hkmg gate

**High-k Metal Gate (HKMG)** — replacing the traditional SiO₂/polysilicon gate stack with hafnium-based high-k dielectric and metal gate electrode, the most significant transistor material change since the invention of the MOSFET. **The Problem (Pre-2007)** - SiO₂ gate oxide scaled to ~1.2nm (just 5 atomic layers) - Quantum tunneling through such thin oxide → massive gate leakage (100 A/cm²) - Couldn't go thinner → hit the "gate oxide wall" **The Solution** - Replace SiO₂ (k=3.9) with HfO₂ (k≈25) - Same electrical thickness (EOT) with 6x physical thickness - Thicker film → exponentially less tunneling → 100x leakage reduction **Metal Gate (Why Not Polysilicon?)** - Polysilicon gate depletes at the oxide interface → adds ~0.4nm to effective oxide thickness - Metal gate has no depletion → every angstrom of EOT counts - Different metals for NMOS and PMOS to set correct $V_{th}$ (TiAl for NMOS, TiN for PMOS) **Replacement Metal Gate (RMG) Process** 1. Build transistor with dummy polysilicon gate 2. Complete S/D, spacers, ILD deposition 3. Remove dummy poly (selective etch) 4. Deposit high-k + metal gate stack into the trench 5. CMP to planarize **HKMG** was introduced by Intel at 45nm (2007) and has been used at every node since — it removed the gate oxide as a scaling limiter and enabled the continued Moore's Law progression.