IMPACT FACTORS OF MANUFACTURING TECHNIQUES ......IMPACT FACTORS OF MANUFACTURING TECHNIQUES ON...

IMPACT FACTORS OF MANUFACTURING

TECHNIQUES ON SUPERCONDUCTING THIN-FILM

COATING

EASITrain-P2-WP3-D3.2

EDMS

Date: 26/11/2019

Grant Agreement 764879 PUBLIC 1 / 34

Grant Agreement No: 764879

EASITrain European Advanced Superconductor Innovation & Training

DELIVERABLE D3.2

IMPACT FACTORS OF MANUFACTURING TECHNIQUES ON SUPERCONDUCTING

THIN-FILM COATINGS

Document identifier:

EASITrain-P2-WP3-D3.2

EDMS 2041954

Due date: End of Month 26 (December 1st, 2019)

Report release date: 26/11/2019

Work package: WP3 (Manufacturing)

Lead beneficiary: INFN

Document status: RELEASED (V1.0)

Abstract:

Report on a suitable thin film coating and manufacturing with a description of the manufacturing key

parameters and quantification of their impacts on yield and quality. The report may, as appropriate, include

guidelines on the optimisation potentials of the process.



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Copyright notice:

Information provided with this document is subject to the H2020 EASITrain – European Advanced

Superconductivity Innovation and Training grant agreement. This Marie Skłodowska-Curie Action (MSCA)

Innovative Training Network (ITN) receives funding from the European Union’s H2020 Framework Programme

under grant agreement no. 764879.

Delivery Slip

Name Partner Date

Authored by

E. Bellingeri

J. F. Croteau

D. Fonnesu

V. Garcia Diaz

T. Koettig

S. Leith

G. Mazars

C. Pira

A. Saba

M. Vogel

CNR SPIN

I-CUBE

CERN

INFN

CERN

USIEGEN

I-CUBE

INFN

CNR SPIN

USIEGEN

30/10/19

Edited by C. Hunsicker

C. Pira

CERN

INFN

17/10/19

30/10/19

Reviewed by J. Gutleber CERN 24/11/19

Approved by M. Benedikt

A. Ballarino CERN 25/11/19



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TABLE OF CONTENTS

1. INTRODUCTION .................................................................................................................................................... 4

2. EFFECT OF DEPOSITION ANGLE ON NIOBIUM THIN FILMS ................................................................. 5

2.1. EXPERIMENTAL TEST STATION AND MEASUREMENT PRINCIPLE .......................................................................... 5 2.2. COATING TECHNIQUES ........................................................................................................................................ 7 2.3. RESULTS ............................................................................................................................................................ 7

3. EFFECT OF THE SUBSTRATE ON THALLIUM BASED FILMS AT SPIN-CNR ....................................... 9

3.1. EXPERIMENTAL PROCEDURE THALLIUM DEPOSITION ......................................................................................... 9 3.2. SUBSTRATE PREPARATION ............................................................................................................................... 10 3.3. CHARACTERISATIONS OF THE FILMS ................................................................................................................. 12

3.3.1. Characterisation of the thin films deposited on the silver substrate ....................................................... 12 3.3.2. Characterisation of the thin films deposited on the SrTiO3 substrate ..................................................... 14

4. EFFECT OF THICKNESS ON NIOBIUM TITANIUM NITRIDE FILMS .................................................... 16

4.1. QUADRUPOLE RESONATOR MOTIVATION .......................................................................................................... 16 4.2. RESULTS FROM NIOBIUM TITANIUM NITRIDE SAMPLES ..................................................................................... 16

5. KEY-PARAMETERS OF OFE-COPPER FORMING PROCESS ................................................................... 18

5.1. HALF-CELL SUBSTRATE FORMING BY ELECTRO-HYDRAULIC FORMING .......................................................... 18 5.2. CHARACTERIZATION OF ANNEALED OFE-CU FOR EHF ................................................................................... 18

5.2.1. Mechanical Properties at Different Strain Rates .................................................................................... 19 5.2.2. Forming Limit Diagram ......................................................................................................................... 20

6. KEY PARAMETERS ON NIOBIUM THICK FILMS DEPOSITION PROCESS ......................................... 22

6.1. COPPER SUBSTRATE PREPARATION ................................................................................................................... 22 6.2. EFFECT ON THE RF PERFORMANCES OF THE SINGLE LAYER THICKNESS ............................................................. 25

7. KEY-PARAMETERS ON NIOBIUM NITRIDE DEPOSITION PROCESS .................................................. 27

7.1. NIOBIUM NITRIDE AT UNIVERSITY OF SIEGEN .................................................................................................... 27 7.2. KEY-PARAMETERS INVESTIGATED .................................................................................................................... 28

7.2.1. Substrate Preparation ............................................................................................................................. 28 7.2.2. Process Pressure .................................................................................................................................... 28 7.2.3. Nitrogen Percentage (Partial Pressure) ................................................................................................. 31 7.2.4. Temperature............................................................................................................................................ 32 7.2.5. Cathode Power ....................................................................................................................................... 32

8. CONCLUSIONS .................................................................................................................................................... 33

9. REFERENCES ....................................................................................................................................................... 34



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1. INTRODUCTION

The objective of work package 3, as reported in the grant agreement, is to optimise the production processes

for superconductors to increase quality and yield, paving the road to cost effective production at large scale.

Key is a thorough understanding and assessment of the impacts of advanced manufacturing on the

superconducting (SC) properties of the studied materials.

This report highlights the principal key parameters that drive the manufacturing processes developed

by the different EASITrain partners involved in the manufacturing of SC coatings for cavities and magnets

applications. For each institution it is first described the coating or manufacturing technique under study, and

then the principal process key-parameters are discussed, focusing on their impact on the SC performance and

on the optimization adopted to increase the quality of SC films.

At CERN (ESR1), a permanent test station for the determination of the critical temperature and additional

SC parameters of thin films was built and 10 different Nb samples coated via DC Magnetron Sputtering

and High Power Impulse Magnetron Sputtering (HiPIMS) was characterized. The key-parameters studied

was the effect on the coating Critical Temperature of different sputtering incidence angles with respect

to the substrate surface at different pulses and substrate biases values.

