Investigation of nanoSQUID designs for practical applications

Abstract

In the recent past, considerable effort was spent on the development of superconducting quantum interference device (SQUID) designs for magnetic detection in the micro- and nano-scale. Where these novel nanoSQUIDs were mostly of a simple format, real applications require more elaborate designs including auxiliary components such as coils and transformers. Therefore, we have developed SQUID designs based on a Nb/HfTi/Nb thin-film technology which offers both, nano-patterning and waferscale manufacturing of complex design structures. Employing e-beam lithography and chemical–mechanical polishing process steps, the area of the superconductor–normal conductor–superconductor Josephson junctions of these nanoSQUIDs has been varied between 150 nm × 150 nm and 200 nm × 200 nm, whereas the thickness of the barrier is about 20 nm. In order to enable real practical applications, nanoSQUID designs with a number of implemented auxiliary components and design features, such as gradiometric feedback loops, gradiometric transformers, and rf filters, have been carefully investigated. This paper combines a summary of recent achievements with a presentation of detailed measurements of the device performance in magnetic fields of up to a few tens of millitesla. This investigation is intended to pave the way for future practical complex nanoSQUID tools.

Transport and Noise Properties of sub-100-nm Planar Nb Josephson Junctions with Metallic Hf - Ti Barriers for nano-SQUID Applications

“…Finally, we note that the advances in deep submicron SNS junction technology presented here offer the opportunity to make full use of the further advantages of SNS sandwich-type junctions for nano-SQUID applications. In particular, this multilayer technology provides the possibility to create much more complex and advanced nano-SQUID architectures, as, e.g., the three-dimensional vector nano-SQUID [33] or nano-SQUID susceptometers [34], including serial gradiometers and auxiliary components, such as gradiometric feedback loops gradiometric transformers and rf filters. This may significantly widen the application potential of nano-SQUIDs in the field of nanomagnetism and high-resolution magnetic imaging.…”

“…Noise measurements on nano-SQUIDs based on our SNS JJs revealed 1/f noise in the flux noise spectra, which could be suppressed by applying a bias reversal scheme. This indicates that the 1/f noise is due to critical current and resistance fluctuations in our JJs [32,34]. To quantify the strength of those fluctuations in the single JJ discussed here, we follow the analysis described in Refs.…”

“…There have been several earlier realizations of nano-SQUIDs based on self-shunted SNS JJs with Hf-Ti barrier and linear sizes appreciably larger than 100 nm. Because of their low-noise performance and relatively compact design, nano-SQUIDs based on these junctions promise great potential as stand-alone devices [31,32] and as building blocks of complex circuits [33,34]. However, there is still a need for further improvements regarding size reduction of the SQUIDs and tuning of their electrical parameters, including I c , R n , and, therefore, V c .…”

Fabrication Process for Deep Submicron SQUID Circuits with Three Independent Niobium Layers

“…In addition, a CMP step is used to planarize the SiO 2 layer between the first and second independent Nb layer. Both technologies are available at the clean room center at PTB Braunschweig and have been applied before in the fabrication of JJ-based circuits [ 29 , 30 , 33 ]. Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5 illustrate the deposition and patterning steps (we note that the layer thicknesses are not to scale).…”

Recent advances in high-throughput superconductivity research

“…Scanning magnetic microscopes, e.g. magnetic force microscope (MFM) [76][77][78], scanning Hall probe microscope (SHPM) [79,80] and scanning superconducting quantum interference device (SQUID) [81][82][83][84], which possess the spatial resolution necessary for highthroughput magnetic imaging, have played and will continue to play an important role in understanding the nature of superconductivity. MFM, with spatial resolution of about tens of nm, is generally based on measuring the force between a magnetized tip and the scanned surface.…”