Mechanism of Niobium Etching in Ar/Cl$_{2}$ Microwave Discharge

Plasma based Nb surface treatment provides an excellent opportunity to eliminate surface imperfections [1] and increase cavity quality factor in important applications such as particle accelerators and cavity QED, as well as Josephson junctions [2]. We used a microwave glow discharge in Ar/Cl$_{2}$ mixture to remove impurities and mechanical damages from the surface of bulk niobium samples. The high etching rates, in the order of 1 $\mu$m/min were obtained using a less than 3{\%}Vol concentration of Cl$_{2}$ gas. The etching rate dependence on discharge parameters such input power density, pressure and Cl$_{2}$ concentration was determined. Simultaneously, plasma emission actinometry was used to estimate the absolute densities of Cl, Cl$^{+}$ and Cl$_{2}$ in the variable plasma conditions. This results, combined with results of discharge diagnostics, were used to determine the plasma etching mechanism. [1] M. Raskovic, et al., Nuclear Instruments and Methods in Physics Research A 569 663--670 (2006). [2] S. Gleyzes, et al., Nature 446, 297 (2007).