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Journal of Applied Physics

Fabrication and measurement of cylindrical Nb3Ge rf SQUIDs

Shinya Kuriki, M. Suehiro, Hajime Konishi

Published Mar 1, 1985• 3 citations
Publisher

Abstract

Nb3Ge thin-film microbridges are fabricated on a cylindrical substrate to form rf SQUIDs. High temperature operations well above 10 K (up to 17.5 K) are obtained. An intrinsic flux noise of the order of 10−4 Φ0/(Hz)1/2 is estimated from the step rise parameter. Comparison with a theoretical noise indicates a nonsinusoidal current phase relation of the microbridge.

DOI
10.1063/1.334449

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Nb3Ge thin-film microbridges are fabricated on a cylindrical substrate to form rf SQUIDs. High temperature operations well above 10 K (up to 17.5 K) are obtained. An intrinsic flux noise of the order of 10−4 Φ0/(Hz)1/2 is estimated from the step rise parameter. Comparison with a theoretical noise indicates a nonsinusoidal current phase relation of the microbridge.
We have developed a technique which permits high-yield fabrication of microbridges and low noise YBa2Cu3O7 superconducting quantum interference devices (SQUIDs) in epitaxial thin films. These SQUIDs operate over a wide temperature range extending from 4 K to close to the superconducting transition temperature. Measurements of an rf SQUID operating at 77 K give a peak-to-peak flux sensitivity of 36 μV/Φ0 and a flux noise at 10 Hz of 1.5× 10−4 Φ0/√Hz. Device yields over 80% have been obtained.
We describe the noise performance of thin-film YBa 2 Cu 3 O 7 RF SQUIDS fabricated on MgO substrates with single step-edge Josephson junctions. A substantial range of noise levels has been observed for devices having similar electrical parameters (inductance, critical current, RF frequency). The best devices have white-noise levels below 10 -4 phi 0 Hz -1/2 when operated at a frequency of 170-200 MHz and have 1/f noise corners of a few hertz. Measurements of the V RF versus L RF characteristics of one SQUID, indicate that intrinsic device noise dominates the measured total noise of these devices. The white noise of all devices exceeds the predictions of the theory of Kurkijarvi at al. by a large factor, apparently because of non-ideal behaviour of step-edge Josephson junctions.
Ultrahigh frequency rf-SQUID magnetometer employing a cryogenic preamplifier and a commercially available thin film rf-SQUID sensor have been designed, manufactured, and tested. The system has been operated at 316 MHz, which is the maximum pump frequency accepted by the thin film sensor. The cryogenic preamplifier has been built around a microwave high-electron-mobility transistor which was made unconditionally stable at ultrahigh frequency band by means of an appropriate circuit design, namely, a cascode configuration. The total flux noise of the magnetometer has been found to be 8.4 × 10−6Φ0/√Hz at 4.2 K and 5.8× 10−6Φ0/√Hz at 1.4 K. The temperature dependence of the fractional step rise α of the current-voltage rf characteristic has been measured to be proportional to the 2/3 power of the SQUID sensor temperature. The noise performance of the system at 4.2 K has been limited by the intrinsic SQUID noise only, in good agreement with theoretical predictions.
SQUIDs employing short (\sim 1 \mu) ultra thin weak link sections of "granular" niobium and niobium nitride have been prepared on cylindrical quartz substrates by rf and rf reactive sputtering, photolithography and anodization. These SQUIDs have demonstrated near ideal intrinsic noise characteristics when biased at 20 MHz. Niobium nitride devices have operated with this nearly intrinsic noise at temperatures up to 12.5 K. Current-phase relation and flux entry versus applied flux were measured on one niobium SQUID. Results indicate a nearly [if not exactly] sinusoidal current-phase relation from Tcto almost a degree below Tcand flux entry noise in good agreement with the intrinsic flux noise theory. Biasing these devices at 9.2 GHz gives a degraded response which can be related to relaxation effects in the weak links. Thermal noise switching between the superconducting and dissipative states was also observed at 9.2 GHz and could be fit by a simple model.
A superconducting quantum interferometer device (SQUID) electronics with tank frequencies near 150 MHz was constructed to enhance the rf SQUID sensitivity. By these means we characterized our YBa2Cu3O7 thin-film rf SQUIDs with step-edge junctions immersed in liquid N2. We obtained transfer function values of over 40 μV/Φ0, at a 50 Ω input impedance, for both hysteretic and dispersive mode of rf SQUID operation. The rms white and 1/f flux noise levels were significantly lower than at the tank frequency of 20 MHz. The best magnetic field sensitivity at 77 K was 0.9 pT/Hz1/2, and the energy resolution 1.4×10−28 J/Hz, down to 0.3 Hz.