Phatal187

EUV scanners gobble up hydrogen. With the first high-NA EUV system shipped, we are starting to see more pictures of what an assembled system looks like. No, I'm not talking about a big white box we were used to seeing inside fabs or CAD cartoon drawings, but real pictures of the inner workings of the most complex commercial machine ever created. Here we have a picture of the backside of the EXE:5000 demo tool in Veldhoven, which will serve as the R&D tool for much of the world that is a paying contributor to imec's semiconductor consortium. I highlight the major tool components many of us are familiar with, notably what will become the three most expensive components for any tool in the fab: the reticle stage (upper), the wafer stages (lower), and between the two, the EUV optics, which consist of nine multi-layer mirrors. What jumps out when looking at this complex machine are the three large gas canisters near the optics. What are these for? Well, the short list of experts who would know the specific mechanical details of this machine in the world is quite few, and I'm not one of them, but if I were to speculate with an educated guess, these would be cylinders of hydrogen gas. But why would large canisters like this need to be incorporated into the tool itself? As it turns out, EUV scanners are very unique in terms of their consumption of hydrogen gas. Most fab tools utilize hydrogen gas, such as epitaxy, deposition, plasma etch, annealing, passivation, and ion implants. These all consume H2 on the order of 100 SCCM. A single EUV scanner, however, requires 100~1000X this amount in SLM quantities (standard liters per minute). The reason H2 is so important for EUV is because EUV photons absorb most gases except hydrogen, which has a very high EUV transmission. This makes it a useful gas for use in these systems. So how is H2 used for EUV? The biggest part of the machine that needs huge amounts of H2 gas is the source. It's used as a shroud to protect the collector mirror from tin contamination. EUV sources work by vaporizing a stream of tiny droplets of tin with a CO2 laser; this creates a plasma that emits EUV light. However, this is a dirty process, so the tin vapor and aberrant droplets are reacted with a high flow rate of H2 to form stannane (SnH4), which is then removed with a vacuum line. H2 can also be used to form plasma that cleans the collector mirror, prolonging its life. So why do we need H2 near the optics and the upper and lower stages? It's often stated that the EUV light path operates in a vacuum. This is not exactly true. It's actually an ambient of very low pressure, and that ambient is H2 gas due to its high transmission properties with EUV light. One interesting aspect of this is that EUV light strikes a H2 plasma throughout the lightpath, and the plasma dynamics are pretty complex. Ok, so why operate this machine with what amounts to a giant H2 plasma? Well, it serves to maintain the purity of this vacuum environment much better than a pure vacuum. By flushing the light path, including the wafer area and reticle, with H2 gas, any impurities that may arise from running the machine 24 hours a day, 7 days a week, are immediately mitigated. It reacts with any stray carbon coming from the photoresist on the wafer stage, forming a gas that can be evacuated. Any stray tin entering the middle section is also reacted to in the same way the collector mirror is maintained. On the reticle stage, pure H2 gas is flowed across the reticle and acts as a gas curtain, preventing any particles from landing on the mask. Furthermore, the high-energy EUV photons can cause the optics to oxidize, which would degrade their performance over time. Hydrogen acts as a reducing agent that helps minimize this oxidation. Altogether, hydrogen extends the lifetime of the mirrors used in the EUV system. This is crucial because the mirrors are extremely expensive and difficult to manufacture and replace. So I'll speculate a bit here as to why hydrogen canisters would be held locally inside the machine instead of just relying on the house supply. I would guess this is because the entire purpose of it is to maintain the purity of the vacuum environment, unlike most other unit process tooling. Another level of purification is performed on the house supply, and this is brought into the tool and stored in these canisters. And there you have it: EUV scanners have a voracious appetite for hydrogen gas. This extends the lifetime of the expensive optics and other components of the machine, enabling Moore's law to continue marching on. @dylan522p