There are two types of oxidation: Dry oxidation and wet oxidation. Dry oxidation occurs as:
Si + O2 => SiO2and wet occurs as:
Si + 2H2O => SiO2 + 2H2As the oxidation grows, it uses up Silicon. However the density of the silicon dioxide is lower, so the wafer expands vertically. This process could take a long time, however, the growth rate of the silicon dioxide is a function of temperature as well. When we heat the wafer to 1200oC, the process only takes a few hours.
Another problem: our growth isn't constant. When we stick a brand new, clean wafer into oxygen, instantly it is attacked by the oxygen to start forming silicon dioxide. As the wafer grows more and more of it, we can get to the point where it is harder for the oxygen to reach the surface of the silicon, so the rate slows in proportion to the concentration gradient. This yields some nasty differential equations that I won't get into. Various companies and researchers are also experimenting with using higher pressure instead of higher temperatures (Jaeger, 36).