On a shelf in the far corner of the clean room are blue and black plastic boxes. Alexandra sets one of them on the table and opens the lid. Several silicon wafers, each 20 centimeters in diameter and less than a millimeter thick, are placed next to each other in slots, just like in a CD tray. Using vacuum wafer tweezers, a kind of flat suction lifter, the researcher picks up one of the thin discs and carefully removes it from the box. The polished front of the wafer is coated with a shiny metal that will be used to fabricate the qubit structures. "I process the wafers with the P5000." The physicist points to a small door on the wall, the entrance to the "P5000", as Alexandra calls the machine she mostly works on. The main part of the machine, however, is not in the clean room, but on the other side of the wall, in the adjacent gray room: "The wafers are only exposed to air in the clean room. But we can also operate the equipment from the gray room. Alexandra explains that one is allowed to be there without a cleanroom suit, and she seems quite happy not to have to work in full gear all the time.

Alexandra moves safely and quickly through the rooms. She wants to use her time as efficiently as possible, so she doesn't dawdle. Arriving at the back of the gray room, one can see the P5000 in its entirety. The system consists of four "chambers," as Alexandra calls them. Three of them look a bit like a waffle iron. In them, different materials are deposited on the wafer, the doctoral student explains. The so-called shower head is located in the lid of the three "waffle irons", while the base is formed by a susceptor, which is used to heat the wafer. "The shower head, which has many small holes, distributes the material evenly over the wafer. The heat is necessary for the chemical process to take place that deposits the material onto the wafer," explains the physicist. A fourth chamber, which looks slightly different, can be used to etch structures into the metal layer. The young researcher is very familiar with the machine. Every move fits. Alexandra goes to the control panel and brings one of the chambers into the wafer lift position. The susceptor moves down, exposing four pins. A robotic arm with a flat end then moves under the wafer, much like a spatula, lifting it slightly and moving it from chamber to chamber. Alexandra adds: "While I'm processing a wafer in the machine, it stays closed. I can control everything from the outside."

Benefit from professional exchange among doctoral students

To make TSVs, cylindrical holes are first etched through the silicon substrate. To create a superconducting connection from the top to the bottom of the chip, Alexandra has to deposit a metallic film on the inner wall of the hole. The proper metallization of these cylindrical holes is the focus of her work: "This metal film is very, very thin. It is very challenging to get it everywhere because this hole has a small opening and a large depth.” It is also very difficult to influence the properties of the metal film because, for example, the gas dynamics in the hole are completely different from those on the surface of the chip, which has an extreme influence on the growth of the metal film, the researcher continues. One of the first challenges in her Ph.D. was to make the metal film superconducting. That didn't work out for a while, Alexandra recalls, but the professional exchange during the Benasque Spring School in spring 2023 finally helped her. She was able to pick up a few tips and tricks there. The event in the small, remote mountain village of Benasque in the Pyrenees, which was attended by doctoral students and postdocs from many different countries, lasted ten days, says the young researcher. Everything revolved around superconducting qubits. Having only entered the field of quantum computing with her Ph.D., Alexandra greatly appreciated the opportunity to delve deeper into the fundamental questions of the fabrication of superconducting qubits and the physics behind it. In particular, the intensive exchange with other doctoral students, all of them working on superconducting qubits, helped her tremendously from a technical perspective. "It was definitely a highlight of the first year of my Ph.D."

When Alexandra describes her "daily business", she talks about process development: "I want to find out which process parameters influence my result." The metallic film for her TSVs must have certain properties, and she now varies process parameters such as pressure or temperature to see how that affects the result. Although Alexandra's focus is on process development for the production of superconducting TSVs, the overall process development must always be kept in mind: "The individual processes that go into producing a superconducting chip must all be compatible with each other, and that is a huge challenge." The most critical structure is the Josephson junction, Alexandra points out – an electronic element, only a few hundred nanometers in size, that is, in a sense, the heart of a superconducting qubit. If, for example, the qubit structures are etched first and the holes for the TSVs are etched afterwards, there is no guarantee that the etching of the holes will not change the Josephson junctions again, explains the researcher. There are many different ways to arrange the sequence of individual processes, and there is always the question of what happens to the various elements on the chip, she summarizes.

Vacuuming is also part of the job

After studying physics in Erlangen, Alexandra made a conscious decision to pursue a Ph.D.: "Understanding interrelationships and delving deeper into a topic is something I really enjoy." However, day-to-day work does not always consist of puzzling over and thinking through complex processes. There is no getting around maintenance work in process development, says the doctoral student. Recently, she had to clean the inside of the P5000 because material fluff had come off during a process and was spread all over the interior. She first used a special cleanroom vacuum cleaner for this. Alexandra laughs: "This vacuum cleaner is awesome." She then had to wipe out every corner of the machine with special cloths. "That was very time consuming." Sometimes, however, maintenance problems arise that are less obvious than a dirty machine and require a certain level of creativity. "I then always have lots of ideas and try everything one after the other until everything works again. I hope I never get into a situation where I no longer know what to try," says the young researcher with a laugh.

But even when this is the case, Alexandra can count on the exchange with her colleagues, whom she appreciates very much. In fact, she appreciates them so much that she is willing to meet them at the bouldering hall once a week at seven in the morning before work. She smiles: "I wouldn't describe myself as a typical early riser, but I really enjoy doing it and if you have an appointment...". In addition to the close exchange with her colleagues, she also finds it inspiring to be part of the larger MQV community: "As an individual, you sometimes lose sight of the big picture quite quickly. What I like about MQV is that we have so many project partners covering a very broad area of quantum computing." By exchanging ideas with other research groups, even small intermediate goals that one achieves can be placed in a larger context.

Published 28 February 2024; Interview 23 October 2023