Spectral characterization of an industrial EUV light source for nanolithography

The emission spectra from an industrial, droplet-based, laser-produced plasma, extreme ultraviolet light source for nanolithography are here presented and analyzed. The dependence of spectral features on the CO2-drive-laser intensity is studied by changing the beam spot size at constant pulse energy and duration. We characterize the spectrum by fitting the results of atomic structure calculations to the short-wavelength region (7-11 nm), where the contributions from various charge states can be resolved, and obtain the relative contributions of charge states Sn9+-Sn15+. These relative contributions are compared to charge state populations as calculated with the non-equilibrium plasma kinetics code FLYCHK. The calculations are shown to be in good qualitative agreement with the results, showing that the effective plasma temperature, and with it, the shape of the unresolved, main emission feature at 13.5 nm, is a remarkably weak function of laser intensity under this source normal operating conditions.