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Superconductor Science and Technology

Design of a 16 kbit superconducting latching/SFQ hybrid RAM

Shuichi Nagasawa, H. Hasegawa, Tatsunori Hashimoto, Hideo Suzuki, Kazunori Miyahara, Youichi Enomoto

Published Nov 1, 1999• 19 citations
Publisher

Abstract

We have designed a 16 kbit superconducting latching/SFQ hybrid (SLASH) RAM, which enables high-frequency clock operation up to 10 GHz. The 16 kbit SLASH RAM consists of four 4 × 4 matrix arrays of 256 bit RAM blocks, block decoders, latching block drivers, latching block senses, impedance matched lines and the powering circuits. The 256 bit RAM block is composed of a 16 × 16 matrix array of vortex transitional memory cells, latching drivers, SFQ NOR decoders and latching sense circuits. We have also designed and implemented an SFQ NOR decoder that is composed of magnetically coupled multi-input OR gates and RSFQ inverters.

DOI
10.1088/0953-2048/12/11/371

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IEEE Journal of Solid-State Circuits•Aug 1, 1979•47 citations
We have designed a 16 kbit superconducting latching/SFQ hybrid (SLASH) RAM, which enables high-frequency clock operation up to 10 GHz. The 16 kbit SLASH RAM consists of four 4 × 4 matrix arrays of 256 bit RAM blocks, block decoders, latching block drivers, latching block senses, impedance matched lines and the powering circuits. The 256 bit RAM block is composed of a 16 × 16 matrix array of vortex transitional memory cells, latching drivers, SFQ NOR decoders and latching sense circuits. We have also designed and implemented an SFQ NOR decoder that is composed of magnetically coupled multi-input OR gates and RSFQ inverters.
We have developed a 256-bit superconducting latching/SFQ hybrid (SLASH) RAM block as the first step in developing a 16-Kbit SLASH RAM, which enables high-frequency clock operation up to 10 GHz. The SLASH RAM is composed of ac-powered latching devices and dc-powered SFQ devices. The 256-bit SLASH RAM block is composed of 16/spl times/16 matrix array of vortex transitional memory cells, SFQ-NOR decoders, latching drivers, latching sense circuits, and address buffers. The 256-bit SLASH RAM block chips were fabricated and tested. We confirmed that the 256-bit SLASH RAM block functioned successfully.
We present the design and test results of components for a superconductor Cryogenic Random Access Memory (CRAM). The 16-Kb RAM design consists of four 4-Kb sub-arrays (blocks) with a 400 ps access time (latency) and a 100 ps cycle time (throughput). Each 4-Kb RAM block comprises a row-accessed 32/spl times/128 memory cell array, bipolar line drivers, row decoders, and column sense circuits. The implementation of specially designed distributed Josephson junctions in the sensing circuits reduces the overall size of the blocks and allows the use of smaller dc control currents.
A Josephson 256-word/spl times/16-bit RAM that includes a power circuit has been developed to enable high-frequency clock operation. This RAM consists of a 4/spl times/4 matrix array of 256 RAM blocks, impedance-matched lines, and signal amplifiers. A power-supply circuit, composed of a transformer and a Josephson regulator, is included in each 256 RAM block. Fail bit maps for the 256 RAM block were measured, and perfect operation with a 100% bit yield was obtained. The 256 RAM block functioned up to a clock frequency of 1.07 GHz. We succeeded in feeding a large high-frequency current of more than 2 A into the entire 256-word/spl times/16-bit RAM. The 256-word/spl times/16-bit RAM therefore functioned up to a clock frequency of 620 MHz.
A new design concept for the Josephson-junction random access memory (RAM) has been developed. In contrast to previous RAMs based on single flux quantization (SFQ), in our system READ and WRITE operations employ ballistic transfer of SFQ pulses along bit lines (either Josephson transmission lines, or passive superconducting microstrip lines, or their combination). The basic memory cells are the single-junction SQUIDs, connected serially by the bit lines and inductively coupled to word lines. READ and WRITE operations are performed by sending SFQ pulses in appropriate directions along bit lines, and DC currents of appropriate polarity into word lines. This approach allows design of very dense memories with n/spl les/2 Josephson junctions per bit, memory cell area smaller than 80 /spl lambda//sup 2/ (where /spl lambda/ is the minimum feature size), and the critical parameter margin well above /spl plusmn/20%. In this paper we present the general structure of the RSFQ RAM, as well as design and results of testing of the basic memory cell and decoder circuitry using Hypres' 3.5-/spl mu/m, 1-kA/cm/sup 2/ Nb-trilayer technology.>
We designed a superconducting random access memory (RAM) in which all component circuits can be operated with dc-bias currents. A dc-powered superconducting loop driver and a dc-powered sense circuit are effectively combined with single flux quantum (SFQ) circuits. We proposed a pipeline structure for the memory cell array composed of the dc-powered loop drivers, the dc-powered sense circuits, passive transmission lines (PTLs), and SFQ gates. This pipeline structure enables a clock operation of 10 GHz even in a large-scale RAM. An effective device structure for the RAM based on a planarized multi-layer device structure was proposed. A dc-power layer and two PTL layers were placed under the ground plane. This structure is indispensable to create the pipeline structure using PTLs. The large inductance formed in the power layer enables low power dissipation of the RAM. We found from the estimations that 10 GHz clock operation with extremely low power dissipation can be achieved even in a large-scale RAM of 1 Mbit.
An experimental memory model for investigating the feasibility of a 16K RAM memory with Josephson junctions was fabricated and tested. There are nearly 4500 Josephson junctions in the design which includes array, line drivers, and address decoders. Storage element is a single flux-quantum (SFQ) cell arranged in a 2K array. Drivers and decoders are based on the principle of current steering in superconducting loops, which is a medium speed but low power approach. The measured read-access time of the model is approximately 10 ns. Power dissipation of the unselected chip is zero, whereas for a read/write cycle time of 30 ns, it amounts to about 10 /spl mu/W. Results indicate that a 16K chip is feasible electrically. The estimated access time and power dissipation are 15 ns and 40 /spl nu/W, respectively.