Surface nanostructuring of NbTi superconducting thin-film resonators for enhanced cryogenic thermometry

Abstract

The rising complexity of cutting-edge cryogenic systems is currently imposing challenging technical constraints to the monitoring of ultra-cold temperatures through standard commercially available sensors. Among different alternative technologies, superconducting microwave resonators have been recently investigated as ideal candidates for performing on-chip cryogenic thermometry, in reason of their intrinsically low power dissipation, typically large temperature sensitivities and excellent sub-mK resolution below 10 K. In such a framework, through this study we aim at demonstrating the possibility to enhance the temperature performance of superconducting microwave resonators by means of surface nanostructuring. More specifically, different arrays of nanogaps are strategically patterned on the inductive line of a 1.3 GHz planar resonator, by partially etching a Nb50Ti50 thin film, in order to tune the critical transition of the material and, therefore, increase the curvature of the fres(T) response. Although the presence of such weak-links introduces larger microwave losses, a 1.5 K decrease of TC is recorded, which directly translates into an enhancement of the temperature sensitivity by a factor 10, with respect to a reference non-nanostructured sensor. In particular, a maximum value of dfres/dT = 62 MHz/K, at 4.2 K, is achieved for the device showing the largest nanogap width of about 350 nm, demonstrating that the surface nanostructuring of superconducting thin-films can be effectively engineered to enhance the temperature response of microwave resonators for high-performance cryogenic thermometry. We believe that similar approaches might be investigated and, eventually, adopted for the near-future development of the next generation of low-temperature sensors.