title: Low-temperature co-sputtering for interlayer-free high performance Nb₃Sn thin film coated copper SRF cavities
Creators: name: Schäfer, Nils
Publisher: name: UNSPECIFIED
Year: 2024

description: Thin film Nb₃Sn is a promising candidate to replace bulk niobium cavities for radiofrequency application. New materials, like Nb₃Sn, can increase cryogenic efficiency and acceleration gradients by lower surface resistivity or higher critical magnetic fields. Nb₃Sn co-sputtering is a technique able to control the sputtering rate and kinetic energy of the elements Nb and Sn separately. This allows precise control of the thin film growth and stoichiometry of Nb₃Sn thin films in a broad range. Using this method, it is possible to achieve a critical temperature Tc,0 (point of fully superconducting sample) of 16.8 K and a critical field Hc1 of 50 mT on copper substrate at only 480 °C and 60 min heat exposure without heat treatments after growth. An excellent homogeneity is demonstrated in surface plane and depth. The used synthesis temperature is shown to be sufficient low to suppress detrimental inter-diffusion of the elements. The produced samples strongly shield the external field in both directions, parallel and perpendicular to the external magnetic field. XRD patterns reveal a high phase purity for fast grown samples with a low defect concentration for synthesis temperatures above 400 °C and a strong texture for samples grown at high sputter power. The sputter power exhibits to promote growth of larger grains. However, it has been shown that unfavorable deposition parameters can be lead to inhomogeneous grain boundary conditions that form weak link networks. The origin is found in tin grain boundary segregations leading to performance limiting off-stoichiometry within the grains. Macroscopically, it leads to a broad transition width from normal to superconducting state and additional intermediate resistances appear within the transition. It is further shown how kinetic energy can contribute to homogenize the film and improve the grain boundary condition. Heat treatment of the Nb₃Sn thin films however, is demonstrated to promote grain growth, but also tin segregation and emphasize the grain boundary limitation. Magnetic flux jumps appear for measurements of thin films coated on fused silica substrates. It is then demonstrated that the flux jump issue can be solved by shunting via well conducting substrate (copper in this case) that fulfills the stability criterion. Deposited films of Nb₃Sn appear to have a surface roughness Rq in the range of 12 nm and grain sizes of up to 200 nm as well as an excellent adhesion shown by scratch and indentation tests. The thickness of Nb₃Sn thin films on copper has shown impact on the physical properties critical temperature Tc,0 and critical field Hc1. While the critical temperature decreases for decreasing thickness, the critical field is enhanced up to 190 mT at a film thickness of 240 nm. The low synthesis temperature together with excellent performance of the sputter-coated films pave the way to highly efficient Nb₃Sn thin film coated copper cavity for lowest power loss and optimal heat removal by the low surface resistivity of Nb₃Sn and high heat conductivity of copper.