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Analysis of Normal and Superconducting Coplanar Waveguides in Radio Astronomy

Kim Ho Yeap

Published Oct 19, 2011• 5 citations
OpenAlex

Abstract

In terahertz radio astronomy, superconducting coplanar waveguide resonators have been commonly applied in Kinetic Inductance Detectors (KIDs) to measure the absorption of photon energy in the millimeter and submillimeter bands. Here, we present an analysis on the performance of superconducting niobium coplanar waveguides (CPWs). To compute the loss in a superconducting CPW, we have incorporated the complex conductivity developed by Matis and Bardeen based on the superconducting BCS theory, into the CPW loss equation. We have made a comparison between the loss in a CPW at room temperature with that below the critical temperature Tc of the superconductor. It can be observed that at frequencies below the gap frequency fg, the low in the superconducting CPW is significantly lower than that in a normal CPW. Above fg, however, the material loses its superconductivity and the loss in both temperatures becomes comparable. In our analysi, we have also shown that the loss decreases as the gap bewteen the strip and groundplane becomes wider. Hence, with careful design, the loss in a CPW can actually be minimized.

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In terahertz radio astronomy, superconducting coplanar waveguide resonators have been commonly applied in Kinetic Inductance Detectors (KIDs) to measure the absorption of photon energy in the millimeter and submillimeter bands. Here, we present an analysis on the performance of superconducting niobium coplanar waveguides (CPWs). To compute the loss in a superconducting CPW, we have incorporated the complex conductivity developed by Matis and Bardeen based on the superconducting BCS theory, into the CPW loss equation. We have made a comparison between the loss in a CPW at room temperature with that below the critical temperature Tc of the superconductor. It can be observed that at frequencies below the gap frequency fg, the low in the superconducting CPW is significantly lower than that in a normal CPW. Above fg, however, the material loses its superconductivity and the loss in both temperatures becomes comparable. In our analysi, we have also shown that the loss decreases as the gap bewteen the strip and groundplane becomes wider. Hence, with careful design, the loss in a CPW can actually be minimized.
We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ∼10 6 . When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.
The sensitivity of microwave kinetic inductance detectors (MKIDs) based on coplanar waveguides (CPWs) needs to be improved by at least an order of magnitude to satisfy the requirements for space-based terahertz astronomy. Our aim is to investigate if this can be achieved by reducing the width of the CPW to much below what has typically been made using optical lithography (>1 μm). CPW resonators with a central line width as narrow as 300 nm were made in NbTiN using electron beam lithography and reactive ion etching. In a systematic study of quarter-wave CPW resonators with varying widths it is shown that the behavior of responsivity, noise and power handling as a function of width continues down to 300 nm. This encourages the development of narrow KIDs using Al in order to improve their sensitivity.
Microwave kinetic inductance detectors (MKIDs) are playing an increasingly role in THz astronomy. Superconducting coplanar waveguide (CPW) lines are commonly adopted in MKIDs for the resonators as well as the readout through line. It is therefore of particular interest to characterize the transmission properities of superconducting CPW lines at low temperatures. A cryogenic through-line method based on two cryogenic RF switches is proposed to characterize Nb and TiN superconducting CPW lines. On-chip calibration has been successfully carried out. Detailed results and analysis will be presented.
Superconducting coplanar waveguide (SCPW) resonators have a wide range of applications due to the combination of their planar geometry and high quality factors relative to normal metals. However, their performance is sensitive to both the details of their geometry and the materials and processes that are used in their fabrication. In this paper, we study the dependence of SCPW resonator performance on materials and geometry as a function of temperature and excitation power. We measure quality factors greater than 2 × 106 at high excitation power and 6 × 105 at a power comparable to that generated by a single microwave photon circulating in the resonator. We examine the limits to the high excitation power performance of the resonators and find it to be consistent with a model of radiation loss. We further observe that while in all cases the quality factors are degraded as the temperature and power are reduced due to dielectric loss, the size of this effect is dependent on resonator materials and geometry. Finally, we demonstrate that the dielectric loss can be controlled in principle using a separate excitation near the resonance frequencies of the resonator.
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 present the RF characterization of 2-µm-wide superconducting coplanar waveguides made of 100-nm-thick niobium (Nb) or niobium nitride (NbN) films fabricated on 200 mm silicon wafers. Measurements of the two-ports S-parameters up to 25 GHz in a 4.7K–14 K temperature range with an added direct current up to 115 mA were done using a cryogenic probe station. With proper RF calibration and de-embedding procedure, both temperature and dc current dependence of the RF superconducting properties are analyzed. We report a kinetic inductance of 0.15 pH/□\square□, a critical current density of 50 MA/cm2^{2}2, and a critical temperature of 8.9 K for Nb as well as 4.27 pH/□\square□, 3 MA/cm2^{2}2, and 13.8 K for NbN. A telegraphist transmission line model taking into account the BCS theory has been developed and the comparison with the measurements is discussed.