AASC, San Leandro, California, USA
NCSU AIF, Raleigh, North Carolina, USA
JLAB, Newport News, Virginia, USA
NSU, Newport News, Virginia, USA
ASC, Tallahassee, Florida, USA
This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma, whose 40-120eV ions enable sufficient surface mobility to ensure that the lowest energy state (crystalline structure with minimal defects) is accessible to the film. Hetero-epitaxial films of Nb were grown on a-plane sapphire and MgO crystals with good superconducting properties and crystal size (10mm × 20mm) limited only by substrate size. The substrates were heated to 700 deg C and coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. Residual resistivity ratio (RRR) values (up to a record RRR-554 on MgO and RRR-328 on a-sapphire) vary with substrate annealing and deposition temperatures. XRD spectra and pole figures reveal that RRR increases as the crystal structure of the Nb film becomes more ordered, consistent with fewer defects and hence longer electron mean free path. A transition from Nb(110) to Nb(100) orientation on the MgO(100) lattice occurs at higher temperatures. SIMS depth profiles, EBSD and SEM images complement the XRD data. Crystalline structure in Nb on amorphous borosilicate substrates has implications for future, lower-cost SRF cavities.
NSU, Newport News, Virginia, USA
AASC, San Leandro, California, USA
JLAB, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
ODU, Norfolk, Virginia, USA
NSU, Newport News, Virginia, USA
An energetic condensation thin film coating technique with an electron cyclotron resonance (ECR) induced plasma ion source is used to deposit Nb thin films on crystalline insulating substrates, such as a-plane and c-plane sapphire (Al2O3) and on magnesium oxide, MgO (100), (110), and (111). Heteroepitaxial Nb films were produced by ECR deposition with regulated substrate temperature. The residual resistance ratio (RRR) of about 1 micron thick films reach unprecedented values (350 - 450) on a-plane (11-20) sapphire substrates. The epitaxial relationship of Nb/crystalline substrate is found to be strongly influenced by the substrate bias voltage (adding to the initial Nb+ kinetic energy), the substrate crystalline orientation, and heating conditions. At low substrate temperature, the Nb films demonstrated non-epitaxial crystalline textures, revealed by XRD Pole Figure technique and Electron Backscattering Diffraction (EBSD). The texture might be caused by “Volmer-Weber” growth mode, i.e. island growth, at low surface adatom mobility. This study shows that the film’s crystal structural character has great impact on its RRR/Tc value.
JLAB, Newport News, Virginia, USA
ODU, Norfolk, Virginia, USA
NSU, Newport News, Virginia, USA
Particle accelerator technologies rely on SRF cavities to create the accelerating gradient for beam lines. Solid niobium cavities are widely employed throughout the community despite high material, fabrication, and operation cost. New thin film technologies are being explored for the suitability of niobium coatings for accelerating cavities. Thin layers of high-quality niobium would be deposited on a base material that has lower material and fabrication cost. Copper is a strong candidate for the cavity base due to availability, cost, machinability, and potentially improved performance characteristics of the niobium SRF surface. Initial results of TEM, EBSD and XRD analyses of niobium thin films grown on copper substrates under controlled conditions are presented to demonstrate the feasibility of the technology and establish lower limits of performance characteristics. Correlation of RRR data with the structure of niobium thin films will demonstrate the importance of thin film structural quality.
AASC, San Leandro, California, USA
NCSU AIF, Raleigh, North Carolina, USA
NSU, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma on crystalline (a- and c-sapphire, MgO) and amorphous (borosilicate) substrates. The crystal substrates were heated to 700 deg C and subsequently coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. The borosilicate substrate was preheated to 700 deg C but coated at 500 deg C. XRD spectra (Bragg-Brentano) and pole figures show a change in crystal structure on c-sapphire from textured (with twin-symmetry) to hetero-epitaxial as the temperature is increased. RRR=43 was measured on c-sapphire which is lower than RRR=200 on a-sapphire and 541 on MgO. On borosilicate, the (110) and (220) planes of Nb show sharper spectra at higher temperatures with an increase to RRR=31 at 500 deg C. The growth of crystalline Nb on an amorphous substrate is driven by energetic (40-120eV) ions from the cathodic arc plasma. The significance of crystal structure on amorphous substrates has implications for future, lower-cost SRF cavities. SIMS data show the role of impurities on crystal growth and RRR.
AASC, San Leandro, California, USA
NCSU AIF, Raleigh, North Carolina, USA
NSU, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
Single (sintered composite Mo3:Re1) and dual targets of Mo/Re were used to grow superconducting films of Mo:Re, using cathodic arc plasmas. Sharp superconducting transitions (at up to 13K) were observed in ~1 μm thick films deposited on a-sapphire and MgO crystals. The measured RRR (defined as the ratio of resistivity at 300K to that at 14K) in the best films was 6, which is higher than measured by others at higher annealing temperatures. XRD (Bragg-Brentano spectra) revealed a single sharp peak of Mo-Re (611) plane, from the composite Mo3:Re1 film. SIMS measurements revealed the role of impurity concentrations on superconducting properties. For the dual-target films, stoichiometry was controlled by varying the current to each cathode. The XRD spectra in this case showed the (330) plane of Mo-Re; hero-epitaxial growth of Mo-Re depends upon the stoichiometry of the film. This dual-target approach allows other compound films (e.g. Nb3Sn, MgB2 etc.) to be grown in a single-step. For SRF cavity applications, the RRR should be increased to >100, which is our next goal.