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Electronics Letters

Two-Josephson-junction interferometer memory cell for n.d.r.o.

H. Beha

Published Sep 29, 1977• 11 citations
Publisher

Abstract

A single-flux-quantum random-access memory cell for nondestructive read-out and write has been designed and successfully simulated. The binary information is stored in an interferometer with two unequal Josephson junctions. No bias current is needed to store the information. Reasonable operation margins are obtained.
DOI
10.1049/el:19770429

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A new Single Flux-Quantum (SFQ) Nondestructive Read-Out (NDRO) random-access asymmetric 2-Josephson-junction interferometer memory cell with only one control line has been investigated and designed. The binary information is stored in the cell nonvolatilely without a bias current. For random access, this is the first known interferometer memory cell without a diagonal line. Therefore, this NDRO SFQ memory cell leads intrinsically to smaller cell area and simpler decoder schemes. A cell design of only 52 lithographic squares is given to demonstrate the potential for high-density memory chips. Wide nominal read and write operating margins are obtained. The NDRO interferometer memory cell proposed is promising for use in a high-performance Josephson computer.
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.
An improved single flux-quantum Josephson interferometer memory cell which brought stable nondestructive read-out (NDRO) operation was proposed. The cell was constructed with an end-fed asymmetric two-junction interferometer, in which the bit operating current level could be set far above the critical point for stable NDRO operation. Damping condition for proper memory operation was studied by computer simulation. The NDRO operation was experimentally obtained for the McCumber damping parameter βc of 300–400.
In single flux quantum interferometer memory cells the information can be stored without bias current, if the interferometer inductance L is chosen sufficiently large (\lambda = 2\piLI_{0}/\Phi_{0} = 2\pi) to allow for three stable states (-1-, 0-, 1-mode) at zero control and gate current. In this case the binary informations are represented by the -1- and 0-mode of the interferometer. The nondestructive read out operation of a binary "1" is performed without a transition into another mode or into the voltage state. The binary "1" corresponds to no sense signal. Nondestructive read out of the binary "0" is achieved in switching to the voltage state and in returning to the 0-mode after the end of the drive currents. In this paper the dynamics of switching back into the zero voltage state are investigated by simulations of trajectories in the order parameter phase plane. It is shown that for realistic fabrication tolerances the interferometer settles in the wanted 0-mode and not in the -1- or 1-mode even under severe gate or control current disturbs, if the maximum Josephson currents of the two Josephson junctions are unequal (2:1) and if the McCumber damping factor is sufficiently large (\beta \approx 100).
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.
This paper presents a novel nondestructive memory cell using a two-junction interferometer. Most of single-flux-quantum memory cells have been the destructive ones, which must rewrite the information for logic "1" after a reading operation. This makes the circuitry of the cell complex. To avoid this disadvantage, a novel self-rewriting technique is introduced by setting a coil between the terminals of the interferometer. The principle of the operation of the cell is described and the simulation for the behaviours of the cell are illustrated in detail. The results of the simulation show that the reliable operation of the cell can be achieved with high operating speed.