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arXiv:2403.12342 · IEEE Transactions on Applied Superconductivity

Investigating Millimeter-Wave Thin-Film Superconducting Resonators: A Study Using Tunnel Junction Detectors

Wenlei Shan, Shohei Ezaki

Published Nov 7, 2024• 2 citations
arXivHTMLPDF

Abstract

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 approximately${\text{7}\pm \text{2} \times \text{10}^{-\text{3}}}$. This factor emerged as the dominant contributor to overall loss at temperatures around$\text{4}\,{\text{K}}$.

DOI
10.1109/tasc.2024.3493212

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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.
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.
We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and sub-mm wavelengths. The microstrips are fabricated from NbTiN, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Sub-mm wave losses are measured using on-chip Fabry-P{é}rot resonators (FPR) operating around $350\ $GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of tan⁡δ=3.7±0.5×10−5\tanδ=3.7\pm0.5\times10^{-5}tanδ=3.7±0.5×10−5 at $6\ $GHz and tan⁡δ=2.1±0.1×10−4\tanδ= 2.1\pm 0.1\times10^{-4}tanδ=2.1±0.1×10−4 at $350\ $GHz. These results represent very low losses for deposited dielectrics, but the sub-mm wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.
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.
In millimeter and submillimeter superconducting receivers, which are widely applied in radio astronomical observations, waveguide-to-planar transmission line transitions are key components connecting waveguide medium and the superconducting planar circuit. Although conventional quartz substrate-based transitions perform well for single-pixel receivers, these waveguide probes are not suitable for imaging arrays because their narrow chip space prohibits a higher degree integration of superconducting circuits. For multi-beam receiving applications, we have demonstrated fully functional monolithic microwave integrated circuit superconductor-insulator-superconductor (SIS) mixers with on-chip silicon membrane-based transitions at 2 mm wavelengths. In the present work, we aim to assess the transition as a stand-alone component and investigate its bandwidth potential. We designed a full-height waveguide transition which covers 125–211 GHz, more than 50% fractional bandwidth. The experimental study was carried out by performing cryogenic measurements of the transmission of a back-to-back probe pair with a mm-wave network analyzer. The measured transmission is well consistent with the simulated one in the frequency range of 125–170 GHz, which is covered by the network analyzer in this measurement.
We have developed a new technique for accurate measurement of the loss of superconducting microstrips at mm‐wave frequencies. In this technique, we optically couple power to slot antenna, which is connected to one port of a hybrid coupler. One of the output ports of the hybrid delivers power to a series of mm‐wave microstrip resonators which are capacitively coupled to a feedline followed by an MKID (microwave kinetic inductance detector) that measures the transmitted power. Two other MKIDs are connected to the remaining ports of the hybrid to measure the total incident optical power and the power reflected from the mm‐wave resonators, allowing |S21|2 and |S11|2 to be accurately determined and resonance frequency fr and quality factor Q to be retrieved. We have fabricated such a Nb/SiO2/Nb microstrip loss test device which contains several mm‐ wave resonators with fr∼100 GHz and measured it at 30 mK. All the resonators have shown internal quality factor Qi∼500–2000, suggesting a loss tangent of ∼5×10−4−2×10−3 for the SiO2 in use. For comparison, we have also fabricated a 5 GHz microstrip resonator on the same chip and measured it with a network analyzer. The loss tangent at 5 GHz derived from fitting the f0 and Q data to the two‐level system (TLS) model is 6×10−4, about the same as from the mm‐wave measurement. This suggests that the loss at both microwave and mm‐wave frequencies is probably dominated by the TLS in SiO2. Our results are of direct interest to mm/submm direct detection applications which use microstrip transmission lines (such as antenna‐coupled MKIDs and transition‐edge sensors), and other applications (such as on‐chip filters). Our measurement technique is applicable up to approximately 1 THz and can be used to investigate a range of dielectrics.
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.