Metal fill (density fill)
Beginner
Extra metal added to empty spots, so the factory can polish each layer flat and even.
Novice
Small unconnected metal shapes added to empty areas of the layout. The factory polishes each layer flat, and areas with too little metal polish unevenly, so every region must have roughly the right amount of metal.
Expert
Shapes inserted so every density window meets minimum and maximum density rules for chemical-mechanical polishing (CMP). Fill adds coupling capacitance, so signoff extraction and timing must include it or model it; timing-aware fill keeps it away from critical nets. Fill also changes DRC results, so physical verification runs after fill.