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IEEE Transactions on Applied Superconductivity

Low Frequency Noise Characteristic in HTS Step-Edge Junction Rf SQUID

S. K. H. Lam, Keith E. Leslie, Jeina Lazar

Published Mar 19, 2022• 0 citations
Publisher

Abstract

Rf SQUIDs have been fabricated on MgO substrate using step-edge YBCO junction. Ion beam etching as a post fabrication trimming process is routinely used to reduce (trim) the critical current of the junction for noise optimization. The white noise of SQUIDs with inductance,Ls= 157 pH can be reduced to 55μΦoHz−1/2after the trimming process. However, 1/f noise at low frequency has been found to be degraded after the trimming. Rf SQUIDs withLs= 130 pH were fabricated and measured. A similar noise performance was achieved in these SQUIDs down to 5 Hz without the need of trimming. The 1/f noise was found to be better than that of the trimmed SQUIDs.

DOI
10.1109/tasc.2022.3160678

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Rf SQUIDs have been fabricated on MgO substrate using step-edge YBCO junction. Ion beam etching as a post fabrication trimming process is routinely used to reduce (trim) the critical current of the junction for noise optimization. The white noise of SQUIDs with inductance,Ls= 157 pH can be reduced to 55μΦoHz−1/2after the trimming process. However, 1/f noise at low frequency has been found to be degraded after the trimming. Rf SQUIDs withLs= 130 pH were fabricated and measured. A similar noise performance was achieved in these SQUIDs down to 5 Hz without the need of trimming. The 1/f noise was found to be better than that of the trimmed SQUIDs.
In this work, various methods of improving the performance and yield of our step-edge junction rf SQUIDs are described. These methods include adjusting the YBCO thin film thickness and trimming of the device parameters by ion beam etching and heat annealing. Significant improvement in SQUID performance and yield was demonstrated by employing these techniques. The effects of passivation of the rf SQUIDs by ex-situ deposition of an amorphous-YBCO thin film were also studied. No adverse effects were observed for the noise performance of the devices due to passivation and long-term stability was 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.
We have fabricated Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.
We report the development of a fabrication process of high-resistance step-edge Josephson junctions on sharp argon ion beam etched substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around and is reflected in reproducible electrical properties of the junctions. Critical current and asymptotic normal state resistance both scale with the junction width w . The current-voltage ( I - V ) characteristics depend on the ratio of film thickness t to step height h . For > 0.85 they are flux-flow-like; for < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and per junction, depending on w . At 77 K we achieve products of more than . The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than are achieved. noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.
We have fabricated RF and DC SQUIDS with step-edge Josephson junctions (SEJ) and YBa 2 Cu 3 O 7 thin films. The low-frequency noise of these SQUIDS has been investigated as a function of temperature, bias current and magnetic field. Typically, the energy resolution in the white noise region of the DC SQUIDS with an inductance of 45 pH was 3*10 -30 J Hz -1 at 77.5 K. Depending on the bias current, the low-frequency spectrum could be of either 1/f type or Lorentzian type. The Lorentzian spectrum was generated by random telegraph signals (RTSS). An Arrhenius plot was obtained from the temperature dependence of the switching time of these RTSS. We have determined two barrier lengths of 0.23 eV and 1 eV of the two-level fluctuator (TLF) generating this RTS noise. From the magnetic-field dependence of the low-frequency noise we have found that the noise is periodic with the flux corresponding to the area of the SEJ. The results were discussed in terms of a model treating the SEJ as a multi-junction interferometer which forms a two-level fluctuator when the junction is placed inside a SQUID loop. The noise is generated by thermally activated switching of the SQUID between the two different states of the SQUID.
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.