Opacity of neutral and low ion stages of Sn at the wavelength 13.5 nm used in extreme-ultraviolet lithography

Current research on sources for extreme ultraviolet lithography (EUVL) has converged on the use of discharge or laser produced plasmas containing xenon, tin, or lithium with tin showing by far the most promise. Because of their density, radiation transport from these plasmas is a major issue and accurate photoabsorption cross sections are required for the development of the plasma models needed to optimize conditions for source operation. The relative EUV photoionization cross sections of Sn I through Sn IV have been measured and from a comparison with the results of many body calculations, the cross section has been estimated to be close to 11 Mb in each species at 13.5 nm (91.8 eV), the wavelength of choice for EUVL.

Figure 1

(Color online). Comparison of experimental (solid, black) spectra recorded at time delays (a) $\Delta t=500$, (b) 300, and (c) 60 ns with TDLDA calculations (dash-dot, blue) for the photoabsorption of Sn I, Sn II, and Sn IV. In the case of (c), the TDLDA calculations are shown for Sn IV as this stage appears to be slightly more dominant over Sn III upon analyzing the experimental spectrum. The shaded area in each of the insets is the required 2% bandwidth. Also included in (a) are the results of Haensel et al. (Ref. 35) (dashed, black).

Figure 2

(Color online). Comparison of experimental absolute photoionization cross sections for Xe I (Ref. 37), Xe II–III (Ref. 38), I II–III (Ref. 39), and Cs II (Ref. 40) (dashed, black) with TDLDA calculations (solid, blue) for the respective ion stages.