Josephson shift register design and layout

Integrated circuit chips were designed and fabricated, based on a Josephson shift register circuit that simulated operation at 25 GHz using the SPICE program. The 6.25-mm² chip featured a twelve-gate, four-stage shift register fabricated with Nb/AlO_x/Nb Josephson junctions with a design value of 2000 A/cm² critical current density. SUPERCOMPACT, a general program for the design of monolithic microwave integrated circuits, was used to model the effects of layout geometry on the uniformity and phase coherence of logic gate bias currents. A layout geometry for the superconductive transmission lines and thin-film bias resistors was developed. The original SPICE-designed circuit was modified as a result of these calculations. Modeling indicated that bias current variations could be limited to 3% for all possible logic states of the shift register, and phase coherence of the gates could be maintained to within 2° at 10 GHz. The fundamental soundness of the circuit design was demonstrated by the proper operation of fabricated shift registers