Slide 1
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10/1/10
Surface and Thin Film Characterization ofSuperconducting Multilayer films for Application in RFAccelerator Cavities
A.T. Zocco, T. Tajima, M. Hawley, Y.Y. Zhang, N.F. Haberkorn, L. Civale, and R.K. Schulze, LosAlamos 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 RFSuperconductivity, Padua, IT October 4-6, 2010
This work has been supported by the Defense Threat Reduction Agencyand DOE Office of Science Nuclear Physics
Slide 2
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
• The RF critical magnetic field HRF in atype-II superconductor is somewherebetween Hc1 and Hc2
• Use thin films with thickness d < λL toenhance the lower critical field
[Gurevich, APL 88 (2006) 012511]
The key idea of using a thin film superconductor is the fact that Bc1increases when the thickness is d< λL (penetration depth)
See Tajima talk for further details
MgB2Coherence length 5 nmPenetration depth 140 nm
Slide 3
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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• AssumptionsHc1(Nb) = 0.17 Tλ(MgB2) = 140 nmξ(MgB2) = 5 nm• Hc1(MgB2) = 355 mT• d = 105 nm• The film thickness needs to be determined so that the
decayed field at the Nb surface is below the RF criticalfield of Nb (~200 mT).
H0 = 355mTHi = 170mT
d = 105 nm
NbMgB2
Eacc ~ 100 MV/m
Dielectricmaterial
See Tajima talk for furtherdetails
Slide 4
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10/1/10
Materials and Deposition Methods:Polymer assisted deposition (PAD) for NbN - LANLSequential reactive coevaporation for MgB2 - STICoevaporation with 2 e-beam sources for MgB2 - Kagoshima UniversityAtomic layer deposition for dielectrics Al2O3, MgO, Y2O3 - ANLFuture CVD and PECVD for NbN and MgB2 - LANL
Characterization Tools:XRDSEMSPM - STM, AFMXPSAuger spectroscopy and sputter ion depth profilingPPMS - TcMagnetometry - Hc1RF power measurements - SLAC
Materials and thin film characterization carried out in concert with depositionmethods is critical for fine tuning synthesis methods and desired superconducting andRF performance properties:Chemistry and phase at surfaces and interfacesInterface mixingFilm thickness
See Tajima talk for further details
This talk
Slide 5
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
B.H. Moeckly and W.S. Ruby, Supercond. Sci.Technol. 19 (2006) L21–L24
Reactive co-evaporation method
Film synthesis methods
Polymer assisted deposition of NbN MgB2
PAD solution:NbCl2, NH4OH, polyethyleneimine, HF,H2O
Spin coat to thin film on substrate -provides basis of thin film structure forstarting material NbCl2
Anneal (~1000°C) in reactive atmosphereto provide oriented growth ofmicrocrystalline domains:NH3 to produce NbNCH4 to produce NbC
Zou, GF, et al., Chem. Comm. 45 (2008) 6022
Slide 6
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10/1/10
XPS high resolution scanNb3d XPS
Before anneal mostly Nb oxideAfter anneal 800°C in UHV, surfaceis mostly Nb metal with a bit ofpartial oxidation (high bindingenergy tailing)
Small amount of oxygen left atsurface after anneal by XPS
Nb substrate conditioning
Required to remove excessivesurface oxide to avoid reactionswith deposited thin films andimproves surface magneticproperties - less dissipation
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Binding Energy (eV)
c/s
Before anneal (red)
After anneal (blue)
Nb2O5
Nbmetallic
small amount ofNb sub-oxide
Slide 7
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Angle Resolved XPS used to determine Nb2O5 oxide layer thickness resulting fromBCP treatment on Nb metal crystal plate
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 toextract intensity data for the Nb in the form of Nb2O5 (oxide overlayer) and Nb in the form of metal (base substrate). The spin orbitcouple peaks were constrained to a ratio of 3/2, expected theoretically. The metal peaks were fit using asymmetric broadeningfollowing theory from Doniac and Suncic, and the oxide peaks were simple Gaussian-Lawrencian.
