Preventing via stacking within a PDN

[stockstephanie]

Hi all,
I've been setting up a PDK with four metal layers and I have begun addressing some DRC issues that I have. One of the big ones I have left is the via stacking inside the PDN. In this process via stacking is not allowed, so I was wondering if there was a good way to get around it without making straps on all four layers that are offset from one another. Thanks!

[maliberty]

@gadfort thoughts?

[gadfort]

I don't think PDN looks at those rules, so adding straps would be the right way to handle it for now. I would need a testcase in order to be able to look at implementing something

[stockstephanie]

I unfortunately cannot provide a testcase, but it is just the overlap and spacing of adjacent vias that are causing an issue. If there was a rule in Sky130 that prohibited via stacking I would imagine that it would cause similar problems just by default. Maybe some sort of flag that could be set within the platform configuration about via stacking being allowed? It feels like a slippery slope though. I can kind of get around it by offsetting the vias within the primary cell instances of the via stacks like the pictures I've attached (Picture 1 being M1/Via1/M2; picture 2 being M2/Via2/M3). Via1 can be under Via3 but not under Via2. It's relatively straightforward for a PDN, but I'm not entirely sure what the best course of action would be for all other stacking issues other than being able to add in LEF-like rule statements for niche spacing/stacking rules like this. There are also spacing rules between contacts and Via1, but I have only seen major issues with contacts in the VSS/VDD straps at the top/bottom of the standard cells for body ties.

[gadfort]

I don't think adding a flag is needed, the LEF should provide these rules, so pdn should handle them without the need for user intervention.

[stockstephanie]

Can you create interlayer spacing rules in LEF? I didn't think you could based on the version reference I've been using, but if I can add in those additional spacing rules in the technology LEF I imagine that would help a good amount.

The PDK's release uses LEF version 5.5 so I've already had to go in and edit some things for enclosure rules.

[stockstephanie]

Okay so updating the technology LEF to version 5.7 with new layer spacing rules helped in a few stray routing cases that were violating DRC but the via stacking is still happening within the PDN. I added the obstruction flag to see if it would help, but it did not.
I'm guessing for via1s being placed too close to contacts within the standard cells, I'd need to use blockages in a Post-PDN to prevent placement of standard cells under the straps?