density within a 100um by 100um window. To meet density, fill shapes on diffusion, poly and metal layers are added - manually or by a fill program - after design completion. Fill can be grounded or floating. floating fill adds less total capacitance but more coupling capacitance to nearby wires, while grounded fill requires routing the ground net to the fill structures. There are two fill types in a layout: grounded metal fill, connected to power or ground through vias, and floating metal fill, with no connection to any signal, power or ground net, both can coexist. During StarRC extraction, fill can be modeled as emulated or real, which give different results depending on accuracy needs - emulation fill is only for early place-and-route stages and should not be used for correlation. In LEFDEF, floating fill polygons go in the FILLS section of the DEF file, while fill tied to power/ground goes in the SPECIALNETS section as special wiring with SHAPE defined as FILLWIRE. During extraction, grounded fill is treated like any power/ground polygon with no special handling. For floating fill, capacitance is computed between signal and fill polygons and between fill polygons; afterward the fill nodes are reduced on the fly to give equivalent signal-to-signal and signal-to-ground capacitance. A floating fill's potential is fixed by setting its net charge to zero - and even though it is not electrically connected, it still introduces capacitive coupling between other nets. On gate and diffusion capacitance: diffusion capacitance scales with the source/drain region size, so wider transistors have proportionally more diffusion capacitance. Increasing channel length raises gate capacitance proportionally but does not change diffusion capacitance.

KEY Density rules keep etch and CMP uniform; metal fill meets them but adds coupling capacitance that extraction must model.

Settings for Fragmented Floorplans

For floorplans that are fragmented or complex and contain narrow routing channels, the following options help:

• Turn on global-route-based high-fanout synthesis for place_opt to improve routability and ease congestion: set_app_options -list