OFHC copper substrates for niobium sputtering: comparison of chemical etching recipes

Abstract

Niobium sputtered on copper has been a popular alternative approach for superconducting radio frequency (SRF) community in the last few decades. Comparing to bulk materials of a few millimeters, high-purity niobium of merely a few microns is sufficient to realize superconductivity on the coated surface. Being niobium thin film, it has been widely acknowledged that surface quality of the substrate plays a vital role in obtaining a superior niobium coating with excellent SRF performance. Therefore, proper chemical treatment of the substrate before coating is crucial and the ultimate goal is to create a smooth and defect-free surface. Prior to the design of a cavity etching system, the mechanism of SUBU as well as two industry-used solutions is studied in detail on samples. Copper samples were first pre-treated by mechanical grinding to remove fabrication damages, obvious defects and visible impurities. Two chemical solutions widely used in industries were subsequently chosen to etch the samples. Finally, the established SUBU solution was used independently on these pre-treated samples for comparison. Surface morphology and etching rate were measured accordingly. Mirror-like copper surface was created by using the SUBU solution thus qualified for subsequent niobium sputtering, while the other two solutions used in industries were less effective with nonideal surface morphology. The chemical reactions, the experimental requisites and the involved processes are extensively elucidated for all three solutions. Limitations for SUBU were examined, and the optimum ratio of the chemical bath volume to sample surface area was also determined. These investigations will serve as an important guidance for the development of a chemical etching system for elliptical copper cavities.