EUCARD2 WP12.2 THIN FILMS PROSPECTIVES
C. Z. ANTOINE,
CEA, Irfu, SACM, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette Cedex,
France

Introduction
Characterization tools
Deposition technique
Physics of SRF and advanced superconductors
Multilayers
Conclusion

Claire Antoine Eucard2 WP12 Meeting @
Saclay
| PAGE 3
2014-04-16

WHY THIN FILMS ? 2 REASONS
Making cheaper cavities :
Bulk like Nb on copper (1-
5 µm)
Nb : l
~50 nm => only a few 100s nm of SC necessary
(the remaining thickness= mechanical support only) =>
Make thin films !
Advantages
Thermal stability (substrate cavity = copper)
Cost
Innovative materials
Optimization of R
BCS possible
Disadvantages
Fabrication and surface preparation (at least) as difficult as for bulk
Superconductivity very sensitive to crystalline quality (lower in thin films for now)
2014-04-16
Overcoming Nb monopoly:
Nb
3
Sn, MgB
2
, Multilayers…
Advantages
Can also be deposited onto copper
Higher Tc => higher Q0
Higher H
SH or H
C1
=> higher accelerating field
Disadvantages
Fabrication and surface preparation (at least) as difficult as for bulk
Superconductivity very sensitive to crystalline quality (lower in thin films for now)
Deposition of innovative (compound) materials is very difficult
Theoretical limit (H
SH vs H
C1
) still controverted
=> choice of ideal material !?
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 4

THIN FILMS CHALLENGES: DEPENDS ON THE STRATEGY
Optimizing structure/composition of the films on samples
Advantages
Structure /composition can be optimized with conventional techniques
Ideal structure and composition can be achieved on model sample (guide for deposition of cavities)
Cost
Disadvantages
RF performances cannot be directly measured
Specific measurement tools need to be developed (sample cavity, magnetometer…)
Ultimately a cavity deposition set-up will be needed, but with a known aimed structure
2014-04-16
Optimizing deposition inside cavities
Advantages
RF testing easy and gives direct performance
Work is done only once, direct cavity production
Disadvantages
Very heavy and lengthy, many parameters
Need to develop a specific cavity deposition set-up
Difficult to optimize set-up and film together
Optimization of the structure/composition of the film is difficult
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 5

2014-04-16
Niobium on copper (µm)
After ~ 20 years stagnation : new revolutionary deposition techniques
Great expectations in cost reduction
No improved performances/ bulk Nb
Higher Tc material (µm)
Based on superheating model.
Higher field and lower Q
0 expected
Higher Tc material (nm), multilayer
Based on trapped vortices model (Gurevich)
Higher field and lower Q0 expected
Recent experimental evidences
Specific characterization tools needed
Better understanding of SRF physics needed
Subtask
2
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 6

Claire Antoine Eucard2 WP12 Meeting @
Saclay
| PAGE 7
2014-04-16

Quadrupole resonnator developed at HZB
See O.
Kugeler’s talk (HZB activities)
2014-04-16
TE011 cavity developped at IPNO
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 8

Measurement of H
C1 on sample without edge/demagnetization effect
(local measurement: field decreases quickly far from the coil: r coil
= 2,5 mm; r sample
~1 cm ~ r coil x 4 )
Differential Locking
Amplifier
0
0 1 2 3 4 5 6 r/r
0
Excitation/Detection coil (small/sample)
0 1 2 3 r/r
0
4 5 6
2014-04-16
= T/Tc
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 9

Claire Antoine Eucard2 WP12 Meeting @
Saclay
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2014-04-16

High-energy deposition techniques line of sight techniques issues: getting uniform thickness/structure limited in complex geometry
Thermal diffusion films limited compositions available non uniform composition
Chemical techniques CVD, ALD conformational even in complex shape very quick for large surfaces issues: get the proper crystalline structure
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 11

DEPOSITION TECHNIQUES: INTERNATIONAL
SITUATION
CERN
Nb bulk like: Magnetron sputtering, HPIMS (collaboration with Sheafield
University)
Nb
3
Sn (diffusion furnace) : in project
Grenoble INP
ML: ALD and CVD
STFC
Nb, ML: PVD and ECR –CVD
Jlab and collaborators
At Jlab : ECR, HPIMS
Almeda Corpn: CED (Plasma)
W&M : magnetron sputtering + Complete material characterization
Cornell
Nb
3
Sn (diffusion furnace)
Temple University
MgB
2
(HP-CVD), ML in collaboration with LANL and FNAL
ANL
ML: ALD
INFN Legnaro
NbN (diffusion furnace)
Large experience on sputtering, Nb
3
Sn…
2014-04-16
Publications on that topic at SRF 2013: 23
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 12

