2.2 Principles of the Oxidation Process

Thermal oxidation is a process where silicon is converted into SiO$_2$ with the help of oxidants in an artificial high-temperature ambient. As oxidant source different oxygen compounds can be used which are supplied by the ambient. Fig. 2.2 illustrates that the oxidants diffuse from the oxide surface through the already existing oxide to the interface. At the interface the chemical reaction takes place where the silicon is converted into the SiO$_2$ [29].

In the interface the silicon is converted in principle atom layer after atom layer. In the interface there is a mixture of silicon, oxygen, and SiO$_2$. The interface thickness is only a few atom layers. Because of the silicon consumption the interface moves constantly from the surface into the silicon substrate during the oxidation process. On the other side the molecule density of SiO$_2$ is with 2.3 $\times$ 10$^{22}$ molecules/cm$^{3}$ less than half of the atom density of silicon.

Figure 2.2: Basic process for the oxidation of silicon.

Because of the different molecule densities of silicon and SiO$_2$, the newly formed SiO$_2$ has 125% volume expansion. If the volume expansion takes place only in one direction, as shown in Fig. 2.3, the thickness of the SiO$_2$ is 225% compared to the original silicon. Without mechanical boundary conditions the SiO$_2$ would like to expand by 31% in all three dimensions to accomodate the oxygen atoms.

Figure 2.3: Moving interface and volume expansion.

In practice not the whole silicon surface is oxidized. So the areas which should not be oxidized are masked, usually by a silicon nitride mask, because this mask prevents the oxidant diffusion to the underlying silicon layer. The oxidants can not diffuse through silicon nitride, because compared with silicon or oxide this material has a high density. However, the oxidation process does not stop at the edge of the mask, because the oxidants are able to diffuse through the already existing oxide into regions under the mask and react there with silicon (see Fig. 2.4). The finally oxide regions are therefore normally larger than the not masked ones, but there are also natural mechanisms which strongly restrict or nearly stop the oxidation process under the mask. In the end the form of the oxide is close to the shape of the mask.

Figure 2.4: Defining the oxidation area by masking.