Fabrication and measurements of Nb/Al-AlOx/Nb tunnel junctions and DC SQUID

Several different critical current density (J_c:30A/cm²~4000A/cm²) of Nb/Al-AlO_x/Nb Josephson junctions were fabricated by standard selective niobium etch process (SNEP) for application of dc SQUID magnetometers. The subgap leakage factor Vm of most junctions is larger than 30 mV. Based on the good tunneling properties of junctions, we designed and fabricated a dc SQUID with loop inductance of 140 pH and a pickup coil of 8.9 mm × 8.9 mm by setting the Stewart-McCumber parameter β_c and the screening parameter β_L as about 1. Each of two Nb/Al-AlO_x/Nb junction with dimensions of 4μm × 4μm and a critical current density of J_c≈50A/cm², was shunted by a 8.4 Ω Molybdenum resistance. The white field noise was measured to be less than 5 fTHz^-1/2.