Games / Command flash cards / Sign-off checks and violations / Density and fill

Density and fill

Each card shows a command to explain. A missed card returns later in the round and starts the schedule over. A card answered correctly moves up the schedule and next appears after 1, 3, 7, then 21 days. Keys 1 to 4 choose an option, h shows a hint, and Enter moves on. A hint keeps the card where it is on the schedule. An ordering card is answered by clicking its items in order, and one wrong click ends it. Time values are in nanoseconds. Progress is stored in this browser.

SourceThese cards describe the checks a design must pass before tapeout, using the rule decks that ship with OpenROAD-flow-scripts: the KLayout runsets for sky130 and ASAP7 and the sky130 layout-versus-schematic runset. Rule names and values are quoted from those files. Every process has its own rules; the families and the reasons behind them are what carry across. Reference: the sky130 design rule documentation (the open rule decks on GitHub).
All 8 cards in this section, with answers
  • a metal density rule, checked in a window
    Each window of the layout must be neither too empty nor too full.

    Hint: Ask what the manufacturing step after deposition does to an uneven surface.

    The limit applies to metal in each window, top and bottom. Chemical mechanical polishing removes material faster where there is less metal, so an uneven layer ends up dished or domed, which ruins the layers above it. The rule is checked over a sliding window, so a design can pass overall and still fail locally. the open rule decks reference

  • density_fill -rules $(FILL_CONFIG)
    Adds unconnected metal shapes where a layer is too empty.

    Hint: The design cannot add real wires. Ask what it adds instead.

    Fill shapes are electrically inert; they exist to satisfy density. They do add capacitance to nearby wires, which is why extraction runs after fill in a careful flow, and why fill is kept away from sensitive nets. The flow turns it on with USE_FILL. the open rule decks reference

  • vpp.5a : max. m3 density in vpp : 0.25
    A local region where metal density must stay below a limit.

    Hint: This one is an upper bound. Ask where a maximum would matter.

    Density rules run both ways: too little metal polishes unevenly, too much can trap stress or interfere with a structure underneath. This rule comes from the sky130 deck and applies inside a parallel-plate capacitor structure, where extra metal3 would change the device. the open rule decks reference

  • detailed_route density_fill extract_parasitics
    Fill is added before extraction, so its capacitance is in the timing.

    Hint: Ask what fill does to the wires beside it, and which step must see that.

    Fill shapes are not connected, but they are metal next to signal wires, so they add coupling capacitance. Extracting before fill gives an optimistic answer. This is why the order in a finishing flow is fixed rather than a matter of taste. the open rule decks reference

  • dummy poly and diffusion fill
    Inert shapes on the transistor layers, added so their density is even.

    Hint: Same idea as metal fill, one layer group lower.

    Lithography and etch depend on what surrounds a feature, so an isolated transistor prints differently from one in a dense field. Dummy shapes make the surroundings uniform. In standard-cell rows this is built into the cells; it matters most at the edges of a block and around macros. the open rule decks reference

  • a density rule checked over a sliding window
    Yes: each window is checked separately, so a local gap fails on its own.

    Hint: Ask whether a design can pass on average and still fail.

    The window is typically tens of microns square and steps across the die, so an empty region next to a dense one fails even when the layer's overall density is fine. Fill is therefore added per region, not globally. the open rule decks reference

  • metal fill added after routing
    It adds capacitance to nearby nets, so extraction must be run after it.

    Hint: The fill shapes sit next to the signal wires.

    Fill is electrically present whether or not it is connected, and a wire running beside a field of it sees more capacitance than the pre-fill extraction assumed. That is why the order is route, fill, extract, then time, and why a late fill change means timing again. the open rule decks reference

  • floating fill, against fill tied to a supply
    Tied fill shields, while floating fill couples through.

    Hint: Ask which one has a defined voltage, and what that does to coupling.

    A floating shape between two wires passes coupling from one to the other and its own voltage is unpredictable. Tying fill to a rail makes it a shield and makes extraction straightforward, at the cost of connecting every piece and of the capacitance it adds to the supply. the open rule decks reference