Nb - Dry Etching

Nb - Dry Etching

Material Name: Nb
Recipe No.: 10321
Primary Chemical Element in Material: Nb
Sample Type: Bulk
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: The microwave cavity discharge system, capable of maintaining discharges at pressures above 1 Torr. In the microwave discharge, the high energy transfer efficiency from the microwave electric field to the gas results in high electron and high radical densities in the plasma. These plasma conditions are more favorable for plasma etching than for sputtering processes. Also, the higher gas temperature in the plasma contributes to a higher rate of chemical reactions and vaporization of Nb chlorides. The Nb samples are placed in the quartz tube of a reaction chamber. Low background gas pressure is achieved using a system of mechanical and turbo molecular pumps, both corrosive gas resistant. The gas in the reaction chamber has high constant flow rates, so that reactive species lost due to the chemical reaction can be replenished and products of the chemical reaction removed away from the sample. The gas flow control is achieved through flow meters connected to a controller. For processes that demand more than one reaction gas, a mixing chamber is placed in front of the reaction chamber to facilitate a better mixing of the gases. The experimental set-up is connected to a spectrometer with the CCD camera for emission spectroscopy measurements. Emission spectroscopy is used as a process monitoring technique as well as a tool to determine the reaction mechanism of the plasma etching process.
We determined the etching rates from the difference in sample mass before and after exposure to Ar/Cl2 discharge. From Table 1, one can see that the etching rate increases with a higher amount of Cl2 in the gas mixture. The dependence of the etching rate on the reactive gas concentration is an indicator of the chemical etching contribution to the Nb removal mechanism.
Procedure (Condition): No data
Note: The preparation of the cavity walls has been one of the major challenges in the superconducting radio-frequency (SRF) accelerator technology. Therefore, constant research and development effort is devoted to develop surface preparation processes that will improve roughness and lower the level of impurities, like hydrogen or oxygen, embedded in bulk Nb, having in the same time reasonable etching rates. Plasma based surface modification provides an excellent opportunity to achieve these goals. We present Ar/Cl2 discharge treatment of bulk Nb where we achieved etching rates comparable to the rates obtained with the electropolishing method without introducing impurities in Nb. The current experiments were performed on disk shaped Nb samples, exposed to plasma produced in a microwave discharge system. Surface composition and topology measurements were carried out before and after plasma treatment. Upon determining optimal experimental conditions on disk shaped samples, we will apply the same procedure on the single cell cavities, pursuing improvement of their RF performance.
Reference: M. Raškovic, et al., PLASMA TREATMENT OF BULK Nb SURFACE IN THE Ar/Cl2 DISCHARGE, Proceedings of SRF2007, Peking Univ., Beijing, China, TUP: Poster Session I, pp. 323-326.

Figure 1: Experimental conditions and etching rates obtained during plasma treatment of disk-shaped Nb samples.