A contaminated layer with a certain thickness commonly exists on the surface of silicon carbide (SiC), which consists of particles such as silicon oxide, carbon-carbon bonds and other metallic impurities, organic substances, metals and native oxide layers. These contaminants severely deteriorate interfacial properties and further impair the performance of SiC-based devices.

During the fabrication of SiC-based MOSFETs, the interface between silicon dioxide (SiO₂) and SiC shifts toward the substrate as the thickness of thermally grown SiO₂ film increases. Traditional wet cleaning can effectively remove organic matter, metals and particulate contaminants from SiC wafers. Nevertheless, residual carbon contaminants and oxides still remain on the surface after wet cleaning. Meanwhile, numerous dangling bonds on the SiC surface make it highly susceptible to re-oxidation.

Reaction Mechanism of Hydrogen Plasma Cleaning

Plasma chemical reactions take place in discharge gases. When gas under normal temperature and pressure is energized by high-temperature accelerated electrons and ions, its molecules dissociate into anions and cations. Electrons and positive ions generated by gas ionization tend to recombine into neutral molecules in a short time. Part of the energy carried by electrons and ions is dissipated in forms such as electromagnetic waves. Free radicals are often produced during molecular dissociation, and the resulting electrons combine with neutral atoms and molecules to form negative ions.

Therefore, plasma is a mixture of electrons, positive and negative ions, excited-state atoms, ground-state atoms and free radicals. All chemical reactions in plasma proceed under highly excited states, which differ fundamentally from conventional chemical reactions. The atomic and molecular properties will be altered accordingly. Even chemically inert gases can gain strong reactivity under plasma conditions. Electrons and ions accelerated in an electric field possess high kinetic energy, enabling efficient removal of oxygen, residual carbon and other impurities on material surfaces.

As an active gas, hydrogen exhibits strong chemical reactivity in its plasma state. The formation process of hydrogen plasma is described as follows:

Equation 1 illustrates that hydrogen molecules absorb external energy and ionize into hydrogen molecular cations, accompanied by the release of free electrons. Equation 2 presents the dissociation of hydrogen molecules into two hydrogen atomic radicals upon energy input. Equation 3 shows that hydrogen molecules are excited into excited-state hydrogen molecules under the impact of high-energy free electrons. Equations 4 and 5 describe the further transformation of excited-state hydrogen molecules. In Equation 4, excited hydrogen molecules return to the ground state and emit ultraviolet photons. In Equation 5, excited hydrogen molecules decompose into two hydrogen atomic radicals. Equation 6 depicts the dissociation of hydrogen molecules into hydrogen atomic radicals and hydrogen cations under the action of excited free electrons. Continuous occurrence of the above reactions sustains the hydrogen plasma. It should be noted that the actual reaction system is far more complex than these simplified equations.

H₂ + e⁻ → H₂⁺ + 2e⁻

H2→2H

H₂ + e⁻ → H₂* + e⁻

H₂* → H₂ + hν

H2 + e⁻ →2 H+e⁻

H2+e⁻→H+H⁺+2e⁻

In addition to hydrogen atoms, hydrogen ions, and electrons, there are also electrically neutral hydrogen atoms or atomic groups (also known as free radicals) in an excited state, as well as the light emitted by the hydrogen plasma during the hydrogen plasma cleaning process. Therefore, it is easy to react with substances on the surface of SiC to form new substances, such as water, hydrocarbons, etc. In addition, hydrogen atoms selectively react with pollutants (such as C, 0, etc.) on the surface of SiC and are eliminated in two volatile forms: CHx and H2O.