  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.
