20 years of EUV lithography innovation
acknowledged by European Patent Office
The European Patent Office (EPO) recently recognized the groundbreaking R&D undertaken by ASML and its partners over the past 20 years. This article reflects on some of the most notable highlights and one extra-special ‘eureka moment’.
Recognition of the innovation came in the form of the European Inventor Award, Popular Prize 2018. It was presented by the EPO to the two main EUV team leaders at ASML, physicist Vadim Banine and engineer Erik Loopstra. The technology of EUV (extreme ultraviolet) lithography is central to the next generation of systems that will be used to manufacture tomorrow’s microchips, and it represents a major advance in the continued miniaturization of these essential components.
The EPO award comes at a time when EUV is on the verge of becoming a proven high-volume manufacturing tool. But it wasn’t always certain that this next-generation lithography system could be realized. In fact, 20 years ago when EUV development began, many scientists thought that the challenges would be too difficult to overcome. Radical improvements would be required in areas such as control stability, microscopic debris mitigation, power scaling, conversion efficiency, and more. That’s why Vadim and Erik, supported by their teams, worked together with world-leading external partners and research organizations. They all continuously pushed the boundaries of technology, and over the years ASML registered thousands of patents, more than 1,000 of which were attributed to Erik and Vadim. We asked these two team leaders to reflect on some of the most significant areas of advancement.
Ancient sword sparks eureka moment
“The vast majority of innovation is a result of inspired thinking over months and even years,” says Vadim. “But there was one classic eureka moment, which came in 2011 while I was watching a TV documentary about Japanese samurai swords. At that time we were trying to develop a thin membrane for a contamination control system. The membrane had to be really thin — just 30 nanometers (nm) in fact. That’s very thin: the diameter of a human hair is around 1,000 times thicker. The membrane had to be thin but also strong and robust. For months we tried different materials and techniques, and after many attempts the chances of success were looking increasingly unlikely. That’s when I saw the TV documentary. It showed how ancient Japanese swordsmiths would repeatedly fold sheet steel to create many layers. This made the metal extremely rigid, perfect for a thin but very strong samurai sword. Before the documentary was even finished I was already thinking about how we could do something similar with our membrane but then on the nanometer scale. Within weeks, together with a research institute, we had made it.”
The sword-inspired membrane was used in a system for ‘debris mitigation’, which in more simple terms means keeping the machine clean. This is a particularly challenging objective because within the closed high vacuum chamber not only are 50,000 droplets of molten tin vaporized every second, there are also dozens of moving mechanical parts, including a 200 kg platform that accelerates faster than a fighter jet — and they all produce microscopic dust. The membrane was part of a removable pellicle placed in front of the lithographic mask. It allowed EUV radiation to pass but blocked contamination particles. This was indeed a major triumph because there are so many different sizes of particle produced in the EUV chamber from 50 to 1,000 nm, and all moving at different speeds, some moving as slow as a few millimeters per second and other racing at hundreds of meters per second. Today, seven years later, the patented removable pellicle with its samurai sword-inspired membrane remains a powerful example of inspired innovation that has been the recurring theme of EUV development at ASML over the past 20 years.
10^5 more powerful
“It’s not all eureka moments though,” says Erik. “But many of the advancements can be summed up in terms of the scale of improvement. For example, at the very beginning of EUV, we could only produce radiation at the required 13.5 nm wavelength in very small amounts — less than one milliwatt in fact. Today, after a myriad of incremental improvements, ASML EUV machines now generate the required 250 W of EUV source radiation, which is a scaled improvement of ten to the power five.”
10^8 longer life
An even more dramatic scale of improvement was made in extending the life of the collector mirror, which is a concave reflector that concentrates the bursts of EUV radiation into one focused beam. This device must be placed to within 20 cm of the high-power laser beam and the resulting tin plasma. With 50,000 molten tin droplets being vaporized every second, the resulting microscopic debris would render the collector mirror useless within a hundredth of a second. Thankfully, after countless different breakthroughs to manage that debris, today’s collector mirrors remain operational for more than 1,000 hours. That’s a ten to the power eight of scaled improvement.
Another highly notable improvement was the ingenious solution to stop unwanted infrared (IR) radiation from reaching the reticle. This IR radiation is a by-product of the tin plasma, and if it were not blocked, it would heat the reticle and negatively affect performance.
At first, antireflection filters were developed and tested, but none of them could block IR and simultaneously allow EUV radiation to pass through efficiently. That’s when the team had the idea of using a multilayered antireflection grid-like coating on the collector mirror. A multilayer coating would diffract the IR at a different angle than the EUV source, and with a different trajectory it could be blocked using ordinary filters. Some people thought at first that this was too ambitious, but with plenty of creative thinking the multilayered coating was successfully engineered and the problem of IR was solved.
All this innovation and more has now been honored by the EPO award. On accepting the award at a special ceremony in Paris, Erik Loopstra said, “We are proud to receive this prestigious award by popular vote. We accept it on behalf of the thousands of researchers, scientists and engineers who worked tirelessly to bring EUV lithography to life.”
13,000 patents and still counting
ASML was founded in 1984, and has always invested strongly in R&D, even during the downturn of 2008. As result, it is today a leading supplier of lithography systems to the global semiconductor industry and holds more than 13,000 patents.