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abrication of QFP Logic Gates Based on YBa2Cu3O7 Step-Edge Junctions

H. Hasegawa, Yoshinobu Tarutani, Uki Kabasawa, Nobuyuki Sugii, Tokuumi Fukazawa, Kazumasa Takagi

Published Jan 1, 1997• 0 citations
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Abstract

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.

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•1 citation
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.
Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (J/sub c/) 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.
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 report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.
The Quantum Flux Parametron (QFP) is a Josephson Junction device using the polarity of a quantum magnetic flux to represent a binary information. A quantum unit of magnetic flux in a QFP can be controlled to realize majority logic operation, consequently AND and OR logics can be realized. Together with negation, which can be realized by a flux-inverting transformer, various computing logic devices can be constructed. In addition, the QFP is also a latching device so that a flux can be confined in the device to realize memory function. Due to the homogenous nature of memory and logic circuits as well as the flux nature of the signal which can be transmitted inductively, an entirely new Josephson supercomputer with memory and CPU pipelining arranged in a 3D setting was conceived. The computer with 40 processors is expected to run at a maximum speed of 20 GFLOPS of scalar processing.
The quantum flux parametron (QFP) is a flux-transfer, flux-activated Josephson logic device which realizes much lower power dissipation than other Josephson logic devices. Being a two-terminal device, its correct operation may be affected by coupling to other QFPs. The problems include backcoupling form active QFPs through inactive QFPs (relay noise), coupling between QFPs activated at different times because of clock skew (homophase noise), and interaction between active QFPs (reaction hazard). Previous QFP circuits worked by wired-majority, which is a linear input logic and has low input margin. A logic gate (D-gate) using a QFP to perform logic operations has been analyzed and tested by computer simulation. Relay noise, homophase noise, and reaction hazard are substantially reduced. Moreover, the inputs have little interaction, and hence input margin is greatly improved. Using only D-gates it is possible to realize any combinational logic function. Important logic functions can be realized using one gate, such as a two-input multiplexer, majority, and parity. Since the QFP has a latching effect, D-gates can realize fully pipelined circuits without additional registers.
The characteristics of a simple logic gate which consists of an rf-Superconducting QUantum Interference Device (SQUID) and dc-SQUID has been investigated in connection with the Single-Flux-Quantum (SFQ) logic operation and Josephson junction characteristics. The Josephson junctions were made of Neodymium Barium Copper Oxide thin film employing Focused Ion Beam technology. We confirmed the generating SFQ in the rf-SQUID and simultaneous read-out by the dc-SQUID up to 50 K. It has been found that the temperature dependence of output voltage level decreased more rapid than that of critical current normal resistance product. The origin of the discrepancy was studied using a phenomenological model of the Josephson junction and numerical simulation.