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arXiv:1501.00958 · Superconductor Science and Technology

Investigation of all niobium nano-SQUIDs based on sub-micrometer cross-type Josephson junctions

Matthias Schmelz, Y Matsui, Ronny Stolz, V Zakosarenko, Thomas Schönau, S Anders, S Linzen, H Itozaki, H-G Meyer

Published Nov 26, 2014• 27 citations
arXivHTMLPDF

Abstract

We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n Φ 0 Hz −1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S μ 1/2 < 7 μ B Hz −1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.

DOI
10.1088/0953-2048/28/1/015004

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IEEE Transactions on Applied Superconductivity•Mar 1, 1993•27 citations
We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n Φ 0 Hz −1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S μ 1/2 < 7 μ B Hz −1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.
Abstract We report on the development of nearly quantum limited SQUIDs with miniature pickup loop dimensions. The implemented high quality and low capacitance cross-type Nb/AlO x /Nb Josephson junctions offer large I C R N -products and therefore enable an exceptional low noise level of the SQUIDs. Devices with loop dimensions of 1 μ m exhibit white flux noise levels as low as 45 nΦ 0 Hz −1/2 corresponding to an energy resolution ε of about 1 h at 4.2 K, with h being Planck’s constant. Moreover, the large usable voltage swings of the devices of about 300 μ V allow highly sensitive and easy single-stage operation while exploring nearly the intrinsic noise of the SQUIDs, beneficial e.g. for sensor arrays in SQUID microscopy.
We have fabricated superconducting quantum interference devices (SQUIDs) with an inductance of 140 pH by using sub- µm Nb/Al–AlO x –Al/Nb edge junctions. The small capacitance of the junction resulted in the very large voltage swing of 210 µV and gradient dV/ dΦ of 940 µV/Φ0. Consequently, the very low flux noise of 0.61 µΦ0/ Hz0.5 corresponding to the energy sensitivity ε of 8.6h at the white noise level was obtained by using a conventional flux-locked loop configuration. The low-frequency noise was also very low, 2.7 µΦ0/ Hz0.5 corresponding to ε= 170 h at 1 Hz. We could obtain very low flux noise for the SQUID which had the practical inductance using only a conventional readout circuit.
We investigated, at temperature 4.2 K, electric transport, flux noise, and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise SΦ1/2=250nΦ0/Hz1/2, corresponding to a spin sensitivity Sμ1/2≥29μB/Hz1/2. For the gradiometer we find SΦ1/2=300nΦ0/Hz1/2 and Sμ1/2≥44μB/Hz1/2. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID's critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nΦ0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck's constant. Simulation of spin sensitivities results in about 1 μB/Hz1/2 for an electron spin positioned directly above the SQUID ring.
We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6 µm. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.
The authors have designed, fabricated, and measured a number of Nb-AlO/sub x/-Nb octagonal washer DC superconducting quantum interference devices (SQUIDs) and miniature DC SQUID susceptometers having minimum feature size down to 0.5 mu m. With SQUID inductance values on the order of 100 pH, typical noise performance is better than 1 mu Phi /sub 0// square root Hz. The small minimum feature size of input coils and pickup loop structures will facilitate tight coupling to a wide variety of systems ranging from submicron particles and structures to conventional approximately mu H input circuits. A single-washer SQUID with an 80-turn 0.5- mu m-linewidth, 630-nH input coil has user-friendly V- Phi (voltage-flux) curves and a coupled energy sensitivity of 20 h at 4.2 K. Susceptometers with pickup loops ranging from 20- mu m to 0.8- mu m across have very user-friendly V- Phi curves and a resolution of order 100 mu /sub B// square root Hz at 4.2 K for devices with the smallest loops.>