Although plasmas produced by lasers have been the subject of intense study over the past 40 years, those that are being considered as sources for EUVL offer new challenges in both design and understanding. The requirements of these sources are unique relative to other interests and applications of laser plasmas. EUVL demands a source that emits strong radiation only in one radiation band. The plasma must be small and be stable in position and light emission. It must be a high-repetition-rate source to satisfy dose stability. Moreover, strong consideration must be given to constraining the particulate and ion emission from the source: protecting the EUV illumination optics from degradation or erosion for long exposure times is one of the principal challenges. This combination of requirements forces consideration of features of laser plasmas outside the focus of most studies. They pose new challenges in the understanding, characterization, and design of laser plasmas.
This chapter addresses one of these challenges. The EUV roadmap requires a bright source in a narrow spectral band in the vicinity of a 13-nm wavelength. This wavelength and the width of the spectral band (≈0.26 nm) are set by the characteristics of the collection and imaging optics employed. The brightness required, which can be expressed in several ways [115 W of inband radiation at ≈13.5 nm, 2% bandwidth, at the intermediate focus (IF), or perhaps, more fundamentally, ≈400 W of inband radiation within 2πsr at the source], challenges the limits of laser and laser-plasma technology. Laser plasmas emit radiation that is both characteristic of the plasma conditions and characteristic of the excited ion states created in the plasma. In satisfying the needs for a source, our challenge from a theoretical viewpoint is to understand and model this emission, at least under conditions that are likely to satisfy the intense brightness requirement in a practical system.
It is for these reasons that in this chapter we concentrate on laser plasmas that have characteristics conducive to the creation of high-brightness, high-repetition-rate point EUV sources. We focus on the theory of laser-light interaction, plasma expansion, and radiant emission, seeking plasmas that are most likely to meet these needs. Thus we consider here spherical, 1D models of laser plasmas dominated by inverse bremsstrahlung absorption (IBA), thermal excitation, and classical energy transport. We focus on plasmas formed from ions that have strong emission lines in the 13-nm region. There are several candidates, including Li, O, Xe, Sn, and others. Low-Z materials such as Li and O have been examined in previous studies, and are now thought to be incapable of providing sufficient emission, or to be too impractical as a source. Here we examine mostly emission from so-called unresolved transition arrays (UTAs), tightly packed emission lines from multitudes of excited-state transitions in multiple ions in materials such as Xe and Sn. Theoretically this is a very challenging problem. Not only does it require a thorough understanding of the plasma state, but more important, and much more difficult to model, is the understanding of the detailed radiation transfer from all these transitions. To our knowledge this has not yet been accomplished completely. Thus, this chapter is a report on progress toward this end.
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