http://www.surfacetreatments.it/thinfilms 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.

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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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