Please upgrade your browser. This site's design is only visible in a graphical browser that supports web standards, but its content is accessible to any browser. Concerns?

SRF Web

• Home
• SRF Overview
• Activities
• Research/Development
• Resources
• History
• SRF Worldwide

Search SRF:

Study of Plasma Etching as an innovative Surface Preparation Process for Niobium Superconducting RF Cavities

-Development of an In-Situ technology-

Motivation:

The proposed study is the investigation of plasma etching as an alternative technique for surface preparation of bulk Nb cavities.

The standard (commonly used) surface preparation techniques for Nb superconducting RF cavities are chemical (BCP) or electrochemical (EP) polishing. Those make use of extremely corrosive acids, which present both safety and environmental concerns. With wet chemical polishing, the formation of an Nb oxide layer is unavoidable since oxidation starts as soon as the chemical process stops. More over it has been demonstrated that these techniques, due to the baths composition, introduce hydrogen in the oxide layer and the bulk material. Acid residues are usually left by phosphoric acid on the surface. These chemical residues are eliminated by High Pressure Water Rinsing (HPWR) with de-ionized water for about 2 hours. Finally, especially in the case of EP, the cavity is baked under vacuum at moderate temperature (90ºC-140ºC) in order to outgas the water adsorbed on the surface and reduce the surface hydroxides.

It has long been suspected that “weak links” formed by intergranular oxidation could be responsible for “Q droop”. It is also suspected that niobium surfaces under certain circumstances will form lossy sub-oxides in addition to Nb 20 5. Testing before and after oxidation would provide extremely valuable insight into this issue.

Plasma etching is one form of dry chemistry providing a unique opportunity to explore oxide-free surfaces by directly testing a cavity surface after processing without exposure to air. This technique allows also “control” on the final oxidation phase that cannot be avoided with SRF cavities (oxidation of the fresh Nb surface after treatment by O 2/N 2 (“dry air”) gas injection in the system).

Advantages of dry etching:

• Takes place under vacuum.
• Allows “control” on the final oxidation phase
• Allows the possibility to avoid final oxidation
• Lower process cost.
• Reduces the number of steps for the final surface preparation.
• Gets rid of the hazard to humans and environment induced by the chemicals used for BCP and EP.
• No acid residues on the surface.
• No hydroxides formation on surface.

Approach:

1. Microwave discharge in a cylindrical cavity at various pressures

   a. Ar plasma with a biased sample

   b. Ar/Cl mixtures

   c. CCl 2 and CF 4 flow

   d. Other etchant gases

2. Capacitively-coupled RF discharge with Nb sample as one electrode, at various pressures

   a. Ar plasma with a biased sample

   b. Ar/Cl mixtures

   c. Plasma process characterization

   d. Surface characterization

3. Application of the process to a generic cavity

This project is collaboration between Jefferson lab and Old Dominion University in the framework of a PhD thesis.