Superconducting quantum sensor goes vertical

Quantum materials do things ordinary materials cannot. They carry current without any loss, or conduct only along their outer edge while the inside insulates. Future quantum computers and quantum sensors will run on materials like these. To improve them, researchers need to see exactly where the currents and the magnetic fields run at the nanoscale.

The sensor that is sensitive enough for that is the SQUID, a superconducting ring that registers the smallest magnetic change. The problem is distance. Magnetic fields fade very quickly, so the sensor has to be able to get close to the material. If such a ring lies flat in the plane of a chip, the rest of that chip soon holds it several micrometres away from the material. That is exactly where the detail is lost. The University of Twente has worked on scanning SQUID microscopy for years, mapping the magnetism of a surface.

“That is why we put the sensor on a pyramid,” says Hans Hilgenkamp. “On top of that pyramid the sensor can be brought right up to the material we want to look at, with nothing else in the way. That lets us image magnetism at the scale where quantum materials do their work.” This opens the door to new materials with unusual functionality. Within the Gravitation programme QuMat, Twente builds the instruments with which it and its partners can measure quantum materials and develop them further.

Two Twente specialisms on a single point

The pyramid is not built up. It is first hollowed out of a slice of silicon, the same material computer chips are made from. Using a liquid, the researchers bite away pits shaped like an upside-down pyramid. Over each pit they lay an extremely thin film of a glass-like material, which ends up slightly thicker in the sharp edges. When they remove that film again, material is left behind only in those edges.

What remains is a frame of extremely fine wires, running precisely along the ribs of the pyramid. The technique is called corner lithography. Erwin Berenschot and Niels Tas developed it in the Twente group Mesoscale Chemical Systems, which specialises in three-dimensional structures at the nanoscale.

The surrounding silicon can then be removed, leaving the pyramid standing upright on the end of a thin flexible arm. In a microscope, that arm scans the surface. Much as the needle of a record player follows a groove. A layer of niobium goes over the wires, a metal that is superconducting at extremely low temperatures. Finally, a fine beam of charged particles cuts two constrictions into the ring at the top. The sensor is then complete.

“We come from superconductivity, our colleagues from 3D nanofabrication,” says Thijs Roskamp, PhD candidate and first author. “Both techniques already worked well. Combine them and you get three-dimensional devices with unmatched possibilities.” The chip industry worked in the flat plane for decades. Stacking is common now, but it still happens layer by layer. Genuinely three-dimensional structures with working electronics inside them are rare. This sensor shows that it can be done.

Still measuring where superconductivity normally gives out

Research on quantum materials often takes place in strong magnetic fields. That is awkward for a superconducting sensor, because a strong magnetic field is precisely what destroys superconductivity. The most sensitive sensors therefore drop out at the moment you need them most. The Twente sensor keeps working above 1 tesla, roughly twenty thousand times the Earth’s magnetic field.

There is a further advantage. The frame on the tip consists of four wires, and only two are needed for the measurement. The other two remain as extra connections. They let the researchers steer the sensor from the tip itself and tune it for sensitivity while it scans. “Our sensor has four connections from the start,” says Roskamp. “We use two to measure and the other two stay free. We can add functions to those without having to make the sensor all over again.”

From handwork to a whole wafer

Other groups are trying to make SQUID-on-tip sensors like these, but so far it is all handwork. A specialist first pulls a glass tube into a fine needle and evaporates superconductor onto it in a few steps. The result works, but it is one sensor at a time, with limited reproducibility and little room for anything extra on the tip.

At the MESA+ NanoLab in Twente, the sensor comes out of the same kind of steps the chip industry uses. Everything happens on a whole silicon wafer at once, so hundreds of pyramids side by side in a single production run. Between 80 and 90 per cent of the tips on the wafer come out usable. The size of the ring is adjustable as well. By changing the hollowed-out pit, the researchers made sensors ranging from several micrometres down to about 100 nanometres.

Colleagues from Bruker took part in the work, a manufacturer of scanning microscopes based in Enschede that works in the MESA+ NanoLab. That considerably shortens the step from an experimental set-up to a sensor other laboratories can simply order. “Our role in QuMat is to build tools,” says Hilgenkamp. “We develop the sensors that let us measure remarkable quantum materials together with others in the consortium.”

Source: University of Twente