At Spin-CNR (ESR6), the electrodeposition was chosen as a technique to grow the thallium based thin

films to study their behaviour for the beam screen of Future circular collider. A key parameter of the process

is the substrate material and in this project silver and strontium titanate are the 2 candidates evaluated as

possible substrate.

At HZB (ESR8) is under development a Quadrupole Resonator (QPR), a dedicated and unique tool able to

perform SRF characterizations of different superconducting materials at 3 different frequencies (~415, 845,

1285 MHz). The role of thickness on RF superconductive properties of NbTiN films coated at Thomas

Jefferson National Laboratory in the USA has been the first test done with QPR during the EASITrain

program.

I-Cube (ESR9) is studying the possibility to apply the Electro-hydraulic forming (EHF) to the

manufacturing of resonant accelerating cavity copper substrates. To predict the material’s behavior in multi-

physics finite element models (FEM), the characterization of the mechanical properties of the copper at

different strain rates was done. A forming limit diagram was also obtained deforming different copper

sheets.

At INFN (ESR10) Niobium thick films on 6 GHz copper substrates have been developed in order to increase

the performances of Nb on Cu films. A multilayer deposition demonstrated the possibility to mitigate the Q-

slope problem that affects this kind of cavities. The R&D on Niobium thick films shows the key roles of the

substrate preparation and of the single Nb layer thickness on the superconductive performances.

At University of Siegen (ESR14), the coating of NbN thin films onto OFHC copper substrates was

extensively studied. The films were coated via Reactive DC Magnetron Sputtering using a high pure Nb target

in the presence of a mixture of argon (or krypton) and nitrogen. The role of the main process key parameters

was investigated, in particular: substrate preparation, process pressure, nitrogen partial pressure.

Considerations were also done on the role of temperature and cathode power.



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2. EFFECT OF DEPOSITION ANGLE ON NIOBIUM THIN FILMS

2.1. EXPERIMENTAL TEST STATION AND MEASUREMENT PRINCIPLE

ESR1’s project focuses on the establishment of a permanent test station at the CERN Central Cryogenic

Laboratory for the determination of the critical temperature and additional SC parameters of thin films of

niobium and novel A15 compounds deposited on copper, towards their future application in resonant cavities

structures. The data collected will be part of the study of possible dependencies between the coating

parameters and film structural features, and the SC performance. The films under test are produced at CERN

(TE-VSC-SCC). More samples will be provided by Vanessa Garcia (INFN-LNL, ESR10) and Stewart Leith

(USIEGEN, ESR14) for cross data analysis.

The critical temperature (Tc) test stand is to this day operational and its measurement principle is verified with

a bulk niobium sample, as shown in Figure 1. The measurement is based on a contactless technique sensitive

to the Meissner effect occurring in the thin film when it is in the superconducting state. As in Figure 2, the

sample is placed between two coils arranged in front of each other, the coil planes having opposite orientations

and being parallel to the surface of the sample. The coil facing the thin film, the drive coil, is excited with a

sinusoidal current, which in turn generates an alternate magnetic field that can be detected as induced voltage

by the coil facing the sample substrate, addressed in this context as pickup coil. The pickup coil remains

passive throughout the measurement.

Figure 1: Transition curve of bulk niobium with the error bars for the temperature measurement, additionally indicating

the determined transition width.



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When the film is superconducting, the external magnetic field is expelled by the intense screening currents

induced on its surface. However, depending on the material and thickness of the substrate, and on the

frequency of the alternate magnetic field, the eddy currents induced on the sample in the normal conducting

state are not strong enough to shield the field, so that the sample can be considered transparent to the AC

magnetic field and a voltage proportional to the change rate of the drive current and the coils’ mutual

inductance is induced in the pickup coil.

The voltage amplitude in the pickup coil is read by a lock-in amplifier and plotted against the recorded

temperature. The Tc of the thin film is extracted as fit parameter from a logistic curve that fits the step-like

data measured during the transition of the film from the superconducting to the normal conducting state, with

Tc being the temperature to which corresponds the half-height of the fit curve.

Figure 2: Picture of the experimental setup indicating the drive and pickup coil assembly with a sample in between.



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2.2. COATING TECHNIQUES

DC Magnetron Sputtering (DCMS) is a classical sputtering technique in which the electrons of the

sputtering gas plasma are confined to an area close to the target surface, thanks to the strong magnetic field

generated by magnets placed behind the target. This leads to a denser plasma and higher growth rate than

conventional diode sputtering, and preventing the electrons to impact on the substrate/film surface. In DC

configuration, the high voltage applied to the cathode (placed right behind the target) and the anode (connected

to the sputtering chamber itself as ground) is kept constant during the deposition. High molecular weight

gases, such as argon or xenon, are usually employed to maximise the growth rate of the deposited film.

High Power Impulse Magnetron Sputtering (HiPIMS) allows to reach an even higher density plasma

without incurring the large heat dissipation that would take place if an increasingly larger voltage was applied

to the cathode in DCMS. The denser plasma is instead created by applying a sequence of short and intense

pulses to the cathode, each one of which can result into very high peak power. Low duty cycles ensure the

dissipated heat and average delivered power to stay within practicable limits.

2.3. RESULTS

A set of 10 samples of niobium films deposited on copper has been tested until July 2019, for which the results

were first presented at SRF Dresden 2019 (TUP071). The samples were provided by F. Avino and A. Sublet

from the Vacuum, Surfaces and Coatings group at CERN (TE-VSC-SCC). The films were deposited via DC

Magnetron Sputtering (DCMS) and High Power Impulse Magnetron Sputtering (HiPIMS) applying different

pulses and substrate biases, and at different incidence angles with respect to the substrate surface. Figure 3

and Figure 4 show the measured Tc and the Focused Ion Beam (FIB) microscope pictures of two niobium on

copper samples both deposited with HiPIMS and -50 V bias voltage at respectively 0 and 90 degrees incidence

angle with respect to the surface normal. The table lists a summary of the results obtained from the set and

allows to infer, at a preliminary stage, that HiPIMS coatings with positive pulse provides good quality

niobium films on copper also at grazing incidence angles. This is an advantage for the coating of

resonant SRF cavities of complex shapes as in the case of the Wide Open Waveguide cavities under

investigation as possible crab cavities for FCC.

