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Exposure of collector mirrors facing the hot, dense pinch plasma in plasma-based EUV light sources remains one of the
highest critical issues of source component lifetime and commercial feasibility of EUV lithography technology. Studies
at Argonne have focused on understanding the underlying mechanisms that hinder collector mirror performance under
Sn exposure and developing methods to mitigate them. Both Sn ion irradiation and thermal evaporation exposes
candidate mirrors tested (i.e., Ru, Rh and Pd) in the experimental facility known as IMPACT (Interaction of Materials
with charged Particles and Components Testing). Studies have led to an understanding of how Sn energetic ions
compared to Sn thermal atoms affect three main surface properties of the collector mirror: 1) surface chemical state, 2)
surface structure and 3) surface morphology. All these properties are crucial in understanding how collector mirrors
will respond to Sn-based EUV source operation. This is primarily due to the correlation of how variation in these
properties affects the reflectivity of photons in the EUV spectral range of interest (in-band 13.5-nm). This paper
discusses the first property and its impact on 13.5-nm reflectivity.
Investigation in the IMPACT experiment has focused on Sn thermal and energetic particle exposure on collector mirrors
(Ru, Pd and Rh) and its effect on mirror performance as a function of incident thermal flux, incident ion flux, incident
angle and temperature. This is possible by a new state-of-the-art in-situ EUV reflectometry system that measures real
time relative EUV reflectivity at 15-degree incidence and 13.5-nm during Sn exposure. These results are then compared
to at-wavelength EUV reflectivity measurements using the newly upgraded NIST-SURF facility. Sn energetic ions at 1-
keV and fluxes of about 1013 cm-2s-1 are used in conjunction with a moderate flux Sn evaporative source delivering Sn
fluences ranging from 1015-1017 cm-2. The temperature of the mirror sample is locally varied between 25 and 200 C
with the chemical state of the surface simultaneously monitored using X-ray photoelectron spectroscopy, and lowenergy
ion scattering spectroscopy. Results demonstrate the balance between energetic and thermal Sn has on the total
Sn surface fraction during exposure and its effect on the structural and reflective properties of the mirror surface.
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