PEALD/CVD for Superconducting RF cavities

Paolo PizzolUniversity of Liverpool / STFC –

Daresbury Lab

• Superconductivity Radio Frequencies and Niobium

• Chemical vapour and Atomic layer depositions

• Experimental results and planned work

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Outline

• Superconducting Radio Frequency (SRF) cavities are manifactured using superconductive materials to reach high quality factors and high acceleration gradients.

• Niobium has the highest Critical Magnetic Field (Hc2) = more magnetic field can be accomodated before the superconductivity breaks down = higher acceleration gradients are possible.

SRF - Cavity

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• Niobium SRF cavities are the state of the art for accelerating charged particles, but:

• copper has a better thermal conductivity than Nb à easier to cool down

• a thin film of Nb requires less material à cheaper

Niobium - Limits

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• Problem:Modern accelerators have reached the maximum

gradient achievable by using Nb as a bulk. Over the limit of » 45 MV/m the superconductivity

breaks down!

• Aim of this study:

PhD Topic

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Find something performing as or better than Niobium. Cheaper than Niobium. Easier to cool than Niobium.

Reliable as or more than Niobium.

• Problem:Modern accelerators have reached the maximum

gradient achievable by using Nb as a bulk. Over the limit of » 45 MV/m the superconductivity

breaks down!

• Aim of this study:

PhD Topic

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......any suggestions?

Send them at paolo.pizzol@stfc.ac.uk

• Problem:Modern accelerators have reached the maximum

gradient achievable by using Nb as a bulk. Over the limit of » 45 MV/m the superconductivity

breaks down!

• Aim of this study:

PhD Topic

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Developing PECVD / PEALD deposition techniques to coat copper cavities with an uniform Niobium superconductive

thin layer

The chemical precursors are introduced together in the reaction chamber

Fast: micrometers thick depositions in a few hours

Difficult to control the film thickness uniformity

Niobium chemistry requires high temperatures to work well

Chemical Vapor Deposition (CVD)

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Precursors

The chemical precursors are introduced sequentially in the reaction chamber

Ideal control over the thickness of the deposited layer

Self saturating: only the free surface of the sample interact with the precursors à high conformality

Slow technique: one cycle can last up to a minute, depositing as little as » 10 nm per hour

Surface – precursors interactions driven: the same precursors can behave differently with different substrates

Difficult to deposit monoelemental films with classic ALD à Use of Plasma allows single element deposition

Atomic Layer Deposition (ALD)

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• Swagelok Ò ALD Valves controlled via a bespoke Arduino unit interfaced with a custom made circuit à tested down to 1 millisecond duration pulses

• «Hot walls»» reactor: the entire facility is constantly heated to 120 °C to avoid condensation

• Gas purification system: the gasses entering the facility are purified to limit the amount of contaminants in the deposition chamber (Carbon, Oxygen and water)

Experimental setup

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The chemical side

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The chemical side

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Precursors under study

Niobium Pentachloride (V):• Chosen to obtain metallic Nb layer• Reacts with plasma of H+ to create thin film• Crystalline solid, vapour pressure at 150 °C to

perform ALD• Very sensitive to moisture, hydrolyzes in NbOCl3

• Requires high substrate temperature to reduce Cl contamination in the film (at least 500 °C)

Tris(diethylamido)(tert-butylimido)niobium (V)• Chosen to obtain NbN layer• Reacts with N2 plasma to create thin film• Liquid, good vapour pressure at 104 °C to perform

ALD• Sensitive to heating, start decomposing at 130 °C• Doesn’t require a high deposition temperature

(250 °C) à Suitable for deposition on copper

• First run: Nb on Si... ...unsuccessful. Too much.

• Second run: Nb on Si... ... unsuccessful. Too little.

...until run 5, when...

Deposition results

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NIOBIUM

NbCl5 doesn’t grow well with CVD/ALD on silica.

Deposition results

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Deposition results

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Deposition results

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Very homogeneous niobium film

Copper recrystallizes in bigger grains structure

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• Near future– Optimize the deposition parameters:

• Gas flows• Pressure• Plasma power• Plasma position

• NOT SO near future– Obtain a «nice» sample: wider area covered– Perform in depth characterization– SRF measurements

Plan of action

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THANK YOU FOR YOUR TIME

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