Sources of EUV Light

In Chapter 1, it was explained that the wavelength of light at which EUV lithography is practiced was chosen largely by the availability of multilayers with high reflectance. This can be contrasted with the situation in optical lithography, which has always been operated at wavelengths where there are strong sources of light with narrow bandwidths. The result is that EUV lithography has suffered from light sources that are relatively weak, and a great deal of R&D has therefore gone into the development of EUV light sources in order to improve the situation. Several methods have been used to generate EUV light. Early researchers used synchrotron light sources, but smaller light sources became needed as a practical matter as EUV lithography moved from research to development. This was true for metrology applications as well as for exposure tools, and stand-alone systems provided greater flexibility for development pilot lines. Moreover, it was eventually determined that synchrotrons of a practical size could not provide sufficient EUV light to be cost-effective, even with the use of undulators or wigglers. The EUV source used currently in manufacturing tools, the laser-produced plasma (LPP) source, is discussed in this chapter, and a source that might be used in the future for wafer exposures, the freeelectron laser (FEL), will also be described. Plasma-based sources used in a number of EUV metrology tools will also be described.