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Applied Physics Letters

A 3 bit single flux quantum shift register based on high-Tc bicrystal Josephson junctions operating at 50 K

B. Oelze, B. Ruck, Erik Sodtke, Alex F. Kirichenko, M. Yu. Kupriyanov, W. Prusseit

Published Feb 3, 1997• 22 citations
Publisher

Abstract

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.

DOI
10.1063/1.118325

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Experimental superconducting shift registers consisting of on-chip clock generators, clock regeneration and distribution circuits, shift register elements, and readout circuits are designed using rapid single flux quantum logic/memory (RSFQ) gates. A 7-b shift register has been tested to 12 GHz and a 17-b to 21 GHz using external triggering clocks with relative delay measurements. Testing with internal clocks generated from Josephson oscillations shows a potential high-speed operation of 45 GHz for the 7-b and 30 GHz for the 17-b shift registers. Two types of magnetically coupled readout gates are discussed. The chips are fabricated using a Nb/AlO/sub /x/Nb Josephson-junction process at a critical-current density of 1000 A/cm/sup 2/. The power dissipation per bit is 3 mu W.>

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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.>
We fabricated a 4 stage single flux quantum shift register with YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) bicrystal Josephson junctions and demonstrated the circuit's low error operation by using a computer controlled digital measurement set-up. The circuit was operated at 65 K. Binary data sequences of "1000", "1010", "1011," and "1111" were successfully loaded and shifted in the circuit. The two read SQUIDs placed next to each side of the shift register were used to sense all the individual data states. By operating the circuit with the proper current pulses, we observed no errors during 16 hours, which is equivalent to 21,000 error-free data shifts. We also found that temperature dependent inductance and junction critical currents limit the operating temperature range of the circuit, and the effective thermal noise temperature can be lower than 100 K.
A rapid single-flux quantum (RSFQ) set-reset trigger circuit has been implemented using YBaCuO grain boundary junctions on an asymmetric 32 degrees Y-ZrO 2 bicrystal. The circuit contains 18 Josephson junctions aligned along the grain boundary and required six film layers: a YBaCuO base electrode, two amorphous SrTiO 3 insulating layers, a superconducting Pb/Au/Cr ground plane and Au/Cr wiring. It consists of two DC-to-SFQ converters, two Josephson transmission lines and an SFQ-to-DC converter. Two pulse trains, 'signal' and 'clock', were applied to change the state of the trigger from '0' to '1' and vice versa, and registered at the SFQ-to-DC: output. All experiments were performed at 4.2 K.
We have implemented a simple circuit of the rapid single-flux-quantum (RSFQ) logic family using a single-layer YBa2Cu3O7−x thin-film structure with 14 in-plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip-flop, and an SFQ/dc converter (readout SQUID). Low-frequency testing has shown that the dc-current-biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.
A balanced current comparator based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The operation of the balanced comparator has been demonstrated in the temperature range from 4.2 to 70 K. The correct sequence of switching of the Josephson junctions has been determined by dc measurements for different values of operating frequency and signal current Ix. The threshold uncertainty ΔIx of the balanced comparator, measured at 40 K, was 60 μA at 40 GHz and 300 μA at 240 GHz.
Experimental superconducting shift registers consisting of on-chip clock generators, clock regeneration and distribution circuits, shift register elements, and readout circuits are designed using rapid single flux quantum logic/memory (RSFQ) gates. A 7-b shift register has been tested to 12 GHz and a 17-b to 21 GHz using external triggering clocks with relative delay measurements. Testing with internal clocks generated from Josephson oscillations shows a potential high-speed operation of 45 GHz for the 7-b and 30 GHz for the 17-b shift registers. Two types of magnetically coupled readout gates are discussed. The chips are fabricated using a Nb/AlO/sub /x/Nb Josephson-junction process at a critical-current density of 1000 A/cm/sup 2/. The power dissipation per bit is 3 mu W.>