point contact tunneling spectroscopy and atomic layer deposition for superconducting rf cavities...
TRANSCRIPT
Point contact tunneling spectroscopy and Atomic layer deposition for superconducting rf cavities
Thomas Prolier, Mike Pellin, Jim Norem, Jeff Elam.
Collaboration:-Jlab: P. Kneisel, G. Ciovati-Fermilab: L.Cooley, G. Wu, C. Cooper
- IIT: J. Zasadzinki
Superconducting Radio-Frequency (SRF)
Department of Energy – Office of Science• DOE-OS is in the particle accelerator business (ILC ($19B), RIA($0.4 B),
NSLS-2($0.5B) , SNS($1.8B), APS, APS-ERL, etc.)
• Orbach to HEPAP 2/22/07 “DOE is committed to continuing a vigorous R&D program of accelerator technology SCRF is a core capability having broad applicability, both to the ILC and to other future accelerator-based facilities as well. Out FY2008 request for ILC R&D and SCRF technology confirms this commitment
1 m15 km • 30 km of ultra pure Nb bellows
• 2 K• very high electrical and magnetic fields
3
Outline
Performance limitations: Point contact spectroscopy: a probe of the surface superconductivity.
Atomic Layer Deposition: synthesizing new materials and application to RF cavities.
Niobium surfaces are complex, important, and currently poorly controlled at the nm level
4
45
nm
RF
dep
thInclusions,
Hydride precipitatesSurface oxide Nb2O5 5-10 nmMagnetic!
Interface: sub oxides NbO, NbO2
often not crystalline (niobium-oxygen
“slush”)
Interstitials dissolved in
niobium (mainly O, some C, N, H)
Grain boundaries
Residue from chemical
processing
Clean niobiume- flow only in the top 45 nm
Probe the surface superconductivity
Point Contact Tunneling (PCT) Spectroscopy – a Surface Probe of Nb superconductivity
5
6 Tesla magnet 1.6-300 K
2-
Ideal BCS superconductor
• Measure of the superconducting gap Δ
• The ZBC value -> Number of normal electron
Normal electrons in gap => dissipation and lower Q
PCT: 1- insight into Mild Baking Procedure Improvement: Small changes in O stoichiometry -> Magnetic Oxide reduction
Unbaked Niobium
T.Proslier, J.Zasadzinski, M.Pellin et al. APL 92, 212505 (2008)
Cavity-grade niobium single crystal (110)-electropolished
ILC-Single crystal cavities P.Kneisel
Qo improvement - 1.6
Average ZBC ratio = 1.6
2-
Ideal BCS, T=1.7KBaked Niobium 120C-24h
Cavities have dissipative losses due to Cooper pair breaking!-> Nb2O5--, NbO2--
7
PCT: 2- Hot and cold spots in SRF cavity (from J-lab)
Anomalous spectrumOnly on hot spots
“Normal” spectrum
Hot spots: show dissipative behaviorHigher ZBC and anomalous spec.lower gap values (1.3<∆<1.55)
Cold spots: “normal” dissipationLow ZBC values Normal gap values (1.5<∆<1.55)
Origin of peculiar spectrum and dissipation?
Correlates with cavities results! (once again)
8
PCT: 2 Hot and cold spots in SRF cavity, Origin
Temp. dep: peak at 0 mV bias increases Killing superconductivity by applying a mag. Field
High bias peak:LOSSES!!
fits with Appelbaum theory -> Magnetic impurities in the oxides !!J>0 -> antiferromagnetic coupling
-First time measured on Nb oxides
-Same behavior observed on unbaked Nb coupons !
IIT: EPR revealed magnetic moments tooFSU: theory, dissipation and magnetism
To be published
How to make better cavities
Add a better dielectric (thanks to Intel) and bake
O2
O
Atomic layer deposition (ALD)
Al2O3(2nm)NbOx
Nb
Heating -> Reduction + diffusion of the oxides
Baking, but now protected form O (Al2O3)
- ∆ (1.55meV = Nb).
- Γ (dissipation)
- 500 oC bake should significantly reduce dissipation
Th.Proslier, J. Zasadzinski, M.Pellin et al. APL 93, 120958 (2008)
11
Cavities used for ALD
Jlab (P. Kneisel) provided four different niobium cavities to ANL for atomic layer deposition: Cavity 1:
Material: RRR > 300 poly-crystalline Nb from Tokyo-DenkaiShape/frequency: Earlier KEK shape, 1300 MHzBaseline: electropolished, in-situ baked
Cavity 2 :Material: RRR > 300 large grain Nb from Tokyo-DenkaiShape/frequency: TESLA/ILC shape, 1300 MHzBaseline: BCP, in – situ baked
Cavity 3:Material: RRR > 300 poly-crystalline Nb from FansteelShape/Frequency: CEBAF shape, 1497 MHzBaseline: BCP only
Cavity 4: Shape/Frequency: CEBAF Single cell cavityBaseline: BCP + 600oC UHV bake.
