SRF Material Science - Fermilab

kitefleaUrban and Civil

Nov 15, 2013 (3 years and 6 months ago)

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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 m

15 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 depth

Inclusions,

Hydride precipitates

Surface oxide

Nb
2
O
5

5
-
10 nm

Magnetic!

Interface: sub oxides
NbO, NbO
2


often not crystalline

(niobium
-
oxygen
“slush”)

Interstitials
dissolved in
niobium (mainly O,
some C, N, H)

Grain boundaries

Residue from
chemical
processing

Clean niobium

e
-

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⸶

䅶敲慧攠婂䌠牡瑩漠r 1⸶

2


Ideal BCS,
T=1.7K

Baked Niobium 120C
-
24h

Cavities have dissipative losses
due to Cooper pair breaking!

-
>
Nb
2
O
5
-

, NbO
2
-



7

PCT: 2
-

Hot and cold spots in SRF cavity (from J
-
lab)

Anomalous spectrum

Only on hot spots

“Normal” spectrum

Hot spots: show dissipative behavior


Higher ZBC and anomalous spec.


lower gap values (1.3<∆<1.55)

Cold spots: “normal” dissipation


Low 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 too

FSU: theory, dissipation and magnetism

To be published

How to make better cavities


Add a better dielectric (thanks to Intel) and bake

O
2

O

Atomic layer deposition (ALD)

Al
2
O
3
(2nm)

NbO
x

Nb

Heating
-
>

Reduction + diffusion of the oxides

Baking, but now protected form O (Al
2
O
3
)

-

∆ (1.55meV = Nb).

-

Γ

(dissipation)

-

500
o
C 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
-
Denkai


Shape/frequency: Earlier KEK shape, 1300 MHz


Baseline: electropolished, in
-
situ baked


Cavity 2

:

Material: RRR
>

300 large grain Nb from Tokyo
-
Denkai

Shape/frequency: TESLA/ILC shape, 1300 MHz

Baseline: BCP, in


situ baked


Cavity 3
:


Material: RRR
>

300 poly
-
crystalline Nb from Fansteel


Shape/Frequency: CEBAF shape, 1497 MHz


Baseline: BCP only


Cavity 4:



Shape/Frequency: CEBAF Single cell cavity


Baseline: BCP + 600
o
C UHV bake.


J Lab Cavity 1:After ALD Synthesis (10nm Al
2
O
3

+ 3nm Nb
2
O
5
), 250
o
C


Only last point shows detectable field emission.


2
nd

test after 2
nd

high pressure rinse. (1
st

test showed field emission
consistent with particulate contamination)

1
0
8
1
0
9
1
0
1
0
1
0
1
1
Q
u
e
n
c
h
@
E
a
c
c
=
3
2
.
9
M
V
/
m
Q
0
E
a
c
c
[
M
V
/
m
]
0
5
1
5
2
0
2
5
3
0
3
5
1
0
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A
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3
+
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O
5
)
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J lab Cavity 2: Large grain,10 nm Al2O3 + 3 nm Nb2O5 (250
o
C)

13

Second coating: 5 nm Al
2
O
3

+ 15 nm Nb
2
O
5

First coating: 10 nm Al
2
O
3

+ 3 nm Nb
2
O
5



Baseline

Test 2

Test 1

J Lab Cavity 3: Annealing 450C/20hrs + Coating: 5nm Al
2
O
3
+15 nm Nb
2
O
5



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]
R
s
[n
W
]
Add. HPR
120C/23h UHV bake
450C/20h HT
Treatment


/kT
c



(nm)

R
res

(n
W
)

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 H
c1

than Nb


Build “nanolaminates” of
superconducting materials


~ 10
-

100 nm layer
thicknesses with 10 nm
Alumina Between.


