Effect of charged-particle bombardment on collector mirror reflectivity in EUV lithography devices

EUV metallic light radiators such as Sn or Li used for lithography will limit the lifetime of collector optics in source devices by both contamination and irradiation. Generation of EUV light requires the use of hot, dense plasma. Pinch dynamics generates fast ions and atoms, such as metallic sources (Sn, Li) with energies ranging from 100 eV up to several keV. The expanding Sn plasma will thermalize and condense in nearby components, including the debris shield and collector optics. The incident distribution of debris onto the collector optics will likely include Sn fast ions. Sn contamination will lead to two different mechanisms. One is condensation and Sn thin-film buildup on the reflective optics surface (i.e., Ru or Pd mirror) from the thermalized Sn plasma. This mechanism will lead to performance failure after about 1-2 nm build up of Sn thin film whereby the at-wavelength EUV reflectivity will decrease 20% in magnitude for grazing incident angles less than 20-degrees. The second mechanism is more complex. Fast Sn ions generated at the pinch will reach the collector optics and induce mixing, sputtering, and implant at depths between 3 and 5 monolayers on the Ru or Pd surface. EUV light can also induce ionization in background Ar or He gas used for debris mitigation. Low-energy Ar or He ions therefore impinge on the collector mirror surface at threshold-level energies between 40 and 100 eV. A steady-state Sn surface concentration will be attained after a given fluence of both Sn debris and low-energy Ar ions. The amount of Sn implanted or deposited will affect EUV reflectivity as a function of ion and/or atom fluence. Sn contamination mechanisms, as well as threshold-level sputtering from inert ion species, are studied in the IMPACT (Interaction of Materials with charged Particles and Components Testing) experiment. Sn exposure conditions include incident singly charged particles between 500 and 1000 eV, oblique incidence and incident fluxes ranging from 10 ¹¹ to 10 ¹⁴ ions/cm ²/s. In-situ surface metrology includes sputter yield diagnosis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, direct recoil spectroscopy and low-energy ion scattering spectroscopy, and at-wavelength EUV reflectivity.