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

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4 Author(s)
Ketchen, M.B. ; IBM Thomas J. Watson Res. Center, Yorktown Heights, NY, USA ; Bhushan, M. ; Kaplan, S.B. ; Gallagher, W.J.

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 Vm'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

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Magnetics, IEEE Transactions on  (Volume:27 ,  Issue: 2 )