ACWWong
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You can improve the matching by means of user defined metal fill patterns, to ensure inter metal thickness uniformity it good.
In Al, you will need to read you process guide as to how metal fill is added to planarise the dielectric, and then design your own fill patterns to get best planarisation around your intra-metal caps... and ensure each cap has the same metal environment, (all levels below, and above)
In Cu, you will need to read your process guide as to the back-laping and "cheesing" process, again to ensure you (not some automatic tool) control the planarisation, and ensure your matched capacitors are given the best chance to have the same cheesing holes/planarisation/dielectric thickness.
As to a number for matching... i'm not sure, but process data with regards to oxide uniformity is usually available. Tolerance wise, oxide thickness & metal thickness can be ~20%, but matching i think i'd have to do a test chip (if of course you have the luxury to do this.)
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