Large crystal grain niobium thin films deposited by energetic condensation in vacuum arc.

This article presents evidence for unprecedented, large crystal grain size in ∼1 μm thick Nb films that were grown on sapphire and copper substrates using a vacuum arc process called coaxial energetic deposition CED™. Most other deposition techniques with low adatom energy produce amorphous or small crystal-grain films. Typically, high substrate temperatures and annealing steps are required to form the large, highly connected grains. The CED™ technique deposits from plasma consisting of a nonequilibrium, high energy (50–150 eV) ion population produced from the ionized source material. At the substrate these fast ions break up columnar structures, intermix with the first few atomic layers of the substrate to improve adhesion, and form dense films at lower substrate temperatures than are typical for low adatom energy techniques, such as physical vapor deposition (PVD). Nanoscale features of the thin films were examined using electron backscatter diffraction (EBSD). The films’ cryogenic state electrical properties were characterized by their residual resistivity ratio (RRR) and superconducting transition temperature (T_c). RRR of ∼77 and T_c∼9.2 K were measured on a Nb thin film deposited on a sapphire substrate. EBSD and x-ray diffraction measurements indicated that the sapphire substrate thin films have single crystal structure, with a Nb {110} crystal plane monolithically aligned and parallel to the sample surface. Nb thin films on an ∼400 °C Cu substrate had average crystal grain size of 50 μm, which is an order of magnitude larger than that which is typical of films grown by PVD. The crystal structure of CED™ thin films is comparable to that of polycrystalline bulk Nb material, which is the state of the art for superconducting radio frequency (SRF) particle accelerators such as at the Thomas Jefferson National Accelerator Facility (JLab). The films’ novel crystal features suggest that CED™ is a promising technique to coat Nb thin films for lower cost SRF particle accelerators. Further studies of the nanoscale grain boundary features would shed light on the role played by these features in determining the performance of SRF cavities using such thin films on Cu. [ABSTRACT FROM AUTHOR]