Universität Siegen
For the last few decades, the material of choice for Superconducting radio frequency (SRF) cavities has been bulk niobium (\(Nb\)), but its RF performance has already approached its theoretical limit in terms of accelerating gradient. In recent years, to improve further performance and economical operation of RF cavities, research interest has shifted towards alternative materials with SRF performance beyond bulk Nb. This research largely delved into the use of superconducting thin films of \(Nb\) or other alternative higher \(T_{c}\) materials. A potential advancement involves coating the inner surface of a copper cavity with a superconducting thin film. This method leverages cost-effectiveness and higher thermal conductivity compared to bulk Nb. However, the use of alternative superconducting materials may not allow high accelerating gradients and quality factors greater than \(Nb\) due to their smaller \(H_{c1}\). Addressing this problem, Alex Gurevich in 2006 proposed a new theory related to alternating multilayer coatings to shield an underlying superconductor from the applied magnetic fields, thereby increasing the maximum accelerating gradient beyond the bulk Nb limits. \(NbTiN\) is one of the most promising alternative materials to \(Nb\), already displayed high quality factors in coated cavities for research. The present work exclusively focuses on the deposition of high \(T_{c}\) \((17.3\ K)\) \(NbTiN\) thin films. We used the industrial coating machine, CC800, to deposit single layers of NbTiN thin films onto silicon (\(Si\)), thick film of \(Nb\), and aluminum nitride (\(AlN\)) substrates, using both DC Magnetron Sputtering and High Power Impulse Magnetron Sputtering (HiPIMS) techniques. The primary focus here is solely on optimizing \(NbTiN\) thin films for potential future use in alternating multilayer layer structures. The impact of various deposition parameters on the microstructure, phase formation, and subsequent superconducting properties of \(NbTiN\) films deposited on various substrates are presented. The results display a change in the microstructure and superconducting properties with the change in deposition parameters. Furthermore, HiPIMS yields films characterized by higher density and fewer voids in comparison to DCMS. Following the successful optimization of \(NbTiN\) thin films, they will be utilized for the development of multilayer coatings.
Abstract
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