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International Conference on Solid State Devices and Materials (SSDM)

Fabrication of DC-SQUIDs and their characteristics for digital applications

S. Hasuo, Hiroyuki Suzuki, Kouichi Dazai

Published Aug 28, 1979• 1 citation
Publisher

Abstract

This paper presents a design theory of DC-SQUID type Josephson tunnel junctions. Discussions are focused on the experimental verification of calculated threshold characteristics of the DC-SQUIDs for various device parameters. DC-SQUIDs designed here have been applied to the flux quantum memory cell and also to the full adder circuit, and they have been successfully operated as expected.

DOI
10.7567/ssdm.1979.c-2-3

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This paper presents a design theory of DC-SQUID type Josephson tunnel junctions. Discussions are focused on the experimental verification of calculated threshold characteristics of the DC-SQUIDs for various device parameters. DC-SQUIDs designed here have been applied to the flux quantum memory cell and also to the full adder circuit, and they have been successfully operated as expected.
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We have fabricated coplanar type dc SQUIDs using NdBa2Cu3Oy superconducting thin films and operated fundamental single-flux-quantum (SFQ) circuits. The Josephson junctions were made by the narrow-focused ion beam irradiation technique. For a 145 μm wide and 10 μm long logic SQUID having a critical current of 105 μA and an inductive parameter (βL) of 28, a store and a restore of the flux quantum have been demonstrated at temperatures of 4.2–30 K. These operations were performed with an input pulsewidth of 5 ns (5 ns was the shortest input pulse width available from our function generating equipment). These results show experimentally the possibility of high speed operation in all high-Tc superconducting digital circuits.
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Superconductor–insulator–superconductor Josephson junctions with an area smaller than 1 µm 2 are increasingly used for many applications because of their low capacitances and short switching times. The characteristics of analogue and digital SQUIDs, for example, can be improved using small-area Josephson junctions. We have developed Nb/Al, AlO x /Al/Nb junctions based on an edge geometry. These low- T c ramp-type junctions allow sub-µm 2 junction areas using micrometre resolution photolithography. The experimental I – V characteristics of the ramp-type junctions showed non-hysteretic behaviour. Using these ramp-type junctions, the first series of dc SQUIDs have been successfully developed. The characteristics of the ramp-type junctions and the dc SQUIDs have been investigated both experimentally and by using numerical simulations. The white flux noise level of a dc SQUID based on ramp-type junctions with an area of 0.2 µm × 12 µm was measured to be .