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The Role Of Thin Film In
Modern Particle Accelerator
Technology
Reza Valizadeh,
on behalf of ASTeC Vacuum Science
Group
Motivation
Higher Accelerating Gradient to shorten the
length of Accelerator
Low outgassing and ESD and hydrogen
diffusion barrier and providing additional
Pumping
Lowering Secondary Electron emisson to
mitigate e-Cloud and Multipacting.
SC Advantages
Power consumption is much less operating cost savings,
better conversion of ac power to beam power
.
•CW operation at higher gradient possible Less klystron power required
capital cost saving
•Need fewer cavities for CW operation Less beam disruption
Superconducting RF Cavities
Nb has the highest Tc among all pure metals and the highest
lower critical field Hc1≈170 mT (2K) among all
superconductors
Advantages of Niobium Thin Film Cavity
Thermal stability
Bulk niobium has a typical heat conductance of about 75W/mK if the purity is high, while the
copper is as high as 300 – 2000 W/mK. For a typical hot spot inside a SC cavity, say, 50-micron
Low cost
The copper is a lot cheaper than niobium, about 1/10 the cost of the niobium. Since the
dimension of the RF cavity is inversely proportional to its resonant frequency, a typical
500MHz cavity needs more than 9 times the material required for a 1,500MHz cavity with a
same cavity thickness.
Thin film is potentially free of undissolved inclusions mostly seen in bulk material
This is mostly a metallurgical issue. During the pressing, rolling and melting, etc., the
niobium sheet or the copper sheet will have some kind of micro-inclusions.
Insensitivity to earth magnetic field trapping
Residual resistance of the niobium surface is caused by many different sources. For an ideally
cleaned cavity, the residual resistance can be caused by trapped magnetic flux, hydrogen
dissolved in the surface layer, surface oxides and even some surface roughness. The residual
resistance of thin film caused by trapped magnetic field can be up to 10 times less than that of a
bulk niobium cavity.
Deposition Facilities and Parameters
Magnetron sputtering from a single
target of correct stoichiometry
(prepared by powder sintering)
Stoichiometry,
Substrate Temperature, (High Ts/Tm,
Ts is the substrate temperature, Tm is
the melting point of depositing
material.)
Deposition Pressure,
Deposition Rates,
Substrate Bias,
Deposition Power
Concurrent Ion Bombardment can be
varied independently
Niobium Pentachloride (V):
• Chosen to obtain metallic Nb layer • Reacts with plasma of H+ to create thin film • Crystalline solid, vapour pressure at 95 – 100 °C to
perform ALD • Very sensitive to moisture, hydrolyzes in NbOCl3
• Requires high substrate temperature to reduce Cl contamination in the film (at least 500 °C)
Tris(diethylamido)(tert-butylimido)niobium (V) • Chosen to obtain NbN layer • Reacts with N2 plasma to create thin film • Liquid, good vapour pressure at 70 °C to perform
ALD • Sensitive to heating, start decomposing at 130 °C • Doesn’t require a high deposition temperature
(250 °C) Suitable for deposition on copper
Initial results samples deposited without a bias
samples deposited with a 50-V bias
Surface Composition and Chemical State
XPS analysis
C 1s
O 1s
Nb 3d5/2
Nb 3d3/2
Nb 3p3/2
Nb 3p1/2
Nb 3s
As received
Ar+ bombarded
x 10 2
10
20
30
40
50
60
70
80 C
PS
600 500 400 300 200 100 0
Binding Energy (eV)
Superconductivity evaluation: RRR
0
5
10
15
20
100 600
Gro
wth
Ra
te (
nm
/min
)
Power (W)
0 V
2 RRR 22 for 70 samples studied.
Samples with RRR ≥ 10 were deposited
with 300 ≤ P ≤ 600 W.
In all cases
RRR is higher for a biased
substrate than with unbiased
However, increasing the bias
further does not always result
with increasing RRR
Film growth rate increases as a function of
power.
Growth rate decreases with biased substrate.
Decrease in film growth with bias is due to
either re-sputtering or fewer voids in denser
films
Superconductivity evaluation: DC SQUID
A typical DC magnetic susceptibility
measurement with the sample parallel
to the magnetic field.
HC2 increases with RRR at T = 6 K
Hc1
Hc2
Hsmp
Substrate
Layer 1: Nb
Layers 2,4,6,8:
MgO
Layers 3,5,7,9:
NbN
Pt
Effect Of Substrate On Epitaxial Growth
Layer 2:
MgO?
Layer 4:
MgO?
Layer 1:
Nb?
Layer 3:
NbN?
Layer 5:
NbN?
Effect Of Substrate On Epitaxial Growth
PECVD Deposition of Nb
Successfull deposition of Nb metal over copper substrate
Preliminary EDX data show lack of Cl in the film, thanks to high deposition
temperature
Low Outgassing And Additional
Pumping Surfaces
What NEG coating does
Reduces gas desorption:
– A pure metal film ~1-m
thick without contaminants.
– A barrier for molecules from
the bulk of vacuum chamber.
Increases distributed pumping
speed, S:
– A sorbing surface on whole
vacuum chamber surface
S = Av/4;
where – sticking probability,
A – surface area,
v – mean molecular velocity
Vacuum NEG Subsurface Bulk
Coating Layers
Quaternary NEG alloy film deposited on Si test
sample from twisted Ti, V, Zr, and Hf wires.
Cylindrical Magnetron: Power = 60 W, PKr > 10-1 mbar, deposition rate = 0.12
nm/s, T = 120°C.
Quaternary NEG alloy film deposited on Si test
sample from twisted Ti, V, Zr, and Hf wires.
Cylindrical Magnetron: Power = 60 W, PKr = 5x10-2 mbar, deposition rate = 0.12 nm/s,
T = 120°C. Very glassy structure.