The Tc measurements presented in Table 1 have been validated by an independent SQUID measurement of

the Tc of the same samples.

Table 1: Critical Temperature (Tc) and Transition with (W) for Nb thin film samples coated at different incidence angle.

Incidence angle

0° 90°

Process Bias [V] Pulse [V] Tc [K] W [K] Tc [K] W [K]

DCMS 0 - 6.5 0.97 No transition

HiPIMS 0 0 9.0 0.08 6.8 1.36

HiPIMS -50 0 9.1 0.3 8.6 0.7

HiPIMS 0 +50 9.16 0.34 7.0 0.74

HiPIMS 0 +100 9.1 0.1 8.7 0.38



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Figure 3: Transition curve of a niobium film deposited on copper at 0°degrees impinging angle with respect to the

substrate surface normal. The film was deposited with HiPIMS and -50 V substrate bias.

Figure 4: Transition curve of a niobium film deposited on copper at 90°degrees impinging angle with respect to the

substrate surface normal. The film was deposited with HiPIMS and -50 V substrate bias.



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3. EFFECT OF THE SUBSTRATE ON THALLIUM BASED FILMS AT SPIN-CNR

3.1. EXPERIMENTAL PROCEDURE THALLIUM DEPOSITION

A variety of techniques have been employed to grow the thallium based thin films to study their behaviour for

the beam screen of Future circular collider. At present, we use electrodeposition method because of its

feasibility, inexpensiveness, fast deposition and possible deposition on nonplanar surfaces.

The thallium based thin films are deposited, on the Silver substrate and on Strontium Titanate, by a

conventional three electrode cell. The reference electrode is Ag/AgNO3 0.1 M in Dimethyl Sulfoxide

(DMSO), working electrode, and counter electrode being platinum grid. To prepare the solution, 0.25g TlNO3,

0.18g Bi(NO3)3·5H2O, 0.18g PbNO3, 2.73g Sr(NO3)2, 1.52g Ba(NO3)2, 1.63g CaNO3·H2O, and 1.33g

Cu(NO3)2 ·H2O are added in 250 ml of 99.9% pure DMSO and then stirred and heated at 110 °C for several

hours. The deposition of the samples is performed between -2.9 V to -3.1 V for 10 minutes.

Figure 5: The three-electrode electrodeposition cell.

The deposition of the thallium-based precursor at constant potential provides a highly reactive precursor of

desired stoichiometry.



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Figure 6: Scanning electron microscope image of the electrodeposited thin film precursor.

Post-deposition processing of the film is performed into two steps. First, the electrodeposited precursor is

dried in vacuum at around 120 °C for 40 minutes. Second is the annealing of the dried precursor at high

temperature around 880 °C for 10 minutes wrapped in the gold capsule in oxygen (1 atm). The gold foil

capsule is used to avoid the thallium escape from the atmosphere, since thallium oxide starts to vaporize

around 700 °C. To get better Tl-1223 phase and thallium pressure inside the gold foil, precursor thin films are

treated at high temperature along with Tl2O3 powder, or a piece of Tl-1223 pellet.

3.2. SUBSTRATE PREPARATION

Although we are preparing and investigating a wide variety of substrates for Tl-1223 film, but, at present, only

two are of major interest. The first Tl-1223 films are grown on the silver substrates and SrTiO3 substrates. To

prepare the silver substrates, we elongate and flatten the pieces of silver rod. We reduce the thickness down

to 50 µm.

Since a substrate plays an important role in the growth of the film, so we deposit thin films on the substrates

with different (50, 80, and 100 µm) to understand the film growth. We also use different thicknesses to find

out the most suitable thickness to grow the most desired thin films.

The melting point of the silver is around 960 °C, close to the treatment temperature required for the formation

of thallium 1223 phase. Therefore, before depositing precursor on the silver substrate, we treat the silver

substrate at 900 °C for 1 hour in the oxygen to reduce the stress and stabilize the substrate. Finally, we deposit

precursor and anneal thin film samples at high temperature around 880 °C to grow the desired coating.



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Figure 7: Mechanically elongated Silver ribbons of different thicknesses.

Figure 8: SEM image of an annealed silver ribbon.



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Some of the thallium based thin films are also grown on strontium titanate (SrTiO3) because the lattice match

of Tl-1223 is reasonable to single crystal SrTiO3.

We use commercially available single crystal STO of dimensions 5×5 mm or 10×10 mm. Since the SrTiO3 is

not conductive and we need an electrode for the electrodeposition, so we deposit silver buffer layer, around

20 nm, on the surface of single crystals (SrTiO3) to make them conductive for a successful electrodeposited

precursor using DC sputtering or laser ablation. Some of the primary thin films deposited on the single crystals

have shown well orientated grains of the thallium 1223 phase.

The thin films deposited on the single crystals have shown the 1223 phase in majority, 1212 phase in minority.

These films also contain CaO and non-stoichiometric oxides. To purify the Tl-1223 phase, we are (I) prepare

electron bath with different recipes, (II) change post-deposition treatment time and temperature and (III) study

of different substrates.

3.3. CHARACTERISATIONS OF THE FILMS

3.3.1. Characterisation of the thin films deposited on the silver substrate

Thallium based thin films deposited on the pre-annealed silver substrates have shown some improved results

and contain enhanced amount of thallium 1223 phase. The microscopic image of the film shows the grains of

the phase 1223.

Figure 9: SEM micro graph showing the Tl-1223 grain on the single crystal.



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Figure 10: Tc measurement from the Thallium on Ag substrate.

In Figure 10, two transitions are identified at approximately 107 K and 75 K, which correspond to the Tl-1223

and Tl-1212 phase, respectively. For the scanning electron microscopy, the remnant field profiles were

mapped with a Hall scanner in an 8 T cryostat setup with micro meter resolution.

Figure 11: Tc Large Tl-1223 grains (30-50 µm) on Ag substrate and inversion of the remnant field shows current flow in grains

and across grain boundaries.



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3.3.2. Characterisation of the thin films deposited on the SrTiO3 substrate

The primary films deposited on the single crystal show well orientated thallium 1223grains growth. The

microstrucure of the measured thin film on STO substrate is investigated by means of SEM and TEM imaging

Figure 12: SEM micro graph showing the Tl-1223 grain on the single crystal.

