Extreme ultraviolet lithography
Extreme ultraviolet lithography (also known as EUV or EUVL) is a next-generation lithography technology using an extreme ultraviolet (EUV) wavelength, currently expected to be 13.5 nm. EUV is currently being developed for high volume use by 2020. The primary EUV tool maker, ASML, projects EUV at the 5 nm node will require a higher numerical aperture than currently available and multiple patterning to a greater degree than immersion lithography at 20 nm node.Immersion lithography is still at least three times faster than EUV, due to EUV source power limitations. As of 2016, a 125W source enables a wafer throughput of 85 WPH at a dose of 20 mJ/cm2; for reference, the leading immersion lithography tool runs at 275 WPH. If the target dose were doubled from 20 to 40 mJ/cm2, the tool stage would have to be slowed to accumulate more energy per unit area, resulting in half the WPH, in which case the EUV tool would be more than six times slower than immersion lithography; conversely, a 250W source would be needed to maintain 85 WPH at 40 mJ/cm2. Current throughput at customer site is 1,200 wafers per day with 80% availability, while conventional tools produce 5,000 wafers per day with 95% availability. Hence, multiple patterning with immersion lithography has been deployed for volume manufacturing, while deployment of EUV is expected in 2018–2020.
While source power is the chief concern due to its impact on productivity, significant changes in EUV mask infrastructure, including blanks, pellicles and inspection, are also under study. Particle contamination would be prohibitive if pellicles were not stable above 200 W, i.e., the targeted power for manufacturing. Without pellicles, particle adders would reduce yield, which has not been an issue for conventional optical lithography with 193 nm light and pellicles. The current lack of any suitable pellicle material, aggravated by the use of hydrogen plasma cleaning in the EUV scanner, presents an obstacle to volume production. Even with a pellicle, the hydrogen interacts with Sn in the light source or resist to form SnH4 which reaches the coatings of the EUV optical surfaces, leaving Sn which is subsequently unremovable. Hydrogen also reacts with metal-containing compounds to reduce them to metal, and/or diffuses through to the multilayer, eventually causing blistering. Hydrogen also reacts with resists to etch or decompose them.
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