Surface and Thin Film Characterization of
        Superconducting Multilayer films for Application in RF
        Accelerato...
The key idea of using a thin film superconductor is the fact that Bc1
               increases when the thickness is d< λL ...
An example: Coating 105 nm MgB2 layer could sustain 355 mT,
           corresponding to ~100 MV/m with Bpeak /Eacc ~ 3.6 m...
Materials and Deposition Methods:
        Polymer assisted deposition (PAD) for NbN - LANL
        Sequential reactive coe...
Film synthesis methods

Polymer assisted deposition of NbN                                                                ...
Nb substrate conditioning
Required to remove excessive
surface oxide to avoid reactions
with deposited thin films and      ...
Angle Resolved XPS used to determine Nb2O5 oxide layer thickness resulting from
BCP treatment on Nb metal crystal plate
 X...
NbN Surface and Thin Film Analysis

 •       NbN intrinsic Tc = 16K
 •       thin superconducting films produced by PAD met...
NbN Surface and Thin Film Analysis - surface morphology by AFM


                        SRF-NbN6-1
                      ...
NbN films - surfaces vs.                                               Comment:        sample NbN3_2, NbN on sapphire produ...
NbN film - profile
Auger survey spectrum taken at 12 nm point
in profile shows O, C, and Al in addition to
the Nb and N. C is...
NbN film - profile
XPS survey spectrum taken at 8 nm point in
profile shows a very clean film.

Oxygen <2% atomic

Tc = 9.5K

...
NbN film - excessive oxygen in film
               5                               NbN4_6.spe
            x 10
      4.5



...
MgB2 Surface and Thin Film Analysis

 • MgB2 intrinsic Tc = 39K
 • thin superconducting films produced by codeposition meth...
MgB2 Surface and Thin Film Analysis
principal component analysis (PCA) in Auger profiling spectroscopy


Separating B and N...
MgB2 Surface Analysis - surface alteration due to air exposure for a thick film (100 nm)




Note:
Ultrathin films show full...
MgB2 film structure




                                                                             surface oxide




    ...
MgB2 film structure




                                                                  surface oxide




               ...
Auger spectroscopy sputter depth profile: peak intensity profile
                with Mg chemical states resolved using pr...
MgB2 + dielectric film multilayers
Intended:
200nm MgB2 on 300nm Al2O3 on Nb substrate
                                    ...
surface Mg oxide
MgB2 + dielectric film




                                                                               ...
MgB2 + dielectric film
multilayers
MgB2 50 nm / ALD MgO 10 nm / Nb

                                                       ...
MgB2 + dielectric film
multilayers
MgB2 50 nm / ALD Y2O3 10 nm / Nb
                                                       ...
Comparison of Auger sputter depth profiles for MgB2 films on
ALD dielectrics on baked Nb substrates




MgB2 50 nm / ALD Al2...
MgB2 + dielectric film many multilayers
 Auger sputter ion profile
                                                         ...
Summary:
•Lots of materials and thin film information available in surface analysis, sputter
depth profiles, and full spectr...
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Shulze - Surface and Thin Film Characterization of Superconducting Multilayer films for Application in RF Accelerator Cavities

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Surface and Thin Film Characterization of Superconducting Multilayer films for application in RF (Roland Schulze - 30')
Speaker: Roland Schulze - Los Alamos National Laboratory | Duration: 30 min.
Abstract
The use of multilayer ultra-thin films on the interior surfaces of Nb superconducting RF cavities shows great promise in substantially improving the performance characteristics of superconducting RF cavities into the 100 MV/m range by increasing the RF critical magnetic field, HRF, through careful choice of new materials and thin film structures. However, there are substantial materials science challenges associated with producing such complex film structures, particularly for conformal application of uniform thin films on the interior surfaces of RF cavities. Here we present surface and thin film analysis of ultra-thin films of two candidate materials, MgB2 and NbN superconductors, deposited through several different methods, along with multilayers produced with alternating superconductor and dielectric films. We report on the analysis methods and techniques, using primarily x-ray photoelectron spectroscopy and Auger spectroscopy with ion sputter depth profiling, and describe results from variety of thin film samples. The materials stability, microstructure, chemistry, and thin film morphology are highly dependent on methods and parameters used in the thin film deposition. From our analysis, important factors for producing quality superconducting and dielectric films include chemical stoichiometry, impurity content, deposition temperature, substrate choice and conditioning, choice of dielectric material, and the nature of the thin film interfaces. These factors will be discussed in the context of the production methods used for these ultra-thin superconducting films.

