One of the biggest advantage of Si is that nature has provided it with excellent native oxide (SiO2). It is thermally stable, has superior dielectric properties and is primarily used for selective opening of windows during the photolithography and etching process. It is also used for final encapsulation or passivation after the entire chip is manufactured.
In thermal oxidation of Si the source of oxygen can be either dry oxygen (O 2) or water vapour/steam (H2O). The process of using dry oxygen is called as the dry oxidation and the processor of using water vapour/steam is called as wet oxidation.
Dry oxidation produces excellent quality oxides with very high dielectric strength low defects, less porosity and good permittivity but the rate of oxidation is quite slow. On the other hand oxide growth rate of wet oxidation is much faster than for dry oxidation but wet oxidation produces inferior quality oxides as compared to dry oxidation oxides. Therefore for thick oxides where quality is not that important (e.g. field oxide for MOSFET) wet oxidation is used where as when quality of oxide is important (e.g. gate oxide of MOSFET) dry oxidation is a must.