JLAB, Newport News, Virginia, USA
Over the years, Nb/Cu technology, despite its shortcomings due to the commonly used magnetron sputtering, has positioned itself as an alternative route for the future of accelerator superconducting structures. Recently, significant progress made in the development of energetic vacuum deposition techniques is showing promise for the production of thin films tailored for SRF applications. Energetic condensation allows to improve film structure on low temperature substrates by adding energy to the film during condensation to compensate for the lack of thermally induced growth processes. Energetic condensation is characterized by a number of processes enabled by the energy of the incoming ions such as desorption of adsorbed species, enhanced mobility of surface atoms, and sub-implantation of impinging ions. All these with the nature and properties of the substrate have an important influence on the nucleation and subsequent growth of the Nb film. This paper will show how the structure and the electron mean free path (represented by residual resistance ratio values) of Nb films can be tailored on various substrates by varying the ion energy and thermal energy provided to the substrate.
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.
ODU, Norfolk, Virginia, USA
JLAB, Newport News, Virginia, USA
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.
JLAB, Newport News, Virginia, USA
NCSU AIF, Raleigh, North Carolina, USA
The College of William and Mary, Williamsburg, USA
NSU, Newport News, Virginia, USA
For the past three decades, bulk niobium has been the material of choice for SRF cavities applications. In the recent years, RF cavities performances have approached the theoretical limit for bulk niobium. For further improvement of RF cavity performance for future accelerator projects, an interesting alternative has been recently proposed by Alex Gurevich with the Superconductor-Insulator-Superconductor multilayer approach, using the benefit of the higher critical field Hc2 of higher-Tc superconductors without being limited with their lower Hc1. JLab is pursuing this approach with the development of multilayer structures based on NbTiN via magnetron sputtering and High Power Impulse Magnetron Sputtering (HiPIMS). Insulators such as, AlN, Al2O3 and MgO are being investigated as candidates for the insulator layers. This paper present the preliminary results on the characteristics of NbTiN and insulator layers and a first attempt of a NbTiN-based multilayer structure on bulk Nb and thick Nb films.
JLAB, Newport News, Virginia, USA
Surface preparation is an essential prerequisite for thin film depositions. Rough or chemically impure surfaces adversely affect the nature of the thin film. Understanding the properties of the substrate and how they influence the quality of the thin film is necessary to transfer thin film deposition technologies to SRF cavity applications. A substrate that is flat, has sufficient grain size, and is chemically pure is the ideal starting point for thin film depositions. A method for copper substrate preparation is reviewed for niobium thin film deposition that provides epitaxy on large and fine grain copper as well as single crystal copper. Preliminary data on niobium and aluminum substrate preparation will also be included.