After Silicon Wafers are loaded into a hightemperature oxidation furnace, oxygen or water vapor reacts with silicon. The dryoxygen reaction is expressed as Si＋O₂→SiO₂; the wetoxygen reaction is written as Si＋2H₂O→SiO₂＋2H₂. Both reactions produce silicon dioxide, differing only in the oxidant used, which results in different growth rates and film quality.

The key term here is “growth”. Thermal oxidation consumes the silicon substrate, which is not equivalent to CVD, where oxide is deposited onto the surface. As an approximation, growing 100 nm of silicon dioxide consumes approximately 44 nm of silicon, with the remaining ~56 nm protruding above the original silicon surface. This volumetric relationship originates from the difference in silicon atomic density between silicon and silicon dioxide.

Thermal oxide layers are used for surface passivation, sacrificial oxidation, doping or etching masking, and can also form the insulating or interfacial layer in MOS structures. While highk materials are widely adopted as the main gate dielectrics in advanced logic devices, a highquality Si/SiO₂ interface remains the foundation for understanding modern gate stacks.