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IEEE Transactions on Applied Superconductivity

A novel approach to chip-to-chip communication using a single flux quantum pulse

Masaaki Maezawa, Hirotake Yamamori, Alexandre Shoji

Published Jun 1, 1999• 8 citations
Publisher

Abstract

A novel approach to chip-to-chip communication for RSFQ technology is proposed. The main idea is to fabricate Josephson junctions on a substrate for multi-chip packaging, which equips the substrate as well as the chip with built-in active drivers and receivers. The driver on the chip is connected directly to the receiver on the substrate through a connector which can be considered as a lumped circuit element. Because the circuit is free from the impedance matching constraint, broadband chip-to-chip data transfer with wide operating margins will be realized. We have designed, simulated and optimized a chip-to-chip single flux quantum (SFQ) pulse transfer circuit. For a connector inductance of 20 pH, the bias current margins and the maximum throughput have been calculated to be /spl plusmn/24% and above 50 Gb/s, respectively.

DOI
10.1109/77.783916

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IEEE Transactions on Applied Superconductivity•Jun 1, 2007•6 citations
A novel approach to chip-to-chip communication for RSFQ technology is proposed. The main idea is to fabricate Josephson junctions on a substrate for multi-chip packaging, which equips the substrate as well as the chip with built-in active drivers and receivers. The driver on the chip is connected directly to the receiver on the substrate through a connector which can be considered as a lumped circuit element. Because the circuit is free from the impedance matching constraint, broadband chip-to-chip data transfer with wide operating margins will be realized. We have designed, simulated and optimized a chip-to-chip single flux quantum (SFQ) pulse transfer circuit. For a connector inductance of 20 pH, the bias current margins and the maximum throughput have been calculated to be /spl plusmn/24% and above 50 Gb/s, respectively.
Chip-to-chip propagation of a single flux quantum (SFQ) pulse has been demonstrated using an active multichip module (MCM) technique in which active transmitters and receivers are integrated on an MCM substrate as well as on a chip. A chip-to-chip SFQ transfer circuit has been designed and implemented by a standard Nb-trilayer process and solder-bumped flip-chip technology. The correct operation of the circuit has been confirmed by low-frequency testing. Experimental margins for the global factor for bias currents have been as large as /spl plusmn/33%.
We have investigated the applicability of Single Flux Quantum (SFQ) and Multiple Flux Quanta (MFQ) approaches to communications between RSFQ logic/memory chips. SFQ pulses can be transmitted between chips if inductive discontinuity L lies below 3-5 pH. For larger L one can employ MFQ pulses which have the same width as the SFQ pulses, but M times larger amplitude. For example, for L=20 pH, M=4 is sufficient. We have developed an MFQ driver and a receiver capable of transmitting the data in 10-20 Gbps/pin range for the use in simple flip-chip MCM packages based on Si substrates.
We report experimental results on chip-to-chip transfer of single-flux-quantum (SFQ) pulses using an active multichip module (MCM). Josephson transmitters and receivers are integrated on both a chip and an MCM substrate. An MCM consisting of a 4.8-mm chip and an 8.2-mm MCM substrate was fabricated using a 1.6-kA/cm/sup 2/ Nb-trilayer process and a solder-bumped flip-chip bonding technology. The correct operation of the circuit was confirmed by low-speed testing. Bit-error rate (BER) of the circuit was measured down to 10/sup -5/, and extrapolation of the error-function fits suggested a very small BER, lower than 10/sup -200/, at the optimum bias point. Experimental margins on the bias voltage were as large as /spl plusmn/34%.
We recently reported communication up to 60 Gb/s between digital superconductor chips mounted on a passive carrier, using a novel driver circuit that produces a double-flux-quantum pulse. Here, we answer various practical questions pertaining to chip-to-chip and on-chip communication in greater detail. On-chip, the traditional single-flux-quantum driver is adequate, although margins and bit error rate degrade significantly at the microstrip resonance frequency. Chip-to-chip, using the double-flux-quantum driver, shows little frequency dependence, in spite of 13% reduction of signal current in the chip-to-chip transitions. Appropriate microstrip impedance is in the range 4-8 /spl Omega/ for a 4 kA/cm/sup 2/ Josephson junction IC process. Data includes bit errors rates for pseudo-random data in the range 3-27 Gb/s.
We have designed circuits to transmit and receive bits generated by Single-Flux-Quantum (SFQ) circuits. Our circuits were fabricated using shunted Nb-AlOx-Nb junctions. The transmitter is a series array of dc SQUIDs driven into the voltage state in response to an input SFQ pulse. The receiver circuit is inductively coupled to the transmitter. Current from the transmitter biases a junction race in the receiver such that a clock pulse either passes (a '1' bit) or does not (a '0' bit). We have measured a maximum peak-to-peak amplitude from the transmitter of 296 /spl mu/V at a clock rate of 2.5 GHz, and 187 /spl mu/V at 5 GHz.
Superconducting single flux quantum (SFQ) technology is well known for digital logic operations below 4.2 K, which are compatible with the Josephson junction qubit. Operation speed and signal sensitivity of circuits in SFQ are also sufficient to control qubits. Therefore, the SFQ circuit is a candidate technology for multiple qubit control. However, the self-heating effect of the SFQ circuit is a potential barrier to development of an SFQ-supported multiple qubit system because the qubit operating temperature is lower than the conventional 4.2 K. The multi-chip approach is a possible solution because the qubit chip can be thermally isolated from the SFQ circuitry. We report on recent progress in SFQ circuit and packaging technology for operation at sub-Kelvin temperatures. As for multi-chip module technology, a test sample of the multi-chip module shows good reliability of electrical connections between chips, and each solder bump has a bandwidth wide enough for signal transmission. We have designed lower power SFQ circuits for sub-Kelvin temperatures. The critical currents of Josephson junctions were scaled down, inductances were scaled up, and bias voltage was reduced to make power consumption lower. Based on our fabrication technology, critical current densities were optimized by considering trade-off relations between fabrication availability and operation speed. We successfully confirmed its correct operations at around 0.35 K. The combination of the multi-chip module and optimized SFQ circuits is very effective way of controlling qubit.