Clean Sweep For Cutting-edge Research

The room hums to the sound of powerful ventilators. In the pale light, Peter Rimpf pulls a full-body coverall with hood over his clothes, steps into clean room shoes and snaps a nitrile glove onto each hand. "A single flake of skin can destroy a dozen chips," says Rimpf, who heads operations at the Cleanroom Facility Basel in ETH Zurich's Department of Biosystems Science and Engineering (D-BSSE). To enter the clean room, researchers pass through an airlock that is kept at a higher pressure than the surrounding environment. There, they carefully wipe the soles of their shoes on a sticky mat and change into clean room garments. For taking notes, Rimpf provides special paper that doesn't release fibres or particles. This is an environment where dirt, lint and dust are decidedly unwelcome.

Micro and nano worlds

The Cleanroom Facility Basel is housed in the basement of ETH's BSS laboratory and research building on the Schällemätteli campus in Basel. As well as fabricating chips for biotech research, its remit includes the production of tiny sensors and prototypes for new semiconductor components. These are complex processes that require a stable environment - hence the constant temperature of 21 degrees Celsius and 45 percent humidity. One line of research here centres on high-tech chips and the traditional silicon wafers used to produce them - pristine disks of pure, crystalline silicon, slightly larger than a CD, which shimmer like a rainbow in the ambient light. Covering 360 square metres, this clean room features all the equipment necessary to fabricate CMOS microelectronic chips that accommodate millions of tiny transistors. CMOS technology powers the image sensors found in most of today's smartphones, cameras and onboard vehicle systems.

Clad in his coveralls, Rimpf exits the airlock into a corridor connecting eight clean room bays and a number of service rooms. Each bay features a variety of equipment used to process and inspect wafers and chips. Two of the clean room bays are sub-let to the University of Basel, and the facility is also made available to industry partners. In total, the Cleanroom Facility Basel can accommodate around 40 people at a time. The labs are kept as far apart as possible from technical equipment to minimise the risk of contamination during operation or maintenance. Likewise, each bay is kept at a higher pressure than its surroundings to keep out particles and dust. "A cubic metre of city air contains millions of particles," says Rimpf. "Our filter systems cut that to just a few thousand." Fresh air is constantly pumped into the rooms via ceiling filters, with 30,000 cubic metres of air filtered and circulated every hour.

Rimpf's next stop is the lithography bay, where photoresist chemicals are applied to wafers. UV radiation can easily damage these light sensitive coatings - hence the yellow light that illuminates this part of the facility. Set into the benchtop beneath a fume hood is a spin coater, which spreads epoxy-based photoresist onto the wafers in a layer just a few micrometres thick. "Unlike industrial clean rooms, only a handful of our processes are standardised and automated," says Rimpf. "We still do a lot of our work by hand." This demands considerable experience and dexterity. After being spin coated, the wafer is moved to a hot plate to allow the solvent to evaporate. The features required for the specific chip are then cured under UV light inside a cabinet the size of a washing machine. Finally, solvents wash away the excess resist, leaving behind a minuscule pattern of microscale or nanoscale structures on the wafer.

The neighbouring bay houses a dry-etch tool, which uses a plasma of ionised gases to etch ultrafine structures into the silicon wafer, including features with very steep sidewalls and sharp edges. This machine enables researchers to carve out trenches, recesses and electrical structures with extreme precision. "The smaller the features, the more powerful the resulting chip," says Rimpf. The smallest structures fabricated here measure just 500 nanometres across - many times thinner than a single human hair.

Complex and demanding

Marina Marti, a chemical lab assistant by training, joined the team at the Cleanroom Facility Basel a year ago. She previously worked in a clean room for the pharmaceutical industry and is well accustomed to operating under ultra-clean conditions. Her task is to make sure the clean room researchers have everything they need as well as to maintain machinery and dispose of rubbish. This also means ensuring a steady supply of materials, such as gloves, solvents and photoresist. Spending the whole day working in the basement isn't a problem, says Marti: "You quickly get used to it." And since the work is generally quite intense, time passes quickly. For administrative tasks, staff members also have access to a room with daylight.

When it comes to servicing and maintaining the high-tech instruments, the operations team works closely with manufacturers in Switzerland and beyond. "The variety and complexity of these machines is simply incredible," says Marti. Aided by the manufacturers' instruction manuals, training video material and colleague support, she is currently learning to operate and maintain the various systems. "Not a day goes by where I don't learn something new. That's what I love about this job!" she says. Some of the clean room instruments cost well over a million Swiss francs, including a latest generation scanning electron microscope, which produces three-dimensional images at a resolution as low as 20 nanometres. Using this instrument, researchers can analyse every last detail of a wafer's features, making it far easier to spot defects or contamination. "It was just amazing being able to use this piece of equipment for the first time," says Marti.