Mapping of the remnant magnetic field by Scanning Hall Probe Microscopy at 5 K was performed on thin

films of Tl-1223 on single crystal SrTiO3. Using Focused Ion Beam (FEI Quanta 200 3D DB-FIB), two TEM

lamellas were prepared, and a protective Platinum layer is deposited on the sample surface, and the thickness

of the lamella is around 100 nm. In the area with high trapped field, the image shows one large grain with a

thickness of 1.8 µm flat on the substrate surface, seen in figure c. whereas there are also some compositions

in the lamella taken from the area with no trapped field, shown in figure d. In this case, we find multiple Tl-

1223 grains, randomly oriented on the substrate surface. So it shows the importance of the grain orientation,

as the current flow is blocked by the misalignment of the Tl-1223 grains.



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Figure 13: SEM image and trapped field of a thin film on STO substrate (a), and two TEM lamellas with a Focused Ion Beam (b).

TEM images of areas with high trapped field (c) and no trapped field (d) show the desired Tl-1223 phase with grains alignment,

respectively.

An area scan (Figure 14 below) shows the trapped field of a larger grain cluster. In this case, the maximum

trapped field amounts to 100 mT at 5 K. Calculation of the Jc distribution gives the same value as on the Ag

substrate on a larger area

Figure 14: Trapped field of the thallium-1223 grain cluster on SrTO3 substrate (a), and corresponding critical current density

distribution (b).

We are able to grow big grains of thallium 1223 phase on the different substrates. The X-ray diffraction results

show thallium 1223 phase in the majority. The films also show the current flow through the grains. Further

steps are being taken to improve the phase (1223) purity and to perform more characterization in order to

understand the potential of thallium based superconductors for the beam screen of the future circular collider.

It is known that a substrate plays an important role in the growth of a thin film. So we are directed at trying to

a b

a c

b d



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set the most suitable substrate and conditions to achieve the desired quality of the film. Additional substrates

(single-crystal silver and silver deposited steel substrates) are under consideration for further studies.

4. EFFECT OF THICKNESS ON NIOBIUM TITANIUM NITRIDE FILMS

4.1. QUADRUPOLE RESONATOR MOTIVATION

Cavities, coated with different superconducting materials aim to replace present bulk Nb cavities in future

projects (such as FCC). Deposition of superconducting materials on Cu and Nb might increase important RF

(radiofrequency) parameters of those cavities and reduce costs of material. But production recipes of those

films require optimisations and more research. For those research we use the Quadrupole Resonator (QPR)

which is a dedicated and unique tool (only 2 institutes worldwide have those devices) that is able to perform

SRF characterizations of different superconducting materials at frequencies ~415, 845, 1285 MHz.

4.2. RESULTS FROM NIOBIUM TITANIUM NITRIDE SAMPLES

In frame of the campaign of NbTiN films research, two films with different thickness have been tested (70 nm

and 2 µm on Nb substrate, more information [1]). The films have been produced by Thomas Jefferson National

Laboratory (JLab) by the same technology and under the same conditions. Here, at HZB the properties of

those films were determined using the Quadrupole Resonator (QPR). The structure of films, reported here,

can be seen from the Figure 15.

a)

b)

Figure 15: Structure of the films: (a) Bulk film (2 µm), (b) Thin film (70 nm).

Films made of NbTiN are interesting because this material has higher critical temperature (~14 K), and,

therefore, can be used more efficiently at higher temperatures, then bulk niobium. In result the test shows

higher growth of the BCS surface resistance (temperature dependent part) for the thin film (see Figure 16).

This happens also because thin film (red line on the plot) is not thick enough to shield the Nb substrate from

the RF field, which has higher temperature dependent resistance, compared to the film. The thick film, on the

other hand, shields the substrate completely.



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a)

b)

Figure 16: Measured surface resistance of films at (a) ~400 MHz and (b) ~800 MHz as a function of temperature. Both

frequencies are the choice for many present and future facilities such as FCC.

Another interesting result shows, that the maximum achievable magnetic component of the RF fields (the

“Quench field”) is much smaller for the thin film (~10 mT, compared to >40 mT for the 2 µm film). It is still

not clear if this is the property of the film itself or the production defect, but, nevertheless, the results show

that thinner films are more sensitive to such effects and the production good thin films is more difficult. Figure

17 shows that the thin film also shows larger “Q-slope”, which is degradation of RF properties of the

superconducting material at higher field values inside the cavity. Both Thin and Thick films showing lower

critical magnetic field compared to the bulk niobium, and it still needs to be improved.

Figure 17: Measured surface resistance of films at ~400 MHz as a function of peak magnetic component of the RF field on the

surface.

Overall, thin films are more interesting for the coating of more complicated structures (such as multilayer

films, etc.), whether thick films are the choice for bulk monolayer coatings on normal conducting materials

(such as copper).



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5. KEY-PARAMETERS OF OFE-COPPER FORMING PROCESS

5.1. HALF-CELL SUBSTRATE FORMING BY ELECTRO-HYDRAULIC FORMING

Electro-hydraulic forming (EHF) is a high-pulsed power high-velocity sheet forming technique, see Figure 18

for a schematic. An electrical discharge is released between two electrodes in a water chamber. The expansion

and collapse of the plasma generated by the electric arc leads to the formation of high-pressure waves that

travel in the fluid and impact on the blank to accelerate it. The blank finally ends its course on a die with the

geometry of interest.

Figure 18: Schematic of a non-confined EHF or explosive forming setup. Adapted from [2].

SRF half-cells fabricated with EHF have been analyzed by Cantergiani et al. [3] for niobium and by Abajo et

al. [4], [5] for oxygen-free electronic (OFE) copper. The reported advantages and differences of EHF on

OFE-Cu half-cell’s inner surface compared with traditional processes are a thinner damaged surface,

a reduction of roughness, larger grains at the surface, in the macroscopic scale, and a homogeneous

distribution of dislocations [4]. The effect of substrate surface roughness on thin film deposition was

presented in deliverable D3.1 by Stewart Leith (ESR 14) on OFE-Cu specimens prepared by Vanessa Garcia

Diaz (ESR 10) and a minimization of surface roughness was preferred. The fabrication of half-cells with EHF

is then promising for the production of high performance SRF cavities. However, the complexity of the high

velocity sheet forming process, involving multi-physics finite element simulations, first requires a

thorough characterization of the mechanical properties of annealed OFE-Cu at different strain rates.

