NEG Coatings as a complex solution
for Particle Accelerators
O.B. Malyshev
ASTeC Vacuum Solutions Group,
STFC Daresbury Laboratory, UK
3rd Topical Workshop on Advanced Low Emittance Rings Technology 2019
(ALERT 2019), Ioannina, Greece
10-12 July 2019
O.B. Malyshev 2
• Introduction
• What is NEG coating?
• Pumping properties
• Desorption properties
• Bombardment induced activation and pumping
• Surface resistance
• Summary
Outlook
What is NEG coating?
O.B. Malyshev 4
Two concepts of the ideal vacuum chamber
Traditional:
• surface which outgasses as little as
possible (‘nil’ ideally)
• surface which does not pump
otherwise that surface is contaminated
over time
Results in
• Complex vacuum chamber shape
• Surface cleaning, conditioning, coatings
• Vacuum firing, ex-situ baking
• Baking in-situ to up to 300C
• Separate pumps
‘New’ (C. Benvenuti, CERN, ~1998):
• surface which outgasses as little as
possible (‘nil’ ideally)
• a surface which does pump, however,
will not be contaminated due to a very
low outgassing rate
Results in
• Simple vacuum chamber shape
• NEG coated surface
• There should be no un-coated parts
• Activating (baking) in-situ at 150-180C
• Small pumps for CxHy and noble gases
O.B. Malyshev 5
What NEG coating does?
1) Reduces gas desorption:
• A pure metal (Ti, Zr, V, Hf, etc.)
film ~1-m thick without
contaminants.
• A barrier for molecules from
the bulk of vacuum chamber.
2) 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 Bulk
Coating
O.B. Malyshev 6
NEG coating is not a single product, this is a
technology with a number of variable parameters:
• Composition:
• Various combinations of transitional metals (Ti, Zr, Hf, V,
Nb) with other (conductive) metals (Ag, Au, Cu, Al, Fe)
• Single, binary, ternary, quaternary alloys
• Film morphology
• Dense or columnar
• Grain size
• Structure
• Single or dual layer
NEG coating’s family
O.B. Malyshev 7
Thin films deposited on Si sample from a single metal wire
Cylindrical Magnetron:
Power = 60 W, PKr = 10-2 mbar,
Deposition rate = 0.14-0.16 nm/s,
T = 120°C.
Average grain size: 100 – 150 nm.
Ti:
Zr: Hexagonal lattice structure
V: Rhombohedral lattice structure
Hf: Hexagonal lattice structure
Ti ZrV
Hf
O.B. Malyshev 8
Thin film deposited on Si sample from two twisted wires
Ti-V Ti-Zr
Cylindrical Magnetron:
Power = 60 W, PKr = 10-2 mbar,
Deposition rate = 0.13-0.16 nm/s,
T = 120°C.
Average grain size:
Ti-V: 50 – 100 nm, Hexagonal lattice structure
Ti-Zr: 50 – 100 nm, Hexagonal lattice structure
Zr-V: 10 – 20 nm, Rhombohedral lattice structure
Zr-V
O.B. Malyshev 9
Ternary NEG film deposited on Si test sample from
twisted Ti, V, Zr, and Hf wires and TiZrV alloy wire
Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.
Average grain size 5 nm. Hexagonal lattice structure.
Ti-Hf-Zr twisted wire V-Hf-Zr twisted wire
Ti-Zr-V alloy wire Ti-Zr-V twisted wire
O.B. Malyshev 10
Quaternary NEG alloy film deposited on Si test
sample from twisted Ti, V, Zr, and Hf wires
Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.
Very glassy structure.
SEM images of films (film morphology)
columnar dense
O.B. Malyshev, R. Valizadeh, J.S. Colligon et al. J. Vac. Sci. Technol. A 27 (2009), p. 521.
Why to use NEG coating?
O.B. Malyshev 13
Comparison of PSD from 316LN and NEG
Stainless Steel (baked at 300C for 24 hrs)V.V. Anashin et al, Vacuum 75 (2004) p. 155.
