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THIN FILMS TECHNOLOGIES FOR SRFEUCARD2 WP12.2 THIN FILMS PROSPECTIVES
C. Z. ANTOINE, CEA, Irfu, SACM, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette Cedex, France
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OUTLOOK
Introduction
Characterization tools
Deposition technique
Physics of SRF and advanced superconductors
Multilayers
Conclusion
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INTRODUCTION
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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)CostInnovative materialsOptimization of RBCS possible
Disadvantages
Fabrication and surface preparation (at least) as difficult as for bulkSuperconductivity very sensitive to crystalline quality (lower in thin films for now)
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WHY THIN FILMS ? 2 REASONS
Making cheaper cavities :
Bulk like Nb on copper (1-5 µm)
Overcoming Nb monopoly:
Nb3Sn, MgB2, Multilayers…
Advantages
Can also be deposited onto copperHigher Tc => higher Q0Higher HSH or HC1 => higher accelerating field
Disadvantages
Fabrication and surface preparation (at least) as difficult as for bulkSuperconductivity very sensitive to crystalline quality (lower in thin films for now)Deposition of innovative (compound) materials is very difficultTheoretical limit (HSH vs HC1) still controverted => choice of ideal material !?
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Advantages
Structure /composition can be optimized with conventional techniquesIdeal structure and composition can be achieved on model sample (guide for deposition of cavities)Cost
Disadvantages
RF performances cannot be directly measuredSpecific measurement tools need to be developed (sample cavity, magnetometer…)Ultimately a cavity deposition set-up will be needed, but with a known aimed structure
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THIN FILMS CHALLENGES: DEPENDS ON THE STRATEGY
Optimizing
structure/composition of the
films on samples
Optimizing deposition inside
cavities
Advantages
RF testing easy and gives direct performanceWork is done only once, direct cavity production
Disadvantages
Very heavy and lengthy, many parametersNeed to develop a specific cavity deposition set-upDifficult to optimize set-up and film togetherOptimization of the structure/composition of the film is difficult
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STRUCTURE OF THE TASK 12.2 (EUCARD2)
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 Q0 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
1
Subtask 2
Subtask
3
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CHARACTERIZATION TOOLS DEVELOPMENTS(TASK 3)
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“SAMPLE CAVITIES”
Quadrupole resonnator developed at HZBSee O. Kugeler’s talk (HZB activities)
TE011 cavity developped at IPNO
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LOCAL MAGNETOMETRY DEVELOPED @CEA
Measurement of HC1 on sample without edge/demagnetization effect (local measurement: field decreases quickly far from the coil: rcoil = 2,5 mm; rsample~1 cm ~ rcoil x 4 )
= T/Tc
1 2 3 4 5 6
Br (
a,u,
)
Bz (a
,u,)
1 2 3 4 5 60r/r0
r/r0
0
0
Excitation/Detection coil (small/sample)
Differential Locking Amplifier
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DEPOSITION TECHNIQUES ISSUES(TASK 1 AND TASK 2)
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3 MAJORS DEPOSITION TECHNIQUES
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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
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DEPOSITION TECHNIQUES: INTERNATIONAL SITUATION
CERN Nb bulk like: Magnetron sputtering, HPIMS (collaboration with Sheafield University)Nb3Sn (diffusion furnace) : in project
Grenoble INPML: ALD and CVD
STFCNb, ML: PVD and ECR –CVD
Jlab and collaboratorsAt Jlab : ECR, HPIMSAlmeda Corpn: CED (Plasma)W&M : magnetron sputtering + Complete material characterization
CornellNb3Sn (diffusion furnace)
Temple UniversityMgB2 (HP-CVD), ML in collaboration with LANL and FNAL
ANLML: ALD
INFN LegnaroNbN (diffusion furnace)Large experience on sputtering, Nb3Sn…
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Publications on that topic at SRF 2013: 23
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HPIMS @ CERN: bulk-like thin films
See G. Terenziani’s talk
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CVD/ ALD @ GRENOBLE INP
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.
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See F. Weiss’s talk
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UNDERSTANDING THE PHYSICS OF SRF(TASK 3)ADVANCED SUPERCONDUCTORS(TASK 2)
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SRF LIMITS : BACK TO BASICS
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Q0 (1/Thermal dissipations) depends on surface resistance … which depends on TcHigher Tc => higher Q0 => lower operation cost
Ultimate limit in Eacc: 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 BRF !!!
HC1Nb
= 180-190 mT
Cavities : Meissner State, no vortex please !!!
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? HSH
SRF : HC1 VS HSH
Cavities : Meissner State, H~ HC1, J~JD (@ HC1), T~2-4 K
Coils : higher TC, butmixed state (low HC1) Generally low magnetic field
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Reaching higher field (Eacc/BRF) => Reach superheating field (metastable Meissner state) : Nb3Sn, MgB2…=> Artificially enhance HC1 : Multilayers
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Nb3Sn : RECENT BREAKTHROUGH
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At 4,2 K Q0Nb3Sn =
20 x Q0Nb !!!
At 2 KQ0Nb3Sn ~
Q0Nb
Limited in Eacc
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Nb3Sn: RECENT BREAKTHROUGH
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HC1Nb3Sn (~27mT)
Recent results from CornellCW
Cornell, 1997pulsed
HC1 is not a fundamental limit for SRFbut
Is it a practical limitation ?NB : HSH is more easily observed :
Close to Tc (cf Yogi)Pulsed RF
Hays. "Measuring the RF critical field of Pb, Nb, NbSn". in SRF 97. 1997.
