Effect of xenon bombardment on ruthenium-coated grazing incidence collector mirror lifetime for extreme ultraviolet lithography

The effect of energetic xenon ion bombardment on the extreme ultraviolet (EUV) reflectivity performance of mirrors is of vital importance for the performance of discharge- and laser-produced plasma extreme ultraviolet lithography sources. To study these effects, we measured absolute and relative reflectivities at the National Institute of Standards and Technology and the Interaction of Materials with Particles and Components Testing facility to quantify the effects of singly ionized Xe ion bombardment on the reflectivity of Ru EUV collector mirrors. Results show that unity sputtering is reached at Xe⁺ energies near 400–500 eV. The Xe⁺-induced sputter yield decreases an order of magnitude with only a 60% decrease in energy. Incident angle-dependent data of Xe⁺ bombardment show that the sputter yield is weakly dependent on angle at energies near 1 keV. Dynamic measurements of in situ EUV reflectivity during Xe⁺ irradiation show that the oxygen state of the reflecting mirror has a significant effect on reflectivity performance. For example, 13.5 nm light reflecting from an oxygen-rich mirror results in over a 40% loss in reflectivity. These studies also found that the surface roughness increased only at the atomic scale (subnanometer scale) when exposed to energetic Xe⁺ and thus did not contribute to EUV reflectivity losses except for cases of very high fluences (≫10¹⁶ cm^-2).