Feasibility study on reactive ion etching occurrence in EUV-induced photoionized hydrogen plasmas based on electron temperature and electron density measurements

Temporal evolutions of electron temperature (Te) and electron density (ne) of photoionized hydrogen plasmas, which were induced by radiation from laser-produced Sn-plasma EUV sources, were measured using the laser Thomson scattering technique. Measured Te and ne ranged from 0.5–2.5 eV to 10¹⁶–10¹⁸ m⁻³, respectively, for hydrogen pressures of 50–400 Pa. The Te of this EUV-induced hydrogen plasma decayed with the thermal relaxation time between electrons and gases. The maximum value of Te in the time variation depended on hydrogen pressure. The lower the pressure, the higher the maximum Te, and it reached approximately 2 eV at 50 Pa. The sheath potential between the EUV-induced hydrogen plasma and the unbiased wall might be exceeded 6 eV at 50 Pa, which is sufficient to enhance the removal of Sn-debris from a Mo/Si multilayer mirror via reactive ion etching processes.