Waveguide-to-substrate transition based on unilateral substrateless finline structure: Design, fabrication, and characterization

We report on a novel waveguide-to-substrate transition with prospective use for broadband mixer design. The transition employs a substrateless finline, i.e., a unilateral finline structure with the substrate removed between the fins. This distinctive feature diminishes the overall insertion loss and facilitates matching with the waveguide. The transition is designed on a thin silicon substrate covered by a superconducting niobium thin layer. An auxiliary Au layer situated on top of the Nb layer provides grounding for the fins and facilitates the mounting process in the split-block waveguide mount. Aiming to compare simulations with measurements, a back-to-back transition arrangement for the 211-373 GHz frequency band was designed, fabricated, and characterized at cryogenic temperatures. The simulation results for the back-to-back structure show an insertion loss of less than 0.6 dB in the whole band, i.e., 0.3 dB per transition. Furthermore, a remarkable fractional bandwidth of 55% with a return loss better than 15 dB is predicted. Experimental verification shows consistent results with simulations

A Nonlinear Transmission Line Model for Simulating Distributed SIS Frequency Multipliers

Superconductor/insulator/superconductor (SIS) jun-ctions have extremely nonlinear electrical properties, which makes them ideal for a variety of applications, including heterodyne mixing and frequency multiplication. With SIS mixers, the SIS junctions normally have circular cross sections, but they can also be fabricated in the form of microstrip transmission lines, known as distributed SIS junctions (DSJs). By using a DSJ as an open-circuit stub, it is possible to create a large SIS junction with a low effective input reactance. This is beneficial for SIS frequency multipliers because their output power is proportional to the area of the junction. It is challenging, however, to simulate the behavior of DSJs because they have to be modeled as transmission lines and the model has to take into account the quasiparticle tunneling current, which is a nonlinear function of the ac voltage. In this article, we present a new nonlinear transmission line model to accurately describe the behavior of DSJs and to simulate the performance of distributed SIS frequency multipliers (DSMs). This model is compared to experimental data from a recent DSM device and good agreement is found between the dc tunneling currents and the output powers at the second harmonic. Based on this success, an improved DSM design is proposed that has a higher output power and a higher conversion efficiency than previous designs.\ua0\ua9 2011-2012 IEEE

Millimeter-Wave Wideband Waveguide Power Divider with Improved Isolation between Output Ports

We present a novel compact wideband waveguide T-junction power divider especially suited for mm-wave and THz frequencies. It incorporates substrate-based elements into a waveguide structure to provide the output port\u27s isolation and matching. The internal port is introduced at the apex of the T-junction formed as an E-probe on a substrate. This facilitates efficient coupling of the reflected energy from the output port to a novel thin-film-based resistive termination integrated with the E-probe onto the same substrate and fabricated by means of thin-film technology. A power divider was designed, simulated, and fabricated for the frequency band 150-220 GHz, to experimentally verify the theoretical and simulated performance. The results showed excellent agreement between the simulations and measurements with the devices demonstrating a remarkable return loss of 20 dB for both the input and output ports for a three-port device with equal split and isolation better than 17 dB between the output ports. Furthermore, the measured insertion loss is less than 0.3 dB and the amplitude and phase imbalance are 0.15 dB and 0\ub0, respectively. Moreover, the divider\u27s remarkable tolerance to the dimensions and sheet resistance of the resistive material of the built-in absorbing load, makes the device a very practical component for mm-wave and THz systems, in particular radio-astronomy receivers

Wideband Slotline-to-Microstrip Transition for 210-375 GHz based on Marchand Baluns

This paper describes the design and cryogenic measurement of a novel slotline-to-microstrip transition based on Marchand baluns. The proposed transition is an attractive solution for numerous THz applications due to its remarkable broadband performance and compactness. For instance, such transition could be considered for wideband devices covering the frequency band 210-375 GHz. The suggested transition is designed on a thin silicon substrate and employs superconducting Nb as the electrode for the slotline and microstrip lines. In order to verify the performance of the designed transition, we fabricated a dedicated test structure consisting of two transitions connected back-to-back and integrated with E-probes at the waveguide interfaces. Due to the inherent bandwidth limitation of the E-probes, two different test structures for 210-295 GHz and 295-375 GHz were employed to characterize the proposed transition over the whole frequency band. The experimental verification performed at cryogenic temperatures showed results consistent with the simulation. Moreover, the cryogenic measurements indicated a remarkable 56% fractional bandwidth with an insertion loss as low as 0.3 dB for the fabricated slotline-to-microstrip transition

Interface Layers of Niobium Nitride Thin Films

Intermediate layers formed by thin NbN films are studied. A surface phase of NbN different from the bulk one under the oxide layer and a layer consisting of NbNx-SiOy between the film and the substrate are found

