Superconducting thin film transformer design for powering Josephson logic

Superconducting step-up transformers, formed by overlaying primary and secondary lines across a hole in a superconducting groundplane, are the key elements in distributing ac current in parallel to the maximum possible number of Josephson LSI logic chips. Since frequencies approaching 1 GHz are required it was found that two side-by-side transformers with serially connected primaries allowed the maximum current transfer while keeping the electrical length of the primary winding below a quarter wavelength. Inductance calculations predict that for an optimum design the two transformers should share a common hole; that the spacing between secondaries should be three times the distance from the secondary to the edge of the hole; and that the transformer will occupy a minimum area when the hole width is three times the secondary width. The width of the secondary is determined by the critical current characteristics of the windings. Affecting the critical currents of the windings is the close proximity of the groundplane edge in the minimum area geometry. The resulting increase in current density at the edge of the secondary winding is predicted to reduce its critical current by 15%. Measurements of thin film transformers fabricated in the lead alloy metallurgy with a niobium groundplane have verified the predicted inductance and critical current characteristics of the double-transformer structures.