<@U016HSAH6AE>: My understanding is that contact ...
# sky130
@User: My understanding is that contact cuts have a lower yield if the metal doesn't extend beyond the cut area. This used to always be specified as a surround rule, but apparently it's okay to have the extension past the cut in one direction only, not a complete surround, which allows tighter rules for routing.
@Philipp Gühring, @Tim Edwards In my previous life (as memory designer) we had exactly this rule for metal vias and contacts. Very small metal lines with minimal extension to contact in the direction perpendicolar to the metal direction, but with a bigger required OV rule at line end. I am not a litho or process expert, so i don't know the exact reason. For sure having more overlap in one direction will mitigate somehow the yield degradation due to mask mis-alignment. in that direction.
I fully understand that an extension helps due to misalignment. What puzzles me is why an asymmetrical extension should help more than a symmetrical extension. I would expect a symmetrical extension to be even more helpful since it works in both directions, than an asymmetrical extension that only (primarily) works in one direction?
@Philipp Gühring Sure a symmetrical overlap helps more since it tolerates X/Y misaligments, however the asymmetrical rule is a trade-off, allowing to have tight pitched metal lines. Requiring more overlap in both directions will reduce metal pitch. I think this applies both for metal-to-metal vias and for metal1/licon to poly/diffusion contacts
[Edit]: reduce metal pitch -> increase metal pitch