Modeling of C 4 F 8 inductively coupled plasmas: effects of high RF power on the plasma electrical properties

Part of the electrical parameters of C4F8 inductively coupled plasmas is analyzed experimentally using a global kinetic model. To perform a relevant study of C4F8 plasmas, we performed Langmuir probe measurements. The latter is based on solving the mass balance equations of all the considered neutral and charged species coupled to the power balance equation and the charge neutrality equation. The model allows the calculation of the densities of neutral and charged species as well as the electron temperature versus the operating conditions. Our study is focused on the analysis of the evolution of C4F8 plasmas by varying the operating conditions, especially for a very large range of RF power. The simulation reveals an unusual phenomenon characterized by an abrupt transition from an electronegative plasma to an electropositive plasma. Such an abrupt transition is translated into high power by increasing the pressure. This transition phenomenon is explained by the fact that for a power lower than the power transition the electron collision processes are mainly done with the primary molecule, C4F8 and the heavy molecules such as C3F6 and C2F4. Under these conditions, excitation, dissociation, and attachment processes are more important than those of ionization. However, beyond the transition power values, the electron collisions appear to favor the ionization process of atomic neutrals, F and C, because of the high dissociation of secondary molecules of C4F8 plasmas. In this case, the light neutrals such as CF2 and CF are also highly dissociated. So, the ionization process to create much electrons becomes the dominant process because of the strong dissociation processes that occur during the transition phase. Thus, the RF power is essentially absorbed through ionization processes leading to the efficient creation of electrons. Under these conditions, the electron energy losses by excitation, attachment, and dissociation processes become lower than the losses due to ionization processes.