Microscopic wave dynamics and discharge structure hierarchy of highly electronegative plasma at quasi-cold ion approximation

In this paper, the discharge structure of an Ar/SF6 inductively coupled plasma at low pressure is investigated by means of fluid simulation at the quasi-cold ion approximation with the room temperature magnitude. It is found that the structure is hierarchal, including (1) the macroscopic characteristics, the stratification and the parabola profile in the stratified core; (2) the microscopic characteristics, the double layer at the interface of stratification and the related acoustic wave that is damped; and (3) the mesoscopic characteristics, the density bump within the parabola profile. In the mesoscopic scale, the density bump in the parabola is given by the self-coagulation behavior under the conditions of ionic recombination loss chemical source (with negative magnitude) and free diffusion. By means of dimensional analysis and the analogy between the ionic recombination chemical loss and an effective physical drift force, the dynamics of self-coagulation paves the way for the anions to achieve the Boltzmann’s relation of them. In the macroscopic scale, the transport dominance of low pressure and the intrinsic high electronegativity condition of Ar/SF6 plasma give rise to the stratification that divides the Ar/SF6 discharge structure into an electronegative plasma region (i.e., internal core) and an electropositive plasma region (i.e., external halo), and in the core, the profiles of ionic densities are parabolas as the theory predicted. In the microscopic scale and at the interface of the core and halo, the simulated double layer is found to be formed by the anion and cation sheaths and the acoustic wave occurs due to the ionic beams generated by the double-layer potential drop. The propagation of ionic vibration is strongly suppressed by means of Landau damp because the temperatures of cations and anions are equal. In particular, Landau damping in the scope of the drift-diffusion approximated fluid model is constructed, based on the quasi-Maxwellian density distribution, and the wave dynamics transition from the acoustic toward Langmuir oscillation and the sheath characteristics on the way toward the chamber wall are revealed. In addition, the influences of the transport coefficient and ion-temperature error on the discharge hierarchy are presented.