18 March 2016 High-radiance LDP source: clean, reliable, and stable EUV source for mask inspection
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Abstract
High-throughput and -resolution actinic mask inspection tools are needed as EUVL begins to enter into volume production phase. To realize such inspection tools, a high-radiance EUV source is necessary. Ushio’s laser-assisted discharge-produced plasma (LDP) source is able to meet industry’s requirements in radiance, cleanliness, stability and reliability. Ushio’s LDP source has shown the peak radiance at plasma of 180 W/mm2/sr and the area-averaged radiance in a 200-μm-diameter circle behind the debris mitigation system of 120 W/mm2/sr. A new version of the debris mitigation system is in testing phase. Its optical transmission was confirmed to be 73 %, which is 4 % lower than that of the previous version and therefore will be improved. Cleanliness of the system is evaluated by exposing Ru mirrors placed behind the debris mitigation system. Ru sputter rate was proven to be sufficiently low as 3~5 nm/Gpulse at 7 kHz, whereas frequency-dependent sputter rate was 1~3 nm/Gpulse at 5~9 kHz as previously reported. Sn deposition remained very low (< 0.05 nm) and did not grow over time. A new technique to suppress debris was tested and preliminary results were promising. Time-of-flight signal of fast ions was completely suppressed and Ru sputter rate of exposed mirrors at 3 kHz was approximately 1.3 nm/Gpulse, whereas the conventional mitigation system (new version) resulted in Ru sputter rate of 0.7 nm/Gpulse. This new technique also allows increasing the radiance efficiency by 30 %. Stability tests were done at several different discharge frequencies. Pulse energy stability was approximately 10 %. Dose energy stability dropped from approximately 2 % to 0.1 % when feedback control was activated. EUV emission position stability was studied at 3 kHz. Deviation of the plasma center of gravity was 6 μm, which is 3 % of plasma diameter and therefore considered to be negligible. Reliability tests were performed on both R and D and prototype machines and up to 200 hours of non-interrupted operation was demonstrated.
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Yusuke Teramoto, Yusuke Teramoto, Bárbara Santos, Bárbara Santos, Guido Mertens, Guido Mertens, Ralf Kops, Ralf Kops, Margarete Kops, Margarete Kops, Alexander von Wezyk, Alexander von Wezyk, Klaus Bergmann, Klaus Bergmann, Hironobu Yabuta, Hironobu Yabuta, Akihisa Nagano, Akihisa Nagano, Noritaka Ashizawa, Noritaka Ashizawa, Yuta Taniguchi, Yuta Taniguchi, Takahiro Shirai, Takahiro Shirai, Kiyotada Nakamura, Kiyotada Nakamura, Kazuya Aoki, Kazuya Aoki, Kunihiko Kasama, Kunihiko Kasama, "High-radiance LDP source: clean, reliable, and stable EUV source for mask inspection", Proc. SPIE 9776, Extreme Ultraviolet (EUV) Lithography VII, 97760L (18 March 2016); doi: 10.1117/12.2219219; https://doi.org/10.1117/12.2219219
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