Fabrication and characteristics of NbN-based Josephson junctions for logic LSI circuits

The characteristics of a 1.5 μm square NbN-based Josephson junction were investigated and improved to make large scale application of these junctions to logic LSI circuits possible. An amorphous-like Si thin film was newly examined in order to define a junction area consistent with the pattern size of the resist mask. A Nb-oxide barrier was formed on a NbN base electrode by rf plasma cleaning and oxidation. The low leakage current (the characteristic voltage Vm, that corresponds to converting the leak into the maximum junction currents, being larger than 40 mV) and the small critical current variation were achieved by low rf-voltage plasma cleaning. By using Si-insulating layers and optimizing surface-cleaning conditions, the maximum critical current variation was reduced within ±12% for 850 junctions. The change in the normal tunneling resistance R nn due to heat-treating below 200 °C was kept within ±5%. To confirm the feasibility of applying the junctions to logic LSIs, a 3K-logic-gate array was fabricated using about 23,000 junctions and its operation was successfully demonstrated.