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Shulze - Surface and Thin Film Characterization of Superconducting Multilayer films for Application in RF Accelerator Cavities

  1. 1. Surface and Thin Film Characterization of Superconducting Multilayer films for Application in RF Accelerator Cavities A.T. Zocco, T. Tajima, M. Hawley, Y.Y. Zhang, N.F. Haberkorn, L. Civale, and R.K. Schulze, Los Alamos National Laboratory, Los Alamos, NM 87545 USA T. Prolier, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 USA B. Moeckly, Superconducting Technologies, Inc., 460 Ward Drive, Santa Barbara, CA 93111 USA The Fourth International Workshop on: Thin films and New Ideas for Pushing the Limits of RF Superconductivity, Padua, IT October 4-6, 2010 This work has been supported by the Defense Threat Reduction Agency and DOE Office of Science Nuclear Physics Slide 1 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  2. 2. The key idea of using a thin film superconductor is the fact that Bc1 increases when the thickness is d< λL (penetration depth) • The RF critical magnetic field HRF in a • Use thin films with thickness d < λL to type-II superconductor is somewhere enhance the lower critical field between Hc1 and Hc2 [Gurevich, APL 88 (2006) 012511] MgB2 Coherence length 5 nm Penetration depth 140 nm See Tajima talk for further details Slide 2 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  3. 3. An example: Coating 105 nm MgB2 layer could sustain 355 mT, corresponding to ~100 MV/m with Bpeak /Eacc ~ 3.6 mT/(MV/m) Simple single layer example Eacc ~ 100 MV/m • Assumptions Hc1(Nb) = 0.17 T λ(MgB2) = 140 nm ξ(MgB2) = 5 nm H0 = 355mT • Hc1(MgB2) = 355 mT Hi = 170mT • d = 105 nm • The film thickness needs to be determined so that the decayed field at the Nb surface is below the RF critical field of Nb (~200 mT). Nb MgB2 See Tajima talk for further details Dielectric material d = 105 nm Slide 3 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  4. 4. Materials and Deposition Methods: Polymer assisted deposition (PAD) for NbN - LANL Sequential reactive coevaporation for MgB2 - STI Coevaporation with 2 e-beam sources for MgB2 - Kagoshima University Atomic layer deposition for dielectrics Al2O3, MgO, Y2O3 - ANL Future CVD and PECVD for NbN and MgB2 - LANL Characterization Tools: XRD SEM SPM - STM, AFM This talk XPS Auger spectroscopy and sputter ion depth profiling PPMS - Tc Magnetometry - Hc1 See Tajima talk for further details RF power measurements - SLAC Materials and thin film characterization carried out in concert with deposition methods is critical for fine tuning synthesis methods and desired superconducting and RF performance properties: Chemistry and phase at surfaces and interfaces Interface mixing Film thickness Slide 4 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  5. 5. Film synthesis methods Polymer assisted deposition of NbN MgB2 Reactive co-evaporation method PAD solution: NbCl2, NH4OH, polyethyleneimine, HF, H2O Spin coat to thin film on substrate - provides basis of thin film structure for starting material NbCl2 Anneal (~1000°C) in reactive atmosphere to provide oriented growth of microcrystalline domains: NH3 to produce NbN CH4 to produce NbC Zou, GF, et al., Chem. Comm. 45 (2008) 6022 B.H. Moeckly and W.S. Ruby, Supercond. Sci. Technol. 19 (2006) L21–L24 Slide 5 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  6. 6. Nb substrate conditioning Required to remove excessive surface oxide to avoid reactions with deposited thin films and 4 x 10 improves surface magnetic 18 properties - less dissipation Nb 16 metallic 14 small amount of Nb sub-oxide 12 XPS high resolution scan Nb3d XPS 10 c/s Before anneal mostly Nb oxide 8 After anneal 800°C in UHV, surface is mostly Nb metal with a bit of 6 partial oxidation (high binding After anneal (blue) energy tailing) 4 Small amount of oxygen left at 2 surface after anneal by XPS Before anneal (red) 0 Nb2O5 220 218 216 214 212 210 208 206 204 202 200 198 Binding Energy (eV) Slide 6 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  7. 