Microstructure investigation of superconducting NbN thin films on copper
The use of superconducting thin films on copper is a proven technology for the application in SRF accelerating cavities, with Nb-coated cavities already used both by LEP and LHC at CERN. NbN offers several advantages over Nb, such as a higher superconducting transition temperature and a higher critical field. In this respect, superconducting NbN thin films deposited onto copper by DC magnetron sputtering have been explored as a means to improve the theoretical limits of cavity performance. Coated-copper accelerating cavities provide a significant cost saving compared to bulk Nb. Moreover, the thermal conductivity of the cavity is strongly improved. To optimize the thin-film microstructure, cross-sectional transmission electron microscopy (TEM) provides essential insight in the structure-property-relationship.
The analysis includes the detailed characterization of the obtained composition and crystal structure of the NbN as well as the interface between the NbN thin film and the pure Cu substrate. Emphasis is placed on the NbN nucleation and morphological evolution during further growth.
Conventional cross-section preparation in conjunction with optimized single-sector Ar-ion milling is applied . For high performance NbN thin films, TEM revealed a well-ordered columnar morphology with a high degree of crystallographic texture. However, the crystallinity and morphology of the NbN thin films are highly dependent on the applied deposition conditions.
The EASITrain project has received funding from the European Union's Horizon 2020 research and innovation programme under grant No. 764879. The information herein only reflects the views of its authors and the European Commission is not responsible for any use that may be made of the information.
Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen.