gate replacement

**Gate Replacement** is the **core process step in the gate-last (RMG) integration flow** — where the dummy polysilicon gate is physically removed by selective etching, and the resulting trench is filled with the actual high-k dielectric and metal gate stack. **How Does Gate Replacement Work?** - **Dummy Removal**: Wet etch (NH₄OH-based for poly-Si) followed by HF for dummy oxide, leaving an empty gate trench. - **High-k Deposition**: ALD HfO₂ (~1-2 nm) conformally coats the trench walls and bottom. - **Work Function Metal**: TiN, TiAl, TiAlC deposited by ALD/PVD to set the target $V_t$. - **Fill Metal**: Tungsten (W) or aluminum (Al) fills the remaining trench volume. - **CMP**: Planarize to remove overburden and isolate individual gates. **Why It Matters** - **Quality**: The gate stack is deposited at low temperature (<400°C) -> no thermal degradation. - **Multi-$V_t$**: Different metal stacks can be deposited in different gate trenches for multiple $V_t$ flavors. - **Complexity**: Requires precise etch selectivity, conformal ALD, and void-free metal fill in ultra-narrow trenches. **Gate Replacement** is **the surgical swap at the heart of HKMG** — removing the placeholder and installing the precision-engineered metal gate that defines transistor performance.