Superconducting Microstrip Line Research Articles

A 220 GHz superconducting titanium transition edge sensor array developed for cosmic microwave background experiments

In this paper, we report on the design, fabrication, and characterization of a 220 GHz superconducting transition edge sensor (TES) array developed for ground-based cosmic microwave background (CMB) experiments. Unlike conventional TES arrays adopting thermometers made of bilayer superconducting films, the TES array has thermometers made of simpler single-layer titanium (Ti) film deposited on a suspended silicon nitride (SiN x ) membrane. The thermal weak link is realized by releasing the TES’s thermal island including the Ti thermometer by dry etching with xenon difluoride (XeF2), giving a typical thermal conductance of 25 pW K−1. Its thermal-conductance mechanism is further studied by fitting the measured TES’s complex impedance with a three-block thermal model. The dark and optical noise equivalent power (NEP) of a superconducting TES are both measured. Despite existing lens reflection and dielectric loss in the superconducting microstrip line, its typical optical NEP reaches 100 aW Hz−0.5, which meets the sensitivity requirement for ground-based CMB experiments.

Fabrication of C-Band High-Temperature Superconducting Microstrip Triplexer with High Accuracy

This article presents a miniaturized C-band triplexer fabricated with high-temperature superconducting (HTS) microstrip lines and to be applied to quantum computers. The triplexer consists of three bandpass filters and branch lines. In the design of the bandpass filters, folded step impedance resonators (SIRs) are firstly used to achieve good stopband performances by harmonic suppression. Secondly, cross-coupling structures are also utilized to further improve the passband edge steepness of the bandpass filters. Finally, the parasitic transmission zeros originating from the assembling box sizes are employed to achieve a deeper out-of-band rejection. Three bandpass filters are designed and connected to the common input port of the triplexer. After that, the triplexer is coarsely optimized by advanced design system (ADS) for time-saving and is finely optimized by Sonnet for better accuracy. The optimized layout of the triplexer is fabricated on a 2-inch diameter and 0.5 mm thickness MgO wafer; here, the MgO wafer is double-sided coated with HTS YBa2Cu3O7 (YBCO) thin films. The measured passband bandwidths (return loss greater than 12 dB) of the triplexer are 6.420-6.680 GHz, 6.780-7.030 GHz, and 7.110-7.380 GHz, respectively, which satisfy the required design specifications.

A Compact 2-D Stochastic FDTD Method for Uncertainty Analysis in Superconducting Transmission Lines

In this article, a stochastic finite-difference time-domain (S-FDTD) method is proposed for analyzing the influence of physical size uncertainty on the time-domain response of superconducting transmission lines. The formulations of the proposed method are, in detail, derived with the two-fluid model and London equation. A typical superconducting microstrip line is simulated to verify the effectiveness of the proposed algorithm. Compared with the Monte Carlo simulation, the proposed method has the same accuracy but much higher efficiency. The computation time of the proposed S-FDTD method is only about 3% of that of the Monte Carlo simulation. Therefore, the proposed method shows a promising prospect on predicting the variability in transmission line's performance caused by variation in superconducting manufacturing.

Design and Performance of the Antenna-Coupled Lumped-Element Kinetic Inductance Detector

Focal plane arrays consisting of low-noise, polarisation-sensitive detectors have made possible the pioneering advances in the study of the cosmic microwave background (CMB). To make further progress, the next generation of CMB experiments (e.g. CMB-S4) will require a substantial increase in the number of detectors compared to current instruments. Arrays of kinetic inductance detectors (KIDs) provide a possible path to realising such large-format arrays owing to their intrinsic multiplexing advantage and relative cryogenic simplicity. In this paper, we report on the design of a variant of the traditional KID design: the antenna-coupled lumped-element KID. A polarisation-sensitive twin-slot antenna placed behind an optimised hemispherical lens couples power onto a thin-film superconducting microstrip line. The power is then guided into the inductive section of an aluminium KID, where it is absorbed and modifies both the resonant frequency and quality factor of the KID. We present the various aspects of the design and preliminary results from the first set of seven-element prototype arrays and compare to the expected modelled performance.

Fabrication of superconducting tunnel junctions with embedded coil for applying magnetic field

We have proposed and demonstrated a superconducting tunnel junction (STJ) with an embedded coil for applying a magnetic field. The STJ was fabricated on the coil, which was embedded in a Si substrate. The coil in the Si substrate consists of superconducting microstrip lines and applies a magnetic field to the STJ to suppress the dc Josephson current. The embedded coil was designed with a line and space of 3µm and a thickness of 120nm. To planarize the coil, we employed chemical mechanical polishing (CMP) in our fabrication process. In this STJ, the maximum current of the embedded coil was 28mA, which corresponded to the maximum magnetic field of 11.76mT.