Controlled Ultra-Thin Suboxide Films Generation in Metal-Oxide Systems by Ar+Ion Irradiation

A method of controlled generation of metal suboxide films is proposed, basing on low-current ion sputtering of native oxides of ultra-thin metallic films and XPS chemical and phase depth profiling. Niobium suboxide ultra-thin films are generated and controlled using this approach

SEPIA345: A 345 GHz dual polarization heterodyne receiver channel for SEPIA at the APEX telescope

Context. We describe the new SEPIA345 heterodyne receiver channel installed at the Atacama Pathfinder EXperiment (APEX) telescope, including details of its configuration, characteristics, and test results on sky. SEPIA345 is designed and built to be a part of the Swedish ESO PI Instrument for the APEX telescope (SEPIA). This new receiver channel is suitable for very high-resolution spectroscopy and covers the frequency range 272- 376 GHz. It utilizes a dual polarization sideband separating (2SB) receiver architecture, employing superconductor-isolator-superconductor mixers (SIS), and provides an intermediate frequency (IF) band of 4- 12 GHz for each sideband and polarization, thus covering a total instantaneous IF bandwidth of 4 \uc3\uc2 - 8 = 32 GHz. Aims. This paper provides a description of the new receiver in terms of its hardware design, performance, and commissioning results. Methods. The methods of design, construction, and testing of the new receiver are presented. Results. The achieved receiver performance in terms of noise temperature, sideband rejection, stability, and other parameters are described. Conclusions. SEPIA345 is a commissioned APEX facility instrument with state-of-the-art wideband IF performance. It has been available on the APEX telescope for science observations since July 2021

Nb/Al-AlN/Nb superconducting tunnel junctions: fabrication process and characterization results

Modern radio astronomy demands for broadband receiver systems. For SIS mixers, this translates into objective to employsuperconducting tunnel junctions with very low RnA and low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely approached their physical limit of minimizing those parameters. It is commonly recognized that it is AlN-barrier junctions, which are needed for further progressing of the broadband instrumentation for radio astronomy. In manuscript we present results of developing a fabrication process for high quality Nb/Al-AlN/Nb junctions and characterization the junctions\u27 DC electric properties and their aging and annealing stability, as well as the junctions\u27 specific capacitance

Nb/Al-AlOx/Nb Junction Properties\u27 Variations Due to Storage and Mounting

We report studies of room temperature aging and annealing of Nb/Al-AlO x /Nb tunnel junctions with a 2. . . 3 sq.\ub5msize. We observed a noticeable drop of the junction normal resistance Rn unusually combined with increase of subgap resistance Rj as a result of aging. Changes of Rj occur at sufficiently shorter time scale than that of Rn. Variation of both Rn and Rj depend on the junction size. An effect of aging history on the junction degradation after consequent annealing was discovered. We suggest that the observed junction aging andannealing behavior could be explained by diffusional ordering and structural reconstruction in the tunnel AlOx barrier. The diffusion driving such structural ordering and reconstruction of the AlO x tunnel layer is enhanced due to the intrinsic stress relaxation (creep) processes in the underlying Al layer. Also, we discuss the influence of dicing the wafer into the single mixer chip on the junction aging behavior

Frequency Multiplication in a Distributed Array of SIS Junctions

We report the experimental study of the off-chip detection of frequency multiplication in a distributed array of Superconductor-Insulator-Superconductor (SIS) junctions. A test device consisting of a series array consisting sixty eight Nb/Al-AlOx/Nb tunnel junctions was designed for this study, and was fabricated using in-house Nb thin-film technology. The test device with SIS array was optimized for the study of second harmonic generation in 182−192 GHz output frequency band. The SIS array was exited with microwave radiation at 3 mm band using a quasi-optically coupled Gunn oscillator and the output response of the device was studied using a double sideband SIS mixer operating in 163 − 211 GHz range with 4−8 GHz. The Josephson-effect for both the SIS multiplier and the detector mixer was carefully suppressed using magnetic field. We observed very sharp second harmonic spectral signals, due to frequency multiplication by the SIS array. We also observed distinct multi-photon process in the SIS array tunnel junction response to the applied microwave signal, and the amplitude of the multiplied signal shows dependence on the bias voltage of the SIS array. We observed that the output power of the multiplied signal increases linearly with the power of the pumping signal up to certain level and them saturates. Increasing the input power beyond this level results in the heating of the chip. When the output of the test device was connected to the LO port of the SIS-mixer, an increase of 10 − 20% in the SIS -mixer dark current was observed when the SIS mixer was voltage biased in the middle of first photon step below the gap voltage. The device, although far from providing sufficient power to pump a practical SIS mixer, may be considered as a first experimental step towards SIS frequency multipliers