H
c1

Enhancement Scales
with:


~T
c,laminate
/T
c,base





For NbN laminate
layers
-
> ~1.5 H
C1

enhancement


50 MV/m
-
> 75
MV/m

Nb, Pb

Insulating

layers

Higher
-
T
c
SC:
NbN, Nb
3
Sn, etc

SEM

XPS

XRR
-
RBS

SQUID

RRR

New materials by Atomic Layer Deposition:


NbF
5

+ Si
2
H
6

= NbSi + reaction product,
copy on : WF
6

+ Si
2
H
6

= W + RP

On Si (100): NbSi superconductor 3.1K

On Quartz: Nb
3
Si
5

On MgO: NbSi
2

Elastic stress in the film
-
mismatch ?

Nb
3
Si superconductor at 18K

Vary substrate and growth conditions

Post
-
annealing studies

Model for A15 compounds:

Nb
3
Ge (20K): NbF
5

+ Ge
2
H
6

= NbGe + reaction products

Etc…

To be published

Fast growth rate:

2.5
Å/Cy


Grows only W

Not on oxides

New material by Atomic Layer deposition

New precursor NbF
5

for NbN, Nb
2
O
5

grows much faster!

19

Zinc pulse growth for NbN and TiN:


NbCl
5

+ NH
3

+ Zn = NbN + ZnCl
2

+
HCl


TiN films: resistivity
ρ
=50 µ
Ω
.cm

for 10 nm films! (350 without Zn)


NbN films: resistivity
ρ
=200 µ
Ω
.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

Nb

a)
New Cavity Designs

b)
Enable Continuity of Superconducting Surface (fewer perfect welds)


(2)
Other

layered

structures

a)
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

Cu

a)
Reward : Significant Cost Reductions for Materials, Fabrication, and
Cooling

b)
Risk: Dissimilar materials require stress management (Cu is bad,
alumina is better)


(4)

Field emission for warm and cold cavities

Particulate 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
m
m / 1
-
4 hours

0

500

1000

1500

2000

2500

3000

3500

4000

0

500

1000

1500

2000

2500

3000

AB Cycles

Thickness (
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.

[(CH
3
)
3
Al // H
2
O]

100
nm

ZnO

ZnO

Al
2
O
3

Al
2
O
3

[(CH
3
CH
2
)
2
Zn

// H
2
O]



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 m
2

/ 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

HfO
2

Epoxy

ALD Thin Film Materials

A Solution? Atomic Layer Deposition
-
>




non
-
dissipative dielectric layer

27

Mike 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

Nb
2
O
5
-


NbO
2

Al
2
O
3

Nb

Al
2
O
3

ALD coated + Baking > 450
°
C

Mild baked before ALD

Test

Cavity 4: to be coated by SiN + NbSi (below 200
o
C)

28

SRF 2009

Understanding Cavity E
acc

and Q

29

Q
-
slope problem

Rs = R
BCS
+ R
res

R
BCS

= C

4

2
l

exp(
-

/kT)

Experimental Goals:


Measure


at the surface


Tunneling Spectroscopy is ideal

P.Kneisel et al. 12 th SRF workshop Cornell 2005

B.Visentin SRF workshop 2003

G.Ciovati, P.Kneisel, A.Gurevich PRST, 10 2007

C.Antoine SRF workshop 2004


H(

)

Nb

NbOx

NbO

Nb
2
O
5
-
δ

NbO
2



B(r)

Surface

Q
-
slope disappears, Q
0

increased


SRF Impedance is a surface effect (

~4㔠湭ⰠN戩
depends on the energy gap
at the surface altered by proximity effects, magnetic scattering.

SRF 2009

1.0E+08
1.0E+09
1.0E+10
1.0E+11
0
5
10
15
20
25
30
35
Q
0

Eacc [MV/m]

1.3 GHz Cavity, KEK Shape

450C for 20 hrs
ALD + HPR


Quench or discharge?

Quench @
Eacc

= 32.9 MV/m

J
-
lab cavity 1 + HT annealing (450
o
C for 20 hrs).

30

SRF 2009

J Lab Cavity 3: Small grain 2 steps Coating, first: 15 nm Al
2
O
3
at 90
o
C

31

SRF 2009