@User It's complicated... Although the exact contributors will depend an exact processes a foundry uses for different steps here are some I can think of: • Etch stop layer and chemistry will depend on which contact/via you are etching. Some etch processes may allow that the via bottom is outside bottom layer and some don't. What I have seen when I was at imec is that typically contact etch to poly/active needs to fall inside the bottom and that it is more relaxed for higher via layers. • The sidewall of a contact hole/via is typically not fully perpendicular but slightly sloped, this combined with the height means that the bottom is smaller than the top. • Width/space rules are actually not very good for describing the 2D spatial filtering of the lithography especially for more aggressive lithography. Minimum pitch of metal1 is typically lower than the middle of line metal layers so the lithography uses a more aggressive illumination. So for metal1 one may get more design rules like forbidden T-shapes, finger shapes, ... • middle of line metal layers are typically used for routing which typically have also a routing direction. So one wants to optimize the minimum pitch of the layer by minimizing overlap in one direction and extra extension in other direction. Metal1 is typically use for connecting inside standard cell which goes in both directions so it needs other kind of optimization. And in the latter case also the minimization of the SRAM cell area needs to be taken into account. • ...