The chemical reaction at the silicon surface for dry and wet oxidation is given as

Oxidation of silicon is not difficult, since silicon has a natural inclination to form a stable oxide even at room temperature, as long as an oxidizing ambient is present. In both cases the oxidizing species diffuses through the growing oxide and reacts with the silicon surface. These oxidation reactions occur at the Si-SiO₂ interface, i.e., silicon at the interface is consumed as oxidation takes place. As the oxide grows the Si-SiO₂ interface moves into the silicon substrate. As a result, the Si-SiO₂ interface will always be below the original Si wafer surface. The SiO₂ surface, on the other hand, is always above the original Si surface. SiO₂ formation therefore proceeds in two directions relative to the original wafer surface as shown in Fig.3.12.