Handbook of High Temperature Superconductor Electronics Part 14 pps

13 High-Temperature Superconductor Electronics: Status and Perspectives Shoji Tanaka Superconductivity Research Laboratory, ISTEC, Tokyo, Japan 13.1 INTRODUCTION After the discovery in 1986, many kinds of high-T c superconductor (HTS) have been found, and the critical temperature was raised up to 130 K in mercury-based compounds. In the first 10 years, however, they have been the objects of material sciences primarily, because they are quite peculiar cuprates and it was necessary to understand the physical properties, chemical properties, and so on. At around 1995, the trends in the applications of HTS became clear. The development of microwave filters and superconducting quantum interference devices (SQUIDs) have been accelerated. Furthermore, the applications of well- known low-T c superconductors (LTSs) were stimulated by the progress of HTS research and developments, and the application of LTS SQUIDs to medical electronics and study of single flux quantum (SFQ) devices started. As for materials for the LTS devices, only Nb-based compounds are used. Over the past 20 years, the progress of the Internet has been remarkable; and it covers the whole world and is changing the structure of our society to form the so-called “ubiquitous society.” Superconductivity electronics must have a great impact on the progress of the Internet, as will be mentioned in this chapter. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. 13.2 MICROWAVE FILTERS High-T c superconductor microwave filters and low-noise amplifiers for mobile telephone base stations have been the first commercialized devices in HTS electronics. HTS-based systems offer an improved quality in wireless communica- tions, with increased area coverage and reduced interference. Currently, the mobile telephone system is connected to the Internet, and service is widely improved (e.g., i-mode). Especially in Japan, third-generation (3G) wireless communication began last year in major cites, which is close to becom- ing a picture phone system. The area of this service is limited at present, but after an extremely large number of users join this system, the HTS microwave filters will be used in order to prevent interference. The number of base stations in Japan is increasing and is expected to become 200 thousand in the year 2010. This may become a large market for the microwave filters. It is not clear at present when a software-defined radio system will be introduced, but we expect that it will be introduced in the fourth-generation communication system (4G) around the year 2010. However, in order to construct the software-defined radio system, it is necessary to develop a high-quality AD converter of 200 MHz and 16 bits using very high-speed SFQ circuits. 13.3 SQUID The SQUID is the most well-known superconducting device and it has a high sensi- tivity for detecting small magnetic fields. Thus, it has many possibilities for applications in various fields. Before 1990, however, real application was limited because LTS SQUIDs must be cooled down to a liquid-helium (He) temperature of 4.2 K. Around 1990, applications to the medical electronics for magnetic diagnos- tics of heart and brain activity started in many countries; in Japan, a project for constructing new systems of observing magnetic brain waves by using 256-channel LTS SQUIDs and also of observing magnetic heart waves by using 36-channel HTS SQUIDs was started. The system for detecting human magnetic brain waves is now used for the study of brain activity in universities and the systems for detecting magnetic heart waves will now be commercialized after a li- cense from the government is obtained. Recently, LTS SQUIDs are used even in magnetic heart wave detecting systems to obtain more precise information. It is hoped that the market for these magneto-cardiographic systems expands rapidly and reaches that of magnetic resonance imaging (MRI). The application of SQUIDs for the precise voltage standard has been per- formed, and the application to nondestructive evaluation systems in materials of airplanes and other constructions is very hopeful. Recently, the SQUID microscope has appeared. In this equipment, the LTS SQUID is used to observe the distribution of weak magnetic fields in a sample with high accuracy; thus, it is possible to observe the distribution of magnetic flux 426 Tanaka Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. quanta in superconducting thin films with a precision of a few microns. There also seems to be a potential market in the application for the observation of biomateri- als, where very fine magnetic particles are attached. 13.4 RSFQ LOGIC DEVICES At present, the developments of rapid-SFQ (RSFQ) logic circuits are the most ex- citing subject in superconducting electronics, as the RSFQ device has an ultrafast operating speed of several hundred gigahertz and a very small power dissipation of the order of 10 nW. The principle of the RSFQ device is very simple; the quantum flux stored in a superconducting loop is used as an individual bit of information. The quality of Josephson junctions included in the loop is the most important factor in constructing an integrated RSFQ circuit. The structures of the RSFQ devices of LTS and HTS are shown in Figs. 13.1 and 13.2 . For LTS, the stacked junction of Nb–A1O x –Nb is used. In the case of HTS, the ramp-edge junction of YBCO–barrier–YBCO is used, and, sometimes, YBCO is used as a counterelectrode to lower deposition temperatures. The most important factor in both cases is the standard deviation (␴) of the critical current of the junction. At present a nearly 1% deviation in LTS and 6% in 100 junctions of a HTS are obtained at 4.2 K. We expect that a 5% deviation will be obtained soon, with 1000 HTS junctions, which means that the sigma–delta AD converter will be made in the yield of 50% if suitable designs are made, as shown in Fig. 13.3. The road maps of the LTS junctions are now presented by many in- stitutions and one of them is shown in Table 13.1. The road map of a HTS SFQ is not available yet, as the HTS SFQ is only 5 years in use. High-T c Superconductor Electronics: Status and Perspectives 427 FIGURE 13.1 Structure of a LTS RSFQ device. