Resistivity measurements on
coated collimator materials
C. Accettura, D. Amorim, S. A. Antipov, A. Baris, A. Bertarelli, N. Biancacci,
S. Calatroni, F. Carra, F. Caspers, E. Garcıa Tabarés Valdivieso, J. Guardia-Valenzuela,
A. Kurtulus, A. Mereghetti, E. Métral, S. Redaelli, B. Salvant, M. Taborelli, W. Vollenberg
COLUSM, 28/02/2020
Acknowledgements: N. Catalan Lasheras and the BE/RF group, F. Di Lorenzo, D. Gacon, R. Martinez, A. Perez-Fontenla, J.
Busom Descarrega and A. Lunt for the metallurgical support, EN/STI and TE/VSC groups
INTRODUCTION
• The impedance of LHC collimators is largely dominant over a wide frequency
range.
• This is mainly due to the collimator proximity to the beam and high resistivity of the
jaw material (CFC AC150K).
• Without any mitigation measure, impedance driven instabilities would limit the
performance expected for the HL-LHC project: octupole current not sufficient to
stabilize the beam.
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570 A octupole limit
INTRODUCTION
• New graphitic jaw materials have been considered to lower the collimator
impedance: presently used is MoGr NB-8304Ng by Nanonker
• IR7 collimators’ jaw made by MoGr, for primaries, and MoGr coated with Mo, for
secondaries, have been selected as baseline option for the impedance reduction.
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INTRODUCTION
• New graphitic jaw materials have been considered to lower the collimator
impedance: presently used is MoGr NB-8304Ng by Nanonker
• IR7 collimators’ jaw made by MoGr, for primaries, and MoGr coated with Mo, for
secondaries, have been selected as baseline option for the impedance reduction.
• Stability would be significantly improved (-250 A octupole current)
• Test in LHC was done with a prototype (TCSPM) to validate the baseline option [1].
[1] S.Antipov et al. “Transverse Beam Stability with Low-Impedance Collimators in the High Luminosity Large Hadron Collider: Status and Challenges”, submitted to PRAB.
Three material under beam test:
1. 5um coating of Mo on MoGr
2. Uncoated MoGr
3. 5um coating of TiN on MoGr
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INTRODUCTION
• Higher than expected Mo resistivity was measured with beam triggering
attention to the coating process and final resistivity measurements.
• SEM observation identified micrometric clusters on the surface
• A dedicated investigation campaign started.
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MEASUREMENTS ON COATED SAMPLES
Two sputtering techniques have been used for Mo coating production on MoGr:
Direct Current Magnetron Sputtering (DCMS) and High Power Impulse
Magnetron Sputtering (HIPIMS)
As a comparison, Mo coating on graphite was performed as well (SGL R4550
and R7550 equivalent grades) with the same techniques.
Systematic resistivity measurements were performed with three different
techniques:
• DC
• Eddy-current (<2 MHz)
• H011 cavity (16.5 GHz)
Systematic FIB-SEM observation were performed and associated to the
resistivity measured.
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DC measurements (thick substrates)
Standard 4-probes measurements were performed on thick (few mm) samples
of MoGr, graphite and CFC for reference.
Voltage is applied on three orthotropic directions 𝑋, 𝑌, 𝑍.
CFC: good only in beam direction,.
Graphite: isotropic.
MoGr: good only in-plane (𝑋 − 𝑍), more resistive through-plane
beam𝒁
𝒀
𝑿
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MoGr in-plane variation along depth
Due to the manufacturing process, the MoGr in-plane resistivity is changing with depth
(measurements done on NB-8304Je grade, similar to NB-8304Ng).
𝒁𝒀
𝑿 𝑿
0.72𝑀𝑆/𝑚(1.38 𝜇Ωm)
0.95𝑀𝑆/𝑚(1.05 𝜇Ωm)
We observe:
• Resistivity variation with depth (follows density profile).
• Not an issue for production jaws (resistivity is the lowest on the surface)
Caveats:
o DC measurements through thick blocks sample the full curve.
o Samples for our analysis are not necessarily taken from surfaces.
Top surface
Bottom surface
Explains the higher
resistivity measured.
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DC measurements (thin substrates)
Modified 4-probes measurements were performed on thin (150 nm) samples of MoGr,
graphite and CFC.
• Thin stripe-electrodes apply voltage on the top surface.
• Substrate resistivity is measured on uncoated samples first.
• Substrate thickness is small enough to give a comparable resistance to the applied
Mo coating (5𝜇𝑚)
• Measurements compatible with previous ones.
• Larger uncertainty due to setup resolution.
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DC measurements (coatings)
Once the substrate is known the same procedure is applied for coated substrates.
• Resistivity of DCMS coating systematically higher then HIPIMS.
