Low noise de squids fabricated in Nb-Al2O3-Nb trilayer technology

Abstract

We have designed, fabricated, and tested all-refractory dc SQUIDs in Nb -A12O3 -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. SQU ID 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 process (SNAP) 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 junction Vm's typically 60 mV. The shunt sheet resistance was varied in the 1–5Ω per square range. The noise was measured with an rf SQUID direct small signal readout scheme. A 50-pH SQUID with 3µm2 junctions and 14–Ω shunt resistors was shown to have an ideally low white noise of lx10-13 Φ20Hz, a white to 1/f crossover frequency at 7 Hz, and a noise level less than 6 x 10-12 Φ20Hz, at 0.1 Hz. © 1991 IEEE