EUV pellicle is ready for fabs and is expected to improve chip yield and size

Foundries began limited use of extreme ultraviolet (EUV) lithography for mass production (HVM) chips in 2019. At that time, ASML’s Twinscan NXE scanners were good enough for production, but not the full EUV ecosystem. One of the things that affected EUV was the lack of protective pellicle for photomasks. This restricted the use of EUV tools and affected yields. Fortunately, thanks to the recent introduction of EUV pellicle ready for production, the pellicle situation has finally improved.

Precious reticle protection

ASML has made significant strides in recent years with Twinscan NXE EUV lithography tools to improve light source performance, availability and productivity. Other companies in the same industry have done a lot to enable mass production (HVM) using EUV equipment. Still, the EUV ecosystem needs to develop further. One of the most notorious challenges in the semiconductor supply chain facing EUV is the development of pellicle, which was not available two years ago. That’s why TSMC and Samsung Foundry needed to invent a way to use EUV scanners without a protective film.

Reference: 16nm TSMC pellicle with reticle

The pellicle protects a 6 x 6 inch photomask (reticle) by sealing it from particles that may land on the surface during the chip manufacturing process. Otherwise, the pellicle will be damaged and the wafer being manufactured will be defective. Since each EUV tool reticle costs $ 300,000, chip makers are committed to protecting the reticle from damage from particles and the EUV radiation itself to reduce costs. On the other hand, mitigating the risks associated with yields is probably even more important.

The need for pellicle depends on the manufacturer and the type of photomask used. Known for its large CPU dies, Intel tends to use single reticle. That is, only one mask defect introduced by the particles will automatically kill the entire die. On the other hand, when using a 25-die photomask, the particle adder yields a 4% lower yield (one dead die). Therefore, small chips and multi-die photomasks can escape without pellicle.

ASML leading the pack.for now

The industry began developing protective films for EUV tools relatively late after discovering that no one could guarantee that ultra-complex EUV scanners were 100% free of harmful particles. As a result, it wasn’t ready for 2019.

Photomask pellicle used in deep ultraviolet (DUV) lithography equipment is common and inexpensive. In contrast, EUV photomasks are different from DUV photomasks (EUV masks are basically stacks with a thickness of 250-350 nm, with 40-50 alternating layers of silicon and molybdenum on the substrate. ), The pellicle of such a reticle is also completely different. In particular, the very short wavelength of EUV means that EUV pellicle has many requirements that are difficult and expensive to manufacture. EUV pellicle is very thin, does not affect the reflection characteristics of the reticle, has a high transmission speed (the higher the speed, the more productive the scanner is), maintains high EUV power levels and reaches extreme temperatures. Must withstand (600ºC to 1,000ºC in the future).

ASML EUV Pellicle (Image Credit: Semiconductor Engineering)

“Most materials absorb very strongly at the higher energy 13.5nm EUV wavelength. Even if you choose the most EUV permeable material, make the film very thin for the transmittance to approach 90%. We need to, “said Emily Gallagher, a key member of our technical staff. At Imec. “Such thin membranes are usually not strong enough to stand on their own in the required dimensions. In addition, the EUV scanner environment is incompatible with many materials, exposing the pellicle to the pump vent cycle. . “

According to SemiEngineering, many EUV pellicle options have appeared so far.

ASML unveiled its first EUV pellicle in 2019 and licensed the technology to Mitsui Chemicals, which is scheduled to begin mass production in the second quarter of 2021. Since then, ASML has improved the pellicle. Imec publishes test results for carbon nanotube-based pellicle. Graphene Square, Freudenberg Sealing Technologies (FST), and some universities are developing their own pellicle.

So far, only ASML has succeeded in creating a commercially viable pellicle for EUV tools that are actually available. ASML’s pellicle is based on 50 nm thick polysilicon. In 2016, they showed a transmission rate of 78% with a simulated 175W source. ASML can now sell pellicle at 88% transmission speed. And soon, Mitsui will start supplying large quantities of such pellicle.

ASML’s latest prototype, made of metal silicide, shows a transmission rate of 90.6%, a non-uniformity of 0.2%, and a reflectance of less than 0.005% at a 400W light source.

“This upgrade will eventually support a roadmap that requires up to 400 watts of power,” ASML’s Pellicle Product Manager Raymond Mars said in an interview with Bits & Chips.nl. “The pellicle heats up to 600ºC at that power level that polysilicon couldn’t stand.”

In contrast, Imec’s prototype pellicle has a transmittance of 97.7%. In fact, in the long run, the availability of more sophisticated light sources will require more sophisticated pellicle. This is where Imec’s carbon nanotube-based pellicle comes in.

“Few materials have the potential for high EUV transmission over 90%, and at the same time even fewer materials are compatible with EUV output above 600W. In addition, pellicle to hang over a large area of ​​the mask. Needs to be strong (~ 110mm x 140mm), “said Imec researcher Joost Bekaert.

Unfortunately, it is unclear when Imec’s carbon nanotube-based pellicle will be ready for prime time.

Overview

TSMC and Samsung Foundry have invented a method for using EUV lithography tools in multi-die photomasks for small chips without pellicle, as particle adders can be a yield-damaging defect. Is dangerous. In addition, such methods are risky for large chips and single-die photomasks, so pellicle is important to enable EUV tools to create large dies. However, regardless of the size of the photomask, pellicle is needed to improve EUV yield and reduce overall risk.

Overall, the use and improvement of EUV pellicle is a step-by-step process. The early pellicle, developed and manufactured by ASML and soon manufactured by Mitsui, is sufficient for some of today’s needs, but at the level of transparency, as evidenced by the next-generation prototypes developed by ASML and Imec. There is room for improvement. As these machines have more powerful sources, better pellicle is needed to accommodate future scanners. Nonetheless, such pellicle has many indisputable advantages and will be used by chip makers to help improve yields at the expense of some productivity.

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