Re: Metal Mask ROM
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 )