Fabrication and measurement of Nb-based SQUID magnetometer

We have developed low-noise direct current-superconducting quantum interference devices (DC-SQUIDs) based on Nb/Al-AlOx/Nb Josephson junctions on thermally oxidized silicon substrate. The SQUID fabrication was realized with a simple 5-mask-level process, in which one single insulting layer (SiO2) was used not only to insulate different metal layers, but also to protect the junction shunt resistor from oxidation or corrosion. In our SQUID design, a washer-type loop with an inductance of 140 pH and a coupled flux transformer with a multi-turn spiral input coil connecting to a pickup coil of 8.5 × 8.5 mm2 were used, thus leading to a SQUID magnetometer with magnetic field sensitivity of 0.7 nT/Φ0. We measured the voltage-flux characteristics and flux noise in superconducting niobium shielding at 4.2 K. The measured white flux noise in a flux-locked loop was 5 µΦ0/√Hz, corresponding to magnetic filed noise of 3.5 fT/√Hz.