• HIPIMS on CFC not performed due to porous structure of the material.
• Mo on graphite are similar on both substrates.
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Eddy current testing (ECT)
Induced currents from a coil can be used to probe material properties.
• Well established technique to find surface defects and material thickness.
• Not so commonly used for thin coating resistivity assessment.
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Eddy current testing (ECT)
Induced currents from a coil can be used to probe material properties.
• Well established technique to find surface defects and material thickness.
• Not so commonly used for thin coating resistivity assessment.
ECT is applied on three configurations:
A: coated surface is close to the coil
B: un-coated surface is close to the coil
C: coil is in air
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ECT for substrates
We measure the change in input impedance between B and C configurations:
Example for graphite
• The same configuration is computed analytically varying the unknown substrate resistivity.
• By least square comparison we derive the resistivity vs frequency
The average resistivity is in-line with DC measurements.
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ECT for coatings
We measure the change in input impedance between A and B configurations:
Applying the same procedure as for the substrates:
• Mo coating DCMS on MoGr exhibits higher resistivity than in HIPIMS which is close to the
theoretical value of Mo.
• Mo coating DCMS on graphite exhibits same relative behaviour as MoGr but larger absolute
values.
Example for graphite
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RF H011 cavity (coatings)
Problem: DC and ECT coating measurements need always to take out the contribution
of the substrate resistivity.
Solution: H011 cavity to probe coating resistivity → 16.5 𝐺𝐻𝑧 (𝛿𝑠𝑘𝑖𝑛~1𝜇𝑚), bulk invisible.
H011 mode is insensitive to contacts -> used to probe different materials.
As for DC and ECT:
• Mo coating DCMS on MoGr exhibits higher resistivity than in HIPIMS which is close to the
theoretical value of Mo.
• Mo coating DCMS on graphite exhibits same relative behaviour as MoGr but larger absolute
values.
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RF H011 cavity (substrates)
Roughness effect can be important at operation frequency: taken into account with
gradient model.
• From optical measurements we can measure the roughness profile.
• We associate a density probability 𝑝(𝑥) and the cumulative density function which represents the
bearing contact area 𝐶𝐷𝐹(𝑥). • Conductivity is assumed to change proportionally to the 𝐶𝐷𝐹 function, 𝜎 𝑥 = 𝜎0𝐶𝐷𝐹(𝑥).
The power loss along depth is computed and the effective resistivity for a smooth surface deduced.
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MoGr
RF H011 cavity (substrates)
Using measured roughness profiles (not for graphite although, 𝑅𝑞 is used) we deduced
the effective resistivity increase on top of the value measured with ECT.
The agreement is satisfactory and the method allows us to get the DC resistivity from
RF measurements*
* Bearing in mind that materials like MoGr exhibit in-plane resistivity variation along depth…
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Summary of three methods (coatings)
DC, ECT and RF are compared for MoGr and graphite, in HIPIMS and DCMS.
1. ECT and RF generally in good agreement. Note that no roughness computation has been done
with the coating -> if any it should be a small contribution.
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Summary of three methods (coatings)
DC, ECT and RF are compared for MoGr and graphite, in HIPIMS and DCMS.
1. ECT and RF generally in good agreement. Note that no roughness computation has been done
with the coating -> if any it should be a small contribution.
2. DC and ECT/RF show lower resistivity → could be related to the very thin (150 um) size of the
samples used in DC (higher temperature reached, coating further annealing).
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Summary of three methods (coatings)
DC, ECT and RF are compared for MoGr and graphite, in HIPIMS and DCMS.
1. ECT and RF generally in good agreement. Note that no roughness computation has been done
with the coating -> if any it should be a small contribution.
2. DC and ECT/RF show lower resistivity → could be related to the very thin (150 um) size of the
samples used in DC (higher temperature reached, coating further annealing).
3. DCMS resistivity is higher than HIPIMS,
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Summary of three methods (coatings)
DC, ECT and RF are compared for MoGr and graphite, in HIPIMS and DCMS.
1. ECT and RF generally in good agreement. Note that no roughness computation has been done
with the coating -> if any it should be a small contribution.
2. DC and ECT/RF show lower resistivity → could be related to the very thin (150 um) size of the
samples used in DC (higher temperature reached, coating further annealing).
3. DCMS resistivity is higher than HIPIMS,
4. Coatings done on MoGr behave better than those on graphite (not the case in DC, see point 2).
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MICROSTRUCTURE OBSERVATIONS
The observed discrepancies triggered additional SEM and FIB-SEM analysis.
DCMS, Mo on MoGr HIPIMS, Mo on MoGr
• Mo coating done on MoGr with HIPIMS looks better connected.
• No surface protuberances, grains barely distinguishable from surface.
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MICROSTRUCTURE OBSERVATIONS
The observed discrepancies triggered additional SEM and FIB-SEM analysis.
