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IEICE Transactions on Electronics

Advances in High-Tc Single Flux Quantum Device Technologies

K. TANABE, H. Wakana, Koji Tsubone, Y. Tarutani, S. ADACHI, Yoshihiro Ishimaru, Michitaka Maruyama, Tsunehiro Hato, Akira Yoshida, Hideo Suzuki

Published Mar 1, 2008• 14 citations
Publisher

Abstract

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-Tc single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with IcRn products above 1mV at 40K and small Ic spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the Jc spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200GHz at 40K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50GHz.

DOI
10.1093/ietele/e91-c.3.280

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We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-Tc single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with IcRn products above 1mV at 40K and small Ic spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the Jc spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200GHz at 40K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50GHz.
Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.
YBa2Cu3O7-x(YBCO) ramp-edge junction technologies have been developed for fabricating high-operating-temperature SFQ(Single Flux Quantum) circuits. The Ic(critical current) spread that is the key parameter for integrating junctions could be decreased to 8% for a 1000-junction array that was formed on a groundplane. The background technologies that enabled this uniform junction characteristic are precise control of the counter-electrode deposition-conditions and deposition of the extremely smooth multiplayer that showed the roughness as small as 2 nm. Based upon these ramp-edge junction technologies, oxide SFQ-circuit-elements were fabricated and their logic operations were investigated. SFQ circuit elements such as a confluence buffer, a set-reset flipflop, and a toggle flipflop have been fabricated and their logic operations were measured by using an SFQ-dc converter. Both toggle flipflops that have one output and two outputs have been fabricated. These circuits could be operated up to the temperature of near 60 K under finite operating margins. These experimental results enable us to further develop fuctional and much more integrated SFQ circuits.
Single flux quantum (SFQ) circuit elements have been designed and fabricated using the YBa 2 Cu 3 O 7-δ ramp-edge junction technology. Logic operations of SFQ circuit elements, such as a toggle flip-flop (T-FF), a set-reset flip-flop (RS-FF), and a 96-junction Josephson transmission line (JTL), were successfully demonstrated, and dc supply current margins were confirmed up to temperatures higher than 30 K. The circuit layout was improved in order to suppress the critical current (I c ) spread that appears during the junction fabrication procedure. By employing the new circuit layout rule, correct operations at temperatures from 27 K to 34 K with dc supply current margins wider than ±7% were confirmed for the T-FF with a single output. Moreover, the maximum operating frequencies of T-FFs were measured to be 360GHz at 4.2K and 210GHz at 41K, which are substantially higher than the values for the circuits with the conventional layout. According to the simulation result, the maximum operating frequency at 40 K was expected to be approximately 50% of the characteristic frequency at a bit error rate (BER) less than 10 -6 .
Recent progress in single flux quantum (SFQ) device technologies in Japan is described, mainly focusing on those developed in the NEDO "Superconductors Network Device Project" which includes developments of both LTS and HTS devices. The advanced Nb process which consists of a planarized structure with nine Nb layers and 10 kA/cm2Josephson junctions (JJs) has been developed. This enables fabrication of chips including 100,000 JJs and operating frequency up to 80 GHz. We have also developed the CAD tool by which one can generate SFQ circuit layouts including about 400,000 JJs from logic descriptions. For demonstration of a small-scale switch system, a 4 times 4 switch chip, a 117 Gbps multi-chip-module (MCM) technology, and a packaging technology with 32-ch 10 Gbps I/Os to an MCM have been developed. By using the multilayer process with three HTS epitaxial layers and interface-engineered ramp-edge JJs, we demonstrated operation of SFQ circuits with up to 200 JJs. A variety of elementary SFQ circuits such as a T-FF and a 1:2 switch required for an A/D converter front-end circuit have been designed based on a new layout method, and their operation at 30-50 K has been confirmed. A prototype sampler system is being developed for demonstration of bandwidth over 100 GHz for optical input signals.
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.
We have fabricated and measured high T/sub c/ superconductor single flux quantum 1-bit flux-counting analog-to-digital converters (ADCs). The ADCs were made with a multilayer all-epitaxial process which incorporates 10 edge SNS (superconductor-normal-superconductor) or step-edge grain boundary (SEGB) Josephson junctions with a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// groundplane. The ADC consists of a quantizer connected to a Toggle flip-flop through a buffer-like stage. Direct readout of the flux state of the T flip-flop was made with a Read SQUID inductively coupled through a hole in the groundplane. The circuits were operated at 65 K and low speeds. The SNS circuits outperformed the SEGB circuits because of their higher readout voltages and tighter critical current spreads.