Some EUV light sources have been promoted that use LPPs, and some that use DPPs (see, for example, Ref. 1). Although each proposed source has significant advantages, the LPP offers an especially attractive prospective application range due to advances in laser technology and tunability of the source emission by target-material selection. The LPP EUV light source also has the advantages of small source size, due to the focusability of the laser, and less debris in that the target is isolated from the energy driver. Many of the sources under consideration use 13.5-nm radiation from multicharged xenon, tin, and other low-Z ions, because Mo∕Si multilayer mirrors have high reflectivity at this wavelength.
One key requirement in the source development is a high conversion efficiency (CE) from the incident laser energy on the target to 13.5-nm EUV emission energy in a 2% bandwidth over 2π sr. Figure 11.1(a) summarizes recent experimental values of the CE for various types of targets, such as the xenon liquid jet, xenon cluster jet, cryogenic xenon, frozen xenon, solid tin, and liquid tin jet. A CE of a few percent is achieved for tin, and a relatively small CE of ≤ 1% is obtained for xenon.
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