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Chapter 6a:
Optics Contamination
Extreme ultraviolet lithography (EUVL), using a 13.5-nm wavelength, is likely to be the most promising next-generation lithographic (NGL) technique. This technology, which is a natural extension of optical lithography, seems superficially very similar to that technique. However, shorter wavelengths, which enable printing of much smaller features, also create new technological challenges. Radiation at these short wavelengths is strongly absorbed by any matter. For this reason, reflective rather than transmissive optics must be used, and the entire EUVL system must be maintained in a vacuum environment. Reflective optics consist of precisely figured substrates coated with alternating layers of molybdenum (Mo) and silicon (Si) (Fig. 6A.1). Such multilayer (ML) coatings, invented by Spiller, enable high reflectivity and wavelength selectivity of EUV mirrors. Residual water and other contaminants in the presence of EUV photons oxidize and degrade the optics surface. Both oxidation and carbon deposition on the optical surfaces reduce the reflectivity of the optics and can introduce wavefront aberrations. The overall effect not only decreases the throughput of the exposure tool but also the printing uniformity.
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