IntermediateQuestion 209 of 222Source: Synopsys ICC2 Implementation User Guide X-2025.06-SP2: Chip Finishing and Design for Manufacturing (Inserting Filler Cells); ICC2 Tool Commands X-2025.06-SP2: remove_stdcell_fillers_with_violation

How do you remove only the fillers that cause DRCs?

From PDVerse PnR Interview Handbook · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

Use remove_stdcell_fillers_with_violation (ICC2), which checks filler instances against routing and deletes only the ones with violations. Run it first with -check_only true (ICC2) to see what it would remove, then in removal mode, and repeat until it reports that it deleted 0 cell instances. Removing one filler can expose a violation on its neighbour, so one pass is often not enough.

A standard-cell row with metal filler cells between logic cells, where two fillers overlapping or too close to a signal route are marked red and removed in the first pass, and a neighbouring filler shows a new violation that only the second pass removes. — Technical Reference DiagramHow do you remove only the fillers that cause DRCs?

Technical Explanation

By default the command considers instances whose names contain xofiller, the string ICC2 gives every inserted filler, and checks them only against top-level signal routing. -name (ICC2) picks a different pattern and -boundary (ICC2) limits the check to rectangles.
-check_between_fixed_objects true (ICC2) widens the check to every neighbour: other fillers, standard cells, PG rails and terminals. Use it when a violation involves something other than a signal wire.
-check_only true (ICC2) removes nothing and writes a report named after the block, ending in _fillers_with_violation.rpt, with each filler and its first violation. The file is overwritten on every check run, so copy it if you want to compare passes.
-shorts_only true (ICC2) removes only fillers that short to a routing shape, which keeps decap fillers that only have a spacing problem. It cannot be combined with -check_only (ICC2).
Fillers whose physical_status is fixed or locked are never removed, even with violations; set them to placed first. Fillers with PG pins not on a PG net are removed if they touch PG objects, so run connect_pg_net -automatic (ICC2) before checking.
Violations inside a filler, between objects of the same cell, are warned about and not removed. Double-patterning rules are checked only when route.common.color_based_dpt_flow (ICC2) is true and the technology file defines DP rules.
Prevention is cheaper. create_stdcell_fillers -rules check_pnet (ICC2) checks power net violations during insertion, at a runtime cost, so fewer decaps are removed later. In an ECO, create_stdcell_fillers -post_eco (ICC2) marks the new fillers and remove_stdcell_fillers_with_violation -post_eco true (ICC2) checks only those.

What To Check

Flow order: metal fillers, connect_pg_net -automatic (ICC2), removal passes, then nonmetal fillers and another PG connect.
The check report from each pass: filler count and violation types.
Decap loss, meaning how many decap fillers were removed and where, since each one takes decoupling away from busy logic.
The final pass printing Deleted 0 cell instances.
Signoff DRC afterwards with signoff_check_drc (ICC2), since the removal command checks routing DRC in ICC2 rather than the foundry runset.

Command Checks & Actions

ICC2 (icc2_shell)connect_pg_net -automatic

Connects filler PG pins first, so fillers touching rails are not removed only because their pins were unassigned.

ICC2 (icc2_shell)remove_stdcell_fillers_with_violation -check_only true -check_between_fixed_objects true

Checking mode with the same object scope as the removal pass: lists fillers that would be removed and removes nothing.

ICC2 (icc2_shell)remove_stdcell_fillers_with_violation -check_between_fixed_objects true

Removal mode, checking fillers against all neighbouring objects instead of only top-level signal routes.

ICC2 (icc2_shell)remove_stdcell_fillers_with_violation -shorts_only true -boundary {{{100 200} {160 260}}}

Removes only fillers that short to routing inside one rectangle (coordinates illustrative).

ICC2 (icc2_shell)save_block

Saves the block, since IC Validator reads it from disk.

ICC2 (icc2_shell)signoff_check_drc

Confirms the remaining fillers are clean against the signoff runset.

Healthy, Suspicious & Hard-stop Results

Healthy (illustrative): Pass 1 removes 52 fillers, pass 2 removes 5, pass 3 reports Deleted 0 cell instances, and signoff DRC shows no filler-related errors.
Suspicious (illustrative): Pass counts that do not fall, such as 40, then 38, then 41, or more than 5% of decap fillers removed in one region.
Hard stop: Signoff DRC still flags filler shapes after a pass that deleted nothing (fixed or locked fillers, internal violations, or a rule the ICC2 check does not model), or fillers left with unconnected PG pins.

Common Mistake

The Trap: Running the command once and moving on. The first pass reports 52 deletions and looks finished, but removing those fillers creates new violations with neighbouring fillers, a limitation the command reference states. A second pass would have removed 5 more; instead they surface as signoff DRC errors two days later, after metal fill, and the fix now needs fill removal and a refill in that area.

What The Interviewer Is Testing

Knowing the command must be repeated until it deletes nothing.
Understanding what it will not remove: fixed or locked fillers and internal violations.
Candidates who connect PG before checking and start with check-only mode show they have used it on a real block.

Follow-up Question & Model Response

"Why not remove all fillers and reinsert them?"

Candidate Model Response: remove_cells [get_cells xofiller*] (ICC2) removes every filler, including thousands of clean decaps, and a full reinsert can put the same violations back because the routing has not changed. Targeted removal keeps clean fillers and their decoupling. Full removal and reinsertion makes sense after a large ECO that moved many cells, where most filler positions are stale anyway. For small changes, marking new fillers with -post_eco (ICC2) keeps the check limited to them.

Practical Example

Tapeout Scenario: After routing, a block receives 18,400 metal fillers, 6,100 of them decaps (illustrative). A check-only pass reports 52 fillers with violations: 44 spacing violations to top-level signal routes and 8 shorts. Pass 1 removes those 52, pass 2 removes 5 neighbours exposed by the first removal, and pass 3 reports Deleted 0 cell instances. 57 fillers are gone in total, 21 of them decaps, about 0.3% of the decap fillers. Nonmetal fillers then fill the gaps, connect_pg_net -automatic (ICC2) runs again, and signoff_check_drc (ICC2) shows no filler-related errors.

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