CO2 Laser Produced Tin Plasma Light Source as the Solution for EUV Lithography

High average power EUV light source has been the “most critical” issue in the research and development of the EUV lithography system in one decade. EUV LLC and International Sematech significantly stimulated the global research community to work seriously to advance plasma technology in achieving the goal of the EUV source, required by the semiconductor industry. It is instructive to look into the EUV lithography source workshop held in October 2001 in Matsue, Japan. MEDEA+ project consortium “EUV source” had started already in June 2001 in Europe including many public and private research organizations (Stamm, 2002). EUV source workshop was organized in Japan several times by ASET in 2000-2001 to evaluate the technical possibility to develop the required EUV source (Okazaki, 2001), which was then succeeded by EUVA project in 2002. ASML and NIKON talked in the Matsue workshop as the required EUV power was more than 80W with faster than 5 kHz repetition rate, assuming the resist sensitivity as 2mJ/cm2. The requirement came from 80 wafers/hour throughput. The wavelength was confirmed at 13.5nm to optimize to the peak reflectivity of Mo/Si coated mirrors. This caused a serious concern on the scaling of the once established method based on Nd:YAG laser irradiated xenon gas plasma, in which the peak conversion efficiency was at 11nm. Nd:YAG laser irradiation of a gas puff target was a typical laboratory method in laser applications like higher harmonics generation, short wavelength generation, fast ion generation and so on (e.g., Fiedorowicz, 1999). TRW had been working closely with EUV LLC from 1997 to develop the first generation EUV light source, based on Nd:YAG laser irradiated xenon gas puff plasma. The obtained conversion efficiency was around 0.2%, with laser power of 500W by a single beam (Ballard, 2002). It was suggested to increase the density of xenon at longer distance from xenon nozzle to improve the conversion efficiency, together with higher laser power of better beam quality. It seemed still possible to work following the proposed direction, but experiments showed practical limitations around this approach. The author discusses in the following sections on each limitation in the history of the EUV source development, and describes each obtained solution, overcoming the limitation to realize the required source performance finally. The major work was conducted during the project of “Extreme Ultraviolet Lithography System Development Association (EUVA)” in Japan from 2002 to 2009.