Explain physical-only cells. What is the need for physical cells?

o   Decap Cells (Decoupling Capacitors): These are essentially capacitors (often MOSCAPs) placed near switching logic.

o    Need: to supply instantaneous current demands during fast switching events, stabilizing the power supply voltage (VDD/VSS) and mitigating dynamic IR drop and voltage noise on the power grid

o   Filler Cells: These cells fill the empty spaces left in standard cell rows after placement and optimization. They typically only contain VDD/VSS connections and substrate/well contacts.

o    Need: Ensure N-well and substrate continuity for proper biasing. Provide continuous power rail connections along the rows. Required for manufacturability (uniform density for CMP - Chemical Mechanical Polishing).

o   Tie Cells (Tie-High/Tie-Low): These cells provide a clean connection to VDD (Tie-High) or VSS (Tie-Low). They connect the input pins of standard cells that require a constant logic '1' or '0' directly to the power rails.

o    Need: Avoids connecting sensitive gate inputs directly to the main power rails, which can cause issues like antenna violations or noise injection. Provides a reliable, characterized source for logic '1'/'0'.

o   Endcap Cells (Boundary Cells): Placed at the ends of standard cell rows or boundaries of blocks.

o    Need: Properly terminate the N-wells and substrate at row ends, ensuring correct potential and preventing DRC violations. Can sometimes include small decap capacitance. Helps in reducing well proximity effect.

o   (Well Tap Cells): Similar to filler cells but specifically designed to provide frequent substrate and N-well contacts within the rows to prevent latch-up and ensure proper body bias. Often combined with filler functionality.