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Applied Physics Letters

YBa2Cu3O7 step-edge dc SQUID with coplanar control lines

H. Hasegawa, Y. Tarutani, Tokuumi Fukazawa, Uki Kabasawa, Katsuaki Takagi

Published Nov 20, 1995• 14 citations
Publisher

Abstract

A dc superconducting quantum interference device (SQUID) with coplanar control lines has been fabricated from a single layer of YBa2Cu3O7 thin film. Step-edge junctions were formed using resist/Si double-mask layers. Current–voltage characteristics of this SQUID at 77 K show Shapiro steps under microwave irradiation and are qualitatively consistent with the resistively shunted junction (RSJ) model. The spacing between the closed loop and the coplanar control lines in the tested SQUIDs ranged from 0.2 to 1.5 μm. The measured output voltage-control current characteristics showed that the experimentally derived mutual inductances were a function of the spacing, in agreement with the calculated results. The typical mutual inductance for a spacing of 1.0 μm was ∼0.5 pH, which is adequate for detecting signals of flux-transfer logic gates in which the typical logic amplitude is ∼0.1 mA. A coplanar structure is thus useful for logic gate applications.

DOI
10.1063/1.115154

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Applied Physics Letters•May 19, 1997•27 citations
A dc superconducting quantum interference device (SQUID) with coplanar control lines has been fabricated from a single layer of YBa2Cu3O7 thin film. Step-edge junctions were formed using resist/Si double-mask layers. Current–voltage characteristics of this SQUID at 77 K show Shapiro steps under microwave irradiation and are qualitatively consistent with the resistively shunted junction (RSJ) model. The spacing between the closed loop and the coplanar control lines in the tested SQUIDs ranged from 0.2 to 1.5 μm. The measured output voltage-control current characteristics showed that the experimentally derived mutual inductances were a function of the spacing, in agreement with the calculated results. The typical mutual inductance for a spacing of 1.0 μm was ∼0.5 pH, which is adequate for detecting signals of flux-transfer logic gates in which the typical logic amplitude is ∼0.1 mA. A coplanar structure is thus useful for logic gate applications.
We have fabricated Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.
We have investigated the electrical properties of high-T c dc-SQUID's formed in a coplanar waveguide, and discussed their applicability to single flux quantum (SFQ) circuits. The planar type Josephson junctions were fabricated in NdBa2Cu3O y (NBCO) superconducting thin films by a narrow-focused ion beam (FIB) irradiation technique. The critical current and normal resistance values of the Josephson junction at 4.2 K were 115 µA and 8 Ω, respectively. The inductance was estimated by applying a direct injection of the control current into the dc-SQUID loop and measuring the periodic voltage response. A loop inductance of 47 pH was obtained for a 25 µm-wide and 20 µm-long dc-SQUID at 4.2 K. It has been clarified that for a stable operation of SFQ logic circuits, a 10 µm-square dc-SQUID's are required.
Abstruct-Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YJ3a2Cu307 thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (I,) of the Josephson junction, was determined by using directly coupled dc superconducting quantum interference devices (SQUIDS). Magnetically coupled dc SQUIDs were used to detect the output signal from the QFP. Simulated waveforms were used to identify the operation modes of the QFP as a function of the circuit parameters. From examination of the output waveforms of the QFP, operation in agreement with simulations was confirmed.
We report the development of a fabrication process of high-resistance step-edge Josephson junctions on sharp argon ion beam etched substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around and is reflected in reproducible electrical properties of the junctions. Critical current and asymptotic normal state resistance both scale with the junction width w . The current-voltage ( I - V ) characteristics depend on the ratio of film thickness t to step height h . For > 0.85 they are flux-flow-like; for < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and per junction, depending on w . At 77 K we achieve products of more than . The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than are achieved. noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.
Autonomous single-layer dc superconducting quantum interference devices (SQUIDs) have been prepared from epitaxial, laser-deposited YBa2Cu3Ox films on step edge SrTiO3 and LaAlO3 substrates. For device patterning, a SiOx inhibit technique as well as conventional ethylenediaminetetraacetic wet etch was used. The Josephson junctions are of grain boundary type. Their widths are 5 μm. The SQUID hole is a square of about 5 μm; the SQUID inductance is estimated to be about 20 pH. We obtained a very regular, nonhysteretic flux to voltage modulation over more than 100 flux quanta (Φ0). The maximum voltage signal is of the order of 15 μV peak to peak and the maximum transfer function dV/dΦ at the appropriate flux bias is 50 μV/Φ0 at 77 K. The best value of the equivalent flux noise as measured in the flux locked loop mode is 1.4×10−5 Φ0/Hz1/2 at 1 Hz and 1×10−5 Φ0/Hz1/2 in the white noise region for f≳5 Hz. This results in an energy resolution εn,w(f≳5 Hz)=1×10−29 J/Hz in the white noise region and 2×10−29 J/Hz at 1 Hz.
We have fabricated direct-injection high-Tc superconducting quantum interference devices (SQUIDs) with integrated YBa2Cu3O7−x (YBCO) ground planes in the upper part of structure. The YBCO/PrBa2Cu3O7−x(PBCO)/YBCO edge junctions with 10-nm-thick PBCO barriers showed RSJ-like current-voltage (I–V) characteristics before and after the ground plane formation. We measured the voltage modulations of these SQUIDs with two different hole sizes to evaluate the line inductance. At a temperature of 30 K, the estimated line inductance under the ground plane was about 1.0 pH/□, while the estimated line inductance without the ground plane was 2.8 pH/□.