2014-04-16
See G. Terenziani’s talk
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 13

Received special R&D ALD set-up
Need to develop a suitable coordination chemistry for the ALD precursors (+ plasma ALD to help)
Process scaling up to cavity deposition will be performed with specific simulation tools.
2014-04-16
See F. Weiss’s talk
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 14

Claire Antoine Eucard2 WP12 Meeting @
Saclay
| PAGE 15
2014-04-16

Q
0
(

1/Thermal dissipations) depends on surface resistance … which depends on Tc
Higher Tc => higher Q
0
=> lower operation cost
Ultimate limit in E acc
: when the SC becomes dissipative!
Transition : when T and/or B↑
Vortices in RF highly dissipative => keep Meissner state
At w
< 3 GHz: we are limited by B RF !!!
H
C1
Nb
= 180-190 mT
2014-04-16
Cavities : Meissner State, no vortex please !!!
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 16

Cavities : Meissner State,
H~ H
C1
, J~J
D
(@ H
C1
), T~2-4 K
SRF : H
C1
VS H
SH
? H
SH
2014-04-16
Coils : higher T
C
, but mixed state (low H
C1
)
Generally low magnetic field
Reaching higher field (E acc
/B RF )
=> Reach superheating field (metastable Meissner state) : Nb
3
Sn, MgB
2
…
=> Artificially enhance H
C1
: Multilayers
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE

2014-04-16
Nb
3
Sn : RECENT BREAKTHROUGH
At 4,2 K Q
0
Nb3Sn
20 x Q
0
Nb !!!
=
At 2 KQ
0
Nb3Sn
Q
0
Nb
~
Limited in E acc
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 18

Nb
3
Sn: RECENT BREAKTHROUGH
Hays. "Measuring the RF critical field of Pb,
Nb, NbSn". in SRF 97. 1997.
Cornell, 1997 pulsed
Recent results from Cornell
CW
H
C1
Nb3Sn (~27mT) but
H
C1 is not a fundamental limit for SRF
Is it a practical limitation ?
NB : H
SH is more easily observed :
Close to Tc (cf Yogi)
Pulsed RF
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 19

2014-04-16
Nb
3
Sn : A LOT OF ROOM FOR IMPROVEMENT
H
P
Nb 0.12 T (H
°
C1
=0.17 T) H
P
Nb
3
Sn 0.03T (H
°
C1
=0.05 T)
1E+12
Nb
3
Sn
1E+11
Nb
1E+10
QUENCH
1E+09
1E+08
0
Epk (MV/m)
1.5 GHz Nb
3
Sn cavity (Wuppertal, 1985)
1.3 GHz Nb cavity (Saclay, 1999)
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 20

Claire Antoine Eucard2 WP12 Meeting @
Saclay
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2014-04-16

Multilayers: Nb / insulator / superconductor / insulator / superconductor … :
Surface screening and low Rs
Thin SC films. d< l
=> artificial enhancement of H
C1
*
The thin layers stand high fields without vortex nucleation
Niobium surface screening: allows higher field in the cavity
R NbN
S

1
10
R Nb
S
=> Q
0 multi >> Q
0
Nb
In principle :
Nb I-S-I-S-
H applied
100
B(mT)
200
1E+12
2014-04-16
H
Nb
Cavity's internal surface → Outside wall

H
Nb
 H appl e  * *
* In theory 20 nm NbN : H
C1 x ~200
** Simplified model from Gurevich
1E+11
1E+10
1E+09
0 20
E acc
40
(MV/m)
60
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 22

Infinite plane
Single layer Thin film in a uniform field
B B B B
Simple Model (Gurevich, 2006) Modified Model (Kubo, 2013) (1 of the reasons why measurements with
Squid are ambiguous)
T. Kubo @ SRF 2013
Made the calculation for exact boundaries condition l, x
: known bulk values (not necessary exact for thin films)
2014-04-16
Applied to NbN, Nb
3
Sn, MgB
2
Similar calculation also proposed by S. Posen for Nb
3
Sn only
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 23

