Thin Film Processing of Complex Multilayer Structures of High-Temperature Superconductors
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and can be fabricated with fewer parasitic constraints. Superconducting integrated circuits have followed the semiconductor pattern of being developed in a hybrid fashion, then transferred to a fully integrated process. There will be a continuing need for hybrid multilayer structures, especially in areas where semiconducting devices are used with superconducting interconnects or active components. However, many applications for active HTS circuits benefit from integration into a thin
Figure 1. Planar view of the three-YBCO-layer integrated SQUID magnetometer on a YSZ substrate. The corresponding vertical structure is shown in Figure 2b. (Courtesy of K. Char, L.P. Lee, M.S. Colclough, and G. Zaharchuk at Conductus Inc.)
film multilayer geometry. This article summarizes some of the knowledge we have gained over the past five years in processing thin films of YBCO for device applications. We begin by discussing the constraints on the design of a fabrication process. Three examples of multilayer structures are used, and fabrication details are given for each. We conclude with a discussion of the unique challenges provided by HTS materials and a summary of current trends in HTS microelectronics. Designing Fabrication Processes The fabrication of an HTS structure starts with the tailoring of growth and patterning to the desired application. In a dc superconducting quantum interference device (dc SQUID) design, for example, the various superconducting layers can have significantly different requirements for their electrical characteristics. The most important HTS film characteristics are usually the transition temperature Tc, the critical current density Jc, the magnetic penetration depth A, the normal state resistivity pN, and in some cases the rf surface resistance Rs. Patterning tools and techniques must be capable of defining the structures needed in a given layer without damaging the layers below. The designer of a microcircuit must be aware of the limitations imposed by her or his choice of fabrication methods. Film growth is the first stage of fabrication. The films may be grown one at a time, with photolithography and patterning between each successive layer. In some cases, multiple layers are grown sequentially without intervening patterning. Circuit structures are then etched into the complete multilayer. Thin films of YBCO can be grown by nearly any technique; the highest quality films require the subset of processes which enable epitaxial layer formation. These growth techniques can be further subdivided into the so-called in-situ and post-annealing processes. In the HTS community, in situ has come to signify a process in which the film or multilayer is completely superconducting before removal from the deposition system. Most current fabrication processes rely on a single technique for growing the HTS films, whether it is laser ablation, sputtering, or evaporation. The cases discussed below use pulsed laser deposition (PLD). In nearly all HTS cases, films are grown over the entire substrate and then sub
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