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.