17
XRD of Film deposited from Ternary and
Quaternary alloy wire as target.
In Both cases there is only one broad peak near 2 = 36.8°
The film is nearly amorphous.
18
TiZrVHf film deposited on Si by cylindrical magnetron using Alloy wire
Region scan of XPS core levels of Ti, Zr, C,Hf and V of a Ti-Zr-V-Hf film
(surface composition and chemical bounding)
Quaternary alloy pumping properties
Ti-Zr-Hf-V is the best
Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and Zr
are comparable
Ti-Zr-V is lower
Zr-V (best binary alloy) has the
lowest activation temperature
140 160 180 200 220 240 260 280 300 3200.01
0.1
1
CO
stic
king
pro
babi
lity
140 160 180 200 220 240 260 280 300 3201 10
4
1 103
0.01
0.1
Ti-Zr-Hf-V
Hf-Zr-V
Ti-Zr-Hf
Ti-Hf-V
Ti-Zr-V
Ti-Zr
Zr-V
Zr
Activation temperature [ C]
H2
stic
king
pro
babi
lity
140 160 180 200 220 240 260 280 300 3200.01
0.1
1
10
CO
pum
ping
cap
acity
Mitigation Of e-Cloud And
Multipacting
Mitigation of the electron cloud built up due to
Photoemission and Secondary electron (beam instability,
beam losses, emittance growth, reduction in beam life time,
or additional heat loads on cryogenic vacuum chamber)
Mitigation of multipacting in RF wave guide and space
related high power RF hardware.
How ?
Reduce The Secondary electron Yield:
By Changing surface Chemistry
(deposition of lower SEY material)
By Engineering the surface
roughness
Mixture of the above
Existing Mitigation method
1. Coating with Low SEY Material
Ca Normal
coating
Ti-Zr-V-Hf Ti-Zr-Hf-V-N a-C at CERN
Existing Mitigation method
• Coating with a low SEY material with
submicron size structure
Ti-Zr-V black Ag plating, ion etched with Mo Mask
I. Montero et.al, Proc. e-Cloud12
Existing Mitigation method
• Modifying the surface geometry (making
mechanical grooves)
By courtesy of Y. Suetsugu
By A. Krasnov and
By L Wang et.al
• By Implementing weak solenoidal fields to trap
the electrons
• Using clearing electrode
• Using combinations of the above
Laser Treated Metal Surface
Aluminium Stainless Steel Copper
Nd:YVO4 Laser
Pulse length =12 ns at Repetition Rate = 30 kHz
For Aluminium
Max Average Power = 20 W at =1064 nm
For Copper
Max Average Power = 10 W at 532 nm
Argon or air Atmosphere
Beam Raster scanned in both horizontal and vertical direction
With an average laser energy fluence of just above the ablation
Threshold of the metal.
SEY Measurments
Is is the secondary electron current including elastic and inelastic
processes,
IP is the primary beam current.
IF and IS are the currents on the Faraday cup and the sample, respectively.
The sum of the sample current IS and the Faraday cup current IF represents
the primary energy current IP.
SEY of Cu as a function of
incident Electron energy
Untreated Laser treated
SEY of Al and SS as a function of
incident Electron energy
δmax
as a function of electron
dose for Al,306L SS and Cu
Sample Initial After conditioning to Qmax
δmax
Emax
(eV)
δmax
(Qmax
)
Emax
(eV) Qmax
(Cmm-2
)
Black Cu 1.12 600 0.78 600 3.510-3
Black SS 1.12 900 0.76 900 1.710-2
Black Al 1.45 900 0.76 600 2.010-2
Cu 1.90 300 1.25 200 1.010-2
SS 2.25 300 1.22 200 1.710-2
Al 2.55 300 1.34 200 1.510-2
900 800 700 600 500 400 300 200 100 0
Binding Energy (eV)
x 10 3
2
4
6
8
10
12
14
16
18
20
CP
S
𝟏. 𝟏 × 𝟏𝟎−𝟐 C·mm-2
electron
conditioned HD
50um Cu
Cu2p3/2
Cu2p1/2
Cu LMM
O1s Cu3s Cu3p
Cu2p3/2
Cu2p1/2
Cu LMM
O1s Cu3s Cu3p
Laser surface treatment
Latest result with laser treated
Copper: 0.58 < < 0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 100 200 300 400 500 600 700 800 900 1000
δ
Primary electron energy (eV)
As-received 50um Cu in Air
Heating to 250 ⁰C 50um Cu in Air As-received 60um Cu in Air
Heating to 250 ⁰C 60um Cu in Air
XPS analysis of Black Copper
900 800 700 600 500 400 300 200 100 0
Binding Energy (eV)
x 10 3
5
10
15
20
25
30
35
40
CP
S
Cu2p3/
2
Cu2p1/2
O1
s
Cu3s Cu3
p
Cu LMM
Cu2p3/
2
Cu2p1/2
Cu LMM
O1
s Cu3s Cu3p
As-received HD
50um Cu
Summary
In modern particle accelerator Thin Film has become an integrated part
of the accelerator lattice matrix.
Quality of the film (morphology, RRR, Hc2) depends on
deposition parameters such as
Substrate temperature,
Ion/atom arrival ratio,
Substrate bias,
Plasma generation at the target
pulsed or not
HiPIMS
Substrate crystallography
As the beam line diameter get smaller and smaller to save cost,ALD
and CVD will pick up where PVD reaches its limits.
NEG as thin film has dual benefit: pumping surface and hydeogen
diffusion barrier
However, Thin film may not be the correct solution in all cases,
alternative and relatively cheaper solution should be considered.
laser blackening of the metal surface is a very viable solution for
reducing the SEY < 1.