In addition to thin films, HTS superconducting electronic technology requires other materials. These include substrates, insulators, and passivation layers. Based on the site visits in Japan (see Appendix B), the WTEC panel notes the following points and trends. Among more conventional substrates (MgO, Al2O3, LaAlO3), no clear favorite has so far appeared in Japan for microwave applications.
ISTEC is unique in the world in championing the use of HTS single crystals as a substrate (and ground plane) for the epitaxial deposition of electronic circuits. Ultimately, these single crystal substrates might themselves be processed to achieve functionality, in analogy to silicon processing. Toward this end, ISTEC is developing growth techniques for large single crystals of various HTS materials. It should be noted that Japanese companies provide all the bicrystal substrates currently used for commercial SQUID products and for research.
Also unique is the effort at Saitama University in the optimization of ceramic dielectrics for bulk HTS dielectric resonators. On the other hand, there is no Japanese equivalent of the recent efforts in the United States to develop magnetic/dielectric materials suitable for active tuning of thin film HTS resonators and filters, even though Japan is traditionally strong in these types of materials. Finally, the panel notes that Japan is only beginning to address the passivation issue, as is the United States.