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IEEE Transactions on Terahertz Science and Technology

Separation of Loss Mechanisms in Nb Superconducting Thin-Film Transmission Lines

Wenlei Shan, Shohei Ezaki

Published Oct 10, 2024• 0 citations
Publisher

Abstract

Transmission losses in Nb superconducting coplanar waveguide (CPW) and microstrip lines were quantified at 2-mm wavelength and 3.3 K using a network analyzer. The loss measurement utilized both half-wavelength resonators and electrically long transmission lines. Validation of the measurement was achieved using a silicon membrane-based waveguide-to-CPW transition with low return loss across a broad bandwidth. Intrinsic losses were measured within a parametric space defined by frequency, temperature, signal power, and magnetic field. The multidimensional information allowed the breakdown of overall losses into major components, including quasiparticle loss, two-level system dielectric loss, vortex flow loss, and radiation loss. The performance of these transmission lines was extrapolated into the terahertz range based on these findings.

DOI
10.1109/tthz.2024.3477914

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Transmission losses in Nb superconducting coplanar waveguide (CPW) and microstrip lines were quantified at 2-mm wavelength and 3.3 K using a network analyzer. The loss measurement utilized both half-wavelength resonators and electrically long transmission lines. Validation of the measurement was achieved using a silicon membrane-based waveguide-to-CPW transition with low return loss across a broad bandwidth. Intrinsic losses were measured within a parametric space defined by frequency, temperature, signal power, and magnetic field. The multidimensional information allowed the breakdown of overall losses into major components, including quasiparticle loss, two-level system dielectric loss, vortex flow loss, and radiation loss. The performance of these transmission lines was extrapolated into the terahertz range based on these findings.
Investigations into the propagation characteristics, specifically loss and wave velocity, of superconducting coplanar waveguides and microstrip lines were conducted at a 2 mm wavelength. This was achieved through the measurement of on-chip half-wavelength resonators, employing superconductor-insulator-superconductor tunnel junctions as detectors. A continuous wave millimeter wave probe signal was introduced to the chip via a silicon membrane-based orthomode transducer. This setup not only facilitated the injection of the probe signal but also provided a reference path essential for differential measurements. The observed resonance frequencies aligned closely with theoretical predictions, exhibiting a discrepancy of only several percent. However, the measured losses significantly exceeded those anticipated from quasi-particle loss mechanisms, suggesting the presence of additional loss factors. Notably, the measurement results revealed that the tangential loss attributable to the dielectric layer, specifically silicon dioxide, was approximately7±2×10−3{\text{7}\pm \text{2} \times \text{10}^{-\text{3}}}7±2×10−3. This factor emerged as the dominant contributor to overall loss at temperatures around4 K\text{4}\,{\text{K}}4K.
Abstract Superconducting low-loss transmission line resonators are crucial for various applications, including quantum computation and photon detection in submillimeter and far-infrared astronomy. Their performance is sensitive to both their geometry and material, with energy dissipation mainly due to two-level systems (TLS) located in amorphous dielectrics. This paper presents a lab-on-chip experiment to measure the TLS losses of sputtered SiO x and AlO x films in superconducting Nb coplanar waveguide (CPW) resonators at sub-Kelvin temperatures. Different CPW geometries are adopted for these experiments. The measurement results provide quantitative insights into TLS contributions in superconducting resonators, providing empirical evidence for material screening and circuit design.
As superconducting kinetic inductance detectors (KIDs) continue to grow in popularity for sensitive submillimeter detection and other applications, there is a drive to advance toward lower-loss devices. We present measurements of diagnostic thin-film aluminum coplanar waveguide (CPW) resonators designed to inform ongoing KID development at NASA Goddard Space Flight Center. The resonance frequencies span f0f_0f0​ = 3.5-4 GHz and include quarter-wave and half-wave resonators with varying coupling capacitor designs. We present measurements of the device film properties and an analysis of the dominant mechanisms of loss in the resonators measured in a dark environment, demonstrating quality factors of Qi−1≈3.64−8.57 ×10−8Q_i^{-1} \approx 3.64-8.57\ \times 10^{-8}Qi−1​≈3.64−8.57 ×10−8. We observe an enhanced level of suppression in the loss contributions from two-level systems (TLS) at intermediate-to-high read powers, and a regime at these powers and low temperatures where contributions from intrinsic processes Qi−1Q_i^{-1}Qi−1​,other dominate the total loss. We also observe deviations from the standard TLS loss model at low powers and temperatures below 60 mK, and use a modified model to describe this behavior.
We have developed a novel technique for making high quality measurements of the millimeter-wave properties of superconducting thin-film microstrip transmission lines. Our experimental technique currently covers the 75–100 GHz band. The method is based on standing wave resonances in an open ended transmission line. We obtain information on the phase velocity and loss of the microstrip. Our data for Nb/SiO/Nb lines, taken at 4.2 K and 1.6 K, can be explained by a single set of physical parameters. Our preliminary conclusion is that the loss is dominated by the SiO dielectric, with a temperature-independent loss tangent of 5.3±0.5×10−3 for our samples.
Flexible interconnects with superconducting (SC) traces and polyimide dielectric can provide low signal loss and low thermal conductivity at cryogenic temperatures, and are expected to find use in densely integrated cryogenic electronics systems. The ultra-low loss of SC interconnects at microwave frequencies can be accurately characterized through the use of weakly coupled SC resonators. The high-quality factor resonators provide sensitive measurements of the aggregate loss properties of the conductor and dielectric, as functions of frequency, temperature, and signal power. In this work, resonators with different linewidths were fabricated and tested to study how the linewidth impacts the overall internal resonator loss, in order to better characterize and separate the individual loss components (i.e., dielectric loss vs. conductor loss). The results of this work are important for understanding loss and transmission properties of similarly designed and fabricated transmission line interconnects.
We present a systematic analysis of the internal losses of superconducting coplanar waveguide microwave resonators based on niobium thin films on silicon substrates. In particular, we investigate losses introduced by Nb/Al interfaces in the center conductor, which is important for experiments where Al based Josephson junctions are integrated into Nb based circuits. We find that these interfaces can be a strong source for two-level state (TLS) losses, when the interfaces are not positioned at current nodes of the resonator. In addition to TLS losses, for resonators including Al, quasiparticle losses become relevant above 200 mK. Finally, we investigate how losses generated by eddy currents in conductive material on the backside of the substrate can be minimized by using thick enough substrates or metals with high conductivity on the substrate backside.