2.2 Fundamentals of the Oxidation Mechanism

Thermal oxidation of semiconductors (e.g., Si or SiC) is usually performed at temperatures between 800°C and 1400°C to artificially enhance the growth of the SiO2 layers [44, 1]. The oxide-semiconductor interface thickness is in the majority of applications in the order of a few layers of atoms [36]. Semiconductor atoms are consumed in the oxidation process, thus the interface moves from the surface into the substrate during the oxidation process [57]. Because of the different molecule densities of semiconductor atoms compared to SiO2, the formed oxide typically expands in volume. If no mechanical boundary conditions are present, SiO2 expands in all dimensions to accumulate oxygen atoms. The oxide practically grows into the wafer and on top of the wafer. For every thickness unit consumed of, e.g., Si, 2.2 thickness units of oxide will grow. Typically, after the oxidation approximately half of the oxide thickness will reside below the initial surface and half above it.

Depending on which oxidant species is used (O2 or H2O), the thermal oxidation of SiO2 may either be in the form of dry oxidation (wherein the oxidant is O2) or wet oxidation (wherein the oxidant is H2O). The reaction for dry (O2 environment) oxidation of Si is governed by the chemical reaction

\begin{equation} \mathrm {Si (solid)} + \mathrm {O}_2 \mathrm { (vapor)} \rightarrow \mathrm {SiO}_2 \mathrm { (solid)}. \end{equation}

During dry oxidation, the Si wafer reacts with the ambient oxygen, forming a layer of SiO2 on its surface. The dry oxidation rate is ≈ 100 nm/h, which results in high-quality oxide films with thicknesses of up to 100 nm. The reaction for wet (H2O environment) oxidation of Si is governed by the chemical reaction

\begin{equation} \mathrm {Si (solid)} + 2\mathrm {H}_2\mathrm {O (vapor}) \rightarrow \mathrm {SiO}_2 \mathrm { (solid)} + 2\mathrm {H}_2\mathrm { (vapor}). \end{equation}

In wet oxidation, hydrogen and oxygen gases are introduced into a torch chamber where they react to form water molecules which then enter the reactor where they diffuse toward the wafers. Oxides in a wet environment grow relatively fast compared to dry oxidation, which is the only advantage of the H2O oxidation. The reason for the fast growth is the higher oxidant solubility limit in SiO2 for wet oxidation compared to dry oxidation. At 1000°C the typical solubility limit value for dry oxidation is 5.2 · 1016 cm-3 and for wet oxidation 3.0 · 1019 cm-3. However, SiO2 grown in a wet environment exhibits lower dielectric strength and more porosity to impurity penetration than SiO2 grown in a dry environment. Therefore, wet oxidation is typically applied for thick (i.e., >100 nm [58]) SiO2, particularly for insulation and passivation layers, where the electrical and chemical properties of the SiO2 layers are not critical [49, 1]. The oxide grown in a dry environment has superior material characteristics and electrical properties, compared with oxides grown in other environments. For these reasons, the focus for the remainder of this work is on dry oxidation.

SiC is a compound semiconductor which can be thermally oxidized in a dry environment to form SiO2, similar to conventional Si [1]. The oxidation of Si is, however, considerably less complicated. The reaction for dry oxidation of SiC is governed by the chemical reaction

\begin{equation} \mathrm {SiC (solid)} + \frac {3}{2} \mathrm {O}_2 \mathrm { (vapor)} \leftrightarrow \mathrm {SiO}_2 \mathrm { (solid)} + \mathrm {CO} \mathrm { (vapor)}. \end{equation}

Thermal oxidation of SiC includes more rate-controlling steps compared to the oxidation of Si [59, 60, 61]. Those steps are: 1) Transport of molecular oxygen gas to the oxide surface, 2) in-diffusion of oxygen through the oxide film, 3) reaction with SiC at the SiO2/SiC interface, 4) out-diffusion of product gases through the oxide film, and 5) removal of product gases away from the oxide surface, shown in Figure 2.3. The last two steps are not involved in the oxidation of Si. The first and the last step are relatively fast and are not considered rate-controlling steps. In addition to the complex chemical reaction of SiC oxidation, SiC-based fabrication processes face many challenges due to SiC-based phenomena which are related to the complexity of the chemical structure and the crystal orientation. Therefore, it is extremely vital to understand and to be able to model and simulate, thus predict, SiC-related oxidation phenomena. For this reason numerous experimental and theoretical studies have been conducted in the recent decades, which laid the ground work for oxidation models, but still fell short in some aspects, e.g., crystal orientation dependence, as is discussed in the following.

Figure 2.3: Schematic representation of the thermal oxidation process steps of SiC. 1) Solution of oxygen in the SiO2, 2) diffusion of oxygen through the SiO2, 3) reaction between SiC and O2 at the SiC/SiO2 interface, 4) diffusion of product gases through the SiO2, and 5) removal of product gases away from the SiO2.