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ACS Nano

Direct-Write Printing of Josephson Junctions in a Scanning Electron Microscope

Tycho J. Blom, Thomas Mechielsen, Remko Fermin, Marcel B. S. Hesselberth, J. Aarts, Kaveh Lahabi

Published Nov 24, 2020• 25 citations
Publisher

Abstract

Josephson junctions are the building blocks of superconducting electronics, with well-established applications in precision metrology and quantum computing. Fabricating a Josephson junction has been a resource-intensive and multistep procedure, involving lithography and wet-processing, which are not compatible with many applications. Here, we introduce a fully additive direct-write approach, where a scanning electron microscope can print substrate-conformal Josephson devices in a matter of minutes, requiring no additional processing. The junctions are made entirely by electron-beam-induced deposition (EBID) of tungsten carbide. We utilize EBID-tunable material properties to write, in one go, full proximity junctions with superconducting electrodes and metallic weak links and tailor their Josephson coupling. The Josephson behavior of these junctions is established and characterized by their microwave-induced Shapiro response and field-dependent transport. Our efforts provide a versatile and nondestructive alternative to conventional nanofabrication and can be expanded to print three-dimensional superconducting sensor arrays and quantum networks.

DOI
10.1021/acsnano.0c03656

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Josephson junctions are the building blocks of superconducting electronics, with well-established applications in precision metrology and quantum computing. Fabricating a Josephson junction has been a resource-intensive and multistep procedure, involving lithography and wet-processing, which are not compatible with many applications. Here, we introduce a fully additive direct-write approach, where a scanning electron microscope can print substrate-conformal Josephson devices in a matter of minutes, requiring no additional processing. The junctions are made entirely by electron-beam-induced deposition (EBID) of tungsten carbide. We utilize EBID-tunable material properties to write, in one go, full proximity junctions with superconducting electrodes and metallic weak links and tailor their Josephson coupling. The Josephson behavior of these junctions is established and characterized by their microwave-induced Shapiro response and field-dependent transport. Our efforts provide a versatile and nondestructive alternative to conventional nanofabrication and can be expanded to print three-dimensional superconducting sensor arrays and quantum networks.
Abstract In this Perspective article, we evaluate the current state of research on the use of focused electron and ion beams to directly fabricate nanoscale superconducting devices with application in quantum technologies. First, the article introduces the main superconducting devices and their fabrication by means of standard lithography techniques such as optical lithography and electron beam lithography. Then, focused ion beam patterning of superconductors through milling or irradiation is shown, as well as the growth of superconducting devices by means of focused electron and ion beam induced deposition. We suggest that the key benefits of these resist-free direct-growth techniques for quantum technologies include the ability to make electrical nanocontacts and circuit edit, fabrication of high-resolution superconducting resonators, creation of Josephson junctions and superconducting quantum interference device (SQUIDs) for on-tip sensors, patterning of high-Tc SQUIDs and other superconducting circuits, and the exploration of fluxtronics and topological superconductivity.
Abstract Superconducting flux memory devices fabricated with nanoscale junctions and high-inductance materials can maintain high-speed access performance while significantly reducing the area required for superconducting loops. In this work, we present a method for preparing nanoscale NbN-based superconductor/normal-metal/superconductor (SNS) Josephson junctions that combines electron beam lithography with chemical mechanical planarization process. Scanning electron microscope images reveal that the junction has a very round shape, while cross-sectional transmission electron microscopy shows that it has a truncated cone structure, characterized by a narrow top and a wide bottom. The minimum diameter of the junction is 51 nm, with the diameter at the interface between the wiring layer and the top electrode being just 27 nm. These nanoscale junctions exhibit excellent Josephson properties with a critical current density J c of 673 ± 64 kA cm −2 and a characteristic voltage V c of 1.63 ± 0.45 mV at 4.2 K. Furthermore, we designed the superconducting memory cell based on the simulated inductance values of the NbN loop and the nanoscale NbN-based SNS junctions. The cell has a total area of 12.6 μ m × 6.5 μ m and contains a radio-frequency superconducting quantum interference device with one junction in the storage loop and a direct-current superconducting quantum interference device with two junctions in the readout loop. Using the flux-locked and feedback measurement method, a clear flux hysteresis curve can be obtained to verify the read-write functions of the device.
We define superconducting constrictions by ploughing a deposited Aluminum film with a scanning probe microscope. The microscope tip is modified by electron-beam deposition to form a nanoplough of diamond-like hardness which allows the definition of highly transparent Josephson junctions. Additionally, a dc superconducting quantum interference device is fabricated in order to verify the junction’s behavior. The devices are easily integrated in mesoscopic devices as local radiation sources and can be used as tunable on-chip millimeter-wave sources.
We demonstrate a direct writing method for the fabrication of planar Josephson junctions from high-quality superconducting niobium titanium nitride (NbTiN) thin films, by creating local disorder using focused He ion beam irradiation. We control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, enabling us to successfully fabricate Josephson junctions with highly tunable weak links ranging from the metallic to the insulating phase, utilizing the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both superconductor-normal metal-superconductor (SNS) and superconductor-insulator-superconductor (SIS) type of devices, and show that we can achieve an exceptionally wide range ( ∼ 5 orders of magnitude) of critical current densities and junction resistances. The SNS-type junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages up to ∼ 1.5 mV , and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that the junctions are suitable for a wide range of applications in superconducting electronics because of the excellent mechanical, electrical, and microwave properties of NbTiN. In particular, as NbTiN has previously been used to fabricate high-quality factor microwave resonators, we see the method as a promising and simple way to realize high-frequency superconducting qubits and other quantum devices using only a single superconducting film.
Superconducting junctions are widely used in a multitude of applications ranging from quantum information science and sensing to solid-state cooling. Traditionally, such devices must be fabricated on flat substrates using standard lithographic techniques. In this study, we demonstrate a highly versatile method that allows for superconducting junctions to be fabricated on a more complex topography. It is based on maskless direct laser writing and two-photon lithography, which allows writing in 3D space. We show that high-quality normal metal–insulator–superconductor tunnel junctions can be fabricated on top of a 20-μm-tall three-dimensional topography. Combined with conformal resist coating methods, this technique could allow sub-micron device fabrication on almost any type of topography in the future.
The Josephson junction (JJ) is the fundamental nonlinear building block of superconducting quantum technologies. Its macroscopic quantum tunneling physics underpins superconducting quantum computing, sensing, and communication, but scaling these platforms to utility-scale architectures places increasingly stringent demands on junction materials, interfaces, and fabrication. Practical quantum computing requires high reproducibility, low power consumption, tunability, a small device footprint, and resilience to noise and defects. This review surveys how advances in materials science, device characterization, and nanofabrication are addressing these challenges and redefining the figures of merit for next-generation JJs. This review also examines the evolution of fabrication strategies, from conventional multi-angle evaporation to foundry-compatible superconducting processes and the integration of emerging junction materials. Progress along these directions will determine how rapidly JJs move from laboratory-scale components to the foundation of industrial-scale quantum processors.