2.2 Thermal Oxidation of Silicon

Thick thermally grown oxide is mainly used for isolation in semiconductor devices. The two types of processes which are used in order to isolate neighboring MOS transistors are LOCOS and STI. The general steps required for LOCOS and STI are shown in Figure 2.5 and Figure 2.6, respectively. One aspect of LOCOS which can be noted from Figure 2.5 is that, as the oxide grows, the nitride mask bends to generate a ``bird's beak'' effect as the oxide is pinched under the nitride mask at the edges. For a deeper understanding of these methods, refer to [177]. It is important to note that, although CVD of oxide is possible, as explained above, the quality of deposited oxide is below that of thermally grown oxides. Quality, in this case, refers to the electrical properties of the silicon-oxide interface and the oxide density. Thermal oxidation is also a highly refined process which can be finely controled for oxides below 10nm, unlike deposited oxides [34].

Figure 2.5: LOCOS processing steps

Figure 2.6: STI processing steps

Thermal oxidation of silicon is a chemical process, whereby oxygen from the ambient interacts with a silicon surface at high temperatures in order to grow silicon dioxide (SiO$_2$). Some oxidation also takes place at room temperature, resulting in a thin oxide growth of approximately 1nm in height. The oxidation cannot proceed further at room temperature, because the oxygen molecules (O$_2$), which are the main contributors of oxygen in the oxidation process, do not have enough energy to diffuse through the $\sim$1nm thick oxide.

When attempting to understand thermal oxidation of silicon, it is important to note that it is a complex process, which includes the diffusion of oxidants through existing oxide, a chemical reaction at the silicon-oxide interface, and a volume expansion simultaneously. These three events must be viewed as one system where:

1. Oxidants from the ambient reach the oxide-ambient interface. When they gather enough energy, they penetrate the surface and begin to diffuse through the existing oxide until reaching the oxide-silicon interface.

2. The oxidant species which reach the oxide-silicon interface can now interact with the surface silicon atoms in order to form more SiO$_2$ using chemical reaction (2.2).

3. Since the oxidant, together with a silicon atom, forms a molecule which has a larger volume than the oxidant plus the silicon atom in a crystalline arrangement, a volume expansion occurs. This volume expansion causes the entire oxide to expand, leading to the increased height of the oxide at the ambient-oxide interface.