Surface modification of superconducting radiofrequency (SRF) cavities is mandatory to further push the limits in future accelerators. One strategy is the deposition of a multilayer of superconducting and insulating materials on top of the inner surface of a SRF cavity. Here we report on a successful low-temperature coating of a SRF cavity with insulating Al2O3 by thermal atomic layer deposition (ALD) without mitigating its maximum achievable accelerating field of more than 40\,MV/m. Furthermore, an improvement of the surface resistance above 30\,MV/m has been observed, which is likely caused by an enhanced oxygen diffusion during the deposition process. Our results show that ALD is perfectly suited to conformally coat the interior of the cavity and to even modify and improve the properties of such devices.

Next-generation superconducting radio frequency (SRF) cavities, based on tailored thin films, would allow for more efficient and sustainable accelerators operating at higher accelerating gradients. In particular, superconductor–insulator–superconductor (SIS) multilayers are proposed as a potential alternative to bulk Nb. In this context, NbTiN stands out as a superconducting candidate. Here, we report our studies on NbTiN thin films grown by plasma-enhanced atomic layer deposition (PEALD) in a supercycle approach on AlN in situ deposited on planar silicon substrates. In detail, different ternary compound compositions and thicknesses have been investigated concerning the elemental composition, the superconducting properties, and the crystallinity of the deposited thin films. Two different post-deposition thermal treatments have been applied to Nb0.75Ti0.25N thin films of different thicknesses. Their effect on the film properties has been evaluated. It has been demonstrated that an optimized post-deposition thermal annealing procedure significantly improves the quality of our PEALD deposited Nb0.75Ti0.25N thin films, achieving the highest superconducting critical temperature (Tc) of 15.9 K obtained for films deposited by atomic layer deposition (ALD) so far and a lower critical field (Hc1) of 213 mT, which overpasses the bulk Nb intrinsic limit of 200 mT. Our studies are a promising first stepping stone on the path toward tailored thin films based SRF cavities.

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