Self-bias Dependence on Process Parameters in Asymmetric Cylindrical Coaxial Capacitively Coupled Plasma

TL;DR

This study investigates how process parameters influence the self-bias potential in asymmetric coaxial capacitively coupled plasmas, which is crucial for surface processing of complex 3D objects like superconducting RF cavities.

Contribution

It provides experimental insights into the dependence of self-bias potential on electrode size and process parameters, aiding plasma sheath control in coaxial configurations.

Findings

Self-bias potential varies with electrode diameter, gas pressure, RF power, and chlorine percentage.

External DC current can neutralize the self-bias potential.

Process parameter adjustments can tailor plasma sheath voltages for specific surface treatments.

Abstract

An rf coaxial capacitively coupled Ar/Cl2 plasma is applied to processing the inner wall of superconducting radio frequency cavities. A dc self-bias potential is established across the inner electrode sheath due to the surface area difference between inner and outer electrodes of the coaxial plasma. The self-bias potential measurement is used as an indication of the plasma sheath voltage asymmetry. The understanding of the asymmetry in sheath voltage distribution in coaxial plasma is important for the modification of the inner surfaces of three dimensional objects. The plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external dc power supply. The dc self-bias potential is measured for different diameter electrodes and its variation on process parameters such as gas pressure, rf power and percentage of chlorine in the Ar/Cl2 gas mixture is studied. The dc current needed to overcome the self-bias potential to make it zero is measured for the same process parameters.