Results from this work, performed by Jean-Francois Croteau (ESR 9), are presented below.

5.2. CHARACTERIZATION OF ANNEALED OFE-CU FOR EHF

It is fundamental to characterize the mechanical properties of the material at different strain rates to predict

the material’s behavior in multi-physics finite element models (FEM). The parameters of a material

constitutive equation, describing the strength of OFE-Cu at different strains, strain rates and temperature, were

identified. FEM is paramount to determine the EHF sequence to form a half-cell, e.g. the number of electrical

discharges, the energy per discharge, the die precise geometry to account for sheet thinning and springback,

and more. The main results obtained from the mechanical characterization and the forming limit of annealed

OFE-Cu are presented in the two sub-sections below.



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5.2.1. Mechanical Properties at Different Strain Rates

First, the mechanical properties of annealed OFE-Cu, a standardized high purity copper (Cu10100), were

obtained with tests performed at different laboratories. Split Hopkinson tests, a compression high strain rate

characterization technique, were performed at strain rates in the order of 103 s-1. Lower and intermediate strain

rate tensile tests, from 10-3 to 100 s-1, and low strain rate compression tests, from 10-3 to 1 s-1, were also

performed to account for slowly deforming regions of the blank, mostly due to the large inertia of copper.

Tensile tests of specimens pulled along and across the sheet rolling direction showed no anisotropy, which is

an indicator of the annealing step before testing and forming. The OFE-Cu sheets were all annealed in vacuum

at 600°C for 2h. Since the initial dislocation density and average grain size of the sheet define the yield stress and hardening parameters, the characterization of a material in the same initial conditions as the blank used in

half-cell fabrication is essential.

An increase in hardening rate was measured for increasing strain rates in tension and in compression (see

Figure 19 for a comparison of compression results at low and high strain rates). From the experimental results

in both stress states, the parameters of the Johnson-Cook (JC) constitutive equation were identified. Finite

element models of the different tests were created and used in an iterative process in the optimization software

of LS-DYNA, called LS-OPT. Since no tests were performed with tensile split Hopkinson bars, a proprietary

test developed at I-Cube Research was used to deform the material at high strain rates [6].

Figure 19: Annealed OFE-Cu mechanical properties at quasi-static and dynamic strain rates.

The JC equation, as defined in LS-DYNA’s user manual is:

𝜎 = (𝐴 + 𝐵𝜀𝑛) (1 + 𝐶 ln𝜀̇

𝜀0̇) (1 − (

𝑇 − 𝑇0𝑇𝑚 − 𝑇0

)𝑚

)

where 𝜎 is the plastic stress in MPa, 𝐴, 𝐵, 𝐶, 𝑛 and 𝑚 are material dependent constants, 𝜀 is the plastic strain, 𝜀̇ is the strain rate in s−1, 𝜀0̇ is the threshold strain rate at which the rate effect are visible, set to 1 s

−1, 𝑇 is the absolute temperature in K, 𝑇0 is the reference temperature in K, and 𝑇𝑚 is the melting point of the material in K. Constant 𝐴 in the first term of the model represents the yield stress of the material, while 𝐵 and 𝑛 are strain



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hardening constants. The second term has a strain rate hardening constant, 𝐶, while the third term has a temperature softening constant, 𝑚.

The JC parameters identified for compression and tension are presented in Table 2. Note that the tensile tests

performed at intermediate strain rates were not used to identify the JC parameters in tension. Those results

will be used with split Hopkinson tensile results in few months to verify the parameters obtained with a

standard method and to compare strain rate sensitivity in tension and compression.

Table 2: Johnson-Cook parameters of annealed OFE-Cu in tension and compression.

Parameter Proprietary and tensile tests Split Hopkinson and compression tests

A (MPa) 34.25 34.25

B (MPa) 487.47 403.78

n 0.5102 0.3845

C 0.09 0.09

m 1.2 1.2

5.2.2. Forming Limit Diagram

A forming limit diagram (FLD), an engineering tool obtained from standardized Marciniak or Nakajima tests,

is used to predict the onset of localized necking, i.e. the forming limit, in a blank. OFE-Cu sheets of different

widths and 1 mm in thickness were deformed until rupture. Digital image correlation (DIC) was used to

measure the major and minor strains, 𝜀1 and 𝜀2 respectively, at the onset of necking and the strain paths in different regions of the blank. A flat-bottomed punch was used and blank carriers of the same material and

thickness as the analyzed blank were used to initiate the rupture on the top surface of the punch, where

measurements were made. Figure 20 shows the experimental results and numerical data for the Swift-Hill and

Storen-Rice models using a Hollomon constitutive equation. The parameters of the Hollomon equations were

identified with quasi-static tensile tests and the Swift-Hill model shows a better fit with the experiment then

Storen-Rice. However, it is important to note that the strain paths of points with a positive minor strain were

not linear. A second study is then currently ongoing to quantify the effect of the bilinear path that should

produce a lower FLC, due to the biaxial pre-strain [7]. The major strain of the new experimental results is then

expected to be slightly higher.

Now that the mechanical properties and forming limits of annealed OFE-Cu are known, a more

accurate prediction of the manufacturing of half-cells and seamless SRF cavities is possible and should

result in higher performing components. Additionally, the forming limit diagram is useful for traditional

low strain rate forming techniques, too (e.g. spinning and deep-drawing, to predict necking, wrinkling and

other forming defects).



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Figure 20: Experimental FLC with fit of numerical models for a Hollomon hardening equation.



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6. KEY PARAMETERS ON NIOBIUM THICK FILMS DEPOSITION PROCESS

6.1. COPPER SUBSTRATE PREPARATION

In the development of the particle accelerators, cavities surface has been an important topic in the

Superconducting Radio Frequency (SRF) cavities field. The defects on the surface of the cavities, Niobium

Bulk or Niobium on Copper cavities, have strong effects on their performances. In the case of the Nb on Cu

cavities, the surface roughness of the substrate had an immediate effect on the film, which during its growth

will reproduce the substrate surface and the smallest defects will be enhanced.

