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Applied Physics Letters

Niobium nanobridge dc SQUID

Richard F. Voss, Robert B. Laibowitz, Alec Nigel Broers

Published Oct 1, 1980• 61 citations
Publisher

Abstract

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.

DOI
10.1063/1.92026

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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.
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.
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.
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 have fabricated and characterized the noise performance of submicron edge junction dc superconducting quantum interference devices (SQUID’s) which exhibit excellent signal properties and extremely high flux sensitivity. At frequencies above about 3 MHz, the SQUID noise is white and is in reasonable agreement with computer simulations for a thermal noise-limited detector. In one device the measured flux energy resolution is 1.6±0.5ℏ at 1.5 K, within a factor of 2 of the quantum-limited value suggested by theoretical modeling.
DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.