Why tap cells, End caps, Tie high/low cells are used in Physical Design flow?

Tap cells are used to tie the nwell to VDD and substrate to VSS.

Endcap cells are inserted to fullfill well tie off specifications for the cell rows. Endcaps don't have signal connectivity. They only have connectivity to power and ground distribution in the design. Endcaps have a fixed attribute and cannot be moved by optimization steps. Endcaps are placed at the end of the cell rows and handle end of row well tie off requirement.


The-hi and Tie-low is usually provided as a standard cell.  The reason these exist is to provide some kind of shielding for the logic 1 and logic 0 metal lines. Trying these nets directly to VDD and GND can cause various issues.