At INFN-LNL, the study of the 6 GHz cavities could be organized in a protocol as follows:

1. Fabrication of the cavities by spinning

2. Mechanical surface treatment

3. Vibrotumbling

4. Chemical surface treatment

5. High Pressure Rinsing (HPR)

6. Cleanroom assembly

7. Baking at 600 °C for 48 hours

8. Deposition process by Magnetron sputtering technique

9. HPR

10. Cleanroom assembly in RF test bench

11. RF characterization at 4,2 K and 1,8 K

With the idea of improving the performances of the cavities, different fabrication techniques of the cavities

are applied in order to avoid defects. At INFN-LNL, 6GHz cavities are fabricated by spinning to avoid

weldings between the half cells. Nevertheless, defects on the surface of the cavities may be produce during

the mechanical treatment of grinding. These defects may remain even after the chemical treatment of

Electropolishing.

Figure 21: High number of defects on inner surface of cavity after chemical treatments.



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Figure 22: Inner surface of a cavity with low number of defects.

Cavities with significant defects on the copper substrate (inner surface) of the cavity (see Figure 21), had

shown poor RF performances during the RF characterization. While cavities with low amount of defect (see

Figure 22) showed better performances (Figure 23).

Figure 23: RF characterization of cavities with low and high defects amount.

To eliminate deep defects on the surface of the cavities, vibrotumbling technique has been applied to the

cavities. With this technique is possible to remove hundreds of micrometres, which represent the stablish

1.E+07

1.E+08

1.E+09

1.E+10

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Q

Accelerating field (MV/m)

Performances of cavities with low and high amount of defects

Defects Low

Defects High



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deepness of the defects produced by the manufacturing techniques. The previous technique used, mechanical

grinding, was not able to remove the defects in an efficient way, due to the fact that it may led to deep scratches

in the cavities surface. The efficiency of other techniques such as the standard centrifugal barrel polishing

(CBP) lies in the high angular velocity of the abrasive material inside de cavity surface, what makes it

unsuitable for 6GHz cavities due to their small geometry. The vibrotumbling system configuration is

composed by an eccentric vibro-motor, a step motor for the rotation of the cavity, and absorbers for the

vibration. The system is shown in Figure 24.

Figure 24: Vibrotumbling system configuration.

The vibrotumbling technique has been study, optimized and applied at INFN-LNL as a part of the cavities

surface preparation protocol in a two-step process. The first process, with a high removing rate of 3,6um/h

approximately, is applied with aluminium oxide (Al2O3) for 8 hours. The second process, coconut powder is

used to give the cavities surface, the finishing surface.

After the manufacturing technique, the mechanical treatment and vibrotumbling, the cavities are chemically

treated and prepared to apply the coating technique, to finally be characterized respect to their RF behaviour.

The chemical treatment consists in:

1. Ultrasonic degrease in GP 1740 for 60 minutes.

2. Deoxidize process with ammonium persulfate ((NH4)2S2O8) and water at 15g/l ± 5g/l for 30 minutes.

3. Electropolish with a solution of phosphoric acid (H3PO4) and butanol (C4H9OH) in 3:2 ratio for 3

hours.

4. Chemical polish by SUBU5 in a solution of sulfamic acid (H3NSO3), butanol (C4H9OH),

Ammonium citrate (C6H17N3O7) and hydrogen peroxide (H2O2) (5g/l, 50ml/l, 1g/l and 50ml/l

respectively) at 72 °C for 5 minutes.

5. Passivation in a solution of sulfamic acid (H3NSO3) at 10 g/l for 5 minutes.

6. Rinsing with distilled water.



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7. Rinsing of ethanol.

8. Dried with N2.

The process mentioned before, is follow by a High Pressure Rinsing (HPR) with distilled water in order to

remove any powder that may remain inside the cavity to prepare the cavity for the deposition technique.

6.2. EFFECT ON THE RF PERFORMANCES OF THE SINGLE LAYER THICKNESS

The SRF cavities installed in most accelerators are made by bulk Niobium. In the last decades, with the need

of cost reductions for particle accelerator construction, Copper cavities coated with a thin film of Niobium of

approximately 3 microns have been an important field of research in the SRF community. Nb on Cu cavities,

has been installed in accelerators at CERN (ISOLDE, LHC, LEP2) and at LNL-INFN (ALPI). Whereas, the

principal characteristic of these type of cavities is the slope in their performances or Q-slope.

In order to improve the performances of the coated cavities, and to reproduce Niobium bulk-like properties,

thick films of 70 microns approximately are deposited on 6GHz cavities. The deposition on 6GHz cavities by

DC magnetron sputtering is applied by long pulsed deposition, in which the thick film is obtained growing

thin films with thickness between 100nm and 500nm one over the other. EBSD analysis on multilayer

deposited cavity shown a columnar grain growth with bigger grains compared with a cavity deposited in a

single layer (Figure 25).

Figure 25: EBSD characterization on thick film coated cavities. Left: One shot deposition. Right: Multilayer deposition mode.

The thick films deposition on 6GHz cavities is done in the configuration shown in Figure 26. The vacuum

chamber where the deposition take place, is surrounded by a coil that provide a magnetic field of 830 Gauss

homogeneously in the cavity. The Niobium source is a cylindrical target inside the cavity, which is surrounded

by an infrared lamp that allows heating the substrate during the coating up to 550 °C.



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Figure 26: 6GHz magnetron sputtering deposition setup.

Cavities with different single layer deposition thickness were deposited and characterized by their RF

performance. The higher performances are attribute to the cavities that were deposited with a single layer

deposition thickness of 500nm and with a total thickness of 75µm (Figure 27). In the majority of the cases,

these cavities reached higher accelerating fields too. Cavities with 500nm single layer thickness and total

thickness of 40µm will be the subject of further investigation.

Figure 27: RF characterization of 6 GHz cavities deposited with different single layer thickness (100 nm-500 nm) at 1,8 K.



