Density and metal fill

Density and metal fill. After each metal layer is patterned it is polished flat by CMP (Chemical Mechanical Polishing). The polish only stays planar where the metal density is even, so the foundry divides the die into windows and requires each window's density to sit inside a band, here 30% to 70%.

• Below the floor (too sparse). In a nearly empty window the polish removes too much, an effect called erosion, and the surface dips. The fix is to add dummy metal fill until the window reaches the minimum.
• Above the ceiling (too dense). A very full window polishes unevenly too: wide metal dishes in the middle. Fill cannot fix this, because fill only adds metal. A window that is genuinely over the ceiling is a routing problem, fixed by spreading the wires out or by slotting (cutting holes into wide metal to remove area), not by fill. So in this lab a window goes over the ceiling only if you over-fill it, and the fix is to remove that fill.

Fill is not free, and it is not "fill every gap". Dummy metal placed next to a signal wire adds capacitance to that net, which slows it and costs power. So fill is added only to reach the density target, kept clear of timing-critical nets, and never packed into every empty site. Fill tied to a power or ground rail (grounded fill) also shields its neighbours; floating fill mainly loads them.

The die below is a grid of cells: dark cells are existing metal, fixed by the design. Thick lines divide it into density windows, and each window's density is the fraction of its cells that hold metal. Click an empty cell to drop in a dummy-fill cell, click a fill cell to remove it. Fill the sparse windows up to the band without pushing any window past the ceiling, then Submit fill.