7. Angle Resolved XPS used to determine Nb2O5 oxide layer thickness resulting from BCP treatment on Nb metal crystal plate XPS intensities for photoemission peaks associated with the oxide overlayer, and the underlying intrinsic metal were used. The intensity, I, of photoelectron emission from each layer, i, can be described by the equation, where Io is the bulk intensity, which is dependent on the atom b volume density and is taken as unity for the base metal and some lower fraction for the oxide o based on material densities. l is the distance that the electron travels through the material before exiting the surface into the vacuum and is described as l=d/sinθ, where d is the thickness I i = I i " # i" &exp($l / % i )dl a of the oxide overlayer, and θ the angle of electron emission relative to the surface plane. λ is inelastic mean free path of the electron in the solid. For the oxide overlayer we integrate from l=0 to l=d/sinθ, and for the base metal we integrate from l=d/sinθ to ∞ for the bulk substrate. ARXPS reveals an oxide layer that is 27-30Å thick resulting from the BCP treatment ! 3 different photoelectron take off angles (TOA) relative to the surface plane: 90°, 45°, and 20°. The Nb3d manifold is curve fit to extract intensity data for the Nb in the form of Nb2O5 (oxide overlayer) and Nb in the form of metal (base substrate). The spin orbit couple peaks were constrained to a ratio of 3/2, expected theoretically. The metal peaks were fit using asymmetric broadening following theory from Doniac and Suncic, and the oxide peaks were simple Gaussian-Lawrencian. Slide 7 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  8. 8. NbN Surface and Thin Film Analysis • NbN intrinsic Tc = 16K • thin superconducting films produced by PAD method • with current deposition and annealing parameters films are N poor • low oxygen content critical for yielding superconductivity • incomplete coverage (pinhole) issues need to be resolved - AFM and XPS • annealing conditions critical in determining micro-nanostructure of films grain size and surface roughness - AFM • relative atomic sensitivity factors in Auger spectroscopy not yet correct - need standard Slide 8 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  9. 9. NbN Surface and Thin Film Analysis - surface morphology by AFM SRF-NbN6-1 1 x 1 µm RMS = 10.6 nm on Al2O3 SRF-NbN6-2 1 x 1 µm RMS = 5.1 nm on SrTiO3 SRF-NbN3-3 4 x 4 µm RMS = 21.6 nm on Al2O3 Topographic Image Phase Image Slide 9 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  10. 10. NbN films - surfaces vs. Comment: sample NbN3_2, NbN on sapphire produced by PAD process bulk film Atomic Concentration Table C1s N1s O1s Al2p Nb3d [0.296] [0.499] [0.711] [0.193] [3.127] 1.30 25.38 17.45 11.58 44.29 XPS spectroscopy measurement on surface and after sputter ion clean of 10 nm (into main bulk of film) shows relatively high oxygen (17.45% atomic) and a small amount of carbon (1.3% atomic). Some of the O signal may be from the incomplete coverage of sapphire. Na, Si, and most of the C at the surface are just surface impurities from processing or air exposure. Nb:N ratio here is measured to be 1.7. The after 10 nm sputter clean NbN films tend to be nitrogen deficient. The balance in the nitrogen deficiency may be made up by the O and C impurity levels. on surface Slide 10 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  11. 