How are double-patterning odd-cycle violations fixed at signoff?

Short Answer

An odd cycle is a loop of an odd number of shapes, each too close to the next to share a mask, so no two-colour assignment works. At signoff you fix all other routing rules first with the double-patterning rules unselected, then run a separate fix pass on only the double-patterning rules with signoff.fix_drc.custom_guidance (ICC2) set to dpt. A final full signoff check confirms both.

Technical Explanation

• Two masks mean every pair of shapes closer than the same-mask spacing must be on different masks. Three shapes each too close to the other two form a triangle, and a triangle cannot be two-coloured.
• The fix is geometric: increase one spacing in the loop, or move or reroute one segment so the loop becomes even or opens.
• The ICC2 guide requires separate fixing runs for non-double-patterning and double-patterning rules when the block uses double patterning.
• Step one: run the normal auto-fix flow with -unselect_rules (ICC2) on both signoff_check_drc (ICC2) and signoff_fix_drc (ICC2), listing the double-patterning rules. Take the rule names from the foundry design rule manual; the commands below hold them in a Tcl list, $dpt_rules.
• Step two: set signoff.fix_drc.custom_guidance (ICC2) to dpt, which makes the fixing run target odd-cycle violations, and use -select_rules (ICC2) with only the double-patterning rules.
• Step three: run full signoff DRC to verify both sets.
• The order matters because fixing spacing, width or via rules moves shapes and can create or remove cycles. Fixing coloring last works on geometry that is already settled.

What To Check

• The double-patterning rule list from the design rule manual, matched to the rule comments in the runset.
• Odd-cycle counts after step one, before any coloring fix.
• That step two only targets the double-patterning rules.
• Full signoff DRC after step three, with coloring and all other rules.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): Other rules reach 0 in pass one, odd cycles fall from 63 to 0 in pass two, and the final full check is clean.
• Suspicious (illustrative): Odd cycles return in the final check after an unrelated late edit, or cluster around one macro edge.
• Hard stop: A pass fixing spacing rules and odd cycles together keeps trading one for the other with no convergence.

What The Interviewer Is Testing

• Can the candidate explain why an odd cycle cannot be two-coloured?
• Knowledge of the two-pass order and the dpt guidance setting.
• Awareness that rule names come from the foundry manual.

Follow-up Question & Model Response

"Why not just swap the mask colour of one shape?"

Candidate Model Response: In an odd cycle every colouring leaves at least one close pair on the same mask, so swapping a colour only moves the conflict around the loop. The cycle has to be broken by geometry, by opening a spacing or moving a segment. Swapping colours does help when the conflict is between shapes whose colours were fixed badly but the graph itself is even. That case is a coloring problem, not an odd cycle.