Proceedings Article | 19 May 2011
J. Allain, B. Holybee, M. List, A. Kacic, M. Nieto-Perez, M. Hendricks
KEYWORDS: Ruthenium, Tin, Rhodium, Reflectivity, Mirrors, Extreme ultraviolet, Particles, Atomic force microscopy, Xenon, EUV optics
Extreme ultraviolet (EUV) light at 13.5 nm is a candidate for advanced optical lithography where sub-32 nm features
will spawn the next generation of microchips. Plasma-based Sn sources have been used to generate EUV light, but
debris contamination and damage limits the life expectancy of plasma-facing collector optics. Although mitigation
systems reduce the amount of vapor, thermal and energetic particles still reach mirror surfaces. Three Rh mirror
samples were exposed to thermal Sn (14.7-nA) and energetic 1 keV Xe+ (12-nA, 120-nA, and 1.2-uA) particles for 36
minutes and three Ru samples were exposed to thermal Sn (14.7-nA) and energetic Xe+ (100eV, 500eV, and 1000eV).
In-situ measurements of EUV reflectivity and surface composition were correlated with ex-situ morphology
investigations. For the Rh mirrors, a combination of 14.7-nA thermal Sn and 1.2-uA energetic Xe+ reduced mirror
reflectivity by only 18.7%, as opposed to the 48.5% and 41.7% decreases observed with 120-nA and 12-nA energetic
Xe+, respectively. For the Ru mirrors, the higher 1000eV Xe+ irradiation lowered the reflectivity by 41.6%, while the
500eV and 100eV energy decreased the reflectivity by 44.9% and 51.4% respectively.
In-situ low-energy ion scattering spectroscopy (LEISS) monitored the Sn surface fraction. For Rh samples irradiated
with 120-nA and 12-nA energetic Xe+, the fraction was 1 throughout the experiment. The third sample (1.2-uA) initially
had a 0.71 surface fraction, which increased towards 1, dropped to 0.48 after 18 minutes, and then continued rising to
0.8 after 36 minutes. The drop suggests a threshold, where perhaps surface structure abruptly changed, that was too high
for the other samples to cross. The results imply that at certain fluences, debris bombardment may not be as detrimental
to collector mirror EUV reflectivity as at others. For the Ru samples, the Sn surface fraction jumped to 100% for the
sample irradiated at 100eV Xe+, to 85% for the 500eV case, and 75% for the 1keV case. The reflectivity was worse with
the 100eV sample, with a drop of 51.4%, while the 500eV and 1keV had drops of 44.9% and 41.6%, respectively.
Morphology analyses with an atomic force microscope at Purdue University's Radiation Surface and Interface Science
Laboratory revealed that the morphology of the mirror surface plays a key role in the reflectivity performance of the Rh
and Ru mirrors.