Improvement of power-handling capability of superconducting filters using 3D-matrix microstrip lines

We examined power-handling capability of superconducting filters using three different types of resonator: type one, a conventional microstrip line, type two, a layered-film microstrip line, and type three, a 3D-matrix microstrip line. The 3D-matrix microstrip line was made by dividing a type two microstrip line into narrow lines. We used NbN thin films as superconducting microstrip lines. The optimal shape of the filters was designed by using a Sonnet electromagnetic simulator, and the power-handling capability of these filters was measured at 6.5 K. The values of a conventional microstrip line, a layered-film microstrip line, and a 3D-matrix microstrip line filters were 21.2, 22.9, and 24.6 dBm, respectively.

Composite arrays of superconducting microstrip line resonators

A novel design of an array of half-wave superconductive microstrip resonators is described. The resonator is intended to be useful for electron spin resonance studies of thin film samples at cryogenic temperatures. It achieves a high quality factor, has a small mode-volume, and creates a uniform magnetic field in a plane above the resonator. The device is made of thin film Niobium on sapphire wafer and is tested with a static magnetic field. Variation of Q-factor versus the magnetic field's strength at different temperatures is reported and is in a good agreement with simulation when the loss due to the vortices is included. Also, the power-dependence response of the resonator is shown in experiments and is verified by capturing the nonlinearity associated with the surface impedance of the superconducting film into the circuit model of the device.

Tunable Wave Propagation in Superconducting Microstrip Line Based on Ferroelectric Thin Films

The microwave propagation characteristics in superconducting microstrip line supported by ferroelectric layer are theoretically investigated. The control of propagation constant and attenuation coefficient of the waveguide with biasing electric field is analyzed using the spectral domain method. The dependence of propagation characteristics on thickness of ferroelectric thin film as well as operating frequency for different applied electric fields is studied. The variation of propagation constant with temperature is also investigated. This method of analysis can be easily implemented in the design of tunable microwave devices.

Full-wave modeling of superconducting microstrip lines including the nonlinearity behavior

This paper describes a new theoretical model to characterize the superconducting microstrip line and carefully studies the effects of the nonlinearity of superconductors, the strip thickness and losses on circuit performances. The microstrip line has been considered as a multilayered structure. The integral equation for the electrical field has been formulated, in the spectral domain, using the exact dyadic Green’s function of bianisotropic planar media. The Galerkin’s technique has been used for solving this integral equation. Obtained results concern the effective permittivity constant and the attenuation constant versus frequency and temperature rate.

Improved Power Handling Capability of Superconducting Microstrip Lines for Microwave Devices

Abstract We examined a microstrip line structure to reduce the current concentration at the outer edge of a microstrip line. We developed three kinds of microstrip line; sliced-microstrip line, layered-film microstrip line, and conventional microstrip line. The first microstrip line is a line divided into the narrow line, and the second one is a line made by a layered thin film. Electromagnetic simulations indicated that the current concentration of the outer edge of the sliced microstrip line and layered film microstrip line was lower than that of a conventional microstrip line. We measured the power handling capability of the filters made by sliced-microstrip lines, layered-film microstrip lines, and conventional microstrip lines. The value of the first and the second filters was better than that of a conventional one. The difference in the characteristics is based on the difference in the current concentration at the outer edge of the microstrip lines.

Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry

Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).

Numerical study of vortex effects in superconducting microstrip lines

A high transition temperature superconducting microstrip structure is modeled using the theoretical approach developed by Coffey and Clem for elucidating the vortex effects in propagation. Impedance type Green’s functions are derived for the electric field around the strip and the propagation characteristics are computed for a wide range of applied field, reduced temperature and superconducting strip thickness in a Galerkin procedure. The increase of static field and temperature result in increased vortex motion, which in turn, causes a corresponding change in the propagation characteristics of the transmission line. Numerical results are presented for propagation parameters and quality factor.

Geometrical control of transverse electromagnetic wave propagation in nonuniform microwave superconducting transmission lines

It is well known that photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. A closed form expression is found for the equivalent characteristic impedance and propagation constant of a uniform transmission line that can accurately model the transverse electromagnetic (TEM) propagation within nonuniform superconducting transmission lines with subwavelength inhomogeneities. It is shown that electromagnetic wave propagation, in particular the group velocity of propagating TEM waves, can be well controlled by introduction of subwavelength (subcentimeter) geometrical variations within the line. This ability is of great importance in design of matching stages. In our numerical examples, quasi-TEM superconducting microstrip lines with a superconductive strip of varying width are considered and the group velocity of the quasi-TEM modes is controlled by duly varying the line width. The obtained results are validated by being compared against the well-known technique of stair-case approximation whereby the considered nonuniform line is sliced into several sections each being small enough to be considered uniform.

Numerical Modelling of High–Tc Superconducting Microstrip Line Using Fdtdtechnique

Finite difference time domain (FDTD) method is formulated and applied to model the loss that occurs during the transmission of an RF signal through superconducting microstrip line. This method is simple in implementation with much less computational requirements. Different parameters, including the attenuation constant, the phase constant and the effective dielectric constant, current density distribution, and the magnetic field, as function of width and thickness, are calculated. In addition, temperature dependence of the surface impedance is calculated. Good agreement is shown between our results and those reported in literature where other models were implemented.

