peald/cvd for superconducting rf cavities paolo pizzol university of liverpool / stfc – daresbury...
TRANSCRIPT
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PEALD/CVD for Superconducting RF cavities
Paolo PizzolUniversity of Liverpool / STFC –
Daresbury Lab
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• Superconductivity Radio Frequencies and Niobium
• Chemical vapour and Atomic layer depositions
• Experimental results and planned work
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Outline
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• 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.
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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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......any suggestions?
Send them at [email protected]
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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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Developing PECVD / PEALD deposition techniques to coat copper cavities with an uniform Niobium superconductive
thin layer
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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
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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
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• 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
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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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