DCMS, Mo on graphite HIPIMS, Mo on graphite
• Less smooth result when coating is done on graphite.
• In both cases we see grains clustering on the surface.
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Grain size observations
FIB-SEM analysis transverse cut shows no major differences in grain size
between HIPIMS on MoGr or gaphite.
HIPIMS, Mo on graphite
HIPIMS, Mo on MoGr
By eye inspection we deduce 0.2 − 0.3𝜇𝑚 grain size:
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Grain boundary effects
The different resistivity behavior can be qualitatively explained accounting for the
current transmission 𝑇 between grain boundaries (Mayadas-Shatzkes model):
• Well connected grains → 𝑇 = 1• Grains separated → 𝑇 = 0
The increase in resistivity with
respect to the bulk metal 𝜌𝑔/𝜌0 is
related to the size of the grains w.r.t.
mean free path 𝐷/𝜆∞ and current
transmission across boundaries 𝑇.
25
Grain boundary effects
The different resistivity behavior can be qualitatively explained accounting for the
current transmission 𝑇 between grain boundaries (Mayadas-Shatzkes model):
• Well connected grains → 𝑇 = 1• Grains separated → 𝑇 = 0
The increase in resistivity with
respect to the bulk metal 𝜌𝑔/𝜌0 is
related to the size of the grains w.r.t.
mean free path 𝐷/𝜆∞ and current
transmission across boundaries 𝑇.
26
Grain boundary effects
The different resistivity behavior can be qualitatively explained accounting for the
current transmission 𝑇 between grain boundaries (Mayadas-Shatzkes model):
• Well connected grains → 𝑇 = 1• Grains separated → 𝑇 = 0
The increase in resistivity with
respect to the bulk metal 𝜌𝑔/𝜌0 is
related to the size of the grains w.r.t.
mean free path 𝐷/𝜆∞ and current
transmission across boundaries 𝑇.
Grain size is not largely changing for DCMS/HIMPIMS on MoGr/graphite:
• The increase in resistivity mainly relates to lower 𝑇• A lower 𝑇 might be a consequence of a rougher substrate
• Enhanced mobility in HIPIMS can partially increase 𝑇
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Transmission effect
Gathering the best match for the transmission parameter we complete the table as
To understand the source of the low transmission in graphite we studied the
substrate surface characteristics.
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Role of substrate….
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MoGr graphite
MoGr and graphite were ion-polished and observed in the through-plane direction:
• While voids on MoGr are filled by Mo, they are empty on graphite.
… on final coating
MoGr graphite
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MoGr and graphite were ion-polished and observed in the through-plane direction:
• While voids on MoGr are filled by Mo, they are empty on graphite.
• Grains growing on them can show detachment from the ones growing on the nearby surface.
Summary and next steps• Stability of HL-LHC beams is strongly relying on the reduction of IR7 collimators by
Mo coating on MoGr.
• The resistivity of Mo coating was measured both on MoGr and graphite produced in
HIPIMS and DCMS.
• Three techniques used: DC, ECT and RF with relative agreement between them.
• It is confirmed the lower resistivity for HIPIMS Mo coating on MoGr (as Mo bulk)
• It has been investigated the worse performance of DCMS w.r.t. HIPIMS and related
to lower current transmission between grain boundaries.
• When performed on graphite the coating is also showing higher resistivity, likely
related to the large voids not present on MoGr.
Further activities are going on:
• Detailed surface roughness characterization of graphite substrate.
• Analysis of Cu coating on Graphite: why it is performing as bulk?
• General follow up of batch production samples.
• Preparation for irradiation tests on 20x20mm samples.
• Publishing an article to Coatings journal (submitted)
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Many thanks!
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BACKUP
33
34
Comparison with Mo
35
Both sample characterized by
discontinuities coming from the bulk
(Gr)Cu Mo
Comparison with Mo
36
Different
structure of
the Mo and
copper
coating
investigated
with FIB
Cu Mo
Cu coating FIB
37
The coating is
continuous, but
some crack are
located in
correspondence
of bulk porosities
Comparison with Mo
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Cu Mo
Grain size of copper coating almost 2-3 times bigger with respect to Mo
Comparison with Mo
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Grain size of copper coating almost 2-3 times bigger with respect to Mo can this justify why we don’t
see the effect of the transmission factor on the coating conductivity?
• If we increase the grain size and we keep the same T of Mo, we reduce the difference in resistivity
between the two T curves from 21.5
From where does it comes the other difference:
• Carbide formation on Mo that we don’t have on Cu coating
• Better transmission on Cu
Observation:
• Cu coating on graphite~Cu coating on MoGr
• Mo coating on graphite~2 times less conductive with
respect to Mo coating on MoGr