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IEEE Transactions on Applied Superconductivity

Measurement of Specific Capacitance for Nb/Al–AlO $_{x}$/Nb Josephson Junctions in Single-Flux Quantum Circuit Applications

Wei Xiong, Wanning Xu, Yu Wu, Guanqun Li, Liliang Ying, Wei Peng, Jie Ren, Zhen Wang

Published Mar 6, 2018• 10 citations
Publisher

Abstract

In this study, we investigate different technologies to measure the specific capacitances C8of Nb/Al-AlOx/Nb Josephson junctions. They involve measuring Fiske mode resonance in long Josephson junctions, superconducting quantum interference device (SQUID) resonance, and Josephson plasma resonance. We find a logarithmic relationship between the junction C8and its critical current density Jcin the range 0.1-2 kA/cm2using the Fiske mode resonance method. Our measured C8values using SQUID resonance and Josephson plasma resonance are compared with the results obtained by other groups via analyzing the major errors in the C8calculation. We discuss the accuracy in evaluating C8for all three techniques and suggest using Fiske mode resonance as a relatively stable method for C8extraction for the future design of our single-flux quantum circuit.

DOI
10.1109/tasc.2018.2812737

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Measurement of Specific Capacitance for Nb/Al–AlO x_{x}x​/Nb Josephson Junctions in Single-Flux Quantum Circuit Applications
In this study, we investigate different technologies to measure the specific capacitances C8of Nb/Al-AlOx/Nb Josephson junctions. They involve measuring Fiske mode resonance in long Josephson junctions, superconducting quantum interference device (SQUID) resonance, and Josephson plasma resonance. We find a logarithmic relationship between the junction C8and its critical current density Jcin the range 0.1-2 kA/cm2using the Fiske mode resonance method. Our measured C8values using SQUID resonance and Josephson plasma resonance are compared with the results obtained by other groups via analyzing the major errors in the C8calculation. We discuss the accuracy in evaluating C8for all three techniques and suggest using Fiske mode resonance as a relatively stable method for C8extraction for the future design of our single-flux quantum circuit.
The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.
Superconducting three-Josephson-junction flux qubits (3JFQB) are possible candidates for the basic elements of a (future) scalable quantum computer. An important design parameter is the capacitance of their Josephson junctions. We estimate the capacitance per junction area (specific capacitance) of junctions typically used in 3JFQBs, i.e., of Al/AlOx/Al junctions with 5–10 Å thick oxide layers. The capacitance is obtained by analyzing resonant voltage steps in the current–voltage characteristics of specifically designed dc superconducting quantum interference devices (SQUIDs). The junction area is deduced from scanning electron microscope images. We find that the specific capacitance of our junctions is Cs=100±25 fF/μm2. Finally we compare this result to capacitance estimates obtained with 3JFQB microwave spectroscopy and find that the SQUID resonance method provides a much higher accuracy.
We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self- and nearest-neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al2Ox-Nb junctions for critical current densities of 0.3–20 kA/cm2. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm2 is possible, but requires submicrometer junctions with dimensions of the order of 0.3 μm.
We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self and nearest neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al/sub 2/O/sub x/-Nb junctions for critical current densities of 0.3-20 kA/cm/sup 2/. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm/sup 2/ is possible, but requires submicron junctions with dimensions of the order of 0.3 /spl mu/m.>
The specific capacitance of high-quality (V/sub m/>50 mV) Nb-AlO/sub x/-Nb tunnel junctions is determined by the following methods: the measurement of zero field step resonances in specially prepared long Josephson junctions, and the definition of the resonant voltages in two-junction interferometers based on Nb-AlO/sub x/-Nb junctions. The results obtained by these methods were compared with each other and with the figures calculated from the measurements of tunnel barrier parameters. The application of the procedure for the fabrication of single flux quantum (SFQ) logic devices is discussed.
We present the latest development of an Nb/Al-AlOx/Nb Josephson junction fabrication process for superconducting integrated circuits at SIMIT. Our fabrication process has four niobium superconducting layers and one Ti/Pd resistor layer with 1.0 Ω/square sheet resistor for junction shunting and circuit biasing. Critical current density Jcof the junctions is controlled from 30 A/cm2to 10 kA/cm2by exposing aluminum layer to pure or mixed O2atmosphere. Correlation between measured Jcand exposure conditions has been established. Average quality factor (Rsg/Rn) of the junctions in our process is larger than 20 with subgap leakage factor (Vm) beyond 30 mV. Critical current uniformity is obtained according to the switching current (Is) distribution of a 1000-JJ array. Measured standard deviation sigma of Isis less than 4% over the 100-mm wafer. Plasma resonance in a large Josephson junction is used to measure specific capacitance (Cs) of the junction. Sheet inductances of superconducting niobium strip-lines are extracted by a SQUID diagnose circuit and the measured results show good agreement with our design values.