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7. KEY-PARAMETERS ON NIOBIUM NITRIDE DEPOSITION PROCESS

7.1. NIOBIUM NITRIDE AT UNIVERSITY OF SIEGEN

At USIEGEN, we have focused on the synthesis, characterisation and optimisation of NbN thin films

deposited onto OFHC copper substrates. This has been realised through the use of Reactive DC Magnetron

Sputtering (R-DCMS). The NbN thin films were deposited onto copper substrates and silicon witness samples

in a commercial high-volume, fully automated coating tool (CemeCon CC800) using a Nb (RRR 300) target

in the presence of a mixture of argon (99.999 Vol-%) or krypton (99.999 Vol-%) and nitrogen (99.999 Vol-%).

The Ar or Kr to N2 ratio was maintained via flow rate control of the two gases.

Prior to deposition, the system was passively baked at 650 °C for 6 hours, to assist in removing any built up

adsorbents, and thereafter evacuated to a base pressure of 5 x 10-7 hPa. Original samples were sputter etched

with Nb in a 1.5 x 10-3 hPa Ar atmosphere, however this resulted in a Nb interlayer of ~80nm being deposited between the copper and NbN film. For later samples, a new etching process was introduced which utilised an

MF etching procedure. This resulted in a clean surface, free of any defects and was utilised for all samples

going forward. Target plasma cleaning was also utilised to prepare the target for coating. Immediately prior to

deposition, the plasma is ignited with the sample turned away from the target for two minutes before the

samples is moved in front of the target. This conditions the target and allows for a homogeneous film to be

deposited. This is made possible with the turntable on which the substrate sits.

NbN exists in a number of different phases, with three superconducting phases displaying different transition

temperatures. Each of these NbN phases exists within a certain window of nitrogen concentration within the

film, which is directly affected by the deposition parameters. As a result of this, the primary goal of this work

is to obtain a recipe which can reliably deposit the correct NbN phase. The phase we look for is the cubic

δ-NbN phase, which has the highest transition temperature of 17K. The formation of normal conducting

oxides, oxynitrides and voids, between NbN grains, is a known issue with NbN film performance and is

thought to lead to the high resistivity and early flux penetration found in NbN films. As a result of this, we

pursued deposition conditions which lead to a dense and void-free film.

Initial samples were deposited as part of an initial screening study, reported earlier in D3.1. Following this, a

second series of experiments were conducted which aimed to narrow down the results obtained during the

screening study. Argon was chosen as the working gas for the optimisation study due to the high Oxygen

content found in films deposited with Krypton in the screening study. Where not stated, the samples were

deposited with a substrate temperature of 600°C, a pressure of 800mPa, a cathode power of 500W, a substrate

bias of 0V and a nitrogen content of 10%. The varied (adjusted) paramters for each series are described in

Table 3. The deposited films had a thickness of 1.19 μm ± 0.08 μm. These films were deposited onto a copper substrate which was first mechanically polished and then electropolished using a mixture of phosphoric acid

(85%) and butanol in a 3:2 ratio. This process resulted in a substrate roughness of 𝑆𝑞 = 46 ± 3 nm.

Table 3: Detailed NbN investigation parameters.

Series Varied Parameter Set point

1 Nitrogen Content (%) 4, 6, 8, 10, 12, 14, 16, 18, 20, 30

2 Pressure (mPa) 600, 1000, 1400, 1800

3 Bias (V) -25, -50, -75, -100, -150, -200, -300



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The results of these studies have provided significant insight into the workings of the NbN material system

when deposited with sputtering. The change in surface preparation has also drastically changed the resultant

NbN thin film. An overview of the effects of each parameter on the resulting NbN film is provided below.

7.2. KEY-PARAMETERS INVESTIGATED

7.2.1. Substrate Preparation

Initial NbN films were deposited onto a fairly rough copper substrate surface. This was due to the final nitric

acid etching step completed prior to deposition. As a result of this increased substrate roughness, the NbN film

itself also displayed an increased surface roughness as it closely mimics the substrate surface. As displayed in

Figure 28 (a), there are significant voids between the film and substrate as well as significant voids within the

film itself. As a comparison, Figure 28 (b) details the same film, except this film is deposited onto a copper

substrate whose preparation is completed by electropolishing. From the figure it is evident the substrate is far

smoother and flatter, there is a complete lack of voids at the interface between the film and the substrate and

there are no visible voids within the film either. A significant improvement in film quality is thus achieved by

reducing the surface roughness of the underlying substrate.

Figure 28: FIB images showing identical NbN films deposited onto (a) Nitric acid etched copper substrate. (b) Electropolished

copper substrate.

7.2.2. Process Pressure

The process pressure is found to have a large impact on the growth mechanism, with high pressure deposition

tending to grow porous and disconnected columnar films, while low pressure deposition leads to dense films

and adherent columns. This is shown in Figure 29. The disconnected columnar structures obtained with a high

deposition pressure can be adjusted to denser films with the use of an applied substrate bias. A high working

pressure has also been related to an increase in film oxygen content. This could be related to the increased

surface area afforded for reaction due to the disconnect between columns seen in films deposited at higher

pressures.

The increasingly columnar structure of films deposited with increasing deposition pressure has also been

linked to a change in colour of the films surface from silver (non-columnar) to gold (columnar). This is

indicated in Figure 30 for two samples deposited with the same parameters except for the pressure. The



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increasing columnar structure is also related to a change in the surface structure of the NbN grains from a

rounded grain (



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Figure 31: SEM images of (a) NbN film deposited at 600mPa and (b) NbN film deposited at 1800mPa. All other deposition

parameters were maintained.

The pressure required to achieve columnar films with a golden colour is also dependent on the working gas

used. When depositing films with Kr, it is possible to deposit columnar films at pressure < 800mPa, while

with Ar, a pressure > 1200mPa is required for a columnar structure. This is also believed to be linked to a

transition from the deposition of hexagonal 𝛿′-NbN, 𝜖-NbN and other higher order nitrides to 𝛿-NbN, based on XRD and superconducting results.

Due to the nature of the overlapping NbN peaks, it is known to be challenging to characterise NbN phases

from XRD data alone. Figure 32 shows three selected samples which correspond to the Tc measurements

described below. Sample (a) features a phase mixture of 𝛿 and 𝜀-NbN. Here the Nb interlayer can be detected and displays alignment along a growth direction of [110], evident through the (110) and corresponding (220)

XRD peaks. Sample (b) and (c) are identified as cubic 𝛿-NbN films. While (b) shows relatively sharp NbN peaks which are oriented along the [111] growth direction, the NbN peaks in (c) are broadened and exhibit a

different growth orientation in [100] direction. The SEM cross sections of sample (b) and (c), not shown here,

are in good consistency to the XRD results. Sample (b) feature a columnar growth with a diameter of more

than 100 nm, explaining the found orientation and sharp peaks, while for (c) the columns are a few nm in

diameter which, in turn, leads to broadened XRD peaks.