11. NbN film - profile Auger survey spectrum taken at 12 nm point in profile shows O, C, and Al in addition to the Nb and N. C is in a metal carbide chemical form. Relatively high O (>5%) and C (~5%) level in bulk of film No superconductivity Slide 11 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  12. 12. NbN film - profile XPS survey spectrum taken at 8 nm point in profile shows a very clean film. Oxygen <2% atomic Tc = 9.5K Slide 12 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  13. 13. NbN film - excessive oxygen in film 5 NbN4_6.spe x 10 4.5 -Nb3d 4 -Nb3p3 -Nb3p1 3.5 3 2.5 -N1s at 10 nm sputter depth c/s 2 -Nb3s File Name: NbN4_6.spe -O1s 1.5 Comment: PAD NbN on sapphire from YYZ sample NbN4-2 -------------------------- 1 -N KLL -Nb4p Atomic Concentration Table - RSF in [brackets] -O KLL -------------------------- 0.5 -Al2p N1s O1s Al2s Nb3d -Al2s [0.499] [0.711] [0.312] [3.127] 0 32.55 9.53 4.43 53.49 1200 1000 800 600 400 200 0 Binding Energy (eV) Al and some of the O signal is from the sapphire substrate due to incomplete coverage (holes) of the NbN film Slide 13 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  14. 14. MgB2 Surface and Thin Film Analysis • MgB2 intrinsic Tc = 39K • thin superconducting films produced by codeposition methods • high quality films are being produced - Tc, stoichiometry, interfaces good, RF performance, Hc1 • some issues with stability and interface mixing (inter reactions) • oxygen from substrate or dielectric may cause chemical interference at interfaces • for Auger spectroscopy and Auger thin film profiling there exists an overlap in the low energy Nb and B Auger peaks. Principal component analysis used to effectively separate signals for these two elements. The Mg chemical states of MgB2 and MgO may also be separated. Slide 14 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  15. 15. MgB2 Surface and Thin Film Analysis principal component analysis (PCA) in Auger profiling spectroscopy Separating B and Nb B Auger peaks B Nb Nb sum to fit experiment Mg in MgB2 Separating Mg in MgB2 and Mg in MgO Mg in MgB2 Auger signals Mg in MgO Mg in MgO sum to fit experiment Slide 15 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  16. 16. MgB2 Surface Analysis - surface alteration due to air exposure for a thick film (100 nm) Note: Ultrathin films show full depletion of B from altered surface layer - see below Slide 16 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  17. 17. MgB2 film structure surface oxide Mg-B oxide Mg-B oxide Mg oxide MgB2 Nb Intended: 100nm MgB2 on 10nm B on Nb x 10 5 substrate 3 C1 O1 Mg2 B1 2.5 Nb1 Thin film structure complicated: 1) Nb substrate 2 2) Thin Mg oxide 3) First layer of thin Mg-B oxide Intensity 1.5 4) Second layer of thin Mg-B oxide 1 5) Thicker MgB2 layer 6) Thin surface oxide layer 0.5 0 0 10 20 30 40 50 60 70 80 Sputter Time (min) Slide 17 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  18. 18. MgB2 film structure surface oxide Mg oxide MgB2 Nb Intended: 1000nm MgB2 on Nb substrate Thin film structure: 1) Nb substrate 2) Mg oxide (MgO) 3) Thicker MgB2 layer 4) Thin surface oxide layer MgO layer relatively thick Substantial mixing at interface of MgO and MgB2 Slide 18 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  19. 19. Auger spectroscopy sputter depth profile: peak intensity profile with Mg chemical states resolved using principal component analysis (PCA) / target factor analysis (TFA) Slide 19 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  20. 