thermal oxidation

**Thermal Oxidation** — growing silicon dioxide (SiO₂) by exposing silicon wafers to oxygen or steam at high temperatures, producing the highest quality oxide used for gate dielectrics and isolation. **Reaction** - Dry oxidation: Si + O₂ → SiO₂ (slow, highest quality — used for gate oxide) - Wet oxidation: Si + 2H₂O → SiO₂ + 2H₂ (faster, lower quality — used for thick field oxide) - Temperature: 800–1200°C **Deal-Grove Model** - Oxide thickness vs time follows: $x^2 + Ax = B(t + \tau)$ - Linear region (thin oxide): Growth rate limited by surface reaction - Parabolic region (thick oxide): Growth rate limited by diffusion through existing oxide **Key Properties of Thermal SiO₂** - Excellent interface with silicon (low defect density ~10¹⁰ cm⁻²) - k = 3.9 (used as reference for equivalent oxide thickness) - Breakdown field: ~10 MV/cm - Historically the reason silicon dominated over germanium and GaAs **Modern Status** - Pure SiO₂ gate oxide replaced by HfO₂ (high-k) at 45nm node (2007) - But thin SiO₂ interfacial layer (~0.5nm) still grown between Si and HfO₂ for interface quality - Thermal oxide still used for STI fill, hard masks, and passivation **Thermal oxidation** is the oldest and most fundamental process in silicon technology — the Si/SiO₂ interface is what made the entire semiconductor industry possible.