Samples coated with Ti-Zr-V at CERN (Switzerland)
Experiments on the SR beam line at BINP (Russia)
TiZrV coated vacuum chambers
(activated at 190C for 24 hrs)
O.B. Malyshev 14
Using these result for the ILC-DR design
5 10 15 20 25 301 10
10
1 109
1 108
1 107
H2
CH4
CO
CO2
Thermal desorption
Required CO pressure
Stainless steel tube, S=200 l/s
L (m)
P (
Torr
)
10 100 1 1031 10
11
1 1010
1 109
1 108
1 107
H2
CH4
CO
CO2
Thermal desorption
Required CO pressure
Distance between pumps L=6 m
S (l/s)
P (
Torr
)
inside a stainless steel tube
Seff = 200 l/s every 5 m
inside a NEG coated tube
Seff = 20 l/s every 30 m
10 15 20 25 30 35 40 45 501 10
13
1 1012
1 1011
1 1010
1 109
1 108
H2
CH4
CO
CO2
Required CO pressure
NEG coated tube, S=20 l/s
L (m)
P (
Torr
)
10 100 1 1031 10
13
1 1012
1 1011
1 1010
1 109
H2
CH4
CO
CO2
Required CO pressure
Distance between pumps L=30 m
S (l/s)
P (
Torr
)
Average pressure after 100 Ahr beam conditioning:
O.B. Malyshev 15
NEG coating for accelerators
• First used in the ESRF (France);
• ELETTRA (Italy);
• Diamond LS (UK);
• Soleil (France) – first fully NEG coated;
• LHC (Switzerland) – longest NEG coated vacuum chamber;
• SIS-18 (Germany); MAX-IV (Sweden); Solaris (Poland)
• and many others.
20 years of using NEG coatings in particle accelerators
bring a confidence that they work well.
O.B. Malyshev 16
What is required for an accelerator
vacuum system design?
• Input data for accelerator design:
• (D,E,Ta), (M,Ta), pumping capacity;
• Surface resistance Rs, SEY;
• Better understanding:
• what and why; practical ‘do’s and ‘don’t’s;
• Further development of this coating:
• lower , Ta, SEY;
• higher (M), pumping capacity;
• lower Rs;
• Narrow chamber coating technology;
• Optimising for an application.
NEG activation
O.B. Malyshev 18
ASTeC activation procedure
Advantages of ASTeC activation procedure:
• better activation (less poisoning by gas from uncoated parts),
• lower electricity cost,
• lower total thermal expansion.
O.B. Malyshev, K.J. Middleman, J.S.
Colligon and R. Valizadeh. J. Vac. Sci.
Technol. A 27 (2009), p. 321.
Pumping properties
O.B. Malyshev 20
NEG pumping properties
Pressure ratio P1/P2 measured during
gas injection is used to estimate:
initial sticking probability and sorption capacity
Pumping properties of some NEG films
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
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
21
Desorption properties
O.B. Malyshev 23
Pressure in the accelerator
vacuum chamber
P
where
- desorption yield (photon,
electron or ion stimulated
desorption)
- sticking probability
• Improving pumping
properties is limited:
1
• 0.005 < H2 < 0.02
• 0.1 < CO < 0.5
• 0.4 < CO2 < 0.6
• Reducing the
desorption yields
in orders of magnitude
was our aim
Reducing the gas desorption
from the NEG coatings
• Main gases in the NEG coated vacuum chamber are
H2 and CH4
• Only H2 can diffuse through the NEG film under
bombardment or heat
• CH4 is most likely created on the NEG surface from
diffused H2 and C (originally from sorbed CO and CO2)
• Therefore the H2 diffusion must be suppressed
• Where H2 come from?
Reducing the gas desorption
from the NEG coatings
Gas molecules are contained
on the NEG coating surface
after exposure to air
minimise exposure to air
inside the NEG coating
trapped during deposition
purity of discharge gas
background pressure
in subsurface substrate layer
substrate bakeout before NEG deposition
in the substrate bulk
vacuum firing
ESD is studied as a function of
• Electron energy
• Dose
• Wall temperature (-5 to +70C)
• Activation/bakeout temperature
Can be used for samples with:
• Specially treated samples
• Vacuum fired, polished, etc.
• Low desorption coating
• No coatings
• NEG coating
• ESD measurements
• Sticking probability
measurements
Electron stimulated desorption facility
O.B. Malyshev, A.P. Smith et al. J. Vac. Sci. Technol. A 28 (2010), p. 1215.
O.B. Malyshev 27
H2 ESD from NEG coated vacuum fired 316LN
O.B. Malyshev, R. Valizadeh, et al.
Vacuum 86, 2035 (2012)
O.B. Malyshev, R. Valizadeh, et al.
