Nanoscale wireframe SQUID on a cantilever by corner lithography

Abstract

We present the fabrication of nanoscale superconducting quantum interference devices (SQUIDs) at the apex of wireframe tips on self-aligned superconducting cantilever probes. The probes are made on silicon wafers using molding techniques in combination with corner lithography, which results in a nanowire frame tip with a tuneable apex structure. A shadow effect deposition using magnetron sputtering of Nb creates self-aligned superconducting wireframes on cantilevers with accompanying device circuitry. Superconducting weak links are realized at the apex of the wireframes with the use of focused ion beam nanopatterning. The realized SQUIDs have effective diameters ranging from several micrometers down to 100 nm and can be operated in magnetic fields up to 1 T. Furthermore, the nanowires in the wireframe can be used to flux modulate the SQUID locally. This fabrication process enables the production of wafer-scale templates for probes based on on-tip superconducting devices.

Figures (5)

• Figure 1: Schematic illustration of the principle of corner lithography. (a)/(d) Template preparation consisting of pits with concave corners $\alpha$, $\beta$, and $\gamma$. (b)/(e) Zoomed-in image of the conformal layer deposition (cross-section) at the pit bottom of (a)/(d). The thickness in the corners ($t_1$ and $t_2$) exceed the conformal layer thickness ($t_0$). After corner lithography, with an isotropic thinning distance $r$, nanostructures are formed in the corners of the template. (c)/(f) Corner lithography is able to make complicated three-dimensional structures by combining the nanostructures formed in different corners of the template with each other. The top (a)/(b)/(c) and bottom (d)/(e)/(f) row highlight that a variation in the initial template results in different nanostructures after the corner lithography process.
• Figure 2: (a) Etched inverted truncated pyramid in Si (100), defined by a rounded Si_RN mask. (b) Etching of electrodes in the Si_RN mask, subsequent corner lithography is performed with an additional conformal layer of Si_RN to create a wireframe with an open loop, which is connected to the electrodes. (c) Deposition of a heterostructure of TEOS/Si_RN/TEOS (second TEOS layer not shown) and subsequent lithography and etching to define the cantilever and chip layout. (d) Anodic bonding of a diced glass wafer to the top TEOS layer, subsequent etching of the Si wafer in TMAH to release the freestanding cantilever probe (chip is flipped upside down with respect to (c)). (e) Magnetron sputtering of Ti/Nb/Pd/Au on the entire chip creates a self-aligned superconducting probe. (f) Cross-section image of the TEOS undercut and shadow effect deposition to electrically separate the Si_RN chip and electrode layers. (g) Scanning electron microscopy (SEM) image of the created self-aligned superconducting cantilever probe with a tip at its end. (h) SEM image of the open-loop superconducting wireframe tip.
• Figure 3: (a) Scanning electron microscopy image of a SQUID at the apex of an open-loop nanowire frame tip, the two Dayem bridges milled with FIB are also visible. (b) Current-voltage characteristics at 6.5 K for different applied fields, on the right a schematic is shown of the measurement circuit. (c) Quantum interference pattern of the SQUID. (d) On-tip flux modulation by a modulation current supplied with one of the nanowires in the wireframe, see the schematic on the right. The SQUID voltage oscillations can be seen for the applied modulation current for various bias currents ($-100µ A$ to $100µ A$ with steps of $10µA$ ) at T = 6 K.
• Figure 4: (a) Scanning electron microscopy image of a completely FIB-made nanoSQUID on a sharp nanowire frame tip. (b) Quantum interference pattern of a nanoSQUID with an effective diameter equal to 114 nm taken at a temperature of 3.25 K. The applied magnetic field is perpendicular to the cantilever probe. (c) SQUID oscillations in the normal state at $I_b = 3µ A$ for a large range of magnetic field.
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