PART III - Evaluation of the Photomask-Photoresist Method of Cryotron Thin-Film Fabrication

This paper discusses the results of a technology-evaluation program to ascertain the feasibility of a piotornask-photoresist technology developed for fabrication of multiple-layer thin-film superconductive circuitry. The uehicle selected for this evaluation was a 120-bil associative data processor requiring a set of nine photomasks to define a similar number of patterned material layers of lead, tin, and polymer insulation. A total circuit area of 4 sq in. involved 2500 actitle and 12,000 passive crossings. Several short-free, electrically continuous specimens were satisfactorily completed under this program. Problems encountered in the preparation and use of the requi-ed set of Photomasks are discussed, as also are the problets of providing suitable external connections. A major retarding factor in the development of the superconducting thin-film cryotron technology for many years was the absence of batch fabrication means yielding necessary device density and count per substrate for practical applications. Early workers in the field desired only limited numbers of devices to permit careful study of discrete devices and representative circuitry integrating logic and memory capabilities. For these purposes mask stencils could be formed to selectively control the placement of materials, sequentially vapor-deposited in vacuum, to form the multilayer structure. In general a distinct pattern was required for each material layer; therefore, mechanical provisions were required to change not only the material source, but the patterning mask as well, before each deposition. All deposition steps were preferably performed in a single evacuation of the deposition chamber. Thus the means for storing, transporting, and registering masks and sources with respect to the substrate existed and were operated within the vacuum environment. In the authors' experience, the approach was and remains serviceable for preliminary device study, but was not extensible to an economical manufacturing method by virtue of the difficulties inherent in mask stencil preparation and use, and the capital investment implicit in the mechanization requirements. Since the commercial attraction of the cryotron technology stems principally from its potential for economical batch fabrication, the authors' program has placed major emphasis on fabrication methods. A highly useful method has evolved through adaptation