JVST A 32, 061601 (2014)
O.B. Malyshev, R. Valizadeh, et al
JVST A 34, 061602 (2016)
Bombardment induced
NEG activation and pumping
O.B. Malyshev 29
NEG Coated Vacuum Chamber: SR Induced Pumping
NEG TiZrV coated surface saturated with CO (i.e. no pumping speed)
exposed to SR
V.V. Anashin et al. Vacuum 75
(2004), p. 155.
The photon
stimulated NEG
activation efficiency
estimated as
= 2×10-5 [CO/]
O.B. Malyshev 30
1 103
0.01 0.1 1
1
10
100
1 103
G1
G4
Surface coverage [monolayers]
Rat
io [
P1
/P2
]
Electron
bombardment 1
Electron
bombardment 2
Electron stimulated NEG activation
Non-activated NEG
The electron stimulated NEG activation efficiency
estimated as 7.9×10-4 < 1 < 2.4×10-3 [CO/e-]1
COCO
e D
Activated at 180°CP1
P2
O.B. Malyshev 31
The electron stimulated NEG
activation efficiency estimated as
5 103
1 104
1.5 104
2 104
1 1010
1 109
1 108
1 107
P2
P1
Time [s]
Pre
ssu
re [
mb
ar]
Electron
bombardment 1
Electron
bombardment 2
3
2 2.2 10CO e
B
Q q CO
k T I e
Electron stimulated NEG activation
O.B. Malyshev 32
• NEG does not pump CH4 and other hydrocarbons
• However, CH4 can be pumped in a presence of SR or
electron bombardment: = 2.310-5 CH4/e-.
CH4 problem
O.B. Malyshev and R. Valizadeh. Further optimisation of NEG coatings for
accelerator beam chamber. Proc. IPAC-5 (2014) p. 2399.
CH4 injection
Q= 5.010-9 mbarl/(scm2)
CH4 injection
Q = 1.210-9 mbarl/(scm2)
O.B. Malyshev 33
• What is on the surface after air went? • an oxide layer,
• physisorbed gas molecules
• few layer of water
• How to avoid this?• Do not went a vacuum chamber after NEG deposition with a
sophisticated engineering (quite complicated)
• Ex-situ activation followed by
• Installing without vent to air (valves on either side)
• Installing in noble gas flow atmosphere
• Long bake (a few days or weeks) to low temperatures (Tb = 80-120C)
to remove water and physisorbed gas molecules
• More studies needed to optimise Tb and duration tb
• Very long pumping (weeks or months?) without bakeout
• To be studied for a necessary duration tp
Could the bakeout be avoided?
Lifetime sorption capacity of
1-m thick NEG film is ~100 ML
Absorption of all these gases
could be poisoning for NEG
O.B. Malyshev 34
• ESD provides good data for comparing different
coatings under accelerator-like environment
• However, the test with SR would bring more
certainty in parameters used for the machine design:
• PSD yields as a function of photon dose
• SR induced pumping for different gases
• SR induced NEG coating activation
Different NEG coatings
Different operation scenarios
Insufficient study on
photon stimulated desorption
NEG surface resistance
O.B. Malyshev 36
• The cavity geometry
consists of two parts:
• a body of the cavity
• a planar sample,
• separated by an air
gap.
• Contactless
• RF chokes in order to
keep the RF power
within the cavity
• f = 7.8 GHz
Surface resistance: method
1
0
cavsam S cS
s
GQ R pR
p
• Modelled with CST Microwave
Studio.
• G = 235 .
• The field ratios pc = 0.625 and
ps = 0.375 for perfect electric
conductor boundary conditions.
O.B. Malyshev 37
• NEG films
• columnar
• dense
• Deposited on:
• polycrystalline copper
• silicon Si(100) substrates.
• The substrate size was
100 mm 100 mm 2
mm
• Sample thickness:
• from 0.7 to 18 m
NEG coatings
O.B. Malyshev, L. Gurran, P. Goudket, K. Marinov, S. Wilde, R. Valizadeh
and G. Burt.. Nucl. Instrum. Methods Phys. Res., A 844, 99-107 (2017)
O.B. Malyshev 38
• The expressions for the surface impedance of a planar
metallic film deposited on a substrate (dielectric or metallic)
are derived by following the standard approach employed in
calculating the transmission and reflection coefficients in
layered media
Analytical model
2
1 1 1 1 1 1
1 2
1 1 1 1 1 1
1 exp 4 2 sin 2 exp 2 for NEG on metal substrate;
1 exp 4 2 cos 2 exp 2S
d d dR R
d d d
1 1 1 1 1 1
1
1 1 1 1 1 1
1 exp 4 2sin 2 exp 2 for NEG on Si substrate.