!
iI = ioI " i# " exp($l / i%
a
b
& )dl
XPS intensities for photoemission peaks associated with the oxide overlayer, and the underlyingintrinsic metal were used. The intensity, I, of photoelectron emission from each layer, i, can bedescribed by the equation, where Io is the bulk intensity, which is dependent on the atomvolume density and is taken as unity for the base metal and some lower fraction for the oxidebased on material densities. l is the distance that the electron travels through the materialbefore exiting the surface into the vacuum and is described as l=d/sinθ, where d is the thicknessof the oxide overlayer, and θ the angle of electron emission relative to the surface plane. λ isinelastic mean free path of the electron in the solid. For the oxide overlayer we integrate froml=0 to l=d/sinθ, and for the base metal we integrate from l=d/sinθ to ∞ for the bulk substrate.
Slide 8
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10/1/10
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 - needstandard
Slide 9
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10/1/10
NbN Surface and Thin Film Analysis - surface morphology by AFM
Topographic Image Phase Image
SRF-NbN6-1 1 x 1 µm
RMS = 10.6 nmon Al2O3
SRF-NbN6-2 1 x 1 µm
RMS = 5.1 nmon SrTiO3
SRF-NbN3-34 x 4 µm
RMS = 21.6 nmon Al2O3
Slide 10
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10/1/10
Comment: sample NbN3_2, NbN on sapphire produced by PAD process 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
on surface
after 10 nm sputter clean
XPS spectroscopy measurement on surfaceand after sputter ion clean of 10 nm (intomain bulk of film) shows relatively highoxygen (17.45% atomic) and a smallamount of carbon (1.3% atomic). Some ofthe O signal may be from the incompletecoverage of sapphire.
Na, Si, and most of the C at the surface arejust surface impurities from processing or airexposure.
Nb:N ratio here is measured to be 1.7. TheNbN films tend to be nitrogen deficient.
The balance in the nitrogen deficiency maybe made up by the O and C impurity levels.
NbN films - surfaces vs.bulk film
Slide 11
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10/1/10
Auger survey spectrum taken at 12 nm pointin profile shows O, C, and Al in addition tothe Nb and N. C is in a metal carbidechemical form.
Relatively high O (>5%) and C (~5%) levelin bulk of film
No superconductivity
NbN film - profile
Slide 12
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10/1/10
XPS survey spectrum taken at 8 nm point inprofile shows a very clean film.
Oxygen <2% atomic
Tc = 9.5K
NbN film - profile
Slide 13
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10/1/10
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Binding Energy (eV)
c/s
-O K
LL
-O1s
-N K
LL
-N1s
-Nb4
p
-Nb3
s
-Nb3
p3 -N
b3p1
-Nb3
d
-Al2
s -A
l2p
File Name: NbN4_6.spe
Comment: PAD NbN on sapphire from YYZ sampleNbN4-2
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Atomic Concentration Table - RSF in [brackets]
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N1s O1s Al2s Nb3d[0.499] [0.711] [0.312] [3.127]
32.55 9.53 4.43 53.49
at 10 nm sputter depth
NbN film - excessive oxygen in film
Al and some of the O signal is from the sapphire substrate due toincomplete coverage (holes) of the NbN film
Slide 14
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
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 inthe low energy Nb and B Auger peaks. Principal component analysis used toeffectively separate signals for these two elements. The Mg chemical states ofMgB2 and MgO may also be separated.
