INVESTIGATION OF NUCLEATION STEP IN THE TWO-STEP
DIFFUSION COATING OF Nb3Sn on Nb
Uttar Pudasaini
11-30-2016
3rd TTC Film Working Group Meeting
Outline
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Motivation
Coating Process
Nucleation Studies
Summary
Motivation
• Nb cavities are approaching the
intrinsic material limit and are
expensive to run.
• Higher Tc and Hsh of Nb3Sn promises
potential cavity operation at 4.2 K
and higher Eacc.
• Researched since 1970s, it is a
challenging material for cavity
fabrication.
Nb
Nb3Sn
Tc
9.2 K
18.3 K
Hsh
~200 mT
~400 mT
S. Posen and M. Liepe, LINAC 2014, TUIOC03
Nb3Sn Coating Process
• Tin vapor diffusion is widely
attempted.
• Two steps: nucleation followed by
deposition.
*schematic
(a)
Nb substrate covered
with oxide layer
(b)
SnCl2 evaporates early
on, depositing tin sites
Deposition
(d)
3 hr
Nucleation
(c)
Oxide layer disappears ,
tin comes in contact with
Nb.
(c)
(b)
1 hr
(a)
Nb3Sn coating at Jlab
(d)
Nb3Sn phase formation
and grain growth
Evolution of Nucleation Step
• Nucleation problem: Researchers at
Siemens AG (1970’s) found Nb spots not
covered with Nb3Sn film.
• Solution: Pre-anodize or use tin halides
• Tin halide (high vapor pressure) initiates
Nb-Sn nucleation early by more availability
of tin
• University of Wuppertal, Cornell, Jlab, etc.,
but no details on:
– What happens after the nucleation step?
– How does it help the final coating?
– Can we alter the final coating by changing
parameters of the nucleation step?
Patchy area with thin coating is
commonly observed.
Experimental Studies
• Nucleation Experiments
– Characteristics of nucleated surface
– Effect of changing nucleation parameters
• Experimental setup
~2 mg/cm2 of SnCl2
0.5 g 99.99%
SnCl2 per pkg.
1 g 99.9999%
Sn per pkg.
Nucleation Time Variation @ 500oC
5 min @ 500oC
SEM/EDS : < 10% Sn
coverage
1 hour @ 400oC
1 hr @ 500oC
4 hr @ 500oC
Nucleation Temperature Variation
1 hour @ 450oC
1 hour @ 500oC
2 mg/cm2 of SnCl2
Nb
Sn
XPS shows more tin ! No Chlorine.
Supplementary Analysis
U45.2.map: Sample U45
2015 Dec 16 10.0 kV 0 nA FRR
Company Name
8.19 min
Sn1
1 hr @
JLAB
500oC
411.3
Sn1
2 µm
More tin !
2 µm
Sn
Particles
SAM tin mapping
0.0

AFM
Tin film + tin particles
Low amount of SnCl2
6 ug/cm2 of SnCl2 instead of 2 mg/cm2
SAM tin mapping
Amount of SnCl2 is not crucial for tin coverage.
Conclusion and Future Work
 SnCl2 deposits tin particles and tin film on the niobium
surface.
 No Chlorine.
 Larger amount of SnCl2 results in bigger tin particles.
 What is role of tin particles vs. tin film?
 Can we modify the final coating by changing nucleation
parameters?
In progress
Acknowledgement




Michael J. Kelley
Grigory Eremeev
Charlie Reece
Anne-Marie Valente-Feliciano
Growth modes
Frank–van der Merwe (FM: layer-by-layer)
substrate
Volmer–Weber (VW: island formation)
substrate
Stranski–Krastanov (SK: layer-plus-island)
substrate
Model Calculation
hν = 1486.7 eV
θ
eSn
Nb
(A)
90%
K.E. (3d5/2)Sn = E(Al Kα) – Eb = 1486.7 – 484.9 = 1001.8 eV
(B)
For simplicity,
Nsn
3.708 x 1022
Monolayer of Sn on Nb
NNb
5.55 x 1022
(3d5/2)
2.316 nm
Monolayer thickness (d)
0.29 nm
10%
For θ = 0; Sn: Nb = 0.089 =>~8% Sn (for surface with monolayer of Sn on Nb)
Total tin = 9/10*8 (A) + 1/10*100 (B) = 18%
XPS measurements: 20-30% Sn
Sn particles on Nb