THIN-FILM PROCESSING OF COMPLEX MULTILAYER STRUCTURES OF HIGH-TEMPERATURE SUPERCONDUCTORS

HTS materials pose a number of problems for thin film patterning. They all require some high-temperature (>600°C) processing, proper oxidation, and fine control of the stoichiometry of up to four metallic elements. The high temperatures can lead to a mixing of elemental components between the superconducting film and underlying or overlying layers. The superconductors are easily damaged and, in the case of YBCO, oxygen diffuses rather freely out of the film. Insulating dielectric layers are also problematic. The layers must be lattice-matched to YBCO if high-quality layers are to be grown on deposited dielectrics. The growth process must be nondamaging to underlying layers. The layers must be pinhole-free over areas large enough to provide adequate yield of complex circuits. Finally, the standard dielectric properties of relative permittivity, loss tangent, and breakdown voltage must be in a usable range for the particular application. A third problem area is just beginning to be addressed. As multilayers become more complicated, the surface relief of the circuits will be an increasingly difficult problem. YBCO lines crossing underlying structures will nucleate grain boundaries even over a sloped crossover edge. Planarization is a standard semiconductor fabrication problem.10 It is possible that the techniques invented by the vastly larger community of Si process engineers will find use in the rapidly developing area of HTS electronics. In spite of these difficulties, many intriguing and potentially useful device structures have already been made, such as the dc SQUID discussed in the first fabrication case study. The initial materials science work in developing buffer layers for semiconductor substrates has moved on to the development of processing techniques for making useful devices incorporating superconductors and semiconductors. In conclusion, the state of the art in HTS circuits is changing more rapidly than a short review can cover. The work of many research groups could not be discussed in the scale of this article. In those groups, as well as those represented here, processes and techniques are constantly being invented, modified, discarded and reinvented. In this rather exciting period, one overriding theme has developed: the need for applications as drivers for the process developer. A defining need for practical devices and circuits provides the incentive (both scientific and financial) for the fabrication developments of the future. © 1992, Materials Research Society. All rights reserved.