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IEEE Transactions on Magnetics

Very low noise all-niobium DC SQUIDs

V. J. de Waal, P. van den Hamer, Johan E. Mooij, T. M. Klapwijk

Published Jan 1, 1981• 8 citations
Publisher

Abstract

All-niobium thin film planar DC SQUIDs have been fabricated, containing submicron tunnel junctions. The fabrication method using photoresist techniques is briefly described. The IcR product of the SQUIDs is 0.3 mV, the inductance 1 nH. They show no changes after repeated thermal cycling or storage over extended periods. The best intrinsic energy resolution obtained so far in a flux-locked loop configuration is 4 . 10-32J/Hz. The noise power is an order of magnitude higher than predicted from the simple RSJ model calculation.

DOI
10.1109/tmag.1981.1061049

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Journal of Applied Physics•Jan 15, 1989•5 citations
All-niobium thin film planar DC SQUIDs have been fabricated, containing submicron tunnel junctions. The fabrication method using photoresist techniques is briefly described. The IcR product of the SQUIDs is 0.3 mV, the inductance 1 nH. They show no changes after repeated thermal cycling or storage over extended periods. The best intrinsic energy resolution obtained so far in a flux-locked loop configuration is 4 . 10-32J/Hz. The noise power is an order of magnitude higher than predicted from the simple RSJ model calculation.
Tunnel-junction dc SQUIDs are being fabricated from Nb films using electron-beam lithography to define linewidths as small as 0.5 μm. The barriers in the Nb-NbOx-Nb junctions are grown in an oxygen plasma discharge, and the Pd2Si resistive shunts are formed by annealing Pd on the Si substrates. These devices have excellent storage and recycling properties. At a measurement frequency of 40 kHz, a SQUID with an inductance of about 1 nH, a critical current of about 4 μA, and a parallel shunt resistance of about 15 Ω had an intrinsic energy resolution of 2.5×10−32 J Hz−1 at 4.2 K and 1.1×10−32 J Hz−1 at 1.6 K, in good agreement with a model of the dc SQUID.
We present the results of an investigation of thin-film dc super conducting quantum interference device (SQUID’s) with very high energy resolution. The SQUID’s were fabricated using integrated-circuit techniques appropriate for superconducting devices. A weakly coupled SQUID with very low inductance (1 pH) had an intrinsic energy sensitivity 6×10−34 J/Hz = 0.9 h. A well-coupled SQUID (coupling constant α = 0.43) had an energy sensitivity referred to the input coil of 4.7×10−32 J/Hz = 71 h.
We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room-temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small-signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux-locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux-locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.
The design, fabrication, and noise properties of an ultra-low-noise tunnel junction dc SQUID (superconducting quantum interference device) are presented. The two resistively shunted Pb-alloy tunnel junctions are each 3.6 μm diam. The SQUID has an inductance L of ≈11 pH, a critical current per junction, I0 of ≈100 μA, and a resistance per shunt R of ≈2 Ω. At 2.3 K, eI0R≈kBT, and simple theory predicts that the minimum intrinsic energy sensitivity should be ≈2h. The actual minimum value of the measured high-frequency (70 kHz) flux noise at 4.2 K is φn=1.3×10−7φ0 Hz−1/2. This corresponds to an intrinsic energy sensitivity, φ2n/2L, of 3.2×10−33 J Hz−1 or 5h. Lowering the bath temperature to 1.8 K gave no improvement in performance.
The noise characteristics at low frequencies are studied for various planar dc SQUIDs made of junctions with different barrier materials of a-Si, aluminum oxide, and niobium oxide. It is found that fluctuations of the parameters of individual junctions contribute to the flux noise of the SQUIDs in a magnitude of the same order with the apparent flux noise. The magnitude of the flux noise due to the parameter fluctuations depends on the barrier material and on the inductance parameter βL . We have observed a telegraph noise in the voltage time trace and a Lorentzian in the voltage power spectrum for a current-biased SQUID. As a source of the parameter fluctuations, capture and emission of electrons at the trapping sites in the barrier are indicated.