Reversal of the Asymmetry in a Cylindrical Coaxial Capacitively Coupled Ar/Cl2 Plasma

TL;DR

This study investigates how geometric modifications of the inner electrode in a cylindrical coaxial capacitively coupled Ar/Cl2 plasma can reverse plasma sheath asymmetry, leading to improved surface etching efficiency.

Contribution

It demonstrates the reversal of negative dc self-bias to positive through specific electrode shape modifications, reducing plasma asymmetry.

Findings

Reversal of dc self-bias observed with DLCS electrode.

Improved etch rate achieved due to reduced asymmetry.

Electrode shape significantly influences plasma sheath behavior.

Abstract

The reduction of the asymmetry in the plasma sheath voltages of a cylindrical coaxial capacitively coupled plasma is crucial for efficient surface modification of the inner surfaces of concave three-dimensional structures, including superconducting radio frequency cavities. One critical asymmetry effect is the negative dc self-bias, formed across the inner electrode plasma sheath due to its lower surface area compared to the outer electrode. The effect on the self-bias potential with the surface enhancement by geometric modification on the inner electrode structure is studied. The shapes of the inner electrodes are chosen as cylindrical tube, large and small pitch bellows, and disc-loaded corrugated structure (DLCS). The dc self-bias measurements for all these shapes were taken at different process parameters in Ar/Cl2 discharge. The reversal of the negative dc self-bias potential to become positive for a DLCS inner electrode was observed and the best etch rate is achieved due to the reduction in plasma asymmetry.