What are filler cells and tap cells in physical design?

Filler cells: Fill gaps between placed cells, provide continuous power/ground rails, and maintain manufacturing continuity (no isolated active regions). Types include: simple fillers, decoupling capacitor fillers (add decap), and metal-only fillers. Tap cells (well taps): Connect n-well and p-substrate to their respective supplies, prevent latch-up by maintaining proper well potentials. Placed at regular intervals (foundry-specified maximum distance, typically 20-30um). Both are inserted post-placement and are essential for DRC-clean, manufacturable, and reliable designs.