20. MgB2 + dielectric film multilayers Intended: 200nm MgB2 on 300nm Al2O3 on Nb substrate SRF45_7.pro 100 O1 Mg2 90 Al2 B1 MgB2 Al2O3 Nb1 80 MgB2 film of ~230 nm thickness shows very low oxygen and close to Mg:B = 70 0.5 stoichiometry Atomic Concentration (%) Layer of MgO at interface which seems 60 fairly sharp Al2O3 layer of ~370 nm thickness 50 Nb shows poor stoichiometry of ~Al1O1 instead of Al2O3 40 Interface of Al2O3 layer with Nb seems 30 to be very broad, indicating interdiffusion of Al2O3 with Nb 20 10 0 0 100 200 300 400 500 600 700 800 Sputter Depth (nm) Slide 20 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  21. 21. surface Mg oxide MgB2 + dielectric film buried Mg oxide aluminum oxide multilayers MgB2 Nb substrate MgB2 50 nm / ALD Al2O3 10 nm / Nb Auger sputter depth profile 100 O1 90 Surface layer >10 nm is fully Mg Mg2 oxide and completely depleted of B Al2 80 B1 MgB2 layer (~40 nm) is slightly B Nb1 70 poor except at 50 nm depth where Atomic Concentration (%) stoichiometry is close to correct 60 Mg oxide layer (~20 nm) 50 Aluminum oxide (~15 nm) 40 The small amount of oxygen (~2%) in the MgB2 film is real 30 Al is actually at ~0 atomic% in MbB2 20 layer - nonzero signal arises from spectral noise 10 0 0 20 40 60 80 100 120 Sputter Depth (nm) Slide 21 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  22. 22. MgB2 + dielectric film multilayers MgB2 50 nm / ALD MgO 10 nm / Nb MgOald4_5.pro 100 surface Mg oxide ALD Mg oxide 90 O1 Mg2 80 B1 MgB2 Nb1 70 Atomic Concentration (%) 60 Nb substrate 50 40 30 20 10 0 0 20 40 60 80 100 120 Sputter Depth (nm) Slide 22 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  23. 23. MgB2 + dielectric film multilayers MgB2 50 nm / ALD Y2O3 10 nm / Nb MgB2Y_6.pro 100 surface Mg oxide buried Mg oxide O1 90 ALD Y oxide Mg2 Y2 80 MgB2 B1 Nb1 70 Atomic Concentration (%) 60 Nb substrate 50 40 30 20 10 0 0 20 40 60 80 100 120 Sputter Depth (nm) Slide 23 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  24. 24. Comparison of Auger sputter depth profiles for MgB2 films on ALD dielectrics on baked Nb substrates MgB2 50 nm / ALD Al2O3 10 nm / Nb MgB2 50 nm / ALD MgO 10 nm / Nb MgB2 50 nm / ALD Y2O3 10 nm / Nb Slide 24 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  25. 25. MgB2 + dielectric film many multilayers Auger sputter ion profile B B B B B Nb Top layer of nominally pure B 10nm plus MgB2 MgB2 MgB2 MgB2 MgB2Nb5_11.pro 4x double layers of MgB2 50nm / B 10nm 100 on O1 Mg2 Nb substrate 90 B1 Nb1 Top layer of nominally pure B approximately 80 10nm in thickness, but shows Mg signal also 70 Individual layers and total film thickness are Atomic Concentration (%) thicker than predicted 60 I believe that the “less than sharp” interfaces 50 and incomplete stoichiometry gain (Mg found in the pure B layers) are due to 40 intermixing of the layers during the deposition process. Not an artifact from the 30 sputtering during analysis - note the relatively sharp interface at the Nb substrate. 20 First MgB2 layer slightly Mg rich, other layers 10 slightly B rich. 65nm 37nm 0 0 50 100 150 200 250 300 350 400 450 500 Sputter Depth (nm) Slide 25 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
  26. 26. Summary: •Lots of materials and thin film information available in surface analysis, sputter depth profiles, and full spectroscopy •Stoichiometry (with proper calibration), film thickness, material interface interactions •In the NbN system, oxygen content in the films is one critical factor in determining proper phase and superconductivity (<5% atomic need) •Stoichiometry to be improved in PAD produced NbN by adjustment of annealing conditions •MgB2 thick films on Nb crystal plate show promising results •Ongoing progress in producing ultra-thin MgB2 dielectric multilayers •Additional methods to produce thin films being investigated - CVD and PECVD towards the primary goal of conformal coatings on RF cavity interiors Slide 26 Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010 10/1/10 LA-UR 10-06339
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