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HP Nb 0.12 T (H°C1=0.17 T)
1E+08
1E+09
1E+10
1E+11
1E+12
0 5 10 15 20 25 30 35
Epk (MV/m)
Qo
QUENCH
10 20 30 40 50 60 70
HP Nb3Sn 0.03T (H°C1=0.05 T)
Nb3Sn
Nb
1.5 GHz Nb3Sn cavity (Wuppertal, 1985) 1.3 GHz Nb cavity (Saclay, 1999)
Nb3Sn : A LOT OF ROOM FOR IMPROVEMENT
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ADVANCED SUPERCONDUCTORS :MULTILAYERS
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Multilayers: Nb / insulator/ superconductor / insulator /superconductor… :
Surface screening and low Rs Thin SC films. d< l => artificial enhancement of HC1
*
The thin layers stand high fields without vortex nucleation Niobium surface screening: allows higher field in the cavity
=> Q0multi >> Q0
Nb RR NbS
NbNS 10
1
0 10 20 30 40 50 601E+09
1E+11
200100
Eacc (MV/m)
B(mT)
Q0
ALTERNATIVE TO BULK SC: MULTILAYERS
(SC with higher Tc than Nb)
**lNdeHH applNb
Cavity's internal surface →Outside wall
Happlied
HNb
Nb I-S-I-S-
* In theory 20 nm NbN : HC1 x ~200** Simplified model from Gurevich
In principle :
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FIELD SCREENING / THIN FIELD
Modified Model (Kubo, 2013)
T. Kubo @ SRF 2013
Made the calculation for exact boundaries condition
l, x : known bulk values (not necessary exact for thin films)
Applied to NbN, Nb3Sn, MgB2
Similar calculation also proposed by S. Posen for Nb3Sn only
Simple Model (Gurevich, 2006) (1 of the reasons why measurements with Squid are ambiguous)
B B B B
Single layerInfinite plane Thin film in a uniform field
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OPTIMUM THICKNESS FOR SL ?
T. KuboSL/ML structures not effective for Nb3Sn
Contrary to simple model, very thin layers are not interestingOptimum 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
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RESULTS FROM OTHER LABS
Compare with what is expected for bulk Nb : ~1300 Oe @ 4.5 K !
[Lukaszew, W&M 2012]
*
**
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MgO (100) substrate
600(*) / 250(**) nm Nb “bulk like”
~ 15 nm insulator (MgO)
~30(*) or 50 nm (**) NbN
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Alternating MgB2-insulator structures have been fabricated on sapphire substrate: 40 nm MgO as insulating layer, sputtered. MgB2 deposited by HPCVD ex situ. 150,
100, and 75 nm in thickness.
Tc near 40 K for 100 and 150 nm films. Lower for 75 nm film.
Multilayer films with thin MgB2 layers show higher HC1 than the 300 nm film even though the total thickness are the same.
SapphireMgB2
MgOMgB2
MgOMgB2
[Teng, Xi – Temple University]
MgB2-MgO MULTILAYER FILMS
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[Tajima, SRF 2011]
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Magnetic screening evidenced for ML4 up to 38 mT, at T~7K
Dramatic transition around 38 mT => rcoil << rsample 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
MAGNETOMETRY EARLY RESULTS
4 5 6 7 8 9 10 11 12 13 14 15 16
0,0000
0,0005
0,0010
0,0015
0,0020
0,0025
0,0030
0,0035
0,0040
0,0045
0,0050
V3
(V)
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
CZ Antoine, JC Villegier, G Martinet, Applied Physics Letters 102 (10), 102603-102603-4
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THERMAL ANALYSIS
Strong indication that RBCS is improved with MLCould probably be improved with the use of thicker layers (complete screening)Rres is higher for ML than for Nb, but strongly influenced by substrate ?Very promising preliminary results
Same calibrationRs rf-ML2<< Rs Nb
NB. Current: ½I in Nb½I in NbN
Polycrystalline Nb substrateLarge grain Nb substrate
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Bulk LG Nb
~ 15 nm insulator (MgO)
50 nm NbN
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CONCLUSIONS AND PERSPECTIVES
Renewed activity on bulk-like Nb films (cost issues) and high HSH SC e.g. Nb3Sn or NbN (higher performances)
ML structures seem to be a promising way to go beyond Nb for accelerator cavities
Look for higher Q0, not only Eacc !
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
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DSMIrfuSACMLIDC2
Commissariat à l’énergie atomique et aux énergies alternativesCentre de Saclay | 91191 Gif-sur-Yvette CedexT. +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
Claire Antoine Eucard2 WP12 Meeting @ Saclay
THANK YOU FOR YOUR ATTENTION
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SPARES
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COAXIAL ENERGETIC DEPOSITION (CED)
• Ions Energy 60-120 eV• Arc source is scalable for large scale cavity coatings• UHV and clean walls important
Substrate RRR
Single crystal insulatorMgO (100) 176MgO (110) 424MgO (111) 197
a-Al2O3 488c-Al2O3 247
Cu large grains 289
Record 585
Nb films grown by Jlab and AASC Almeda Applied Science Corporation. Balk like RRR values
Ch. Reece; JLab
Coaxial Energetic Deposition (CEDTM)
Cathodic arc plasma.
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T
J
H HC2
HSH
HC1