Studies on Plasma-Enhanced Atomic Layer Deposition of Superconducting- Insulating-Superconducting Multilayers for Use in SRF Cavities

Abstract

Superconducting-Insulating-Superconducting (SIS) multilayers offer a promising approach to surpass the accelerating gradients of standard bulk Nb superconducting radio frequency (SRF) cavities, while enabling more efficient and sustainable accelerators. This dissertation contributes to the pursuit of higher accelerating gradients and improved quality factors by tailoring SRF cavities, focusing on the synthesis and characterisation of SIS multilayers to pave the way for plasma-enhanced atomic layer deposition (PEALD)-based SRF cavities. One of the biggest challenges in developing SIS-based SRF cavities is achieving conformal and uniform coatings on the cavity shape. PEALD is the most promising technique, as it allows for conformal coating of highly structured, three-dimensional substrates without shadowing effect and with sub-nm thickness precision. Furthermore, it is a mature technology already established in industrial processes. The SIS multilayers studied consisted of AlN-NbTiN deposited on planar substrates: on Nb, representing cavity-relevant conditions, and on Si to offer insights into the intrinsic material properties. The optimisation of the SIS multilayers synthesis by PEALD was carried out by studying the superconducting properties of the NbTiN thin films. Fundamental studies were conducted using various material analysis techniques to assess the film microstructure, composition, crystal structure, and superconducting performance. The PEALD-deposited thin films were found to be smooth, which is essential for sustaining high fields in SRF cavities by minimizing field enhancements and preventing early quenching or losses. The study emphasised the interface between the superconducting and insulating films. The NbTiN composition (ratio of Nb to Ti within the ternary) was confirmed to be precisely tailored by the PEALD deposition process. The superconducting performance of NbTiN films is related to the composition and crystal structure of the film. The ratio of Nb to Ti was adjusted to 3:1 for an improved superconducting transition temperature. The PEALD-deposited NbTiN films formed the phase of interest δ-NbTiN. Post-deposition thermal procedures were found to promote crystal growth and impurities outgassing, significantly enhancing the superconducting performance of NbTiN thin films. The post-deposition annealing process was optimised, with higher annealing temperatures resulting in greater enhancements. The impact of annealing on SIS multilayers—microstructure, morphology, composition, crystal structure, and superconducting performance—was evaluated. SIS multilayers produced by PEALD met the necessary criteria for potential implementation in SRF cavities and demonstrated suitability for cavity preparation processes. Nevertheless, RF performance tests are required to assess the effectiveness of SIS coatings.