TY  - THES
A3  - Jiang, Xin
AB  - Thin film coated Superconducting Radio Frequency (SRF) cavities are an attractive substitute for the current industry standard of bulk Nb. As such, the SRF community has, in recent years, engaged in a significant amount of research into the use of alternative materials and fabrication techniques. This dissertation forms part of this body of work and presents the findings of experiments aimed at improving the current operational performance of thin-film-based SRF cavities through the use of improved deposition techniques, alternative materials and multilayer Superconductor-Insulator-Superconductor (SIS) thin film coatings. This work focused on the deposition of individual Nb and NbN thin films, as well as multilayer SIS film coatings onto Oxygen Free High Conductivity (OFHC) Cu substrates. The SIS system studied here consisted of a single tri-layer of Nb/AlN/NbN and required optimisation of each of the individual material systems. The films were deposited using both Direct Current Magnetron Sputtering (DC MS) and High Power Impulse Magnetron Sputtering (HiPIMS) techniques, allowing for a direct comparison of the two as well as observation of the improvements offered by energetic deposition techniques, such as HiPIMS.
An assortment of Cu surface preparation techniques and procedures, including mechanical polishing, chemical polishing and electropolishing, have been explored as part of a surface-preparation optimisation study. The resultant topography of the Cu sample surface, following the use of the optimised process, typically exceeded current industry standards.
Two separate series of NbN films have been deposited onto Cu samples, using either DC MS or HiPIMS techniques. The relationship between the different deposition parameters and the morphological, crystallographic and superconducting properties of these films has been examined. The performance of the NbN films is strictly related to the formation of the correct NbN crystallographic phase. An interplay between the deposition pressure, cathode power and nitrogen-gas flow rate was observed for both DC MS and HiPIMS films, with the optimisation of these parameters of primary importance. The substrate bias was found to play a significant role in the HiPIMS deposition process, with too high a substrate bias resulting in a phase change of the NbN films. Nevertheless, the use of HiPIMS led to substantial improvements to both the density and topography of the NbN films, resulting in a corresponding increase in the superconducting performance of the HiPIMS NbN samples compared to the DC MS NbN samples.
DC MS Nb coatings typically suffer from poor interface adhesion and low film density. One way to improve this is to utilise an energetic deposition technique, such as HiPIMS. The morphological, crystallographic and superconducting properties of Nb films deposited with
HiPIMS were analysed in terms of the changing deposition parameters. A significant reduction in the number of voids at the interface between the HiPIMS-deposited Nb films and the Cu substrate, compared to DC MS-deposited Nb films, was observed. In terms of their superconducting performance, Nb films characterised by bulk-like crystallographic properties were found to be vastly superior. Three specific deposition parameters: the substrate temperature, substrate bias and film thickness were identified as the most significant deposition parameters in this regard.
The SIS film coatings were divided into three separate series: DC MS SIS films, a combination of HiPIMS Nb and DC MS NbN SIS films and finally, HiPIMS SIS films. Microstructurally, the layers were coherent and epitaxially grown for all films, with morphological and topographical improvements of both the Nb and NbN constituent layers provided by the use of HiPIMS. The entry field of the outer shielding layer, NbN in this instance, and the topography of the base Nb layer were found to be critical to the performance of the SIS film structure. The transition to the use of HiPIMS, for both the Nb and NbN layers, led to significant performance improvements compared to DC MS-deposited SIS films. These results provide evidence for the efficacy of SIS film coatings on Cu substrates. Nevertheless, further optimisation of the SIS film coating procedure is required to fully realise the potential of SIS film coatings.
This study culminated in the deposition of the three separate SIS film coatings on a series of QPR samples for additional comparisons. This provided further insight into the use of these coatings in conditions more similar to those found in SRF cavities. The test results provided valuable feedback and some promising outcomes.
AU  - Leith, Stewart
DA  - 2021
DO  - 10.25819/ubsi/10031
KW  - Supraleiter
KW  - Superconductivity
KW  - Thin fIlms
KW  - Sputtering
KW  - Particle accelerator
KW  - Dünnschichttechnik
KW  - PVD-Verfahren
KW  - Teilchenbeschleuniger
LA  - eng
PY  - 2021
TI  - Development of novel superconducting thin films for use in superconducting radio frequency cavities
TT  - Entwicklung neuartiger supraleitender Dünnschichten für supraleitende Radiofrequenz Hohlraumresonatoren
UR  - https://nbn-resolving.org/urn:nbn:de:hbz:467-21088
Y2  - 2024-12-26T21:10:38
ER  -