Krishnan - Energetic Condensation Growth of Nb films for SRF Accelerators
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Krishnan - Energetic Condensation Growth of Nb films for SRF Accelerators



Energetic Condensation Growth of Nb films for SRF accelerators (Mahadevan Krishnan - 30')
Speaker: Mahadevan Krishnan - Alameda Applied Sciences Corporation | Duration: 30 min.
AASC, Jefferson Lab and NSU conduct research into new SRF thin-film coatings by first characterizing the materials properties such as morphology, grain size, crystalline structure, defects, and impurities, then measuring properties such as Tc and RRR and following this with ‘in-cavity’ RF measurements of the Surface Impedance of the films at cryogenic temperatures. These progressive steps are essential to the eventual design of SRF accelerator structures and to measure Q-slope and other performance parameters at high fields.
This paper describes recent results from pure Nb thin-films grown on a-plane and c-plane sapphire, MgO as well as on amorphous substrates. Substrate preparation is shown to be critical to good electrical properties of the film. The sapphire and MgO substrates were heated up to 700 deg C and subsequently coated at 300, 500 and 700 deg C. Film thickness was varied from ~0.25µm up to >3µm. RRR and Tc were measured. The XRD data yielded pole figures, intensity vs. 2-θ and intensity vs. φ plots. These data were complemented by EBSD and SEM images. RRR values ranging from ~10 up to ~333 have been measured and correlated with the XRD data. Good crystallinity is associated with high RRR. Single crystalline (110) epitaxial layers of Nb films are grown well on a-plane sapphire substrates at different temperatures. Nb films have also been grown on Cu substrates, as well as on MgO and borosilicate substrates. The significance of crystalline structure observed on amorphous substrates is discussed in light of its implications for future, lower-cost SRF cavities.



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Krishnan - Energetic Condensation Growth of Nb films for SRF Accelerators Krishnan - Energetic Condensation Growth of Nb films for SRF Accelerators Presentation Transcript

  • Energetic Condensation Growth of Nb films for SRF Accelerators *
    • M. Krishnan, E. Valderrama, C. James, B. Bures and K. Wilson Elliott
    • Alameda Applied Sciences Corporation (AASC), San Leandro, California 94577
    • X. Zhao, L. Phillips, B. Xiao and C. Reece
    • Thomas Jefferson National Accelerator Facility (Jefferson Lab), Newport News, Virginia 23606
    • K. Seo
    • Norfolk State University (NSU), Norfolk, Virginia 23504
    • presented at
    • Thin Films and New Ideas for Pushing the Limits of RF Superconductivity
    • October 4 - 6, 2010
    • This research is supported at AASC by DOE SBIR Ph II Grant DE-FG02-08ER85162.
    • T he JLab effort was provided by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177, including supplemental funding provided by the American Recovery and Reinvestment Act.
    • K. Seo/NSU is a subcontractor to AASC.
  • Outline
    • Alameda Applied Sciences Corporation: an Introduction
    • Motivation
    • AASC’s Energetic Condensation Coaters
    • RRR Measurements
    • Compare AASC’s coater with others
    • Film Crystallinity (XRD and EBSD)
    • Future Plans
  • Alameda Applied Sciences Corporation
    • Founded in 1994, privately held CA Corporation
    • 8 employees, ~$2 million 2009 revenue
    • Develop/license IP via contract R&D
    • Four Pre-commercial areas:
      • Cathodic arc coatings CED TM
      • Fast pulsed x-ray sources for calibration
      • Fast pulsed neutron sources for WMD and HE detection
      • Space micro-propulsion thrusters
    CED ™ coating inside of furnace tubes Anti-coking coating on furnace tube Benefit: extended interval between de-cokings 10 cm Uncoated Coated Cathodic Arc Coatings (CED TM ) Superconducting Thin Films RRR ~300, T c =9.27K Diamond Radiation Detectors UV and soft x-ray ≤ 15 keV 20W / 1kg/ 100mN / 2000s Micro-propulsion Pulsed Neutron Source 2.5 and 14 MeV neutrons DPF-2
  • Motivation
    • NP facilities :
      • The CEBAF facility at TJNAF (undergoing a 12GeV upgrade)
      • Facility for Rare Isotope Beams (FRIB).
