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

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

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.

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

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.

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 paolo.pizzol@stfc.ac.uk

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

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

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

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)

The chemical side

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

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

NIOBIUM

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

Deposition results

Deposition results

Very homogeneous niobium film Copper recrystallizes in bigger grains structure

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

Thank you for your time