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IEEE Transactions on Applied Superconductivity

Integration of Optical Waveguides With Single Flux Quantum Circuits

Yoshiki Arita, Nobuyuki Yoshikawa, Baba Toshihiko

Published Jan 7, 2011• 1 citation
Publisher

Abstract

Single flux quantum (SFQ) circuits and optical circuits are expected to be used in next-generation integrated circuits. This study investigates basic technologies for integrating optical circuits with SFQ circuits to exploit the advantages of both technologies. We calculated, designed and evaluated metal-clad waveguides, which can be fabricated by the ISTEC standard process (STD2). Light propagates along an 830-μm-long waveguide with a propagation loss of 16 dB/mm. We also designed and evaluated an optical-electrical (O-E) converter that uses a Josephson junction, which can be fabricated by the same process used to fabricate the SFQ circuits. When an optical signal with a peak power of 1.15 mW at 1.548 μm is input, the Josephson O-E converter generates SFQ pulses.

DOI
10.1109/tasc.2010.2095813

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Single flux quantum (SFQ) circuits and optical circuits are expected to be used in next-generation integrated circuits. This study investigates basic technologies for integrating optical circuits with SFQ circuits to exploit the advantages of both technologies. We calculated, designed and evaluated metal-clad waveguides, which can be fabricated by the ISTEC standard process (STD2). Light propagates along an 830-μm-long waveguide with a propagation loss of 16 dB/mm. We also designed and evaluated an optical-electrical (O-E) converter that uses a Josephson junction, which can be fabricated by the same process used to fabricate the SFQ circuits. When an optical signal with a peak power of 1.15 mW at 1.548 μm is input, the Josephson O-E converter generates SFQ pulses.
Single flux quantum (SFQ) electronics is extremely fast and has very low on-chip power dissipation. SFQ VLSI is an excellent candidate for high-performance computing and other applications requiring extremely high-speed signal processing. Despite this, SFQ technology has generally not been accepted for system implementation. We argue that this is due, at least in part, to the use of outdated tools to produce SFQ circuits and chips. Assuming the use of tools equivalent to those employed in the semiconductor industry, we estimate the density of Josephson junctions, circuit speed, and power dissipation that could be achieved with SFQ technology. Today, CMOS lithography is at 90?65?nm with about 20 layers. Assuming equivalent technology, aggressively increasing the current density above 100?kA?cm?2 to achieve junction speeds approximately 1000?GHz, and reducing device footprints by converting device profiles from planar to vertical, one could expect to integrate about 250?M Josephson junctions cm?2 into SFQ digital circuits. This should enable circuit operation with clock frequencies above 200?GHz and place approximately 20?K gates within a radius of one clock period. As a result, complete microprocessors, including integrated memory registers, could be fabricated on a single chip.
We developed an optical input module with a superconducting single-flux-quantum (SFQ) circuit for application of ultrafast photonic networks in the near future. The optical input module, consisting of a built-in photodiode (PD) with a coplanar waveguide transmission line, was fabricated on an InP substrate and flip-chip bonded with the SFQ circuit on a Si substrate. The fabricated PD showed photosensitivities of more than 0.2 A/W for the wavelength range 1480 to 1530 nm at 4.2 K. SFQ pulses were generated by less than 1 mW optical pulse input.
We report low-power single-flux-quantum (SFQ) circuits fabricated with a lowered critical current density process and low-voltage design. We selected 250 A/cm2^{2}2to obtain 2–10 μA Josephson junctions (JJs) easily using conventional facilities. Our numerical simulation shows that the expected operating frequency is 10 GHz and that we can reduce the bias voltage to 0.1–0.5 mV; power consumption can be reduced to around 1 nW per JJ. We newly developed a fabrication process featuring four planarized Nb layers and a Pd resistor layer. We obtained the correct operation of several basic SFQ circuit elements designed with the lowered critical currents to 1/2 of the conventional circuits at 4.2 K. We also evaluated the Josephson junction critical currents, specific capacitance, resistance, and inductance at 3 K and 300 mK. We observed an increase of 7% in the critical currents at 300 mK, while the other parameters were unchanged. We demonstrated a 0.5-mV Josephson transmission line where we lowered the critical currents to 1/10. We expect such low-power SFQ circuits to operate at 20 mK, providing classical, general-purpose digital processing near quantum bits.
We developed an optical input module consisting of built-in metal-semiconductor-metal photodiodes (MSM-PDs) and co-planar waveguide transmission lines fabricated on an InP substrate, which is flip-chip bonded with a single-flux-quantum (SFQ) circuit fabricated on a Si substrate. To improve photosensitivity of the MSM-PD for a wavelength above 1530 nm at 4 K, we tested the MSM-PDs made of lattice mismatched In1-xGaxAs films on InP substrates. By employing In1-xGaxAs with x=0.44, an apparent reduction of photosensitivity for a wavelength above 1530 nm was suppressed. We also tested newly designed DC/SFQ converters with higher input current sensitivities. The observed current sensitivity was improved to 41 μA, while that in the conventional DC/SFQ converter was 140 μA.
The Superconductivity Research Laboratory has successfully fabricated large quantities of single flux quantum (SFQ) large scale integrated circuits, including several thousands of Josephson junctions (JJs). Using a Jc = 2.5 kA cm−2 process in which the number of Nb layers was four and the minimum JJ size was 2 µm square. We developed a new advanced fabrication process that produced a Jc = 10 kA cm−2, nine Nb layers and a minimum JJ size of 1 µm square. The increase in the number of Nb layers was achieved by using a planarization technique. The target of our next generation process is a Jc = 40 kA cm−2 with a 0.5 µm square for the minimum junction size. This specification will be achieved by using advanced semiconductor technologies. This process will enable SFQ circuits to be produced with one million JJs on a chip and achieve a clock frequency greater than 100 GHz.
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.