The current status of epitaxial technology towards high Tc electronics is summarized. Properties and processing maturity were compared for various oxide single crystals to give hints for the choice of substrate. Such problems in YBa2Cu3O7 (YBCO) film growth as precipitate formation and cracking were quantitatively investigated and shown to be solved by carefully preparing the substrate surface and by optimizing buffer layers and growth conditions. It is now possible to perform ideal layer-by-layer growth of various oxide films on atomically flat substrates by laser MBE to explore not only high Tc electronics but also a possible new field of oxide electronics.
An overview of present Josephson junctions based on high-temperature superconductors (HTSs) is described. I indicate the importance of the critical current-normal resistance (IcRn) product, and show the experimentally-obtained values of the IcRn products for the HTS Josephson junctions reported so far. The junctions include grain boundary junctions, edge junctions, and sandwich junctions. Considering the charge transport mechanism across the junctions, effective ways to increase the IcRn product are proposed.
The cellular communication market is expanding dramatically, with bands near 1-2GHz being auctioned for use. The frequency spectral regions allocated for cellular communication are rather narrow, so they are certain to have a high density of communication channels. Thus, proper filtering of transmitting and receiving signals into the allocated bands, and precise channelization of the multiple simultaneous signals within each band, are essential for providing a high standard of customer service. HTS filters are expected to be used on circuit systems of the base station as the means to maximize frequency use. Bandpass filters are typically realized by appropriately coupling a set of resonant elements which may be in the form of three-dimensional cavities, bulk rods, or resonant structures. Though planar circuits like microstrip are especially attractive because of their compactness and low cost, even cooled copper metals are unable to achieve a Q value of even 1000. This low Q value is insufficient to support the low insertion loss and sharp filter skirts needed for selecting communication frequency bands. To receive circuit systems of the base station, a nine-pole, 3.7% bandwidth HTS filter designed by microstrip resonator configuration would have a dissipation-induced insertion loss of only 0.4dB including connectors at 898MHz and would have much better out-of-band rejection. However this HTS microstrip-type filter typically needs a single 2-inch wafer. The cooling system requires small-sized cooled components for long-life reliability. There have been some ideas devised for small-sized HTS filters. Filter circuits, one-third the previous size, have been successfully designed by using hair-pin circuits. In addition, a two-pole, 0.5% bandwidth HTS hair-pin-type filter has an insertion loss of less than 0.2dB including connectors loss at 5GHz. A more difficult challenge is to realize the transmission of HTS filters, for the filter must have a power-handling capability more than 10W. In cellular base station, combined different channel signals are combined and amplified to the required power level. A stringent linearity of the power amplifier is required for low intermodulation distortion. However such an amplifier would not be able to satisfy the intermodulation regulations outside the provider's licensed frequency band and it may reduce the spurious-free dynamic range of the base station receiver. HTS filters can improve this intermodulation problem by utilizing a sharp filter skirt and by conserving in-band power through low loss. This low-loss property makes the amplitude gain and size of the power amplifier small, and prevents energy loss caused by heating. Two typical filters were designed by using wide microstrip lines having low impedance and by an elliptic disk circuit pattern having two dipole resonating modes. For the HTS disk filter of two-pole Tchebyscheff type, 42dBm power transmission in 2% bandwidth and 0.3dB of insertion loss was confirmed at 5GHz.
Recent developments of SQUID technology are reviewed. First, key elements for the development of high Tc SQUID are briefly shown. Due to these developments, performance of high Tc SQUID has been much improved, and system application is now becoming possible. Second, development of an electronic circuit which makes SQUID robust against environmental noise, is shown. With this circuit, the requirement for magnetic shielding can be reduced. As a result of these developments, constraints in using SQUID, i.e., cooling and shielding, can be relaxed, which is helpful for the extension of the application-field of the SQUID system. Requirements for the SQUID system are briefly mentioned in the application to biomagnetic measurement, nondestructive testing of materials, SQUID microscope and precise measurement. It is emphasized that the requirement for the system is different according to the types of applications. Therefore, it is necessary to develop the system in order to optimize the requirements, including the choice of high Tc or low Tc SQUID. As an example, a nondestructive test of materials using SQUID is shown.
Superconductive LSI with Josephson junctions have features such as low-power dissipation and high switching speed. This paper describes the recent progress of Josephson digital LSI technology. Firstly, we reviewed our developed 4-Kbit RAM with vortex transitional memory cells to illustrate the operation of the superconductive LSI using Josephson junctions. We developed a fabrication process tehnology for the 4-Kbit RAM. In the 4-Kbit RAM, a suitable moat structure was designed to reduce the influence of the trapped magnetic flux. The RAM is characterized by 380 ps access time, 99.8% bit yield, and 9.5mW power dissipation. Furthermore, we discuss GHz testing which is one of the most significant issues concerning Josephson digital LSIs. Finally, we discuss a future application for the Josephson digital LSI. The Josephson digital LSI has a special operating feature at a clock frequency of several GHz. We propose a superconductive ring pipelined network between processor elements at such a high-speed application. The prototype chip was designed and estimated to be able to operate at approximately 10GHz. A clock with a frequency of several GHz clock is an impontant feature for future Josephson digital LSI technology.
In this paper, the author describes studies on electronic devices and circuits utilizing both of superconductors and semniconductors. First, a design of an interface circuit composed of Josephson junctions and MOSFETs is described. The Josephson-MOS interface circuit will be used for realizing high-speed data communication between a Josephson digital circuit and a semiconductor one. Second, fabrication and characterization of MOSFETs with superconductive NbN gate electrodes are described. The use of superconductive gates makes it easy to hybridize MOSFETs with Josephson junctions. Finally, studies of Josephson-CMOS hybrid memory circuits are introduced.