Extreme Ultraviolet Spectroscopy of a Laser Plasma Source for Lithography

Spectra from various target materials from a KrF-laser plasma source have been investigated in the Extreme UV spectral range between 12 and 17nm using an off-Rowland grazing-incidence spectrograph. The electron temperature T e and mean ionization stages Z have been measured to amount to T e = 80eV and Z = 10–12, respectively. Additional calibration and measurement of the spatial and temporal characteristics of the plasma has been done using a combination of a multilayer mirror and XUV diode or fiber image carrier system. The maximum yield at 13.5nm (2.4–3 × 10 15 phot/srnm) has been observed for targets with atomic number Z a = 32, 50, 73–75, of which the radiative transitions have been identified as 3p–3d, 4d–4f and 4f–5d transitions respectively. The corresponding maximum conversion efficiency at 13.5nm was 0.43% in a 1% bandwidth. An option for further optimization of the KrF laser plasma source for application in EUV lithography is discussed.