A short wavelength light source for lithography using a regenerative liquid metal bismuth target

We demonstrated a water-window (WW) soft x-ray (SXR) source using a regenerative liquid bismuth target irradiated by a solid-state laser. A tank filled with the solid Bi particles was heated by a band heater to make liquid Bi, and then it was pushed out from the nozzle by the nitrogen gas. A 1064-nm Nd:YAG laser with a pulse duration of 150 ps and a repetition rate of 10 Hz was irradiated to the liquid Bi target. We observed the time-integrated spectrum of SXR with a wavelength shorter than 6 nm using a spectroscopy and energy spectrum of the suprathermal ions emitted from the WWSXR source using a Faraday cup. The number of photons was observed to be about 1 ×10¹³ photons/nm·sr·pulse at a peak wavelength of 3.9 and 4.2 nm, and 0.4 ×10¹³ photons/nm·sr·pulse at a peak wavelength of 2.4 nm. The total number of photons emitted in 2.3 – 4.4 nm was about 1×10¹³ photons/sr·pulse. Suprathermal ions were also emitted with a maximum energy of 140 keV from the hot, dense Bi plasma. These results have the potential to use a short wavelength light source for next-generation lithography systems.