Focused Ion Beam fabrication of superconducting scanning Probes
Our vision is to enable a new era in scanning probe microscopy (SPM), in which nanometer-scale sensing devices – specifically superconducting devices – can be directly patterned on-tip and used to reveal new types of contrast. To realize this vision, we will use focused ion beam (FIB) techniques to produce sensors with unprecedented size, functionality, and sensitivity directly on the tips of custom-designed cantilevers. The key to this undertaking will be the unique capability of FIB to mill, grow, or structurally modify materials – especially superconductors – at the nanometer-scale and on non-planar surfaces. Our FIB-fabricated probes will include on-tip nanometer-scale Josephson junctions (JJs) and superconducting quantum interference devices (SQUIDs) for mapping magnetic fields, magnetic susceptibility, electric currents, and dissipation. Crucially, the custom-built cantilevers, on which the sensors will be patterned, will enable nanometer-scale distance control, endowing our probes with exquisite spatial resolution and simultaneous topographic contrast. The resulting imaging techniques will significantly surpass state-of-the-art SPM and help us to unravel poorly understood phenomena in physics, chemistry, and material science, which are impossible to address with today’s technology.
By virtue of their combined sensitivity and spatial resolution, our imaging tools will provide new types of contrast, giving insight into two-dimensional (2D) materials and a range of emerging van der Waals (vdW) heterostructures. Layer-by-layer stacking and twisting of these structures has recently opened a new area of materials engineering with huge potential for both fundamental discovery and applications. These materials can be metals, semimetals, topological insulators, semiconductors, insulators, and magnets. Correlation phenomena such as superconductivity, Mott insulating states, and magnetically ordered states have recently been observed and are just beginning to be understood [1-6]. There is now an urgent need for sensitive and high-resolution SPM to zero-in on the nanometer-scale mechanisms behind these phenomena and to understand the role of inhomogeneity and lateral confinement effects in these new materials.
For this reason, we will focus on imaging magnetic field and dissipation in 2D and vdW materials to map transport on the nanometer-scale, shedding light on edge currents and correlated electronic states. In vdW layered magnets, we will measure stray magnetic field together with topography, allowing us to correlate magnetic state with the number of layers or lateral size. Our non-invasive probes will have spatial resolution down to 10 nm and the ability to collect fully independent topographic contrast. They will operate at up to 80 K, allowing sample temperatures to be even higher. Because of these capabilities, this new type of SPM will surpass state-of-the-art techniques and help us to reveal the local properties of these emerging materials.
Today, SPM – although spectacularly successful – is typically carried out with rather simple probes, e.g. sharp conducting, insulating, or magnetic tips, limiting the possible types of measurements. Although attempts at realizing nanometer-scale superconducting sensors on cantilever tips have been made, none have succeeded, due to the difficulty of patterning on high-aspect-ratio tips. We believe that FIB patterning will finally provide a solution to this technical challenge. This unconventional patterning technique, combined with state-of-the-art device-ready cantilevers, will allow us to overcome this long-standing obstacle. Using FIB milling, deposition, and material modification, we will pattern JJ sensors for dissipation mapping, SQUIDs for magnetic field imaging, and SQUID susceptometers for susceptibility measurements, directly on device-ready cantilevers. These device-ready cantilevers will be developed starting from specialized cantilevers with integrated on-tip heaters designed by IBMZ for the well-known “Millipede” data storage project and commercially available via the Nanofrazor Technology from SwissLitho AG.
Ultimately, our cantilevers will have all the necessary electrical contacts and will be designed to host FIB-patterned superconducting sensors. The combination of advanced cantilever fabrication based on standard lithographic technology on a Si wafer with advanced FIB techniques for fabricating nanosensors has not been attempted before and will provide the possibility to develop new device architectures, e.g. implementation of microwave structures for dispersive readout of nanoSQUIDs. Instead of limiting ourselves to on-tip JJs and SQUIDs made from Pb, as used in the best SOTs so far , we will pursue a number of alternative superconducting materials and fabrication methods. These include the production of Nb devices by thin-film sputter deposition followed by FIB milling. We will also produce on-tip WC1-x (WC) JJs and SQUIDs by FIBID, in which superconducting nanowires of finely controlled thickness are deposited using the FIB and patterned to include geometrical Dayem bridge JJs. The group of Prof. De Teresa at the Institute of Nanoscience and Materials Science of Aragon (INMA), has already demonstrated the fabrication of superconducting WC nanowires by He-FIBID . Finally, using He+-FIB irradiation, we intend to produce YBa2Cu3O7 (YBCO) JJs and nanoSQUIDs on cantilevers through the ions’ structural modification of the YBCO film . Experiments, performed at the University of Tuebingen in the group of Prof. Koelle, have already shown that JJs and SQUIDs with controllable electric transport properties can be realized by this technique on planar YBCO films grown on various substrates .
In particular, there is a need for SPM probes, which combine high resolution, high sensitivity, and non-invasiveness – just what we seek to develop. Our ability to finely control tip-sample distance and obtain simultaneous topographic contrast will go significantly beyond recent SOT-based imaging, which showed how surface defects and sample inhomogeneity strongly perturb the correlated states . Combined SPM of magnetic field and topography will also allow us to investigate confinement effects in 2D magnets, which may have implications for memory applications.
In short, the major breakthrough, around which we have designed FIBsuperProbes, is the realization of tailor-made on-tip superconducting nano-sensors using FIB-based fabrication on advanced cantilevers. This new fabrication paradigm will drastically improve the imaging resolution and sensitivity of magnetic and thermal SPM. Ultimately, these probes will play a key role in unravelling the nanometer-scale mechanisms behind the formation of quantum states, which are often too sensitive to the local environment to survive on macroscopic scales.
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