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IEEE Transactions on Applied Superconductivity

A single flux quantum shift register operating at 65 K

Martin G. Forrester, John X. Przybysz, John Talvacchio, J.H. Kang, A. Davidson, J. R. Gavaler

Published Jun 1, 1995• 34 citations
Publisher

Abstract

We report the fabrication and quasi-static testing of a two-stage, high-temperature superconducting, Single Flux Quantum shift register. The five-junction circuit was fabricated using a single YBCO film, with step-edge grain boundary junctions. Storage of flux, and its motion in response to LOAD and SHIFT signals, was demonstrated at 65 K.>

DOI
10.1109/77.403322

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We report the fabrication and quasi-static testing of a two-stage, high-temperature superconducting, Single Flux Quantum shift register. The five-junction circuit was fabricated using a single YBCO film, with step-edge grain boundary junctions. Storage of flux, and its motion in response to LOAD and SHIFT signals, was demonstrated at 65 K.>
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A 3 bit single flux quantum (SFQ) shift register based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The circuit consists of 26 bicrystal Josephson junctions and includes the shift register itself, two dc-SFQ converters, one readout superconducting quantum interference device, serving as a SFQ-dc converter, and three Josephson transmission lines. The correct operation of all circuit components has been demonstrated by low frequency testing at a temperature of 50 K.
A Josephson SFQ (single flux quantum) shift register circuit operating under a two-phase power was designed and tested. The test cell was fabricated using the Nb/AlO/sub x//Nb junction SNEAP process. The main data latch is DC biased; the data storage is accomplished by the transfer of a single flux quantum in or out of a SQUID (superconducting quantum interference device) superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended.>
Testing on a single-flux-quantum (SFQ) shift register fabricated with a SFQ Nb-AlO/sub x/Nb Josephson junction process indicates that a stable flux quantum can be stored in a rapid SFQ (RSFQ) shift register stage and be shifted by SFQ pulses. Design simulations showed that logic errors can occur in SFQ shift registers even though the circuit passes the average voltage test. Three-junction-per-stage shift registers had an advantage over the two-junction-per-stage shift register in handling data with successive 1's and in incorporating resistors to increase tolerance of trapped magnetic flux. Input and output buffers for data and clock pulses were added to get an RSFQ shift register to obtain correct performance. The highest error-free operation speed for the data with successive 1's is about half of the speed for propagation of a lone data 1. Error-free operation of SFQ shift registers with three junctions per stage could be obtained up to speeds of 30-60 GHz with Josephson junctions having I/sub c/R/sub n/ (critical-current-normal-resistance product) values of 600 mu V.>
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.
Shift registers have been demonstrated in YBaCuO operating at 77 K using from 64 to over 1000 junctions. These are some of the larger scale integrated circuits demonstrated to date using YBaCuO Josephson technology. The circuit is a modified rapid single flux quantum design in which a single trigger pulse causes a one bit shift of the entire word of 32-512 b in length. Two different junction technologies, electron-beam defined nanobridges and epitaxial edge junctions, have been used with parameter spreads ranging from 11% to 22%. Correct operation has been verified with low speed random word tests and circulating data tests while pseudo random bit sequence demonstrations are underway. A practical amount of time to shift between cells has been measured to be about 10 ps.>