Influence of Circuit Pattern Layout on Characteristics of Interface-Modified Ramp-Edge Junctions

We have developed a fabrication process for single flux quantum (SFQ) circuits including a superconducting ground plane and interface-modified ramp-edge Josephson junctions. We found that the critical current density (J_c) for individual junctions in a circuit was influenced by the circuit pattern layout. The dependence of the I_c value for 5-mum-width junctions on the base- electrode size (width and length) and the contact-hole area was carefully examined. We obtained 1-sigma J_c spreads less than 8% for junctions in actual circuits by employing a new circuit pattern layout, containing separated base-electrodes with almost the same size. We also employed connection of the separated base-electrodes by the counter-layer to reduce parasitic inductance. By employing new layouts based on the separated base-electrode layout (SBL) method, control of I_c values by changing the junction width was readily achieved.