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.