dummy gate

**Dummy Gate** is the **sacrificial polysilicon gate structure used in the gate-last (RMG) process** — serving as a placeholder during FEOL processing (spacer formation, S/D implant, silicidation) and later removed and replaced with the real high-k/metal gate stack. **What Is a Dummy Gate?** - **Material**: SiO₂ (thin dummy oxide) + poly-Si or amorphous-Si (gate body). - **Purpose**: Defines the transistor channel length, spacer offset, and S/D geometry. - **Removal**: Wet etch (NH₄OH for poly-Si, HF for dummy oxide) after CMP planarization exposes the gate top. - **Result**: An empty trench (gate cavity) lined by spacers -> ready for high-k/metal gate deposition. **Why It Matters** - **Process Compatibility**: All existing FEOL modules (implant, anneal, salicide) work as-is with the dummy gate. - **Critical Etch**: Dummy gate removal must be perfectly selective to spacer and channel — any damage to the channel is fatal. - **Scaling**: Dummy gate height and profile directly determine the replacement gate trench geometry. **Dummy Gate** is **the theatrical understudy** — performing all the rehearsals (high-temperature processing) so the star (real metal gate) can make a fresh, undamaged appearance for the final show.