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Chapter 5:
Atomic Physics of Highly Charged Ions and the Case for Sn as a Source Material
Projection lithography schemes based on 13.5-nm multilayer mirrors (MLMs) have been under intense scrutiny since the mid-1990s. Stearns and coworkers first demonstrated a reflectivity of 61% at this wavelength using Mo∕Si multilayer optics, a figure that has been improved in the intervening period to a present value close to 72%. Ceglio and Hawryluk proposed that a suitable scheme would require a 1% conversion efficiency (CE) of incident laser pulse energy to soft x rays in a 0.3-nm band centered at this peak. Kauffman et al. attained this figure using 7.5-ns, 300-mJ frequency-doubled Nd:YAG pulses focused to a power density of 2 × 1011 W cm−2 onto a tin target. This figure has now been revised upwards because of the increased costs of producing a suitable exposure tool, to better than 3% CE into 2% bandwidth, because this is necessary for the successful commercialization of such a source. Because of the debris problems associated with the use of tin, other target materials were sought with strong emission in the 13–14-nm region. Jin and Richardson used mass-limited water ice targets that emitted O VI lines near 13 nm to limit the debris. Shevelko et al. subsequently undertook an extensive study of the spectra of a large number of elements from LPPs with a KrF excimer laser focused to a power density of 1012 W cm−2 onto planar targets, including tin, and found that the maximum intensity was in fact obtained for Ge and Re targets under these conditions. However, all of their work was with solid targets that produced significant debris in the form of both ions and particulates of various sizes. The need to reduce particulate emission led to the choice of xenon, which as an inert gas should provide a debris-free source. There is a line group in the spectrum of xenon that has been shown by a number of researchers to arise from 4d8-4d7 5p transitions in Xe XI. These lines have recently been measured to an accuracy of 0.01 nm by Churilov et al., who have identified all of the lines in this range by comparison with atomic structure calculations using the suite of codes developed by Cowan. The population of this ion stage needs to be optimized to attain maximum intensity. Note that, depending on the type of source and hence ion density, optimization is not always necessarily the same as maximizing a given ion population because of opacity effects.
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