gate oxide formation

**Gate Oxide** — the ultra-thin insulating layer between the gate electrode and the silicon channel, the most critical film in a MOSFET. **Thermal Oxidation** - Grow SiO2 by exposing silicon to O2 or H2O at 800-1000C - Produces highest quality Si/SiO2 interface - Self-limiting: growth rate decreases as oxide thickens - Gate oxide thickness at 90nm node: ~1.2nm (just 5 atomic layers of SiO2!) **High-k Dielectrics (45nm and below)** - Problem: SiO2 thinner than ~1nm has excessive tunneling leakage (quantum mechanical) - Solution: Replace SiO2 with high-k material (HfO2, $\kappa$~22 vs SiO2 $\kappa$~3.9) - Physically thicker film provides same capacitance with far less leakage - Equivalent Oxide Thickness (EOT): ~0.5-0.9nm at advanced nodes **HKMG Process** - Gate-first: Deposit high-k, then metal gate, then anneal - Gate-last (replacement metal gate): Build dummy polysilicon gate, complete source/drain, then replace dummy with high-k + metal gate - Gate-last produces better interface quality — standard at 28nm and below **Gate oxide quality** directly determines transistor performance, leakage, and reliability.