Oxidation is a process that is performed to create an oxide film on a wafer.
Characteristics and Roles of Oxidation
First, let's learn about the characteristics of oxide. The oxide we're talking about here is SiO2. First, since the wafer's raw material is Si, it's easy to form. Furthermore, its excellent insulating properties and interfacial properties with Si make it suitable for use as a gate oxide in MOS devices. Furthermore, it serves as a high-quality mask, exposing only the necessary areas during diffusion processes like doping.
When Si is converted to SiO2 through oxidation, its volume increases. The substitution ratio is Si:SiO2 = 0.46:1, which can be considered to be almost doubled.
During the oxidation process, impurities or defects on the Si surface are included inside the oxide film, which has the advantage of stabilizing the quality of Si and the interface with the oxide film.
The characteristic of Si expanding in volume when oxidized is also used in the isolation process to prevent interference with the surroundings. SiO2 becomes thicker only in the parts where Si is exposed, and this is called the Local Oxidation of Silicon (LOCOS) process. This has the advantage of creating a part with a gentle slope, making subsequent processes easier. However, these days, processes are becoming increasingly refined, and the STI (Shallow Trench Isoation) process is said to be used instead of the LOCOS process.
Rather than utilizing the volume expansion of oxidation, this is a process that creates a trench through a selective etching process using nitride and fills that space with oxide, which is an insulator.
Oxidation method
There are two main types of oxidation processes: dry oxidation and wet oxidation. The difference between the two lies in the use or non-use of water, which in turn leads to differences in oxidation rate and oxide film quality.
Dry oxidation
Even when left in air, silicon (Si) forms a native oxide layer of approximately 1 to 2 nm. However, semiconductor processing requires a thicker, higher-quality oxide layer, so oxidation is performed in an oxygen atmosphere at high temperatures (900 to 1,200 degrees Celsius).
Si(s) + O2(g) → SiO2(s)
Wet oxidation
Wet oxidation uses water vapor to carry out oxidation.
Si(s) + H2O(g) → SiO2(s) + 2H2(g)
In both dry and wet oxidation, source atoms pass through the oxide film and react with Si at the Si–SiO2 interface, resulting in oxidation. However, because H2O is approximately 1,000 times more soluble in SiO2 than O2, H2O reaches the Si interface more quickly. This means that wet oxidation is significantly faster than dry oxidation. However, while wet oxidation is faster, its lower density results in a lower film quality, which lies at the very edge of this divide.
References: Samsung Electronics
