resistivity measurements on coated collimator materials

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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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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3. DCMS resistivity is higher than HIPIMS,

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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










26










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

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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

resistivity measurements on coated collimator materials

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