How do you control the decap mix and leakage when inserting fillers?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Decap fillers add decoupling but also leak, so you decide how much of the empty space becomes decap and which VT flavour it uses. create_stdcell_fillers -type_utilization (ICC2) sets an insertion percentage per filler group, -fill_remaining (ICC2) fills what is left from the -lib_cells (ICC2) cells that are not in those groups, and -leakage_vt_order (ICC2) makes the tool choose the lowest-leakage filler that legalizes.
Technical Explanation
- By default the command fills all empty space with the
-lib_cells(ICC2) list, tried in the order given, largest to smallest for best results.-utilization(ICC2) sets the maximum percentage of free space to fill, default 100. -type_utilization(ICC2) takes pairs of a cell group and a percentage, and the percentages must add up to 100 or less. The guide's example asks for 70 percent ULVT decap and 30 percent LVT decap.- Legalization can leave space unfilled when you use groups.
-fill_remaining(ICC2) fills it with cells in-lib_cells(ICC2) that are not named in-type_utilization(ICC2). It works only together with-type_utilization(ICC2), and the command fails if every library cell sits in a group and nothing is left to fill with. - Within a group, cells are picked randomly by default.
-prefer_type_ordering(ICC2) inserts them in list order, left to right, which helps cells that are hard to legalize; it needs the advanced legalizer and can add runtime. -leakage_vt_order(ICC2) lists threshold voltage layers in order of decreasing leakage, and the tool picks the lowest-leakage filler that legalizes. It needsplace.legalize.enable_advanced_legalizer(ICC2) set to true and VT layers defined in the technology file with maskType implant, and the list must include every layer the-lib_cells(ICC2) fillers use.-leakage_vt_check(ICC2) inserts nothing. It reports fillers that could be replaced by a lower-leakage one without a legalization error, 100 at most by default, whichchf.create_stdcell_fillers.max_leakage_vt_order_violations(ICC2) changes.- The trade-off is dynamic IR against leakage: decap near busy logic supplies fast current pulses, but every decap adds leakage, more so on lower-VT layers. Plain fillers add neither.
- The power net violation check is off by default.
-rules check_pnet(ICC2) turns it on at a runtime cost and avoidsremove_stdcell_fillers_with_violation(ICC2) later removing many decaps.
What To Check
place.legalize.enable_advanced_legalizer(ICC2) is true before you rely on leakage ordering or type ordering.- Achieved share per group against the target, since legalization rarely hits it exactly.
- The violation count from
-leakage_vt_check(ICC2) after insertion. - Decap count inside the hotspots on the dynamic IR map, not only block totals.
- Block leakage after filling compared with the number before fillers.
Command Checks & Actions
set_app_options -name place.legalize.enable_advanced_legalizer -value trueEnables the advanced legalizer that leakage ordering and type ordering require.
check_legalityConfirms the block is legal before filler insertion.
create_stdcell_fillers -lib_cells $FILLER_CELLS -type_utilization {{*/DCAP8*ULVT */DCAP4*ULVT} 70 {*/DCAP8*LVT */DCAP4*LVT} 30} -fill_remainingAsks for 70% ULVT and 30% LVT decap by insertion percentage, then fills leftover space with the list cells outside those groups.
create_stdcell_fillers -lib_cells $FILLER_CELLS -leakage_vt_order {ULVT LVT SVT} -leakage_vt_checkChecks the inserted fillers and reports any that a lower-leakage filler could replace; inserts nothing (layer names illustrative).
Healthy, Suspicious & Hard-stop Results
- Healthy (illustrative): ULVT and LVT decap land within 2 points of the 70/30 target,
-leakage_vt_check(ICC2) reports 0 violations, and filler leakage stays inside its allowance. - Suspicious (illustrative): Decap share far from target in dense regions, or a leakage check report showing exactly 100 violations, the default cap, so the true count may be higher.
- Hard stop: The command fails because every library cell is in a
-type_utilization(ICC2) group while-fill_remaining(ICC2) is set, or filler leakage alone breaks the block leakage budget.
Common Mistake
The Trap: Putting every filler, plain ones included, into -type_utilization (ICC2) groups and then adding -fill_remaining (ICC2). No cells are left outside the groups, so the command fails, and on a block where insertion takes hours that failure lands in tapeout week.
What The Interviewer Is Testing
- Knowing decaps trade dynamic IR against leakage and are not free filler.
- Knowing which cells
-fill_remaining(ICC2) draws from and when it fails. - Knowing the prerequisites for leakage ordering: the advanced legalizer and implant VT layers in the technology file.
Follow-up Question & Model Response
"How do you pick the decap split?"
Candidate Model Response: I start from the dynamic IR map and the leakage budget, not from the example numbers. In regions with high switching I want decap, and I let the library data decide which flavour gives the most decoupling per unit of leakage. Elsewhere I shift the split toward the lower-leakage group or plain fillers. Then I rerun dynamic IR to confirm the hotspots still pass and report leakage to confirm the budget holds.
Practical Example
Tapeout Scenario: A block has 1.2 mm² of empty row area after placement (illustrative). A 70/30 ULVT/LVT decap target with -fill_remaining (ICC2) lands at 66% ULVT, 28% LVT and 6% plain fillers where decaps would not legalize, so 100% of the space is filled. Filler leakage comes to 4.1 mW against a 3 mW allowance. Changing the groups to 50/30, with plain fillers taking the rest, drops filler leakage to 2.8 mW. A vectorless dynamic rerun moves the worst drop from 71 mV to 74 mV against an 80 mV limit, so the new mix is accepted.
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