Numerical simulation of inductively coupled Ar/O<sub>2</sub> plasma

Abstract

In the inductively coupled plasma (ICP) discharge, surface processes, such as reflection, de-excitation, and recombination, can occur when active species arrive at material surfaces, which accordingly influences the plasma properties. In this work, a fluid model is used to study the Ar/O<sub>2</sub> plasma generated by ICP reactors made of different materials. In simulation, sticking coefficient is employed to estimate the surface reactions on different materials. As the reactor material changes from stainless steel to anodized aluminum to Cu, the sticking coefficient of surface reaction O→1/2O<sub>2</sub> decreases accordingly. It is found that the reactor material has a great effect on species density. In the stainless steel reactor, the density of O atoms at grounded state and excited state are much lower because more O<sub>2</sub> molecules are generated from the surface reaction, yielding a much higher density of <inline-formula><tex-math id="M5">\begin{document}$ {\text{O}}_2^ + $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M5.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M5.png"/></alternatives></inline-formula> molecular ions which are mainly created from the ionization process of O<sub>2</sub> molecules. Similarly, the high density of O<sub>2</sub> molecules also enhances the production of <inline-formula><tex-math id="M6">\begin{document}${{{\mathrm{O}}} _2}\left( {{{\mathrm{a}}^1}{\Delta _{\mathrm{g}}}} \right)$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M6.png"/></alternatives></inline-formula> molecules through the excitation process and O<sup>–</sup> ions through the dissociation attachment reaction. On the contrary, more electrons are consumed via the collisions between electrons and O<sub>2</sub> molecules or <inline-formula><tex-math id="M7">\begin{document}$ {\text{O}}_2^ + $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M7.png"/></alternatives></inline-formula> molecular ions. Therefore, the electron density obtained in the Cu reactor is highest. The density of Ar<sup>+</sup> ions and Ar<sub>m</sub> atoms also increase with sticking coefficient decreasing. The density of O<sup>+</sup> ions and <inline-formula><tex-math id="M8">\begin{document}$ {\text{O}}_2^ + $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M8.png"/></alternatives></inline-formula> molecular ions peak below the coil in the stainless steel reactor, whereas the radial uniformities are improved in the Cu reactor. In the three reactors, the electrons distribute evenly at the reactor center region. The O density and <inline-formula><tex-math id="M9">\begin{document}${{{\mathrm{O}}} _2}\left( {{{\mathrm{a}}^1}{\Delta _{\mathrm{g}}}} \right)$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M9.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240436_M9.png"/></alternatives></inline-formula> density significantly peak at the reactor center, while the maximum value of Ar<sup>+</sup> density and Ar<sub>m</sub> density are below the coil. As for O(<sup>1</sup>D), the maximum density below the coil region moves toward the reactor center as the reactor material changes from stainless steel to Cu. Finally, the effect of sticking coefficient of O→1/2O<sub>2</sub> is studied. The results show that the O atom density decreases with the sticking coefficient increasing, but the opposite trend is observed in O<sub>2</sub> molecular density. It is noticed that the sticking coefficient has little effect on species density when it is higher than 0.5.