Characterization of Argon and Ar/Cl 2 Plasmas Used for the Processing of Niobium Superconducting Radio-Frequency Cavities

Abstract

The plasma processing of superconducting radio-frequency (SRF) cavities has shown significant promise as a complementary or possible replacement for the current wet etch processes. Empirical relationships between the user-controlled external parameters and the effectiveness of Reactive Ion Etching (RIE) for the removal of surface layers of bulk niobium have been previously established. However, a lack of a physical description of the etching discharge, particularly as the external parameters are varied, limits the development of this technology. A full understanding of how these external parameters affect both the amount of material removed and the physical properties of the plasma would aid researchers in the development of a controllable and customizable cavity processing technique. While the RIE of integrated circuits on semiconductor wafers has been studied extensively, the unique properties of the discharge with the application of RIE to a complex 3-D metallic waveguide make comparisons difficult. Thus, electrical probe and spectroscopy techniques were applied to a coaxial cylindrical capacitively coupled plasma designed for the plasma processing of SRF cavities with the intention of finding pertinent relationships between the external parameters and the important plasma parameters. A comparative analysis of two popular spectroscopy techniques was conducted in this aim. The density of the metastable and resonant levels in Ar was measured in both Ar and Ar/Cl2 discharges to properly characterize the unique discharge system and aid in the development of a cavity etching routine. The first method, deemed the ``branching fraction method'', utilizes the sensitivity of photon reabsorption of radiative decay to measure the lower state (metastable and resonant) densities by taking ratios of spectral lines with a common upper level. This method has been gaining popularity as it does not require any a priori knowledge about the electron energy distribution. The second method is a tunable diode laser absorption spectroscopy technique that measures the thermal Doppler broadening of spectral lines, from which the neutral gas temperature and lower state density of the transition can be evaluated. The two methods were conducted in tandem while external parameters that were empirically determined to be important to the etching mechanism of SRF cavities were varied. Relationships between the excited state densities and the external parameters are presented for both spectroscopy methods. In particular, the relationship between the first four excited state densities and the added DC bias indicate an increase in the plasma density and excited state ionization. The results found from the spectroscopic studies were applied to a collisional radiative model (CRM) to determine quantities related to the electron energy distribution. This is because electrons facilitate the vast majority of the collisional processes in the plasma bulk, leading to ionization and excitation. Results from the CRM were compared…