Theories predict that Superconducting-Insulating-Superconducting (SIS) multilayers delay vortex penetration allowing for operation gradients more than twice of bulk Nb cavities and significantly higher Q-values [1]. The University of Hamburg focuses on Atomic Layer Deposition (ALD) as the most promising technique to coat SIS multilayers. A proof-of-principle experiment to coat cavities with an insulator has been successfully carried out, and the complex coating process was numerically modelled, which resulted in a further process time reduction while maintaining the high film quality [2,3]. For SIS multilayer deposition, plasma-enhanced ALD (PEALD) is used to deposit AlN and NbTiN as dielectric and superconducting material, respectively. The deposition process and post-deposition treatments have been optimized by studying the superconducting properties of the NbTiN thin film [4]. Moreover, properties such as flux-trapping behaviour and thermal transmittance of SIS multilayers have been measured. Furthermore, various material characterization techniques were applied to investigate the contribution of vacancy densities, recrystallization eVects due to the annealing past the deposition and the impact of the insulating layer on the properties of SIS multilayers. This talk will show the aggregated results of all those measurements and present the status of the PEALD single-cell cavity coating device at the University of Hamburg.

A program of quantitative measurements of magnetic flux expulsion on flat macroscopic samples has been used to assess and categorise magnetic expulsion efficiency. The measurement setup is a magnetic flux lens based on closed-topological heating/cooling through the material’s superconducting transition. This offers systematic and repeatable expulsion measurements for bulk, thin film and multilayer samples. Of particular interest is the magnetic response of superconductor-insulator-superconductor (SIS) multilayer structures, which can exhibit a response that is characteristically different to that of bulk Niobium, if thermally manipulated in a specific way - this process we term “flux ratcheting”. Flux ratcheting is the incremental expulsion of trapped magnetic flux with repeated, controlled thermal cycles on a SIS sample, such that the trapped flux is incrementally moved (“ratcheted”) out, with limited magnetic relaxation. Measurements indicate flux ratcheting is particular to the SIS structure, and requires the Tc of the surface thin film to be greater than that of the substrate. To assess the impact of flux ratcheting on cavity performance, the application of an SIS structure to a 1.3 GHz bulk Nb cavity has been prepared, and referenced to the baseline performance of the bare Nb cavity. The RF performance with and without flux ratcheting is compared, and first implications of magnetic flux ratcheting to RF cavity performance are discussed.

Superconducting–Insulating–Superconducting (SIS) multilayers offer a promising approach to surpass the accelerating gradients and quality factors of standard bulk-Nb SRF cavities†. Plasma-enhanced atomic layer deposition (PEALD) stands out as a key technique for the next-generation thin-film-based SRF cavities, providing conformal coatings on highly structured, three-dimensional substrates without shadowing effects and with sub-nm thickness precision. This poster contributes to thin-film SRF R&D through dedicated material studies. The results presented correspond to Nb–AlN–NbTiN multilayers grown by PEALD, focusing on the S–I and I–S interfaces. Depth-resolved X-ray photoelectron spectroscopy (XPS) and cross-sectional energy-dispersive X-ray spectroscopy (EDX) are employed to assess the film stoichiometry and detect any interdiffusion or deposition residues. Side effects induced by high-temperature post-deposition annealing–required to obtain high-Tc NbTiN‡–are systematically investigated. Lastly, complementary studies on Superconducting–Superconducting (SS) Nb–NbTiN bilayers–grown without the AlN interlayer–underscore the crucial role of AlN as an effective diffusion barrier.