How does isolated-via fixing choose a fix, and when can't it fix everything?

Short Answer

signoff_fix_isolated_via (ICC2) looks for a neighbour within the range you set per via layer, and if none exists it adds a fixing via using dummy fill as the landing. It tries three methods in a fixed order: a via between an existing non-wide net shape and fill, then a line-end extension plus via, then a via between a wide metal shape and fill. On very congested or very sparse blocks it may not fix them all, and by default it also skips clock and PG nets.

Technical Explanation

• An isolated via has no neighbouring via within the distance the technology requires. You set that distance per via layer in signoff.fix_isolated_via.isolated_via_max_range (ICC2), using mask names such as via1 and microns. It is the one required option.
• Before fixing, the command inserts track-based dummy fill around each isolated via, ignoring existing fill. If track fill is already in, -update_track_fill true (ICC2) with -track_fill_runset_include_file (ICC2) pointing at track_fill_params.rh (ICC2) reuses those shapes.
• Fix priority is fixed: first a via from an existing non-wide net shape to a fill shape, then extending a non-wide line end and dropping a via to fill, and last a via from a wide metal shape to fill. Each candidate honours routing rules including double patterning and nondefault spacing.
• Unused dummy fill is removed afterwards, so the net change is only the fixing vias and the shapes they need.
• The avoid-net-types app option of the isolated via fixer defaults to clock and pg. Isolated vias on clock and PG nets are left alone unless you change it, so they will still appear in the report.
• Congestion is the common failure: with no free track inside the range there is nowhere to put fill or an extension without a spacing violation. The guide also names very sparse blocks as a case where not every via gets fixed.
• The block is not saved by default. Run save_block (ICC2) or use -save_design true (ICC2), then read the summary of isolated vias per layer before and after, and signoff_fix_isolated_via.err (ICC2) in the error browser.

What To Check

• Isolated vias per via layer before and after, from the summary report.
• Remaining isolated vias on clock and PG nets, which are skipped by default.
• Local congestion around each via that could not be fixed.
• New DRC violations near fixing vias after the run.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): Isolated vias on via2 drop from 3,420 to 12, the 12 all sit in one congested channel, and incremental DRC is clean.
• Suspicious (illustrative): Hundreds remain on clock nets because clock and pg are still the default avoid list, and nobody has decided whether that is acceptable.
• Hard stop: Fixing vias that create DRC violations, or a large remaining count with no plan for manual repair.

What The Interviewer Is Testing

• Can the candidate recite the three-step fix priority and explain why a new via on an existing thin shape comes first?
• Awareness of the clock and PG default exclusion.
• Knowledge that the command does not save the block by default.

Follow-up Question & Model Response

"Why is extending a line end ranked below a direct via to fill?"

Candidate Model Response: A direct via to fill adds one cut and leaves the existing wire untouched. A line-end extension changes the net's geometry, adds a little capacitance, and has to satisfy end-of-line spacing to its neighbours, so it is more likely to disturb something. A via from a wide metal shape comes last, probably because wide-metal via rules are stricter. The tool picks the least disruptive option that satisfies the rules.