        • NSAC report states that as a result of technical advances, a world-class rare isotope facility can be built at ≈ half the cost of the originally planned Rare Isotope Accelerator (RIA), employing a superconducting linac
    • Commercial applications:
      • More than 1000 particle accelerators worldwide; most use normal cavities
      • Superconducting RF (SRF) cavities offer a ~10x improvement in energy efficiency over normal cavities, even accounting for cryogenic costs at 2K
      • Operating at higher temperatures (~10K) would further improve accelerator energy efficiency as the cryogenic cooling becomes less demanding and moves away from liquid He and towards off the shelf cryo-coolers such as those used in cryo-pumps
      • Replacing bulk Nb with Nb coated Cu cavities would also reduce costs
      • The ultimate payoff would be from cast Al SRF cavities coated with higher temperature superconductors (Nb 3 Sn, MoRe, MgB 2 , oxypnictides)
    Our thin film superconductor development is aimed at these broad goals
    • JLab has begun a three-year DOE funded project that focuses on thin-film development for future SRF applications
    • AASC has received a companion, three-year grant from DOE
    • AASC also has DOE funded SBIR grants to study Nb, Nb 3 Sn and MoRe
    • AASC and JLab have established a collaborative research agreement, under which AASC develops thin-films using its energetic condensation tools, while JLab uses its array of sophisticated metrology tools to study the film morphology and electrical properties in depth
    • Prof. Kang Seo is part of this collaboration via subcontracts from AASC
    • Together, we aim to advance the technology towards more defect-free SRF coatings
    The AASC-JLab/NSU collaboration
    • Because Enzo demands that we do (he did on Monday and again at the banquet)
    Why do we collaborate? A great American Patriot (Thomas Paine) said during the American Revolution: ( this is for Enzo to use in the future ) Men, we must hang together, or assuredly, we will all hang separately Funding for thin-film R&D hangs by a thin thread; so let’s hang together!
  • Approach of the AASC-JLab/NSU collaboration
    • Use CED TM and FCAD techniques to coat sapphire and Cu coupons
    • Use surface analysis techniques at JLab/NSU to characterize morphology
    • Measure RRR and T c from Nb coated coupons (on sapphire, MgO, amorphous materials …), to evaluate substrate crystal structure effects
    • Use SIC facility at JLab to measure impedance of films in cavity
    • Improve our understanding of the relationships between surface characteristics and superconducting properties
  • Coating Facilities available at AASC
  • Three different Coaters at AASC Coaxial Energetic Deposition (CED TM ) Cathodic Arc Deposition (CAD) Filtered Cathodic Arc Deposition (FCAD)
  • CED TM : Helical arc rotates in a weak B-field CED TM may be used to coat full cavities in the future B Field Trigger Mo Anode Mesh Nb Cathode
  • CED TM : Grows a monolayer (≈4Å) of Nb in ≈2ms
  • Energetic Deposition is different from sputtering and PVD Comparison of Stress build-up in low energy deposition, energetic condensation, and energetic condensation plus high voltage bias Relief of compressive stress by pulsed ion impact [*] [*] M. M. Bilek, R N. Tarrant, D. R. McKenzie, S H. N. Lim, and D G. McCulloch “Control of Stress and Microstructure in Cathodic Arc Deposited Films” IEEE TRANSACTIONS ON PLASMA SCIENCE , VOL. 31, NO. 5, OCTOBER 2003 (A. Anders) (A. Bendavid, CSIRO) (M.M. Bilek) Nb
  • CAD: Allows variation of growth rate from 0.1-3 monolayers/pulse
    • The new CAD apparatus provides flexibility:
      • Pulse Forming Network is easily adjusted to vary growth rate from 0.1-3 monolayers/pulse
      • PFN can drive dual targets to grow compound films
      • Target may be biased, to vary incident ion spectrum, both DC and pulsed bias possible
    Cathodic Arc Deposition (CAD)
  • CAD Coater for compound films
    • We aim to demonstrate single-step growth of Nb 3 Sn superconducting films
  • Recent results from Nb thin-films coated using the CED TM coater
    • Residual Resistance Ratio (RRR), defined as R 300K /R 10K , is a figure of merit for superconductors
    • RRR is a ‘normal’ state measurement, but is related to the electron mean free path in the superconducting state
      • bulk Nb cavities in superconducting RF accelerators show RRR~300 for high-Q, high field performance
    • Over two decades of development, RRR in thin film Nb superconductors has slowly increased from ~10 to ~100
    • Our recent work has pushed RRR in Nb thin films to >300, matching bulk Nb cavities
    Our CED TM coater has shown RRR>300 in 1.5µm thick Nb films! This could be a major breakthrough for thin film SRF See next talk by Anne-Marie on RRR of 223 in biased ECR films
  • RRR of Nb thin films on a-sapphire
    • Nb thin films grown on a-sapphire crystals have demonstrated record levels of Residual Resistance Ratio (RRR)
  • MgO gives the highest RRR ever measured in Nb thin films
    • Historically, MgO has not been used for Nb film growth because its fcc structure has a 10.9% lattice mismatch for the bcc Nb [110] direction and 21.6% for the Nb [100] direction. We included MgO because its fcc structure was expected to better match to Cu
    An APL is in preparation on these results Next steps are RRR vs. film thickness and RRR for biased coatings
  • RRR as measured is an average over the film thickness (recall Larry’s talk on Monday?) RRR of uppermost layer (London depth) is higher than <RRR> Substrate bias improves RRR further ( see Anne-Marie’s talk to follow )
  • Why is our RRR so high relative to other sources?
