A Review of Niobium (on Copper) Sputtering Technology
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A Review of Niobium (on Copper) Sputtering Technology
S. Calatroni
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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• Why films: advantages / disadvantages• Residual resistance: topics for further R&D :
– Effect of roughness– Effect of film structure– Effect of hydrogen– Effect of surface oxidation
• Conclusions and perspectives
Introduction
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Why films
• Advantages (primary objectives)– Thermal stability– Cost– Innovative materials
• Advantages (learned from experience)– Optimisation of RBCS at 4.2 K (sputtered niobium films)
– Reduced sensitivity to earth magnetic field
• Disadvantages (known from the beginning)– Fabrication and surface preparation (at least) as difficult as for bulk
• Disadvantages (learned from experience)– Deposition of innovative materials is very difficult
– Steep Rres increase with RF field (sputtered niobium films)
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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The surface resistance can be written in the form:
RRs s (H(HRFRF, H, Hextext, T) = R, T) = RBCS BCS (H(HRFRF, T) + R, T) + Rfl fl (H(HRFRF, H, Hextext, T) + , T) + RRres res (H(HRFRF))
The dependence of RBCS (0,T) on has been verified by changing the sputter gas
RBCS (HRF, T) has an intrinsic dependence of HRF
Rfl (HRF, Hext, T) has a dependence on similar to RBCS(0,T)
The surface resistance
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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RBCS at 4.2 K Nb bulk: ~900
nNb films: ~400
n
RBCS at 1.7 K Nb bulk: ~2.5
nNb films: ~1.5
n
1 10
350
400
450
500
550
600
650
700
750
800
850
900
2 5 20
RB
CS
(4.2
K)
[n
]
1+0/2
Theoretical and experimental BCS resistance at zero RF field
Nb bulk
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Fluxon-induced losses at 1.7 K are characterized as RRflfl = (R = (Rflfl00 + R + Rflfl
11 H HRFRF) H) Hextext The minimum values are obtained using krypton as sputter gas:
RRflfl00 = 3n = 3n/G R/G Rflfl
11 = 0.4 n = 0.4 n/G/mT/G/mT
Triangles: bulk NbSquares: coatings on oxide-free copperCircles: coatings on oxidized copper
1 10
10
100
Rfl
0 [n
/G]
(a)
1 10
1
10
Rfl
1 [n
/G/m
T]
(b)
Fluxon-induced losses I
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Nb/Cu films – categories
• There are two categories of films
– Films which are intrinsically films• Thin, small grains, microstrained, under stress• Problems: defects & microstructure, (impurities), surface state• Examples: magnetron sputtered films on oxidised copper
– The trend among workers is to aim for films which are bulk-like• Thick, large grains• Problems: hydrogen, surface quality• Examples: high-energy deposition techniques, annealed films,
(Nb Cu-clad)
Of course a film from one family may as well present all the problems typical of the other family…
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Research lines around the world
• Effect of substrate: roughness, thermal impedance– Non uniform coating, H enhancement (demagnetization), increased
granularity. Thermal feedback Optimisation of substrate preparation (electropolishing), study of
angle-of-incidence effects, conformal coatings. Measurements of K
• Film structure – defects– Hc1 reduction, hysteretic losses
Towards a bulk-like film: bias sputter deposition, high-energy deposition techniques, high-temperature annealing of films
• Effect of hydrogen– Hydrides formation Measurements of H2 contents, outgassings
• Oxidation– Localized states, corrosion of grain boundaries Al2O3 cap layers
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Electropolishing – Polarization curve
0
500
1000
1500
2000
2500
0
0.05
0.1
0.15
0.2
0.25
0 1 2 3 4 5 6
Cur
rent
den
sity
[A
/m2 ] P
olishing rate [µm
/s]
Cathode potential [-V]
Average roughness R
a [µm
]
Production of Cu(OH)2 on the surface!
Production of O2 bubbles!
