Shorter wavelength EUV source around 6.X nm by rare-earth plasma

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission. We also have investigated the dependence of the spectral behavior and conversion efficiencies of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6 × 1012 W/cm2 at an operating wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.