http://www.surfacetreatments.it/thinfilms Energetic Condensation Growth of Nb films for SRF accelerators (Mahadevan Krishnan - 30') Speaker: Mahadevan Krishnan - Alameda Applied Sciences Corporation | Duration: 30 min. Abstract 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.

8. Coating Facilities available at AASC

9. Three different Coaters at AASC Coaxial Energetic Deposition (CED TM ) Cathodic Arc Deposition (CAD) Filtered Cathodic Arc Deposition (FCAD)

10. 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

11. CED TM : Grows a monolayer (≈4Å) of Nb in ≈2ms

12. 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

15. Recent results from Nb thin-films coated using the CED TM coater

19. 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 )

21. XRD and EBSD measurements show hetero-epitaxial growth of single crystal Nb on a-sapphire and on MgO

28. 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

30. 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

31. 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

32. T c vs. substrate heating conditions: MgO