Electrical resistance seen by the polishing current:
Diffusion layer ~ 0.1 Bath volume ~ 0.1
(Nb EP bath: 10 times less)Polishing
Standard CuStandard CuElectropolishing:Electropolishing:55% vol. H3PO4
45% vol. butanol
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Numerical modelling of the cathode by simulation of the entire EP process with the Elsy 2D/3D computer code(www.elsyca.be)
Electropolishing – Cathode design
Cathode activeregion
Current density is uniform over all the cell surface
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Measurement of pinholes
Irregularities on the substrate surface shadowing effect film inhomogeneities
2pSQ
C
film
S
p1 p2
Substrate disk
Machining and
cleaning
Film deposition
Substrate removal
He leak rate experiment ‹inc› fraction of leaky
film surface
- equator 9° 4.4 ppm- (~iris 50° 25 ppm)
- equator 9° 0.1 ppm
CP
EP
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Defects in Cu substrate
Electropolished copper surface
• average roughness: 0.02 µm
• A few defects still appearing
Cross section of a copper cavity
• Defects are present inside !
• Not an artifact of the preparation
Thanks to: G. Arnau-Izquierdo
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Variation of properties with incidence angle
Std.Dev. of grey levels of SEM images (CERN 1999)
XRD spectraAFM roughness
From: V. Palmieri, D. Tonini – INFN-LNL
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Incidence angle and residual resistance in low- cavities
Correlation between the incidence angle of the film and the residual resistance, measured on 352 MHz Nb/Cu cavities
0
10
20
30
40
50
60
70
0
50
100
150
200
250
300
35 40 45 50 55 60 65 70 75
Rs
0(4.2K)
Rs
0(2.5K)
Rs
1(4.2K)
Rs
1(2.5K)
Rs0 [
n]
Rs 1 [n
.m/M
V]
Incidence angle [º]It seems there is a “threshold” effect
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Angle of incidence – post magnetron – conformal coating
Niobium cathode
Cavity
B
Magnetic field lines follow the cavity shape
V. Palmieri, R. Preciso, V.L. Ruzinov, S. Yu. Stark, “A DC Post-magnetron configuration for niobium sputtering into 1.5 GHz Copper monocells”, Presented at the 7th Workshop on RF Superconductivity
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Angle of incidence in spherical cavity
20 40 60 80
10
20
30
40
50
60
70
80
PACO cavity INFN Genova
10 20 30 40 50
10
20
30
40
50
60
70
80
Standard 1.5 GHz cavity
Ave
rag
e in
cide
nce
angl
e of
th
eN
b co
atin
g [d
egre
es]
Length along the cavity axis [mm]
Length along the cavity axis [mm]
Ave
rag
e in
cide
nce
angl
e of
th
eN
b co
atin
g [d
egre
es]
The cathode is not point-like
The incidence angleis always >0
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Surface resistance of spherical cavityR
s [n
Ohm
]
Rs
[Ohm
]
1.7 K
Standard 1.5 GHz Nb/Cu cavity
Reducing the angle of incidence does not change Rs(E). However, the angle is always greater than zero, and whether this is creating any effect is only matter of speculation – for the time being
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Thermal impedance film-substrate
Cu (RRR=100) EP 4300 ± 200 Wm-2K-1
Cu EP + 1.5 µm Nb 4100 ± 200 Wm-2K-1
Nb (RRR=180) EP 1200 ± 200 Wm-2K-1
Nb EP + 1.5 µm Nb 1000 ± 200 Wm-2K-1
The overall thermal impedance has been measured for pure Nb and Cu, and for Nb/Cu and Nb/Nb films, on 2-mm thick disks.
Nb (RRR=670)(extrap.) 2500 ± 200 Wm-2K-1
(Still lower than Nb/Cu, but Nb cavities performs better at high field !!)
