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Superconductor Science and Technology

Low-TcSQUIDs with a built-in heater module to remove flux trapping

Kôichi Yokosawa, Daisuke Suzuki, Keiji Tsukada, Seiichi UGAI, Toshiaki Suzuki, Yasushi Shimizu

Published Jul 27, 2000• 4 citations
Publisher

Abstract

We have fabricated low- T c SQUIDs (superconducting quantum interference devices) that have a built-in heater module next to the Josephson junctions to remove trapped flux by using only a low heating power. To suppress the generation of magnetic fields, the heater module comprises a double-layer consisting of a heater, which is a narrow Nb film, and an Al electrode. The module can be fabricated together with the SQUID without complicating the process. The Nb heater is transformed to normal metal with sufficient resistance when a current above the critical current flows through it. We demonstrated that the heater module could reliably remove trapped flux with 24 mW without increasing any noise of the SQUID or causing re-trapping.

DOI
10.1088/0953-2048/13/8/401

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We have fabricated low- T c SQUIDs (superconducting quantum interference devices) that have a built-in heater module next to the Josephson junctions to remove trapped flux by using only a low heating power. To suppress the generation of magnetic fields, the heater module comprises a double-layer consisting of a heater, which is a narrow Nb film, and an Al electrode. The module can be fabricated together with the SQUID without complicating the process. The Nb heater is transformed to normal metal with sufficient resistance when a current above the critical current flows through it. We demonstrated that the heater module could reliably remove trapped flux with 24 mW without increasing any noise of the SQUID or causing re-trapping.
We have fabricated dc superconducting quantum interference devices (SQUIDs) incorporating Nb/Al-oxide/Nb Josephson junctions in both stripline and washer geometries. Low-frequency noise in excess of that predicted by the resistively-shunted junction model is present in both geometries and is demonstrated to be flux noise. This flux noise is not environmental. Improvements in fabrication processing over the past four years have reduced the level of this flux noise. SQUIDs are now fabricated with PdAu resistors, Nb wiring layers, and SiO/sub 2/ interlayer dielectric. In our best well-coupled SQUIDs, the white-noise energy sensitivity is 5/spl times/10/sup -31/ J/spl middot/s, with a 1/f knee below 0.1 Hz. We believe further reduction in the flux noise might be obtained with the use of on-chip flux shielding and/or trapping structures.
The direct current superconducting quantum interference device (DC-SQUID), using controlled Josephson junction technology, provides a mechanism to modify the characteristics of the device post-fabrication. We report on the fabrication and measurement of a micron sized DC-SQUID using two Dayem bridge weak-link Josephson junctions with integrated “heaters.” The weak link critical current is controlled by hot phonons from the current biased titanium, normal metal heater. By using the heaters, control over the critical current oscillations of the SQUID was observed at 4.2K.
A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.
Nano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5 K and magnetic flux densities up to over 20 mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.
Superconducting quantum interference device (SQUID) amplifier is best known for its low input impedance, low noise and low power consumption. Nowadays it is widely used for detecting the weak signals. Compared with other methods, the Nb/Al-AlOx/Nb structure Josephson junction based SQUID has the advantages of high transition temperature, high voltage flux modulation index and good heat recycle ability, wide critical voltage range, so it is a very good option for making SQUID amplifier. In this work, we fabricate the overdamped Josephson junction and washer dc SQUID, and test the I-V characteristics at He3 3 K stage temperature and calculate the current resolution of SQUID. The result of SQUID modulation property is good. The magnification becomes larger after increasing the input line number of loops, and the system noise becomes smaller after the join of the LC filter. This work is very important for designing and manufacturing transition edge sensor readout circuits.