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IEEE Transactions on Magnetics

Single flux-quantum memory cells

Pierre Guéret, Th. O. Mohr, P. Wolf

Published Jan 1, 1977• 28 citations
Publisher

Abstract

Detailed investigations have been carried out on two-junction interferometers. These devices have potential as memory elements. Information is stored as single-flux quanta (SFQ cells) in overlapping vortex modes and is destructively read out by switching from a vortex to the voltage state. The devices are fabricated with a lead alloy and the junction oxide is formed by rf oxidation. Most investigations have been done on devices with an area of about 1000 μm2, but storage and reading have also been demonstrated in our smallest interferometers having a size of about 150 μm2. Computer studies of cell properties, especially of the vortex transitions, have given good agreement with experiments. It has also been found that the cell behavior is little affected by loads such as would exist in an array environment.

DOI
10.1109/tmag.1977.1059439

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Detailed investigations have been carried out on two-junction interferometers. These devices have potential as memory elements. Information is stored as single-flux quanta (SFQ cells) in overlapping vortex modes and is destructively read out by switching from a vortex to the voltage state. The devices are fabricated with a lead alloy and the junction oxide is formed by rf oxidation. Most investigations have been done on devices with an area of about 1000 μm2, but storage and reading have also been demonstrated in our smallest interferometers having a size of about 150 μm2. Computer studies of cell properties, especially of the vortex transitions, have given good agreement with experiments. It has also been found that the cell behavior is little affected by loads such as would exist in an array environment.
The talk is based on an overview which was published recently (1). For this reason, only an abstract is given here, which includes Later results not contained in the aforementioned overview. The presently-known Josephson memory cells store information as persistent currents in superconducting loops, or what is equivalent as a magnetic flux in the area of the ring. This method of storage is nonvolatile and has no power consumption in the storing state. In a superconducting ring system, the magnetic flux is quantized, and cells have been built which operate either with only one flux quantum, or with many of them. For the Latter cells with many flux quanta (ring cells), quite a number of designs exist. They have one or two Josephson junctions in the ring, which allow writing. An external junction which is controlled by the ring current serves for non-destructive reading of the information. The smallest cell realized had an area of 900 ?m2 with a 2 ?m minimum line width. Switching speed was ? 80 ps. Cells which store information as a single-flux quantum (single flux-quantum cells = SFQ cells) in the ring, usually contain two Josephson junctions used for reading and writing. Read-out in most of these cells is destructive, but methods of reading non-destructively have also been proposed. The smallest device made had an area of ? 150 ?m2 with a minimum line width of 2 ?m. The read signal had a rise time ? 100 ps.
The design and experimental results for a novel type of Josephson memory cell are presented. The cell has a simpler construction than the conventional Henkels-type, using a one-junction SQUID (superconducting quantum interference device) capacitively coupled to a single junction. Binary data are stored as single flux quanta and are destructively read out by sensing a pulse caused by a vortex-mode transition. Computer simulations suggest that the read operation could be performed with a bias margin for the sense junction of +or-28% and that the rise time of the sense current in a 16-bit memory cell would be as fast as 30 ps, despite having to sense a vortex-mode transition. Results are presented for a memory cell fabricated with Nb-AlO/sub x/-Nb junctions.>
Single flux quantum two junction interferometers appear as attractive memory cells for constituting dense and low dissipating arrays. In this paper an investigation of cell parameters leading to Non-Destructive Read Out (NDRO) operation and non volatile storage is presented. Basic requirements are defined, various possible designs are studied through static and dynamic simulations, experimental results for one of them are given. Finally, memory array concepts involving statistical analysis on electrical parameters spreads are proposed.
A proposal vortex transitional nondestructive read-out Josephson memory cell is successfully fabricated and tested. The memory cell consists of two superconducting loops in which a single flux quantum is stored and a two-junction interferometer gate as a sense gate. The memory cell employs vortex transitions in the superconducting loops for writing and reading data. The vortex transitional memory operation of the cell contributes to improving its sense discrimination and operating margin. The memory cell is activated by two control signals without timing control signals. Memory cell chips have been fabricated using a niobium planarization process. A±21% address signal current margin and a ±33% sense gate current margin have been obtained experimentally. Successful memory operations of a cell driven by two-junction interferometer gates has been demonstrated. The single flux quantum operations of this memory cell makes it an attractive basic element for a high-speed cache memory.
Josephson-junction interferometers have previously been proposed as memory devices in which information is stored as a single flux quantum, (SFQ). To investigate the feasibility of making large arrays of SFQ cells suitable for the memory of a cryogenic computer, arrays containing 80 interferometers have been made and measured. The prime objective was to determine the distribution of the dc Josephson current i1 within the arays and to compare it with the design value required for successful operation of all cells in high density arrays. The results show that a substantial fraction of the arrays has standard deviations in i1 less than the maximum allowable, thus demonstrating for the first time that such a memory can be made with present technology. The fabrication, measurement technique and a detailed analysis of the current distributions on the arrays are presented.
A new vortex transitional NDRO memory cell has been proposed for use as a high-speed cache memory. The memory cell consists of two superconducting loops with one Josephson junction and a two-junction interferometer gate. The superconducting loop stores single-flux quantum. The interferometer gate operates as a sense gate. The memory cell employs a vortex transition in the superconducting loop for the writing and reading of data. The sense gate current margin increases to ±38% since the coupling magnetic flux is amplified by the vortex transition for nominally designed cell parameters. The memory cell operates with a ±33% address signal current margin. This paper describes the memory cell design and the dynamic behavior of computer simulations.