The Tc results for the same selection of samples as described above in the XRD section are presented in Figure

32. All the investigated samples were found to be superconducting, with Tc ranging from 8.2 K to 14.8 K, with

the larger Ar/N2 ratio films proving to be superior. Sample (a) presents two distinct transition points at 8.2 K

and 10.8 K, which indicates the presence of two separate superconducting phases, as confirmed by the XRD

analysis. Sample (b) shows the highest Tc which is in compliance with the above discussed XRD and SEM

results. Crystal size and/or orientation seems to lead to a decrease in the Tc, as revealed by sample (c).



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Figure 32: (Left) XRD scans of three NbN thin film samples indicating the general results of all films. (Right) m(T) measurements

of NbN thin film samples. The determined Tc values are stated.

7.2.3. Nitrogen Percentage (Partial Pressure)

The N2 % has not been found to have a significant influence on the growth mechanisms of NbN films within

the studied parameter window, even though the deposition rate is significantly decreased by increasing the

N2 % during deposition.

Based on the NbN phase diagram, the formation of different phases of NbN is reliant on certain bands of

N2 %. The 𝛿-NbN phase generally occurs in a nearly stoichiometric ratio, however its Nb:N ratio band overlaps with several other hexagonally structured phases. Chemical composition results from the RBS

analysis completed are detailed in Table 4. From the results we can conclude that films coated with10 % ≥𝑥 ≤ 20 % N flow rate are under stoichiometric and within the range defined for 𝛿-NbN while those deposited with > 20 % are over stoichiometric, however the maximum N2 % flow studied of 30 % N2 still leads to the

deposition of films with a Nb:N ratio within the 𝛿-NbN band. The high fraction of N2 within the films indicates a lack of the non-superconducting 𝛽-NbN and 𝛾-NbN phases, which occur in lower bands of N2, for these deposition parameters.

Table 4: Elemental composition of NbN thin films deposited with differing N2 flow rates.

N2 flow

%

Nb

(at.%)

N

(at.%)

O

(at.%)

x

10 48.8 46.2 5 0.95

15 48.75 45.17 6.07 0.93

20 48.30 45.75 6 0.95

25 45.71 51.37 2.90 1.12

30 48.70 50.86 0.43 1.04



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A decrease in Oxygen content with increasing N2 % was found by EDX and confirmed by RBS. Due to the

presence of metallic Nb in under stoichiometric NbN films, as mentioned above, the reaction with oxygen

leads to the measured higher content of oxygen in these films.

7.2.4. Temperature

In order to deposit a dense film, a high substrate temperature is preferred, but not mandatory provided that a

substrate bias and high cathode power is applied. The use of a high deposition temperature has also been linked

to an improved normal state resistivity by other researchers. The densification of the film, brought on by the

use of a higher deposition temperature, likely decreases the formation of oxynitrides between the NbN

columns, thereby improving the normal state resistivity.

7.2.5. Cathode Power

A high cathode power in nearly all cases is needed for a dense film. However, an applied substrate bias can

compensate for a lack of power, provided that there is a high enough mean-free-path (low pressure). Too high

a cathode power though, leads to defective films due to the increased atom flux to the substrate and the

decreased time allowed for diffusion. An increased cathode power also decreases the reaction time between

Nb and N at the substrate surface and thus the N2 % should be adjusted to account for this.



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8. CONCLUSIONS

The present report focuses on key parameters that have the major impact on the superconductive thin film

coating manufacturing. The RF accelerating cavities are the principal application field, but also other aspects

of the accelerators are covered, as for example the development of low resistance materials for the beam

screen. Each EASITrain participant is developing a different superconducting material coating, using different

deposition techniques. As a result, it is possible to identify at two similarities among all the projects:

One key impact factor is the substrate. The coatings performances are strongly influenced by the substrate,

and, for that reason, it’s extremely important study the effect of different material substrates, improve the

substrate forming processes, and develop new methods to prepare the substrates prior to deposition.

Another important finding is the high number of variables that during the coatings manufacturing that

have an impact: substrate material, substrate manufacturing, substrate preparation, deposition technique

coating thickness, gas process pressure, partial pressure, temperature, bias voltage, etc. The high number of

free parameters increases the difficulties in the R&D of the coating process, but at the same time allows

the possibility to obtain higher performances than the corresponding bulk material, and makes the thin

film manufacturing an extremely interesting and active field of research.



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9. REFERENCES

[1] “D. B. Tikhonov, et al., ‘Superconducting Thin Films Characterization at HZB with the Quadrupole

Resonator’, presented at the 19th Int. Conf. RF Superconductivity (SRF’19), Dresden, Germany, 2019,

paper TUP073.”

[2] S. Kalpakjian and S. R. Schmid, Manufacturing engineering and technology, Seventh edition. Upper

Saddle River, NJ: Pearson, 2014.

[3] E. Cantergiani et al., “Niobium superconducting rf cavity fabrication by electrohydraulic forming,” Phys.

Rev. Accel. Beams, p. vol. 19, no. 11, Nov-2016.

[4] C. Abajo, G. Favre, J. Nolin, and E. Cantergiani, “Surface quality and improvements on the SRF cavity

manufacturing by electrohydraulic forming.”

[5] C. Abajo et al., “First results of large size SRF cavity fabrication by Electro-Hydraulic Forming,”

presented at the FCC Week, Berlin, 2017.

[6] A.-C. Jeanson, “Identification du comportement mécanique sous sollicitations dynamiques extrêmes :

Développement d’une stratégie innovante appliquée au magnétoformage et au formage

électrohydraulique,” thesis, Paris Sciences et Lettres, 2016.

[7] J.-L. Geoffroy, J. Goncalves, and X. Lemoine, “Adequately used FLC’s for Simulations,” presented at the

International Deep-drawing Research Group (IDDRG 2007), Győr-Hungary, 2007.

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