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European Solid-State Circuits Conference (ESSCIRC)

Memories with Josephson Junctions

Peter Wolf

Published Sep 1, 1978• 0 citations

Abstract

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.

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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.>

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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.

Over the past few decades, superconducting circuits have been used to realize various novel electronic devices such as quantum bits, SQUIDs, parametric amplifiers, etc. One domain, however, where superconducting circuits fall short is information storage. Superconducting memories are based on the quantization of magnetic flux in superconducting loops. Standard implementations store information as magnetic flux quanta in a superconducting loop interrupted by two Josephson junctions (i.e., a SQUID). However, due to the large inductance required, the size of the SQUID loop cannot be scaled below several micrometers, resulting in low-density memory chips. Here, we propose a scalable memory consisting of a voltage-biased superconducting ring threaded by a half-quantum flux bias. By numerically solving the time-dependent Ginzburg-Landau equations, we show that applying a time-dependent bias voltage in the microwave range constitutes a writing mechanism to change the number of stored flux quanta within the ring. Since the proposed device does not require a large loop inductance, it can be scaled down, enabling a high-density memory technology.

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.

Abstract Superconducting memory cells that use flux quanta as their storage medium can achieve ultra-fast access times with ultra-low power consumption. However, the data signal generated by a flux quantum memory (FQM) cell is usually too weak and too fast to be measured directly. Here, we present a method to characterize the real-time operation of an FQM cell. The storage loop of the FQM cell, configured with a Nb/NbN X /Nb Josephson junction, was proven the capability to store multiple flux quanta. The readout was demonstrated by a superconducting quantum interference device composed of underdamped Nb/Al-AlO X /Nb Josephson junctions. The writing and reading operations were achieved by a short pulse ranging from 0.1 ns to 2.5 ns, and a constant bit error rate of ∼2.46% was measured for the fabricated FQM cell. The method presented here can be used to study real-time operation of an FQM cell in a direct manner.

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.>