This paper presents a method for measuring the capacitance of superconductor-insulator-superconductor (SIS) junctions with a 4-K probe station. The probe station can be used to perform on-wafer characterization of sample lots at frequencies ranging from dc to microwave. In our study, we evaluated SIS junctions that had a variety of areas, and estimated the capacitance after subtracting the parasitic elements of the probe contact pad. The results show that the RNC products for the evaluated junctions were uniform within +/-5% across the wafer.
This paper presents a method for measuring the capacitance of superconductor-insulator-superconductor (SIS) junctions with a 4-K probe station. The probe station can be used to perform on-wafer characterization of sample lots at frequencies ranging from dc to microwave. In our study, we evaluated SIS junctions that had a variety of areas, and estimated the capacitance after subtracting the parasitic elements of the probe contact pad. The results show that the RNC products for the evaluated junctions were uniform within +/-5% across the wafer.
Superconductor-Insulator-Superconductor (SIS) tunnel junctions are widely used as the mixing component in mm and sub-mm receivers for radio astronomy and environmental science. SIS mixers offer superior noise performance close to the quantum limit for frequencies up to 1 THz. An important property of an SIS junction is the intrinsic specific capacitance (Cs), which is defined by the geometrical capacitance (Cj) divided by the junction area (A). In order to prevent loss of the RF signal, a tuning circuitry is required to resonate out the Cj. The RF bandwidth of an SIS mixer is determined by the quality factor of the resulted resonator-like tuning circuitry, Q = ωRnCj, where Rn is the normal resistance of the junction and ω is the angular frequency. In order to increase the RF bandwidth of the mixer, junctions with lower specific resistance (RnA) and hence thinner insulator layer are required. For the practical SIS mixers, junctions with RnA 100 Ω.µm2 (often as low as 20 Ω.µm2) are typically used. In this RnA range, the published data on the Cs vs. RnA are often spread and not consistent between different sources. Therefore, at this limit, the design of an accurate tuning circuitry becomes challenging. Hence, there is a well-recognized demand for accurate measurement of Cs.\nSo far, all the developed characterisation methods measured the specific capacitance effect indirectly, e.g., in a resonant circuit. In this context, the specific capacitance was extracted by fitting the measurement data into a theoretical model. In this thesis, a new method was developed, which directly measures the junction capacitance at microwave frequencies (1 - 7 GHz) with estimated uncertainty varying from ±2% to ±11% depending on the junction area. In this measurement method, an original calibration approach was suggested and implemented.\nUsing this method, the capacitance of 34 junctions with various RnA values ranging from 8 to 68 Ω.µm2 was measured. The observed scatter in the specific capacitance is discussed. We argue that, it is the tunnel barrier (thickness) nonuniformity, which is responsible for the observed spread of the Cs data. The nonlinear capacitance as a result of the Kramers-Kronig transformation of the dc IV characteristic of the SIS junction was shown to be a significant contribution for RnA 40 Ω.µm2 even at frequencies as low as 1-7 GHz. The extracted specific capacitance from the measurements was corrected for the nonlinear capacitance. The obtained Cs(RnA) data agrees well with the model relation Cs = a/ln(RnA), where a =0.227 for the RnA 40 Ω.µm2. \n
The need for operating frequencies well into THz region with higher sensitivity and wider bandwidth pushes superconductor-insulator-superconductor (SIS) technology towards junctions with more transparent barrier and higher current densities. Obtaining accurate knowledge of the specific capacitance, which is related to the transparency of the junctions, leads to a precise design of the tuning circuitry. Previously, characterization of the SIS junction’s specific capacitance involved complex measurements providing data relying on various model fitting. Herein, we present the characterization of the specific capacitance by directly measuring the impedance of the SIS tunnel junction at microwave frequencies (~3 GHz).
In this paper, we present an investigation of the contribution of quantum susceptance in capacitance measurements of superconductor-insulator-superconductor tunnel junctions. We directly measured the one-port S-parameter S11by utilizing a 4-K probe station in combination with a network analyzer. We observed a bias voltage dependence of the measured junction susceptance which demonstrates the contribution of quantum susceptance. Calculation results performed, based on a simple equivalent circuit model, were in excellent agreement with the measurements. Experiment and analysis methods suggested here enable the quantitative verification of the contribution of quantum susceptance and are an effective way of extracting the junction's geometric capacitance.
We have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.
In this paper, we describe a measurement setup to characterize the intermediate frequency (IF) impedance of cryogenically cooled superconductor-insulator-superconductor (SIS) mixers. A setup based on a commercial vector network analyzer (VNA), a circulator and a low noise amplifier (LNA) has allowed to increase the dynamic of the VNA and to perform accurate one-port measurements across a high dynamic range of the SIS mixers IF band. The mixers were biased in three different regions of their unpumped IV curve to obtain three known impedances which are close to the classical calibration SOLT standards (Short, Open, Load, Thru): the reference plane was located at the SIS junction itself and the calibration procedure allowed to calibrate out all IF circuitry of the measurement setup, including the on-chip SIS mixer intrinsic capacitance and inductance without requiring to thermally cycle and open the cryostat to locate and measure the different calibration standards. Thus, the devised method provides a simple and direct measurement of the SIS mixers IF impedance which can be used for quick, accurate and highly repeatable IF characterization of any type of millimeter and sub-millimeter wave SIS mixers. The measurement method is described and the experimental results of the IF output impedances of 3 mm band DSB and SSB mixers with 4 GHz wide IFs (across 4-8 GHz) are presented and compared with simulated predictions obtained from combining Tucker's SIS mixer theory and accurate electromagnetic modeling of the mixer structure.
The junction specific capacitance (Cs) is an essential parameter in designing tuning circuitry for superconductor- insulator-superconductor (SIS) mixers. However, our knowledge of the junction capacitance only relies on the few available empirically obtained Csversus specific normal resistance (RnA) relations, which are inconsistent especially at low RnA values, RnA2. In this paper, we report the Nb/Al-AlOx/Nb SIS junction capacitance data from our recently presented direct microwave (4 GHz) measurements at 4 K for junctions with various RnA values ranging from 8.8 to 68 Ω·μm2. New insight is provided into the extraction of the true geometrical specific capacitance of SIS junctions. We show that even at such low microwave frequencies, the so-far-neglected nonlinear susceptance is significant, especially for junctions with low RnA values. This susceptance originates from the real part of the response function, IKK, which can be calculated through the Kramers-Kronig transform of the dc tunnel current. The new specific capacitance, which accounts for this contribution, is presented as a function of RnA. We provide an improved and more accurate Cs(RnA) relation, which can be a reliable and useful tool for circuit designers. The obtained Cs(RnA) relation is compared with those available in the literature, and the possible reasons giving rise to the disparity among these relations are discussed. By comparing the modeled and the measured noise temperature of the APEX SHeFI band 3\(385-500 GHz) double sideband mixer, we show that the new Cs(RnA) relation offers a great potential for improving the performance of SIS mixers.