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. 428 Tanaka FIGURE 13.2 Structure of a HTS RSFQ device. FIGURE 13.3 Relation between the I c standard deviation and the number of junctions in a circuit. Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. The possibility of LTS RSFQ circuits has been discussed in relation to its use in the future peta-flops computers, mainly in the United States. Recently, a new design of the Microprocessor Unit (MPU) of 16 bits and 25 GHz, the Flux Chip, was proposed by the SUNNY group and its production is in progress at TRW. As for the HTS RSFQ, fundamental circuits for future logic circuits were made already, and they proved to operate in a very high speed of a few hundred gigahertz. These are the toggle flip-flop (9JJs) operated at 270 GHz at 4.2 K (SRL), the ring oscillator (21JJs) at 30 GHz at 30 K (Toshiba), and the sigma-delta mod- ulator (11JJs) at 100 GHz at 20 K (Hitachi). The high-speed sampler is shown in Fig. 13.4, which was developed recently by NEC. It consists of 17 JJs and shows a beautiful wave form of 15 GHz. We expect that it will soon accomplish observa- High-T c Superconductor Electronics: Status and Perspectives 429 TABLE 13.1 Superconducting LSI (Large Scale Integration) Technology Road Map Year 1999 2005 2010 Fabrication process Junction size (µm) 2.0 1.0 0.8 Junction current Density (kA/cm 2 ) 2.5 10.0 15.6 Number of junctions 10 K 100 K 1 M Memory Memory density (bits/chip) 256 16 K 1 M Clock frequency (GHz) 5 10 10 Logic Clock frequency (GHz) 50 50 Performance 16-bit MPU 64-bit MPU F IGURE 13.4 Sampler circuit with JTL buffers. (a) HTS sampler circuit with 6-stage JTL buffers using 17 Josephson junctions; (b) measured 18 GHz waveform. (a) (b) Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. tions higher than 50 GHz. These results proved the very fast operations of the HTS RSFQ circuits, and, we hope, circuits with more than 100 JJs will appear very soon. 13.5 IMPACT OF THE INTERNET The progress of the Internet creates new possibilities for the RSFQ devices. In Japan, optical fibers will be equipped in offices and homes (Fiber To The Homes) this year, and communication at 100 Mbps will become very popular. In such a very fast communication, communication nodes, servers, and routers must be operated in a rate of more than 10 Tbps, which exceeds the operation speed of semiconductor devices. Thus, very fast and very powerful conservative RSFQ circuits are necessary. Furthermore, it is expected that in the mobile communication system of the fourth generation, the communication speed will be 100 Mbps also. Such a “broad band and wireless” technology requires the suitable combinations of optical fibers, semiconductors, and superconductors. Therefore, it is believed that the development of the RSFQ circuits of both LTS and HTS must be accelerated. 13.6 SUMMARY In the coming 10 years, the primary goal worldwide must be the very fast progress of the communication technology of the Internet. The era of picture communication is coming soon, for which a great amount of information will be exchanged at a very high speed of 100 Mbps. However, the progress of the information pro- cessing technology is rather slow compared to that of the communication technology, due to the saturation in the developments of information storage systems and semiconductor devices. This mismatch between the two important technologies will result in the substantial dissipation of electric power in society as a whole. Therefore, the role of the RSFQ circuits of very high-speed operation and very low power dissipation will become very important. The circuits of very high integration of the LTS RSFQ could be used in every node of the Internet. The circuits of medium-scale integration of the HTS RSFQ will be used in the base stations of mobile communication systems in offices and homes. The developments of future RSFQ technologies must be accelerated in order to realize such expecta- tions, and it will also expand the applications of superconductivity electronics in many fields. 430 Tanaka Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. . progress of the communication technology of the Internet. The era of picture communication is coming soon, for which a great amount of information will be exchanged at a very high speed of 100. circuits of very high integration of the LTS RSFQ could be used in every node of the Internet. The circuits of medium-scale integration of the HTS RSFQ will be used in the base stations of mobile. structures of the RSFQ devices of LTS and HTS are shown in Figs. 13.1 and 13.2 . For LTS, the stacked junction of Nb–A1O x –Nb is used. In the case of HTS, the ramp-edge junction of YBCO–barrier–YBCO