Investigation of Chemical/Mechanical Polishing of Niobium

Abstract

A chemical/mechanical method for polishing flat niobium sheets to a mirror finish was developed. Various polishing slurries with different open circuit potentials and pH values were considered. All slurries fell within the niobate region of the Pourbaix diagrams, indicating that slurries are in a thermodynamically stable region. Oxidation characteristics of the niobium in the various slurries were determined by XPS and confirmed previously published work that niobium forms various layers of stable niobium oxides roughly 4.5–4.7 nm in thickness on the surface. A multi-step polishing method that relies on mechanical abrasion of the surface proved to be effective, and particles of different hardness and size were explored. Niobium wafers with initial peak-to-valley (PV) surface roughness of 3 to 7 μm were polished. The multi-step process utilized a slurry containing 1 μm diameter alumina particles to polish this initial roughness down to a submicrometer level. The final polish was provided by a slurry containing smaller particles. The oxide slurry with 70 to 100 nm silica particles gave the best mirror finished surface, with PV = 235 nm, Ra = 32 nm, and RMS = 39 nm. While polishing caused some disorder in the niobium metal, using the oxide slurry gave results closer to those obtained by buffered chemical polish (BCP), which exhibits the highest degree of atomic order based on XPS studies. A polishing process starting with mechanical abrasion, followed by a two-step mechanical polish, is successful for obtaining smooth niobium surfaces on flat wafers.

KEY WORDS:

• Polishing
• Surface Roughness
• XPS
• Niobium
• Chemical Mechanical Polishing
• Buffered Chemical Polishing
• Superconducting Radio Frequency Cavities

ACKNOWLEDGEMENTS

This work was sponsored in part by H.C. Starck, LLC, (Newton, MA) and in part by the National Science Foundation (Award # NSF-0425826). Part of this work was carried out in the facilities of the George J. Kostas Nanoscale Technology and Manufacturing Research Center at Northestern University (NEU). The authors would like to acknowledge the discussions with Mr. Peter Jepson (H.C. Starck), Professor E. Podlaha-Murphy (Chemical Engineering Department, NEU), and Dr. Nam-Goo Cha (NSF-Center for High-rate Nanomanufacturing, NEU) during the course of this work, and thank Professor A. Sacco (Chemical Engineering Department, NEU) for the use of the scale and scanning electron microscope.

Review led by Fred Higgs

Notes

¹Oliver (14) ,

²Pace Technologies (15)

³Not available.

*The original pH values are not changed.