Slide 15
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10/1/10
MgB2 Surface and Thin Film Analysisprincipal component analysis (PCA) in Auger profiling spectroscopy
Separating B and NbAuger peaks
Separating Mg inMgB2 and Mg in MgOAuger signals
B
NbB
sum to fitexperiment
Nb
Mg in MgB2
Mg in MgO
Mg in MgB2
Mg in MgO
sum to fitexperiment
Slide 16
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10/1/10
MgB2 Surface Analysis - surface alteration due to air exposure for a thick film (100 nm)
Note:Ultrathin films show full depletionof B from altered surface layer -see below
Slide 17
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10/1/10
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Sputter Time (min)
Inte
nsity
C1O1
Mg2B1Nb1
surfa
ce o
xide
MgB2
Mg-
B o
xide
Mg-
B o
xide
Mg
oxid
e
Nb
Thin film structure complicated:
1) Nb substrate
2) Thin Mg oxide
3) First layer of thin Mg-B oxide
4) Second layer of thin Mg-Boxide
5) Thicker MgB2 layer
6) Thin surface oxide layer
MgB2 film structure
Intended:
100nm MgB2 on 10nm B on Nbsubstrate
Slide 18
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10/1/10
MgB2
surfa
ce o
xide
Mg
oxid
e
Nb
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 ofMgO and MgB2
MgB2 film structure
Intended:
1000nm MgB2 on Nb substrate
Slide 19
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10/1/10
Auger spectroscopy sputter depth profile: peak intensity profilewith Mg chemical states resolved using principal componentanalysis (PCA) / target factor analysis (TFA)
Slide 20
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
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Sputter Depth (nm)
Atom
ic C
once
ntra
tion
(%)
O1Mg2Al2B1
Nb1
MgB2 film of ~230 nm thickness showsvery low oxygen and close to Mg:B =0.5 stoichiometry
Layer of MgO at interface which seemsfairly sharp
Al2O3 layer of ~370 nm thicknessshows poor stoichiometry of ~Al1O1instead of Al2O3
Interface of Al2O3 layer with Nb seemsto be very broad, indicatinginterdiffusion of Al2O3 with Nb
MgB2 + dielectric film multilayers
Intended:
200nm MgB2 on 300nm Al2O3 on Nb substrate
MgB2 Al2O3
Nb
Slide 21
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
Auger sputter depth profile
Surface layer >10 nm is fully Mgoxide and completely depleted of B
MgB2 layer (~40 nm) is slightly Bpoor except at 50 nm depth wherestoichiometry is close to correct
Mg oxide layer (~20 nm)
Aluminum oxide (~15 nm)
The small amount of oxygen (~2%)in the MgB2 film is real
Al is actually at ~0 atomic% in MbB2layer - nonzero signal arises fromspectral noise
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Atom
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once
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(%)
O1Mg2Al2B1Nb1
MgB2
surf
ace
Mg
oxid
e
buri
ed M
g ox
ide
alum
inum
oxi
de
Nb substrate
MgB2 + dielectric filmmultilayers
MgB2 50 nm / ALD Al2O3 10 nm / Nb
Slide 22
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
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100MgOald4_5.pro
Sputter Depth (nm)
Atom
ic C
once
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tion
(%)
O1Mg2B1Nb1MgB2
surf
ace
Mg
oxid
e
ALD
Mg
oxid
e
Nb
subs
trat
e
MgB2 + dielectric filmmultilayers
MgB2 50 nm / ALD MgO 10 nm / Nb
Slide 23
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
Sputter Depth (nm)
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Atom
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once
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(%)
O1Mg2Y2B1Nb1
MgB2
surf
ace
Mg
oxid
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ALD
Y o
xide
Nb
subs
trat
e
buri
ed M
g ox
ide
MgB2 + dielectric filmmultilayers
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 2010LA-UR 10-06339
10/1/10
Comparison of Auger sputter depth profiles for MgB2 films onALD 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 25
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
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Sputter Depth (nm)
Atom
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once
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(%)
O1Mg2B1
Nb1
B
MgB2
B B B B Nb
MgB2 MgB2 MgB2
65nm 37nm
Auger sputter ion profile
Top layer of nominally pure B 10nmplus4x double layers of MgB2 50nm / B 10nmonNb substrate
Top layer of nominally pure B approximately10nm in thickness, but shows Mg signal also
Individual layers and total film thickness arethicker than predicted
I believe that the “less than sharp” interfacesand incomplete stoichiometry gain (Mgfound in the pure B layers) are due tointermixing of the layers during thedeposition process. Not an artifact from thesputtering during analysis - note therelatively sharp interface at the Nb substrate.
First MgB2 layer slightly Mg rich, other layersslightly B rich.
MgB2 + dielectric film many multilayers
Slide 26
Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339
10/1/10
Summary:•Lots of materials and thin film information available in surface analysis, sputterdepth profiles, and full spectroscopy
•Stoichiometry (with proper calibration), film thickness, material interfaceinteractions
•In the NbN system, oxygen content in the films is one critical factor indetermining proper phase and superconductivity (<5% atomic need)
•Stoichiometry to be improved in PAD produced NbN by adjustment of annealingconditions
•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 PECVDtowards the primary goal of conformal coatings on RF cavity interiors