Characterization of Antenna-Coupled TES Bolometers for the Spider Experiment

We describe the transition-edge-sensors (TES's) to be employed on the Spider experiment, a balloon-based observatory whose goal is detecting the imprint of gravitational waves by mapping the polarization of the cosmic microwave background (CMB). The devices consist of Ti and Al thermistors in series sitting on a thermally isolated suspended membrane. Also on the membrane is a termination resistor coupled through a superconducting microstrip line to an on-chip, polarization sensitive, 150 GHz slot-array antenna. Several important parameters were measured. Transition temperatures were deduced by measuring the Johnson noise in the Ti thermistor. The thermal conductance between the isolated TES islands and substrate was measured by obtaining current-voltage measurements at various temperatures. The Electrical noise equivalent power was measured to sub-Hertz frequencies with varying sample geometries including those with and without normal metal bars (zebra stripes). Finally, the time constant of the devices was measured within the Al and Ti transitions where electrothermal feedback speeds up the bolometer response. This time response is compared with the natural time constant measured just above the Ti transition temperature. The results of these measurements are within the design specifications for Spider.

3D simulation of superconducting microwave devices with an electromagnetic-field simulator

High-frequency microwave applications, such as filters, delay lines, and resonators, are quite important for superconducting electronic devices. In order to design the superconducting microwave devices, circuit parameters should be precisely extracted from the physical structure of the devices. A 3-dimentional electromagnetic-field simulator is very useful for designing microwave devices. However, designing of superconducting microwave devices using a conventional 3D electromagnetic-field simulator is difficult because most of commercially available 3D electromagnetic-field simulators can not exactly characterize electromagnetic phenomena of superconductors. In this study, a novel calculation method of superconducting microwave devices, which can be applicable to a conventional 3D electromagnetic-field simulator, has been proposed. Calculation results of characteristic impedance of superconducting microstrip lines show very good agreements with the theoretically calculated vales. The frequency response of a superconducting Nb microwave filter designed by the proposed calculation method agrees well with the experimental results. This calculation method enables us to precisely estimate microwave characteristics of superconducting devices.

Terahertz Detection with TwinSuperconductor–Insulator–Superconductor Tunnel Junctions

Terahertz detection with twin superconductor–insulator–superconductor(SIS) tunnel junctions, which are connected in parallel via an inductivethin-film superconducting microstrip line, is mainly studied. Firstly,we investigate the direct-detection response of a superconductingtwin-junction device by means of a Fourier transform spectrometer.Secondly, we construct a direct-detection model of twin SIS tunneljunctions. The superconducting twin-junction device is then simulated interms of the constructed model. The simulation result is found to be ingood agreement with the measured one. In addition, we observe that thedirect-detection response of the device is consistent with the noisetemperature behaviour.

SUPERCONDUCTING MICROSTRIP LINE MODEL STUDIES AT MILLIMETRE AND SUB-MILLIMETRE WAVES

This paper presents results of superconducting microstrip transmission line simulation for the frequency range 100–1100 GHz. The simulation is used to calculate the characteristic impedance and the wave propagation constant in a superconducting microstrip line having different geometry. Indeed, modelling provides the only tool for designing superconducting microstrip-based circuits at millimetre and sub-millimetre wavelengths because no direct measurements of such a line can be made at this wavelengths and at cryogenic temperature of 4 K with ultimate accuracy. Niobium, as the most commonly exploited superconducting material, was used for the modelling, though the same approach would work for any different BCS superconductor. In order to evaluate the model accuracy, we have made an extensive comparison study of the superconducting microstrip models known from earlier publications, performed numerical simulations using 3D EM — solver, HFSS, and used a new model introduced in this paper with all simulation result plotted in the paper.

Terahertz imaging with a direct detector based on superconducting tunnel junctions

We demonstrated terahertz imaging using a direct detector based on niobium superconducting tunnel junctions (STJs). The detector is composed of linearly distributed junctions placed on a superconducting microstrip line and is integrated on two wings of a log-periodic antenna. We succeeded nondestructive imaging for an integrated-circuit card and dry material using the detector around its sensitivity peak (∼0.66THz). The dynamic range was measured to be higher than 4×107 (76dB). Thus, the STJ detector is applicable to high-sensitivity and high-speed terahertz imaging for various nondestructive inspection applications.

A waveguide-coupled millimetre-wave TES bolometer suitable for 2-D arrays

We are developing a single-pixel antenna-coupled bolometric detector for use in low-background millimetre-wave astronomy. The device consists of a radial probe in a rectangular waveguide coupled to a Ti/Au transition edge sensor (TES). The signal propagates from the antenna along a superconducting microstrip line into a metal resistor on a thermally isolated Si 3N 4 island, where the TES is also located. We have designed and built prototype pixels optimised for 150 GHz band measurements. We discuss our progress in the development as well as the first results from dark measurements.