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IEEE Transactions on Magnetics

Ultra low noise Nb DC SQUIDs

Richard F. Voss, Robert B. Laibowitz, Alec Nigel Broers, Stanley I. Raider, C. M. Knoedler, J. M. Viggiano

Published Jan 1, 1981• 48 citations
Publisher

Abstract

Electron-beam lithography has been used to produce ultra low noise Nb dc SQUIDs (Superconducting QUantum Interference Devices) with Josephson elements consisting of either small area tunnel junctions or very narrow variable thickness bridges (nanobridges). Detailed voltage noise and transfer function measurements have been made as a function of temperature and flux and current bias. These measurements allow a computation of the intrinsic energy resolution ε vs bias parameters as well as comparison with models of intrinsic SQUID noise. The best ε obtained was of order Planck's constant h. Moreover, the detailed characteristics of the SQUID voltage noise are in agreement with a small signal analysis that depends only on the current noise in each of the Josephson elements and measurable parameters.

DOI
10.1109/tmag.1981.1060901

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Electron-beam lithography has been used to produce ultra low noise Nb dc SQUIDs (Superconducting QUantum Interference Devices) with Josephson elements consisting of either small area tunnel junctions or very narrow variable thickness bridges (nanobridges). Detailed voltage noise and transfer function measurements have been made as a function of temperature and flux and current bias. These measurements allow a computation of the intrinsic energy resolution ε vs bias parameters as well as comparison with models of intrinsic SQUID noise. The best ε obtained was of order Planck's constant h. Moreover, the detailed characteristics of the SQUID voltage noise are in agreement with a small signal analysis that depends only on the current noise in each of the Josephson elements and measurable parameters.
For the first time high-resolution electron beam contamination lithography has been used to produce active sensing devices. dc SQUID’s (Superconducting quantum interference devices) with linewidths as small as 30 nm have demonstrated a minimum intrinsic energy resolution of 3h. This is the best value for any SQUID to date and approaches the limit ≈ h set by the uncertainty principle. The dependence of voltage noise on SQUID parameters is in agreement with a new small-signal model of SQUID noise.
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.
For the fabrication of nanoscale SQUIDs, for example having loop diameters of order 1 µm or smaller, the conventional trilayer window-type Josephson junctions are too large. Instead, we consider the use of nanobridge junctions with dimensions of order 100 nm. The problem of hysteresis in the junctions' current–voltage characteristic (IVC) is avoided by depositing a gold overlayer, which serves as both a thermal and electrical shunt. Noise measurement of dc SQUIDs which incorporate such Nb nanobridges as the Josephson junctions are reported. Devices having different IVCs were fabricated and characterized. The best device has a flux noise of 5 × 10 −6 Φ 0 Hz −1/2 at 4.2 K, and there is no significant increase in the noise level in a static magnetic field up to 2 mT. Techniques for increasing the voltage modulation depth, and hence improving the flux noise performance, are discussed.
We report on the low-frequency noise performance of niobium dc superconducting quantum interference devices (SQUIDs), which contain nanobridges fabricated by focused ion beam lithography as the active Josephson elements. The devices have feature sizes down to 70 nm. We have measured devices of different loop sizes in two readout configurations: nano-scale loop sizes in small signal mode using a series SQUID array as a low-temperature pre-amplifier, and larger micron-scale loop sizes in a conventional flux-locked loop. We investigate the different contributions to the low frequency noise and report on electrical measurements made in applied magnetic fields of up to 0.5 T (in-plane) and 0.1 T (perpendicular to the plane) at operating temperatures around 7 K. We compare the measurements with the existing theories of noise in a dc SQUID.
Abstract We have fabricated Nb-based dc-SQUIDs with sub-micrometer planar Nb/HfTi/Nb junctions in order to investigate their noise performance. The SQUIDs are of simple coplanar design, their nominal inductance is ca. 15 pH. Electron beam lithography and chemical-mechanical polishing have been used to realize junctions with cross sections areas as low as about 100 × 100 nm 2 . The SQUIDs exhibit pronounced excess noise increasing towards lower frequencies. This apparent flux noise arises from fluctuations of the junction critical currents or resistances. The Nb/HfTi/Nb junction critical currents are found to be strongly temperature dependent. Upon cooling below ca. 4 K the SQUIDs start to show current–voltage characteristics with negative differential resistances, and their flux noise increases significantly. Estimation of the HfTi barrier electron temperature indicates that the degradation of the SQUID properties towards lower temperature is caused by self-heating effects.
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.