    • RRR increases with substrate temperature, thickness and ion energy; dep. rate effect?
  • XRD and EBSD measurements show hetero-epitaxial growth of single crystal Nb on a-sapphire and on MgO
  • Nb on a-sapphire thin films: crystal structure Hetero-epitaxial, single crystal growth is correlated with higher RRR RRR=10, 150/150 RRR=31, 300/300 RRR=155, 700/500 Al 2 O 3 110 Nb Al 2 O 3 110 Nb Al 2 O 3 110 Nb Polycrystal Polycrystal Monocrystal
    • XRD Pole figures and Bragg-Brentano spectra show improved crystal structure of Nb (110) on a-sapphire as temperature is increased
  • Nb on MgO thin films: crystal structure
    • XRD Pole figures and Bragg-Brentano spectra show change in crystal structure of Nb from (110) to (100) on MgO as temperature is increased
    Nb crystal planes shift from (110) to (100) as temperature (& RRR) increase: An APL is in preparation on this fascinating trend (Kang Seo et al) RRR=7, 150/150 RRR=181, 500/500 RRR=316, 700/700 200 Nb 110 Nb 200 110 110 200 MgO 200 MgO 110 Nb 200 MgO Polycrystal Monocrystal Monocrystal
  • Nb on MgO: temp. driven transition in crystal orientation
    • EBSD data shows intermixed Nb (110) and Nb (100) for lower RRR samples
    • High RRR samples show pure Nb (100) for the entire surface
    EBSD shows 110 to 100 transition (Pole figure captures the 200 plane): An APL is in preparation on this (Kang Seo et al) Nb (100) RRR = 333, 600/500 Nb (110) Nb (100) RRR = 196, 500/500 100µm 1mm 1mm
  • Nb on MgO: macro-particles show same orientation as flat surface
    • High RRR has been achieved despite macroparticles
    • EBSD demonstrates that macroparticles have the same orientation as the Nb (100) thin film elsewhere
    RRR = 316, 700/700 Nb (100) Macroparticle 35µm
  • Nb thin films grown on c-sapphire and on Borosilicate
    • Nb thin films grown on c-sapphire crystals and on amorphous borosilicate show a similar trend of higher RRR with higher substrate temperature
  • Nb thin films on c-sapphire: crystal structure
    • XRD Pole figures and Bragg-Brentano spectra show change in crystal structure of Nb from textured (with twin symmetry ) to hetero-epitaxial, as temperature is increased
    Textured structure (twins) disappears at higher temperature (RRR) RRR=16, 700/150 RRR=43, 700/700 200 110
  • XRD measurements show polycrystalline Nb with more coherent texture grown on amorphous borosilicate This opens the possibility of Nb superconductors on cast Al cavities of the future Nb crystals grown on amorphous borosilicate
  • Nb thin films on borosilicate: crystal structure
    • XRD Pole figures and Bragg-Brentano spectra show improvement in crystal structure of Nb as temperature is increased
    Nb crystal growth (low texture) despite amorphous substrate; however, EBSD shows grain size is too small, so need to improve this (biasing?) RRR=10, 150/150 RRR=31, 700/500 Intensity increases (higher crystallinity) (110) Nb (220) Nb (211) Nb (110) Nb (220) Nb (110) Phi (rotation) Psi (tilt) (110) plane (211) plane 150/150C
  • SIC measurements at JLab Rs vs. T for thin film Nb on Cu sample TF-AASC-CED-Nb-Cu-103; EP, annealed at 750 C, then EP again
  • The AASC/JLab/NSU team hopes to continue our methodical investigation of Nb, Nb 3 Sn, MoRe and other thin film SRF candidates, culminating in high field cavity tests after better understanding
  • T c vs. substrate heating conditions: MgO