Despite all this high-tech infrastructure, the Cleanroom Facility Basel is accessible to all the department's students and staff. The only condition is that they attend an induction session and safety training and have some experience in working in a clean room. Rimpf and his colleagues introduce candidates to the basic concepts, show them how to use the different systems and outline the basic safety principles. They also explain how to keep the infrastructure as clean as possible. "Many of them will never have set foot in a clean room quite like this," says Rimpf. "We're their first point of contact and can help them put their ideas into practice." His goal is clear: to use his knowledge to support and inspire the next generation of experts in micro- and nanotechnology.

Chip-based brain cells

"Preserving the clean room's status as a user facility is essential," agrees Andreas Hierlemann, Professor of Biosystems Engineering and D-BSSE delegate responsible for overseeing the clean room. "Researchers need a basic understanding of how a clean room works, but they don't have to be experts in the equipment in order to use it. We're all for encouraging people to go ahead and gain some experience." The same philosophy extends to teaching: during their microtechnology internship, Master's students in biotechnology are able to use the clean room, which gives them their first experience of chip design and fabrication. "That's less common than you might think," says Hierlemann. "Not many universities let their Master's students do work in the clean room."

His own research group designs chips for drug testing. These advanced in-vitro systems can replace animal experiments. The microelectronic chips are seeded with heart or brain cells derived from human adult stem cells cultured in the lab. Using these chips, they can measure the electrical signals the cells produce or stimulate the cells with electrical pulses. Since these signals are characteristically altered by certain diseases, the team can compare the electrical activity of healthy and diseased brain cells on these chips and test whether and how well newly developed drugs work. The work of Hierlemann's group relies on a well-run clean room. "Around half of my group work here on a regular basis, so it's a key facility for our research," he says.

From idea to chip design

Alongside the Cleanroom Facility Basel, ETH also operates two clean rooms in Zurich. The FIRST-Lab facility on the Hönggerberg campus houses a 400-square-metre clean room and is kitted out with instruments primarily tailored for work in physics, electrical engineering and materials science. Its smaller sibling, FIRST-CLA, is located on the ETH Zentrum campus. Consisting of a 250-square-metre clean room, this facility caters to the needs of researchers in mechanical and process engineering. "Without these clean rooms, you would have ten or more groups looking for somewhere else to conduct their research," says Giacomo Scalari, Professor of Quantum Electronics and FIRST-Lab coordinator. "For many researchers at ETH, facilities of this kind are absolutely essential."

Scalari's group is developing new semiconductor lasers, which are used in telecommunications, for example, or for facial recognition in smartphones. These lasers emit different wavelengths depending on which semiconductor materials have been used in the chips. Scalari says his nine doctoral students spend roughly 40 percent of their time in the FIRST-Lab: "They arrive with an idea and a design and leave with a chip prototype. They take this to the lab and test it, optimise the design and then head back to the clean room."

This process generally takes more than one attempt. Multiple iterations are often needed to customise the chip to the required specifications, and the cycle can stretch over months. Some designs require modifications to the equipment itself, so the manufacturer's own technicians are frequently called in to help make fabrication possible. As a result, the clean room equipment is constantly being upgraded. Scalari emphasises the advantage of having the FIRST-Lab just a five-minute walk from his office and his group's lab: "Having it practically on our doorstep means we can put new ideas straight into practice and then start testing them. That's really efficient."

At the same time, says Scalari, the clean room has the knock-on "water-cooler" benefit of sparking conversation between colleagues working on different research questions. "That back-and-forth is tremendously valuable," he says. "A good tip can save months of work!" Like Hierlemann, Scalari emphasises the important role that access to these facilities plays in teaching and the advantage it gives graduates when they enter the job market. "Our students are well versed in using this kind of microtechnology infrastructure," he says. "And that's something industry really appreciates." Most of his graduates are snapped up by the optoelectronics industry, he adds, while others have set up their own spin-off companies, including Versics and SCALE Lasers.

The value of teamwork

Back in the airlock, which separates the clean, controlled and largely dust-free environment from the dirty outside world, Peter Rimpf pulls off his coveralls and gloves. Twenty years have passed since he first set foot in a clean room, back when he began working in microtechnology for ABB. Yet his passion for the precision and teamwork it involves has never flagged. "The smaller and more complex the structures, the broader the range of expertise you require," he says. "Going it alone isn't an option. You need people who can work together."