Coating the most precise mirrors in the world

Based in Jena, the historic capital of the optics industry in Germany, optiX fab carries out its extraordinary work in an ordinary-looking old factory building. But looks can be deceptive. Thanks to the building’s history, it is the perfect place for Dr. Feigl and his team to work. Built in 1934 by Zeiss, the German manufacturer of optical systems and optoelectronics, it is one of the most stable industrial buildings in Germany. Each square meter can bear a load of 3.5 tons with no vibrations – nothing moves.

These are exactly the conditions optiX fab needs to house and operate its four NESSY coating machines, provided by Bühler. The NESSY is a magnetron sputtering system which optiX fab uses to coat the mirrors for EUV lithography. Each of the 12-ton machines fits neatly into the spaces between the pillars of the old building and can operate faultlessly for hours on end. “It is almost as if Zeiss knew back then what we would need to carry out EUV lithography today,” says Dr. Feigl with a smile.

optiX fab is a spin off from the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF). Dr. Feigl had been at the head of a working group at IOF that made extreme ultraviolet (EUV) coatings for the semiconductor industry and had already worked with NESSY, the first of which was delivered to IOF by Leybold Optics (now Bühler) in 2003.

The machine was developed by the two teams working together to meet the high requirements of the rapidly changing technology and market. As demand grew for chips that were faster, with more memory and more power at the same size, the industry began using light at shorter wavelengths to print smaller structures on the chips, shifting from 248 nanometers (nm) to 193nm.

“By the mid-1990s, it was not clear what technology the next generation of chip would require,” says Dr. Feigl. “That was when the industry decided to explore EUV – extreme ultraviolet.” There were several challenges with using this new wavelength. This meant a major shift in the hardware required – and NESSY was the result. In 2013, Dr. Feigl decided to take the technology and commercialize it. 

As the light passes through each layer, due to absorption it is reflected less and less the lower the layer – as little as 0.1% . The expertise is in building up the layers in such a way that together they create sufficient reflectance. The key to this is using the right materials, at the right thickness, in the right places – and this is where single-digit picometer accuracy is required. “We have to develop a coating recipe for each mirror. That’s a process that can take anything up to 2 years. When we get it right, we are able to achieve over 70% reflectance,” says Dr. Feigl.

The largest mirror that optiX fab has coated to date is a 662-millimetre diameter collector. Weighing 40 kilograms, it took a year to develop the coating recipe. There were 300 layers on the substrate. The angle of incidence varied from 6 degrees at the center to 36.5 degrees at the edge, requiring a variation in thickness of the layers from 6.883 nm to 8.459 nm – all at an accuracy of less than 10 picometers. 

Today they achieve 1, 2, or 3 picometers accuracy. For a recent customer from Ireland, the team worked with wavelengths of 2.745 nm. This meant building up 2,000 layers that were between 0.1 nm and 0.6 nm thick each. One of the key parameters was to ensure that the first layer in the multilayer stack has exactly the same thickness as the last one. “The machine needs to be stable over 12 hours to achieve this. With NESSY, Bühler made that happen. It is an engineering masterpiece,” says Dr. Feigl. 

For Klaus Herbig, head of the precision optics market segment at Bühler, it is still astonishing what the optiX fab team achieves – even though he has worked with them since the beginning. “Their technology input is at the physical edge,” he says. “We know how to build the machine that does the coating with unmatched levels of layer precision. They know how to combine the different materials and to keep them stable over time. It is not easy. They have to understand the process well to do it properly. The success relies on their skill.” 

In the old Zeiss building, optiX fab has installed the four NESSYs in a row in the machine room. A load lock separates the machine room from the clean room. The process starts in the clean room, where the substrate to be coated is put in a “flow box”. In the clean room, there is next to no dust or vibration. In the flow box, the air is even cleaner than in the rest of the clean room. The substrate is installed in the coating holder, then transferred to the load lock using so-called “touchless tools”. In the load lock, the air pressure is pumped down and the substrate is then transferred to the coating chamber of NESSY, which works under ultra-high vacuum conditions.

Today it is an established part of the industry, but it was a big step to take at the beginning. “When I placed my first order for a NESSY from Bühler, we had no long-term contracts. Now we have four NESSYs and demand is growing so much we will soon need another,” says Dr. Feigl.  As optiX fab celebrates its 10th anniversary, the need for connectivity between people and things continues to grow. Furthermore, to many people’s surprise, Moore’s law shows no signs of stopping – the number of transistors per chip continues to grow, and the race goes on to produce the smallest and most efficient microchips in the world. 

“It is still a growing market. With e-mobility and autonomous vehicles, the requirement for advanced chips will skyrocket. And with the growth of artificial intelligence, the market is far from saturated.” Herbig agrees. “To exploit many of these opportunities, we need computer chips with ever greater capacity. The need for EUV lithography is set to grow, and Bühler and optiX fab are driving in the same direction,” he says.

The optiX fab team has the expertise and the experience to achieve this, but Dr. Feigl says they could not do it without NESSY. “Our brain power and NESSY together create the outcome. We are like Michelin star chefs working with top-notch kitchen equipment – that is what NESSY is for us.”