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For more than a decade, research on plasma light sources in the EUV spectral range has been the subject of intense efforts all over the world. This extensive work is motivated by the planned 2009 introduction of EUVL to high-volume chip manufacturing, operating at a wavelength of 13.5 nm. The scientific and technical challenges are indeed tremendous in achieving sources that are both powerful and clean and that also present very small source spots of less than 1 mm in diameter. Since the start of the development of EUV sources in the early 1990s, the requirements in terms of EUV power needed by the industrial lithography tools have been raised from a few watts to more than 100 W, and the lifetime of the critical components such as the collector optics must today exceed 1011 shots. From an industrial point of view, EUV sources must present very high conversion efficiencies (CEs) between the incoming exciting power of the emitter plasma and the outcoming EUV power in order to keep the heat load of the system at an acceptable level. Other important constraints are linked to the economic viability of the proposed source concepts and to their adaptability to run at high repetition rates with exceptional reliability.
Two main families of EUV sources are currently developed for industrial EUVL tools, which mainly differ in the plasma-exciting mechanisms employed. On one hand, DPP sources use electrical sparking in a low-density gas target to achieve EUV generation. On the other hand, LPP sources use very dense targets that are irradiated by tightly focused laser light. Each of the two concepts presents its own advantages and drawbacks, which will be discussed later, but it is clear that so far neither has yet reached the required specifications, in particular with regard to EUV power and critical component lifetime. Actually, tremendous efforts are still necessary to overcome these important challenges, and the risk of failure is not negligible, since the unavoidable time-to-market law governs the introduction of innovative technologies into industrial high-volume production.
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