T. Kubo
SL/ML structures not effective for Nb
3
Sn
Contrary to simple model, very thin layers are not interesting
Optimum thickness around 100 nm for NbN ?
How does that compare with exp. Measurements ?
=> Series of SL with various thicknesses (50 nm to 150 nm) is needed
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 24

[Lukaszew, W&M
2012]
~30(
*
) or 50 nm (
**
)
NbN
~ 15 nm insulator (MgO)
600(
*
) / 250(
**
) nm Nb
“bulk like”
MgO (100) substrate
RESULTS FROM OTHER LABS
*
**
Compare with what is expected for bulk Nb : ~1300 Oe @ 4.5 K !
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 25

2
MgB
MgO
2
MgB
MgO
2
MgB
2
Sapphire
Alternating MgB
2
-insulator structures have been fabricated on sapphire substrate:
 40 nm MgO as insulating layer, sputtered.
 MgB
2 deposited by HPCVD and 75 nm in thickness.
ex situ . 150, 100,
T c near 40 K for 100 and 150 nm films. Lower for 75 nm film.
Multilayer films with thin MgB
2 higher H
C1 layers show than the 300 nm film even though the total thickness are the same.
2014-04-16
[Teng, Xi
– Temple
University]
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 26

2014-04-16
[Tajima, SRF 2011]
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 27

CZ Antoine, JC Villegier, G Martinet, Applied Physics Letters 102 (10), 102603-102603-4
0,0050
0,0045
0,0040
0,0035
0,0030
0,0025
0,0020
0,0015
0,0010
0,0005
0,0000
4 5 6 7 8 9 10 11 12 13 14 15 16
Température (K)
0,2
0,9
18,56
3,7
7,5
1,2
15
18,6
22
26,5
36,6
36,7
52,4
52,5
52,6
52,6
61,3
61,3
Magnetic screening evidenced for ML4 up to 38 mT, at T~7K
Dramatic transition around 38 mT => r coil
<< r sample not valid anymore ?
Magnetometer is effective up to 1500 mA
(equivalent field 150 mT) Tp ° 2-40 K
Use of larger samples/smaller coil is mandatory
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 28

50 nm NbN
~ 15 nm insulator (MgO)
Bulk LG Nb
Polycrystalline
Nb substrate
Large grain Nb substrate
Same calibration
Rs rf-ML2<< Rs Nb
NB. Current: ½I in Nb
½I in NbN
Strong indication that R
BCS is improved with ML
Could probably be improved with the use of thicker layers (complete screening)
R res is higher for ML than for Nb, but strongly influenced by substrate ?
Very promising preliminary results
2014-04-16 Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 29

2014-04-16
Renewed activity on bulk-like Nb films (cost issues) and high H
SH e.g. Nb
3
Sn or NbN (higher performances)
SC
ML structures seem to be a promising way to go beyond Nb for accelerator cavities
Look for higher Q
0
, not only E acc
!
WE ARE ON THE EVE OF A TECHNOLOGICAL REVOLUTION
FOR SRF CAVITIES !
Few labs involved
EUCARD 2 : only large scale program in Europe in this domain, big opportunity
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE 30

THANK YOU FOR YOUR ATTENTION
| PAGE 31
Claire Antoine Eucard2 WP12 Meeting @
Saclay
2014-04-16
Commissariat à l’énergie atomique et aux énergies alternatives
Centre de Saclay | 91191 Gif-sur-Yvette Cedex
T. +33 (0)1 69 08 73 28 | F. +33 (0)1 69 08 64 42
Etablissement public à caractère industriel et commercial | RCS Paris B 775 685 019
DSM
Irfu
SACM
LIDC2

Claire Antoine Eucard2 WP12 Meeting @
Saclay
| PAGE 32
2014-04-16

Substrate RRR
Single crystal insulator
MgO (100) 176
MgO (110)
MgO (111) a-Al
2
O
3 c-Al
2
O
3
424
197
488
247
Cu large grains
289
Cathodic arc plasma.
Record 585
• Ions Energy 60-120 eV
• Arc source is scalable for large scale cavity coatings
• UHV and clean walls important
Ch. Reece; JLab
Nb films grown by Jlaband AASC AlmedaApplied
Science Corporation. Balk like RRR values
Coaxial Energetic Deposition
(CED TM )

2014-04-16
J
H
H
C2
H
SH
H
C1
Claire Antoine Eucard2 WP12 Meeting @ Saclay | PAGE
T