1. PEALD/CVD for Superconducting RF cavities
Paolo Pizzol
University of Liverpool / STFC – Daresbury Lab

2. Outline Superconductivity Radio Frequencies and Niobium
Chemical vapour and Atomic layer depositions
Experimental results and planned work

3. SRF - Cavity
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.

4. Niobium - Limits
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

5. Over the limit of » 45 MV/m the superconductivity breaks down!
PhD Topic
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:
Find something performing as or better than Niobium. Cheaper than Niobium. Easier to cool than Niobium. Reliable as or more than Niobium.

6. PhD Topic
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:
...
...any suggestions?
Send them at

7. Over the limit of » 45 MV/m the superconductivity breaks down!
PhD Topic
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:
Developing PECVD / PEALD deposition techniques to coat copper cavities with an uniform Niobium superconductive thin layer

8. Chemical Vapor Deposition (CVD)
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
Precursors

9. Atomic Layer Deposition (ALD)
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

10. Experimental setup
SwagelokÒ 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)

11. The chemical side

12. The chemical side 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

13. Deposition results First run: Nb on Si... ...unsuccessful. Too much.
Second run: Nb on Si...
... unsuccessful. Too little.
...until run 5, when...

14. NIOBIUM

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

16. Deposition results

17. Deposition results

18. Very homogeneous niobium film
Copper recrystallizes in bigger grains structure

21. Plan of action Near future NOT SO 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

22. Thank you for your time