The thermal impedanceof the film (if existing) has no effect
on Rres at high RF field
Thanks to: G. Vandoni, J-M Rieubland, L. Dufay
T2
T1
H2
Conflat flange
Conflat ring
Sample
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Nb/Nb
1
10
100
0 5 10 15 20 25 30 35 40
Q0 [
109 ]
Eacc
[MV/m]
1p5.1 Nb bulk cavity
1p5.2 Nb/Nb (quench limited)
Thanks to: V. Palmieri, D. Reschke, R. Losito
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Al substrate (better thermal conductivity)
10
100
1000
104
0 1 2 3 4 5
Rs
[nO
hm]
Eacc
[MV/m]
4.2 K
1.7 K
Al 99.999% purity: 10X thermal conductivity of Cu at 4.2 KSpinning + chemical polishing + coating
Thanks to: V. Palmieri, G. Lanza
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Film structure – FIB cross sections
Standard films Oxide-free films
0.5 µm0.5 µm 0.5 µm0.5 µm
Courtesy: P. Jacob - EMPA
Grain size with Focussed Ion Beam micrographs
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Grain size ~ 100 nm
Fibre textureDiffraction
pattern:powder diagram
Grain size ~ 1-5 µmHeteroepitaxy
Diffraction pattern:zone axis [110]
500nm
500nm
TEM views I – plan view
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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(110) fibre texture
substrate plane
HeteroepitaxyNb (110) //Cu(010) Nb (110) //Cu(111)Nb (100) //Cu(110)
500nm
500nm
TEM views II – cross section
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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200K
100K
Crystallographic defects
TEM views III - defects
~100 nm
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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High-energy deposition techniques
• Crystalline defects, grains connectivity and grain size may be improved with an higher substrate temperature which provides higher surface mobility (important parameter is Tsubstrate/Tmelting_of_film)
• However the Cu substrate does not allow heating The missing energy may be supplied by ion bombardment
– In bias sputter deposition a third electron accelerates the noble gas ions, removing the most loosely bound atoms from the coating, while providing additional energy for higher surface mobility
“Structure Zone Model”
– Other techniques allow working without a noble gas, by ionising and accelerating directly the Nb that is going to make up the coating
– These techniques allow also to obtain“conformal” coatings that followthe surface profile better filling voids.
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Nb/Cu bias deposition – First SEM images at CERN
No bias Bias -60 V
Bias -80 V Bias -100 V
5 µm
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Biased sputtering at LNL
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Evaporation + ECR (JLAB)
• Niobium is evaporated by e-beam, then the Nb vapours are ionized by an ECR process. The Nb ions can be accelerated to the substrate by an appropriate bias. Energies in excess of 100 eV can be obtained.
From: A.-M. Valente, G. Wu
Generation of plasma inside the cavity 3 essential components:Neutral Nb vaporRF power (@ 2.45GHz)Static B ERF with ECR condition
Why ECR?No working gas High vacuum means reduced impuritiesControllable deposition energy,90-degree deposition flux (Possible to help control the crystal
structure)Excellent bonding No macro particlesFaster rate (Conditional)
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Evaporation + ECR: results on samples
• Obvious advantage: no noble gas for plasma creation
• Sample tests: good RRR and Tc, 100-nm grain size, lower defect density and smooth surfaces
60eV
90eV 4000X4000 µm2
3-D Profilometer ImagesTEM Images
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Application to cavities (JLAB)
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Plasma Arc (INFN)
• In the plasma arc an electric discharge is established directly onto the Nb target, producing a plasma plume from which ions are extracted and guided onto the substrate by a bias and/or magnetic guidance
• Magnetic filtering (and/or arc pulsing) is also necessary to remove droplets
• A trigger for the arc is necessary: either a third electrode, or a laser
• Arc spot moves on the Nb cathode
at about 10 m/s• Arc current is 100-200 A• Cathode voltage is ~ 35 V• Ion current is 100-500 mA on the
sample-holder (2-10 mA/cm2)• Base vacuum ~ 10-10 mbar• Main gas during discharge is
Hydrogen (~ 10-7 mbar)• Voltage bias on samples 20-100 V From: R. Russo, A. Cianchi, S. Tazzari
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Plasma Arc – Need for filtering
Arc source
Nb droplets
5 µm
10 µm
Magnetic filter
Nb droplets
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Planar arc – cavity deposition set-up
New ideas are put forward for using a planar arc for cavity coating
5 µm
10 µm
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Planar arc – RF measurements on samples!
Cu samples with Nb ARC-coating. Used as a baseplate of 6 GHz cavity operating in the TE011 mode. At low field, the surface resistance is in the range 3÷6 µOhm, as compared to the BCS Rs of 0.22 µOhm at 2.2 K and small mean free path. The Q remained constant up to a field of 300 Oe.
