The dominant position of YBCO as the material of choice for electronic applications is evident in Japan, as in the United States. This is no doubt due to it having being the first HTS material with a Tc above liquid nitrogen temperature that could be made into thin films with good electrical properties. It cannot be argued that this choice resulted from a thorough examination of its pros and cons as a superconducting electronic material. The race so far is going to the first horse out of the starting gate. On the other hand, Japan and the United States appear to have different favored candidates for practical materials with higher transition temperatures. In Japan, the Hg compounds are favored; in the United States, the Tl compounds are preferred. However, whereas the efforts in Japan with the Hg compounds are largely exploratory, DuPont and STI in the United States have put considerable effort into the development of practical thin films of the Tl compounds.
In Japan as in the United States, the most common approaches to thin film deposition for applications are sputtering and pulsed laser deposition (PLD). Here again, this is due to the success of these techniques early in the development of the technology. Among other approaches, reactive co-evaporation and metallorganic chemical vapor deposition (MOCVD) are both represented in Japan and the United States.
Indeed, reactive co-evaporation and MOCVD appear to be growing in importance because of their greater suitability for manufacturing and in particular for composition control. On the other hand, concerted efforts at liquid phase epitaxy (LPE) are only currently underway at ISTEC in Japan. Also, there is much more evidence in Japan than in the United States of molecular beam epitaxy (MBE)-grade research systems with elaborate surface characterization capabilities.
In the more complex thin film structures, there is steady systematic progress in Japan in the growth and study of multilayers, both in the form of periodic multilayers and in the context of circuit crossovers. Elementary HTS circuits are also now appearing. Not surprisingly, multilayer growth is a stated goal of ISTEC as it looks to the second decade of its existence (Fig. 3.1). While more complicated crossovers and circuits have been fabricated in the United States than in Japan, the Japanese are more systematically exploring the various materials combinations, interface properties, and deposition/processing approaches.
Because of the lack of a clear market pull, Japan has not yet focused on manufacturing scale-up issues. Nor has it identified a favored thin film deposition approach for manufacturing, although the challenge of depositing over large areas while maintaining good composition control is now recognized by leading groups. The United States is two or more years ahead in this area for passive microwave applications (both film area and numbers of wafers), at the expense, however, of broad-based exploratory work on alternative materials.
The United States also appears to be unique in its efforts to develop new and advanced process control, deposition process modeling, and related instrumentation for HTS thin films, thanks largely to the HTMA program of the Defense Advanced Research Projects Agency (DARPA). Interestingly, building on the DARPA HTMA program, the United States also appears to be ahead in tackling the scale-up issues associated with manufacturing high-H, high-Jc YBCO tapes using vapor deposition techniques.