1 exp 4 2cos 2 exp 2S
d d dR R
d d d
O.B. Malyshev 39
The surface resistance RS of dense and columnar NEG coatings on
copper and silicon substrates as a function of film thickness
The bulk conductivity
was obtained with the
analytical model:
• 𝜎𝑑 = 1.4×104 𝑆/𝑚for the columnar
NEG coating
• 𝜎𝑑 = 8×105 𝑆/𝑚 for
the dense NEG
coating
O.B. Malyshev 40
The surface resistance RS as a function of NEG
film thickness on Cu at various frequencies
O.B. Malyshev 41
• New materials has been
developed in ASTeC to increase
the NEG conductivity without
compromising any vital NEG
characteristic such as:
• Low Activation Temperature,
• High Pumping properties (sticking
probability and pumping capacity),
• Low PSD and ESD.
NEG bulk conductivity increased
by a factor 6
Pumping properties are
comparable to dense NEG
High electric conductivity NEG coating
O.B. Malyshev 42
• 6-10-mm tubes coated by Andre Andres at LBNL in
2015
• 6-mm tube coated at SAES in 2016
• ASTeC coated with NEG vacuum chamber with
ID 5 mm without compromising any vital NEG
characteristic such as:
• Low Activation Temperature,
• High Pumping properties (sticking probability and pumping
capacity),
• Low PSD and ESD.
• It is very hard to coat tubes with ID< 5 mm with PVD
Narrow vacuum chambers
O.B. Malyshev 43
Conclusions• NEG coating is a technology that allows to meet UHV/XHV vacuum specification
win long narrow vacuum chambers.
• PSD and ESD After NEG activation at 180°C the initial(316LN)/(Ti-Zr-V) =
• =20 for H2, =1000 for CH4 and =200 for CO.
• Vacuum firing => an order of magnitude lower ESD
• (Ti-Zr-Hf-V) < (Ti-Zr-V).
• Best results is for the dense and dual layer NEG activated at 180 C
• Often the only vacuum solution
• Lower cost of pumping system
• NEG film requires activation at 150-180 C in stead of 250-300 C usual bakeout:
• Shorter bellows or less number of bellows
• Wider choice of material for vacuum chamber and components
• SR (or electron bombardment) induced activation/pumping:
• NEG can be partially activated or re-activated by irradiation/bombardment
• NEG can pump CH4 molecules during irradiation/bombardment
• The bulk conductivity:
• 𝜎𝑑 = 1.4×104 𝑆/𝑚 for the columnar NEG coating
• 𝜎𝑑 = 8×105 𝑆/𝑚 for the dense NEG coating
• It can be further increase at least factor 6 with new NEG materials
O.B. Malyshev 44
Acknowledgments
ASTeC
• Dr. R. Valizadeh
• Mr. A.N. Hannah
• Mr. B.T. Hogan
• Mrs. R.M.A. Jones
• Mr. A.P. Smith
• Dr. K.J. Middleman
• Dr. K. Marinov
• Dr. P. Goudket
• Mr. L. Gurran
• Dr. S. Wilde
• Dr. S. Wang
MMU / Huddersfield Uni.
Prof. J.S. Colligon
Prof. V. Vishnyakov
Lancaster University
Dr. G. Burt
Liverpool University
• Dr. V. Dhanak
ASTeC / ISIS
Dr. S. Patel
O.B. Malyshev 45
Deposition method
Commonly used planar magnetron deposition
Cylindrical magnetron deposition for vacuum chambers
HiPIMS
O.B. Malyshev 46
• Columnar layer:
• Activated at lower temperature
• Provides higher sticking
probability and pumping
capacity
• Dense layer:
• Provides lower ESD
• Dual Layer:
• Combines benefit of both
Dual layerVacuum
Columnar NEG Coating
Dense NEG Coating
Bulk metal
O.B. Malyshev 47
Dual layer
O.B. Malyshev, R. Valizadeh and A.N. Hannah. JVST A 34, 061302 (2016)
O.B. Malyshev 48
(Ee-) for different gases for NEG coating