A baseline of 2.2 µOhm is measured with this cavity with a solid Nb plate
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Liner arc for cavity deposition (Soltan Institute)
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HPPMS: advantages
From W. Sproul, AEI
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Principle: high power pulses of short duration– Peak value typically 100 times greater than conventional
magnetron sputtering– Pulse width of 100 - 150 µsec, repetition rate ~50 Hz– Peak power densities of 1-3 kW/cm2
– Discharge voltages of 500-1000 V– Peak current densities ~ A/cm2
Consequences– High degree of target material ionization– High secondary electron current– Promotes ionization of sputtered species– Can approach 100%, vs. ~1% for conventional sputtering– An applied bias allows attracting ions to the substrate
Operating principle
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Ion density
From W. Sproul, AEI
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HPPMS coatings in trenches (valid for any ion coating technique)
J. Alami et Al, J.Vac.Sci.Technol. A23(2005)278
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First CERN results
Pulse voltage (-500V/div)
Pulse current (100A/div)
Current on sample (-100 V bias, 1A/div)
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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First SEM pictures (all films ~100 nm thick)
Substrate floating
Substrate grounded
Substrate biased -100V
Note: poor substrate preparationUnfortunately no new experiments were possible since last year
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Films as a bulk – Hydrogen becomes a problem!
~1 µm grain size, RRR 28
~5 µm grain size, RRR 55
0.1 µm grain size, RRR 11
H2 content is ~ 0.1 at. % for sputtered Nb/Cu films (in niobium bulk it is0.02 at.%) and it is picked up from vacuum system during deposition.
A possible solution: high-temperature annealing, but it does not work with copper cavities. Proposal (L. Hand, W. Frisken): molybdenum cavities.
There are more differences between these Nb/Cu films than those listed,
this is just a basis for reflection
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Film as a bulk – H2 measurements
From: L. Hand, Cornell U. – W. Frisken, York U.
There are new results on measurements of H2 content by measuring the lattice parameter and the total impurity content
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Grain boundaries and surface oxidation
Famous drawings by Halbritter. Several effects might take place: ITE, flux penetration, Hc1 depression, lower Tc, etc.
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Possible solution – Al2O3 cap layers
• Technique routinely used for S-N-I-S Josephson junctions: a 5-nm thick Al layer is deposited onto the Nb base electrode, and let oxidize in air. Most of it is transformed to Al2O3 but some remains metallic.
• It is important to prevent any surface contamination of Nb prior to Al coating, to reduce the coalescence of the Al atoms.
• Other possible solution: NbN overlayer (J. Halbritter)
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
Thickness (nm)
Elem
ent "
conc
entra
tion"
(at.
%)
Al 2p
Nb 3d5
O 1s
EAl = 8.7 nm
EO = 2.4 nm
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
Thickness (nm)
Elem
ent "
conc
entra
tion"
(at.
%)
Al 2p
Nb 3d5
O 1s
EAl = 12.8 nm
EO = 2.3 nm
5 nm Al (nominal) 10 nm Al (nominal)
XPS depth profile
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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State-of-the-art 20 years ago
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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State-of-the-art at 1500 MHz – 1.7 K – single cell
1
10
100
1000
0 5 10 15 20 25
Rs
[nO
hm]
Eacc
[MV/m]
Q 1x1010 @ 15 MV/m
Q 3x109 @ 20 MV/m
28 MV/m reached in a large LEP cryostat (He evaporation at large power)
9 October 2006 Sergio Calatroni - CERN - Sputtering Technology
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Conclusion
• In an ideal world niobium films would be the best solution for accelerating cavities at any beta.
• However they are presently not competitive for reaching the highest fields because of the increase of Rres: WE MUST STRUGGLE AND UNDERSTAND WHY!
• Films are still a valuable option for lower fields and operation at 4.2 K
• The technique of choice is at present sputter deposition: a prerequisite for it is substrate design and its preparation
• Four proposed research lines:– Substrate effects– New deposition techniques (“energetic”)– Hydrogen effects– Cap layers
• Several of the above research lines are interdependent
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