J Lab Cavity 1:After ALD Synthesis (10nm Al2O3 + 3nm Nb2O5), 250oC
Only last point shows detectable field emission. 2nd test after 2nd high pressure rinse. (1st test showed field emission
consistent with particulate contamination)
108
109
1010
1011
Quench @Eacc = 32.9 MV/m
Q0
Eacc [MV/m]0 5 15 20 25 30 3510
Atomic Layer Deposition (10 nm Al2O3 + 3 nm Nb2O5)
Previous Best Cavity Performance (Initial Electro-Polish and Bake)
Cavity As Received For Coating
Single Cell Cavity Test (J Lab 6/27/08)Argonne Cavity Coating Procedure
J lab Cavity 2: Large grain,10 nm Al2O3 + 3 nm Nb2O5 (250oC)
13
Second coating: 5 nm Al2O3 + 15 nm Nb2O5
First coating: 10 nm Al2O3 + 3 nm Nb2O5
Baseline Test 2 Test 1
J Lab Cavity 3: Annealing 450C/20hrs + Coating: 5nm Al2O3+15 nm Nb2O5
14
High Temp. baking: T maps and Rs(T)
T-map at the highest field measured during the test after 120 °C, 23 h UHV bake.
T-map at the highest field measured during the test after 450 °C, 20 h heat treatment
10
100
1000
0.22 0.27 0.32 0.37 0.42 0.47
1/T [1/K]
Rs [
nW]
Add. HPR
120C/23h UHV bake
450C/20h HT
Treatment D/kTc ℓ (nm) Rres (nW)
Add. HPR 1.866 ± 0.018 19 ± 44 16.0 ± 0.8
120 °C/23 h bake 1.879 ± 0.005 18 ± 55 16.3 ± 0.5
450 °C/20 h HT 1.911 ± 0.026 58 ± 17 93.8 ± 0.2
Ohmic losses
But HT baking: Improve the super. properties
Preliminary Conclusion and High temp annealing
The ALD process is compatible with SRF cavity processing
Promising if one thinks about multi-layer coatings ( A. Gurevich).
development of the process is necessary
The appearance of multipacting in cavity 1 and 2 is concerning, but can
be overcome by additional coating.
Baking doesn’t improve cavity performance: cracks can appear due to
strong Nb oxide reduction -> path for oxygen injection -> Ohmic losses
need a in-situ baking + ALD coating set up.
16
17
ALD Can Produce Layered SRF Structures with significantly higher Hc1
than Nb
Build “nanolaminates” of superconducting materials
~ 10- 100 nm layer thicknesses with 10 nm Alumina Between.
Hc1 Enhancement Scales with:
~Tc,laminate/Tc,base
For NbN laminate layers -> ~1.5 HC1 enhancement
50 MV/m -> 75 MV/m
Nb, Pb
Insulating layers
Higher-TcSC: NbN, Nb3Sn, etc
SEM XPS XRR-RBS
SQUIDRRR
New materials by Atomic Layer Deposition:
NbF5 + Si2H6 = NbSi + reaction product, copy on : WF6 + Si2H6 = W + RP
On Si (100): NbSi superconductor 3.1KOn Quartz: Nb3Si5On MgO: NbSi2Elastic stress in the film-mismatch ?
Nb3Si superconductor at 18KVary substrate and growth conditions Post-annealing studies
Model for A15 compounds: Nb3Ge (20K): NbF5 + Ge2H6 = NbGe + reaction products
Etc… To be published
Fast growth rate:2.5 Å/Cy
Grows only WNot on oxides
New material by Atomic Layer deposition
New precursor NbF5 for NbN, Nb2O5 grows much faster!
19
Zinc pulse growth for NbN and TiN:
NbCl5 + NH3 + Zn = NbN + ZnCl2 + HCl
TiN films: resistivity ρ=50 µΩ.cm for 10 nm films! (350 without Zn)
NbN films: resistivity ρ=200 µΩ.cm (450 without Zn -> Tc= 5.5 K), same ρ for sputtered film with Tc=16K!
To be measured:
-Superconducting properties with Zn pulse -> Multilayers-Vary the substrate (Sapphire’s) to match lattice parameter (epitaxial growth?)-Post annealing in controlled atmosphere
No studies of superconductivity by ALD and interactions substrate-films Phase space of parameter to study is large
Magnetic impurities as a possible explanation for RF dissipation: Mild baking effect Hot spots Origin = Oxides, vacancies?
