The SQUID is the most well-known superconducting device and it has a high sensi-tivity for detecting small magnetic fields.. Around 1990, applications to the medical electronics for magn
Trang 1High-Temperature Superconductor
Electronics: Status and Perspectives
Shoji Tanaka
Superconductivity Research Laboratory, ISTEC, Tokyo, Japan
After the discovery in 1986, many kinds of high-T csuperconductor (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 de-vices (SQUIDs) have been accelerated Furthermore, the applications of
well-known low-T csuperconductors (LTSs) were stimulated by the progress of HTS research and developments, and the application of LTS SQUIDs to medical elec-tronics 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
Trang 213.2 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 elec-tronics 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 ser-vice 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 intro-duced, but we expect that it will be introduced in the fourth-generation communi-cation system (4G) around the year 2010 However, in order to construct the soft-ware-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
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 applica-tions 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 sys-tems 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
Trang 3quanta 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
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 quan-tum flux stored in a superconducting loop is used as an individual bit of informa-tion 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–A1Ox–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
F IGURE 13.1 Structure of a LTS RSFQ device.
Trang 4F IGURE 13.2 Structure of a HTS RSFQ device.
F IGURE 13.3 Relation between the I cstandard deviation and the number of junctions in a circuit.
Trang 5The 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 gi-gahertz 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-T ABLE 13.1 Superconducting LSI (Large Scale Integration) Technology Road Map
Year
Fabrication process
Junction current
Memory
Logic
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.
Trang 6tions 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
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 semiconduc-tor devices Thus, very fast and very powerful conservative RSFQ circuits are nec-essary 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 develop-ment of the RSFQ circuits of both LTS and HTS must be accelerated
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 communica-tion is coming soon, for which a great amount of informacommunica-tion 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 tech-nology, due to the saturation in the developments of information storage systems and semiconductor devices This mismatch between the two important technolo-gies 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 cir-cuits of medium-scale integration of the HTS RSFQ will be used in the base sta-tions 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