also get same result and the code could be metal, via or contact depends on design, basically we will use the last mask layer to reduce the production lead time
Anyway, compare to implant code, the area of metal code is much larger ( source / drain contact rule )
For serial rom ( implant code ), all the non-selected gates are active ( on ) and the selected gate is off, once the gate is coded, it will be on, no matter the gate voltage.
In same condition but replace the ion-implantation by a metal which short the source and Drain of selected gate can also get same result and the code could be metal, via or contact depends on design, basically we will use the last mask layer to reduce the production lead time
Anyway, compare to implant code, the area of metal code is much larger ( source / drain contact rule )
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