High temperature baking works on samples but not yet on cavities ALD a tool for building new materials
Compatible with RF cavities NbN, NbSi, TiN etc… Plasma ALD
Summary
New task force:- Postdocs and students -> Accelerate the process
outlook
21
(1)Nb deposition on Nba) New Cavity Designs b) Enable Continuity of Superconducting Surface (fewer perfect welds)
(2)Other layered structuresa) Reward : Performance far beyond NbN b) Risk: New ALD Synthesis Methods Need to be developed with
semiconductor impurity levels.
(3) Nb deposition on alumina coated Cua) Reward : Significant Cost Reductions for Materials, Fabrication, and
Coolingb) Risk: Dissimilar materials require stress management (Cu is bad,
alumina is better)
(4) Field emission for warm and cold cavitiesParticulate tolerant?
ALD Reaction Scheme
• ALD involves the use of a pair of reagents.• each reacts with the surface completely• each will not react with itself
• This setup eliminates line of site requirments
• Application of this AB Scheme• Reforms the surface• Adds precisely 1 monolayer
• Pulsed Valves allow atomic layer precision in growth
• Viscous flow (~1 torr) allows rapid growth• ~1 mm / 1-4 hours
0
500
1000
1500
2000
2500
3000
3500
4000
0 500 1000 1500 2000 2500 3000AB Cycles
Th
ick
nes
s (Å
)
Ellipsometry Atomic Force Microscopy
• Film growth is linear with AB Cycles• RMS Roughness = 4 Å (3000 Cycles)• ALD Films Flat, Pinhole free
Mixed Oxide Deposition: Layer by Layer
Mixed Layer Growth• Layer by Layer• note “steps”• atomic layer sequence
“digitally” controlled
• Films Have Tunable Resistivity, Refractive Index, Surface Roughness, etc.
[(CH3)3Al // H2O]
100 nm
ZnO
ZnO
Al2O3
Al2O3
[(CH3CH2)2Zn// H2O]
• Mixed Layers w/ atomic precision• Low Temperature Growth• Transparent• Uniform• Even particles in pores can be
coated.
ALD: The Only Viable Method for SRF Surface Control!
25
Niobium is from a surface scientists point of view a difficult material to deal with.– Extremely reactive.– Native Oxide is complex and passivates poorly
Semiconductor Industry – a clue– Silicon is reactive but oxide is simple and
passivates well (but has a low dielectric constant)– Gate dielectric oxides are now being used on Si
metal (and being produced by ALD
20 m2 / batch) Grow a dielectric oxide with superior properties to the
Niobium Oxides– Simple - non-interactive with the sc layer– Passivating (stable surface, protective of the Nb
metal underneath)– Parallel Growth Method Entirely adaptable to SRF
Si
HfO2
Epoxy
ALD Thin Film Materials
A Solution? Atomic Layer Deposition -> non-dissipative dielectric layer
27Mike Pellin
1. Use Atomic Layer Deposition (ALD) to synthesize a dielectric diffusion barrier on the Nb surface
2. Bake cavity to “dissolve” the O associated with the Nb layer into the bulk
Nb NbO
Nb2O5--
NbO2
Al2O3
Nb
Al2O3
ALD coated + Baking > 450°C
Mild baked before ALD
Test
Cavity 4: to be coated by SiN + NbSi (below 200oC)
28SRF 2009
Understanding Cavity Eacc and Q
29
Q-slope problem
Rs = RBCS + Rres
RBCS = C-4-2l exp(--/kT)
Experimental Goals:• Measure - at the surface• Tunneling Spectroscopy is ideal
P.Kneisel et al. 12 th SRF workshop Cornell 2005B.Visentin SRF workshop 2003G.Ciovati, P.Kneisel, A.Gurevich PRST, 10 2007
C.Antoine SRF workshop 2004
H(-)
Nb NbOx NbO Nb2O5-δNbO2
-
B(r)
Surface
Q-slope disappears, Q0 increased
SRF Impedance is a surface effect (-~45 nm, Nb) depends on the energy gap at the surface altered by proximity effects, magnetic scattering.
SRF 2009
0 5 10 15 20 25 30 351.0E+08
1.0E+09
1.0E+10
1.0E+11
1.3 GHz Cavity, KEK Shape
450C for 20 hrs ALD + HPR
Eacc [MV/m]
Q0
Quench or discharge?
Quench @ Eacc = 32.9 MV/m
J-lab cavity 1 + HT annealing (450oC for 20 hrs).
30SRF 2009
J Lab Cavity 3: Small grain 2 steps Coating, first: 15 nm Al2O3 at 90oC
31SRF 2009