Beginning in 2019, fabs began to use extreme ultraviolet (EUV) lithography to a limited extent for high-volume manufacturing (HVM) of chips. At that time, ASML’s Twinscan NXE series lithography machines were able to meet the basic production needs of customers, but the entire EUV ecosystem was not ready for everything. One of the factors affecting EUV was the lack of protective films for photomasks (protective pellicles for photomasks), which limits the use of EUV tools and affects yield.
Fortunately, thanks to the recent introduction of production-ready EUV fenders, the pellicle situation has finally improved, and the situation is expected to improve over the next few years.
Having made great strides in recent years with its Twinscan NXE EUV lithography tool, ASML has improved the light source performance, availability time and productivity of EUV lithography machines. Its industry peers have also done a lot of work to enable high-volume manufacturing (HVM) using EUV equipment. Nonetheless, the EUV ecosystem still needs further development. One of the most notorious challenges the semiconductor supply chain faces with EUV is the development of pellicles, which were not available two years ago, which is why TSMC and Samsung Foundry had to invent how to use them Method for EUV scanner without protective film.
A 16nm TSMC Pellicle With Reticle
Pellicle protects 6 x 6 inch photomasks (reticles) during the chip production process by isolating them from particles that may fall on their surfaces, which could otherwise damage them during the process or cause damage during production cause defects to the wafer. Each EUV tool reticle costs $300,000, so chipmakers are desperate to find new ways to protect their wafers from particles and even the EUV radiation itself, as it reduces costs. At the same time, it may be even more important to reduce the risk associated with yields.
Meanwhile, the demand for pellicles varies according to the manufacturer and the type of photomask used. Known for its large CPU dies, Intel tends to use a single die, meaning that just one mask defect introduced by a particle automatically kills the entire die. At the same time, if a 25-die photomask is used, the particle adder will “only” result in a 4% reduction in yield (one dead die), which is why for smaller chips and multi-die photomasks, no guards need to be used The reason why the pellicles can get away with it.
The industry started developing protective films for EUV tools after learning that no one can guarantee that the ultra-complex EUV scanners are 100% free of harmful particles, which is relatively late, which is why they are not yet ready in 2019 reason.
Photomask pods for use with deep ultraviolet (DUV) lithography equipment are common and inexpensive. In contrast, since EUV photomasks are different from DUV photomasks (EUV masks are essentially 250 to 350 nm thick stacks with 40 to 50 alternating layers of silicon and molybdenum on the substrate), Therefore, the protective film of this kind of marking is also very different. In particular, EUV light has a very short wavelength, which means its protective film has many requirements, making it difficult and expensive to produce. The EUV pellicle must be very thin, should not affect the reflective properties of the reticle, should have high transmittance (the higher the transmittance, the higher the productivity of the scanner), should maintain high EUV power levels and withstand extreme temperatures ( from 600°C to 1,000°C in the future).
“Most materials are very absorptive at the 13.5nm EUV wavelength, and even with most EUV-transparent materials chosen, the films would have to be very thin to achieve 90 percent transmission,” said IMEC’s Emily Gallagher. Such films often do not maintain sufficient strength to stand alone at the required dimensions. In addition, the EUV scanner environment is incompatible with many materials, subjecting the pellicle to the action of a pump-exhaust cycle.”
According to SemiEngineering, to date, a number of EUV protective film options have emerged including:
ASML launched its first EUV pellicle in 2019 and licensed the technology to Mitsui Chemicals, which plans to start volume sales in the second quarter of 2021. ASML has since improved the protective film;
Imec has published test results of its carbon nanotube-based pellicle;
Graphene Square, Freudenberg Sealing Technologies (FST) and some universities are developing their own protective films;
So far, only ASML has managed to create a commercially viable protective film for a practical EUV tool. ASML’s pellicle is based on polysilicon with a thickness of 50 nm. Back in 2016, they demonstrated a 78% transmission rate on a simulated 175W light source. Currently, ASML can sell protective films with 88% transmittance. Soon, Mitsui will start supplying such protective films in large quantities.
The latest prototype from ASML, made of metal silicide, demonstrated 90.6% transmittance, 0.2% non-uniformity, and less than 0.005% reflectance under a 400W light source.
“This upgrade supports our roadmap, which will eventually enable power supplies up to 400 watts,” Raymond Maas, ASML’s product manager for pellicle films, told Bits & Chips.nl. “At that power level, the film is heated to 600°C. , and polysilicon can’t afford it.”
In comparison, Imec’s prototype pellicle had a transmittance of 97.7 percent. In fact, in the long run, when more advanced light sources can be used, more complex pellicles will be required, and this is where Imec’s carbon nanotube-based pellicles will come into play.
“Few materials have the potential for high EUV transmittance over 90%, and even fewer are compatible with EUV powers over 600W at the same time. Also, the pellicle must be strong enough to hang over a large area reticle (? 110mm x 140mm),” said researcher Joost Bekaert from Imec.
Unfortunately, it’s unclear when Imec’s carbon nanotube-based shield will be ready.
Now, TSMC and Samsung Foundry have invented a way to use EUV lithography tools to produce multi-die masks for smaller chips without the need for a pellicle, but this approach is risky as any particle additives may become a defect that leads to a decrease in yield. Furthermore, such an approach is risky for larger chips and single-die photomasks, so pellicles are critical for the fabrication of large dies using EUV tools. That said, regardless of the size of the photomask, pellicles are required to increase EUV yield and reduce risk across the field.
Overall, the use and improvement of EUV protective film will be a gradual process. The initial pellicle developed and manufactured by ASML, and soon to be manufactured by Mitsui, is adequate for some of today’s needs, but its transmission levels still have room for improvement, as evidenced by next-generation prototypes from ASML and Imec. Since these machines will have more powerful light sources, better protective films will also be needed for future scanners. However, because such protective films have many indisputable advantages, they will be used by chipmakers because they can help increase yield even at the cost of some productivity.