Effects of the thin-film thickness on superconducting NbTi microwave resonators for on-chip cryogenic thermometry

Abstract

Superconducting microwave resonators have recently gained a primary importance in the development of cryogenic applications, such as circuit quantum electrodynamics, electron spin resonance spectroscopy and particles detection for high-energy physics and astrophysics. In this work, we investigate the influence of the film thickness on the temperature response of microfabricated Nb50Ti50 superconducting resonators. S-shaped split ring resonators (S-SRRs), 20 nm to 150 nm thick, are designed to be electromagnetically coupled with standard Cu coplanar waveguides (CPWs) and their microwave properties are characterized at temperatures below 10 K. The combined contributions of the kinetic inductance LK(T) increase and the decreasing loaded quality factor QL, for thinner films, induce an optimum condition on the temperature sensitivity and resolution of the resonators. A noise equivalent temperature (NET) as low as 0.5 uK/Hz^(1/2), at 1 Hz, is reported for 100 nm thick resonators at 4.2 K. We also asses the possibility of implementing a multiplexed frequency readout, allowing for the simultaneous temperature tracking of several sensors along a single CPW. Such results demonstrate the possibility to perform a distributed cryogenic temperature monitoring, with a sub-mK resolution. Thus, the application of superconducting S-SRRs, eventually benefiting from an even higher LK(T), for a further miniaturization, as well as a back-end integration directly on-chip, can be envisioned for the accurate monitoring of localized temperature of devices operating in cryogenic conditions.