Digital Etching of Single-Crystalline Silicon Fin Barriers Down to Sub-10 nm

In the quest to build quantum computers, scientists are trying to create tiny, fragile circuits that can hold information in a state of superposition, existing in multiple states at once. One of the most promising designs for these circuits is the transmon qubit, a device that relies on a special component called a Josephson junction. This junction acts as a gatekeeper for electrical current, but it is notoriously sensitive to its surroundings. If the materials inside the junction are disordered or messy at the atomic level, they create tiny defects that steal energy from the qubit, causing it to lose its information before any calculation can be finished. For years, researchers have used amorphous materials, which are like frozen liquids with atoms arranged randomly, to build these barriers. However, these random structures are full of the very defects that kill quantum performance. The ideal solution would be to use a perfect crystal, where every atom sits in a precise, orderly row, but making a thin, perfect crystal barrier inside a complex electronic device has proven to be an immense engineering challenge.

A team of researchers has now developed a new method to carve these perfect crystal barriers out of a single piece of silicon, thinning them down to a scale that was previously out of reach. They started with a standard silicon wafer, a material used in almost all modern electronics, and used a combination of chemical baths and heat treatments to sculpt it into a tall, narrow wall, known as a fin. The goal was to make this wall so thin that it could serve as the barrier for a Josephson junction, allowing electrons to tunnel through it in a controlled way. The challenge was that the initial chemical process used to shape the silicon left the walls uneven, tapering from thick at the bottom to thin at the top, and the standard methods for thinning the silicon further tended to break the delicate structures or leave them too thick to work.

To solve this, the researchers first used a wet chemical etch, dipping the silicon into a potassium hydroxide solution that eats away the material in a specific direction, leaving behind smooth, angled walls. This created a fin roughly seventy-four nanometers thick, but they needed it to be less than ten nanometers to function as a quantum barrier. They then turned to a technique called digital etching, which works like a very slow, controlled peeling process. They exposed the silicon to a gas that turns the very top layer of atoms into a thin skin of oxide, and then used a vapor of hydrofluoric acid to wash that skin away. By repeating this cycle of turning the surface to oxide and washing it off, they could remove the silicon layer by layer. In their initial tests, they managed to thin the fins down to about nineteen nanometers, but the process had a flaw: the protective mask sitting on top of the fin would detach and fall off once the fin became too thin, likely because the liquid acid was eating away at the glue holding the mask in place.

To get past this limit and reach the sub-ten-nanometer goal, the team switched from liquid acid to a vapor-based process and changed how they heated the silicon. They used rapid thermal annealing, a method where the silicon is heated very quickly in a furnace, to grow the oxide layer. By carefully controlling the temperature and the mix of gases in the furnace, they could tune exactly how much silicon was removed in each cycle. They found that heating the silicon to very high temperatures removed material quickly, which was useful for the thicker parts of the fin, while lower temperatures allowed for fine-tuning as they approached the final, ultra-thin dimensions. This approach allowed them to strip away the silicon without damaging the mask, eventually creating fins as thin as six nanometers.

However, the fins still suffered from the tapering problem, where the top was significantly thinner than the bottom, which would make the electrical properties of the junction inconsistent. The researchers realized that the way the silicon was heated was causing the top to oxidize faster than the bottom, creating a gradient in thickness. To fix this, they slowed down the heating process and changed the atmosphere inside the furnace from pure oxygen to a mixture of nitrogen and oxygen. This subtle change prevented the top of the fin from reacting too quickly before the whole structure reached the target temperature. The result was a dramatic improvement in uniformity. On a fin that was 1.2 micrometers tall, the difference in thickness between the top and the bottom was reduced from seventeen nanometers down to just three nanometers.

The final structures were examined using powerful electron microscopes, which revealed that the team had successfully created single-crystal silicon walls that were only about six nanometers thick at the top and fifteen nanometers at the base, with a remarkably straight profile. This level of precision suggests that it is now possible to build Josephson junctions using perfect, single-crystal silicon barriers, which could eliminate the energy-robbing defects found in older materials. By proving that these ultra-thin, uniform fins can be made using standard semiconductor tools, the researchers have opened a clear path toward building more stable and reliable quantum computers, turning a theoretical advantage of perfect crystals into a practical reality.