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Journal of Vacuum Science and Technology

Studies on arrays of Josephson tunnel junction interferometers

Ronald F. Broom, Theodor O. Mohr

Published May 1, 1978• 8 citations
Publisher

Abstract

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.

DOI
10.1116/1.569529

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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.
Measurements on large arrays of Josephson interferometers were carried out to obtain individual device parameters and their spread. Each array contained 784 identical interferometers with junction diameters of 5 and 7 μm and was deposited on a 6.35×6.35-mm Si chip. Threshold currents Im(0), subgap resistances Rj, and Vm —the product of Rj and Im(0) —were investigated. Statistical analysis indicated that the standard deviation σ of Im(0) and Rj ranged between 6 and 18% and that of Vm ranged between 5 and 7%. A systematic spatial dependence of Im(0) and Rj on the chips was obtained which is attributed as being due to nonuniformities in the rf oxygen discharge used to form the tunnel barrier. With improvements in the process, namely, eliminating these nonuniformities, the standard deviation in Im(0) (and Rj) can be reduced to about 5%. From the observed variations in the junction areas it is estimated that the random variation in the Josephson current density is about 4%. Large scale applications of these interferometers in the design of computer memory and logic thus seem feasible.
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.
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.
Abstract In this paper we analyze a cryogenic memory cell circuit based on a small coupled array of Josephson junctions. All the basic memory operations (e.g., write, read, and reset) are implemented on the same circuit and different junctions in the array can in principle be utilized for these operations. The presented memory operation paradigm is fundamentally different from conventional single quantum flux operation logics (SFQ). As an example, we demonstrate memory operation driven by a SFQ pulse employing an inductively coupled array of three Josephson junctions. We have chosen realistic Josephson junction parameters based on state-of-the-art fabrication capabilities and have calculated access times and access energies for basic memory cell operations. We also implemented an optimization procedure based on the simulated annealing algorithm to calculate the optimized and typical values of access times and access energies.
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.