Electrochemical Polishing of Chemical Vapor Deposited Niobium Thin Films

Combining chemical vapor deposition (CVD) with electrochemical polish (EP) operations is a promising route to producing performance-capable superconducting films for use in the fabrication of cost-effective components for superconducting radiofrequency (SRF) particle accelerators and superconducting quantum computers. The post-deposition EP process enables a critically necessary reduction in surface roughness of niobium thin films to promote optimal superconducting surface conditions. In this work, surface morphology, roughness, and crystal orientation of the CVD-grown and EP-polished niobium films were investigated. The grain growth and polishing mechanisms were analyzed. The CVD films were found to comprise steps, kinks, and pyramidal features, resulting in undesirable large peak-to-valley distances. The electrochemical polish was demonstrated to significantly diminish the height of pyramids and effectively minimize the overall surface roughness. In contrast to buffered chemical polishing (BCP), EP results showed a probable dependence on crystal orientation, suggesting this process was influenced by locally enhanced current density and thickness variations of oxide dielectrics. These understandings identify the EP principles tied to CVD-grown Nb films that allow further refinement of surface profiles for film-based SRF applications