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Physica C Superconductivity

Fabrication and characteristics of SFQ circuit elements made of oxide thin films

Y. Tarutani, Yoshihiro Ishimaru, H. Wakana, Seiji Adachi, Ken‐ichi Nakayama, Yasuo Oshikubo, Osami Horibe, Koji Tsubone, Y. Morimoto, K. Tanabe

Published Aug 9, 2005• 2 citations
Publisher
DOI
10.1016/j.physc.2005.02.158

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Research on superconducting signal processing devices. Final technical report

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An understanding of the role of alloying elements in lead thin films for use as Josephson junctions was achieved, and thermally cyclable devices were made. Gold is one of the elements which strongly inhabit the movement of grain boundaries and thus prevent the growth of hillocks and whiskers on the film which would puncture the thin oxide barrier. A new technology in which thin silicon membranes supported by a surrounding frame can be used for device fabrication was developed. This includes a double-etching scheme to achieve even submicron size membranes having thickness of no more than a few hundred angstroms. We have shown that advantages can be had by fabricating Schottky and super-Schottky diodes on such membranes. Also, Josephson junctions made this way show promise of great design flexibility and may develop into the best Josephson device for mixing and detection. Two ways of making double weak superconducting links for possible use as switching elements were tested. It appears that the impedance level is lower than would be desired for that application for the dimensions used so far but fabrication by electron lithography would increase it. The semiconductor barrier is analyzed, as a possible Josephson junction switching element and itmore » appears to have some advantages. One of these is a lower Q for the junction cavity; our analysis shows about two orders of magnitude improvement in this regard over oxide-barrier junctions. (GRA)« less

•Jan 1, 1975•0 citations
YBa2Cu3O7-x(YBCO) ramp-edge junction technologies have been developed for fabricating high-operating-temperature SFQ(Single Flux Quantum) circuits. The Ic(critical current) spread that is the key parameter for integrating junctions could be decreased to 8% for a 1000-junction array that was formed on a groundplane. The background technologies that enabled this uniform junction characteristic are precise control of the counter-electrode deposition-conditions and deposition of the extremely smooth multiplayer that showed the roughness as small as 2 nm. Based upon these ramp-edge junction technologies, oxide SFQ-circuit-elements were fabricated and their logic operations were investigated. SFQ circuit elements such as a confluence buffer, a set-reset flipflop, and a toggle flipflop have been fabricated and their logic operations were measured by using an SFQ-dc converter. Both toggle flipflops that have one output and two outputs have been fabricated. These circuits could be operated up to the temperature of near 60 K under finite operating margins. These experimental results enable us to further develop fuctional and much more integrated SFQ circuits.
Latching-type driver circuits integrated with high-temperature superconductivity (HTS) junctions and two types of resistor were fabricated. These circuits could successfully perform latching operation by means of a single-flux-quantum (SFQ) signal input. Ramp-edge-type HTS junctions were fabricated by interface engineering and showed a hysteretic current-voltage characteristics under a temperature of 50 K. The critical current (I/sub c/) of these 5-micron-wide junctions was about 0.2 mA and the I/sub c/R/sub n/ product was about 1.7 mV at 4.2 K. The circuits included two types of resistors, which were made of RF-magnetron-sputtered indium-tin-oxide (ITO) and Au films. A driver with a parallel 2-junction-stack could convert an SFQ pulse, which was transformed through several Josephson transmission lines (JTL's) from a DC/SFQ circuit, to a 2.3-mV level signal.
An understanding of the role of alloying elements in lead thin films for use as Josephson junctions was achieved, and thermally cyclable devices were made. Gold is one of the elements which strongly inhabit the movement of grain boundaries and thus prevent the growth of hillocks and whiskers on the film which would puncture the thin oxide barrier. A new technology in which thin silicon membranes supported by a surrounding frame can be used for device fabrication was developed. This includes a double-etching scheme to achieve even submicron size membranes having thickness of no more than a few hundred angstroms. We have shown that advantages can be had by fabricating Schottky and super-Schottky diodes on such membranes. Also, Josephson junctions made this way show promise of great design flexibility and may develop into the best Josephson device for mixing and detection. Two ways of making double weak superconducting links for possible use as switching elements were tested. It appears that the impedance level is lower than would be desired for that application for the dimensions used so far but fabrication by electron lithography would increase it. The semiconductor barrier is analyzed, as a possible Josephson junction switching element and itmore » appears to have some advantages. One of these is a lower Q for the junction cavity; our analysis shows about two orders of magnitude improvement in this regard over oxide-barrier junctions. (GRA)« less