Chemical-mechanical polishing (CMP)
Beginner
Polishing the chip flat after each layer is built, a bit like sanding, so the next layer goes on a smooth surface.
Novice
A step that polishes the wafer flat with a rotating pad and a chemical slurry. How fast it removes material depends on how densely the layer underneath is patterned, so an uneven layout polishes unevenly.
Expert
Thickness after polishing follows an effective local density, a weighted average over a neighborhood set by how much the pad bends. Uneven density leaves thickness variation (dishing, where wide metal is scooped out, and erosion, where dense areas wear down), which changes wire resistance and capacitance.
Explained in GDS & tapeout (Design Flow).
See also: Metal fill (density fill), Density window.