Low noise DC SQUIDs fabricated in Nb-Al2O3-Nb trilayer technology

The authors have designed, fabricated and tested all-refractory DC SQUIDs in Nb-Al₂O₃-Nb trilayer technology that have noise performance comparable to the best previously reported for any technology. A variety of SQUID designs were incorporated as part of a trilayer process development test vehicle. SQUID inductance, junction area, and resistive shunt geometry were varied in matrix fashion to give SQUIDs with near-optimum parameter values for a factor of five range in Josephson current density and shunt sheet resistance. The devices were fabricated using a selective niobium anodization with a minimum feature size of 2 μm. The base electrode and Nb wiring were patterned with dry etching, and the junction areas were defined by anodization: the Ti resistors were patterned with a lift-off process. Current density on different wafers was varied from 400 to 1000 A/cm² with typical junction V_m´s of 60 mV. The shunt sheet resistance was varied in the 1-5-Ω/□ range. The noise was measured with an RF SQUID direct small-signal readout scheme. A 50-pH SQUID with 3-μm², 16-μA junctions and 14-Ω shunt resistors was shown to have an ideally low white noise of 1×10^-13 Φ₀²/Hz, a white to 1/ f crossover frequency at 7 Hz, and a noise level less than 6×10^-12 Φ₀²/Hz at 0.1 Hz