Sn-Induced Surface Modification of EUV Plasma-Facing Collector Mirrors
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Sn-Induced Surface Modification of EUV Plasma-Facing Collector Mirrors Jean P. Allain, Ahmed Hassanein, Martin Nieto, Vladimir Titov, Perry Plotkin, Edward Hinson, Daniel Rokusek, Christopher Chrobak, Matthew Hendricks, William Klimowych Argonne National Laboratory, Argonne, IL, USA 60439 ABSTRACT Generation of extreme ultraviolet (EUV) light for nanolithography requires the use of hot, dense plasmas. Gas discharge produced plasma (GDPP) and laser produced plasma (LPP) are the primary configurations used to generate these plasmas. The in-band EUV spectrum composes about 1% of the total light generated and efficient collection requires the use of thinfilm grazing incidence mirrors or near-normal incidence multi-layer mirrors for GDPP and LPP, respectively. Both GDPP and LPP collector mirror systems face serious issues regarding lifetime and EUV light reflectivity performance. For both configurations debris, fast ions, fast neutrals, and condensable EUV radiator fuels (Li, Sn) interact with nearby collector mirrors. Sn, in particular is used due to its high conversion efficiency in the 13.5-nm in-band spectrum. In addition to debris, collector mirrors are exposed to impurities (H,C,O,N), off-band radiation (depositing heat) and highly-charged ions leading to their degradation and consequently limiting 13.5 nm light reflection intensity. This work presents results of Ru and Rh-based collector optics irradiated by Sn ions at 1.3 keV and exposed to Sn thermal atoms at room temperature. In-situ metrology monitors the effects of ion implantation on ion-induced erosion and EUV reflectivity changes using lowenergy ion scattering spectroscopy and X-ray photoelectron spectroscopy. Results find that Sn implants at the first few monolayers leading to a mixed layer near the surface. Sn thermal atoms deposit on top of the mirror surface leading to 10-20% loss of collector mirror EUV light reflectivity. INTRODUCTION Ion-induced mechanisms can change optical properties of mirrors exposed to environments dominated by energetic particles. This is the case for EUV lithography source devices, which expose collector mirrors to a dense pinch plasma that generates fast singly and multi-charged particles [1]. Another example is first mirrors used in diagnostic systems in reactor-grade fusion devices that are also exposed to deleterious environments including light fast and slow ions leading to mirror degradation [2]. Generation of EUV light for high-resolution lithography requires the use of hot, dense plasmas. Current technology focuses mostly on either laser produced plasmas (LPP) or gas discharge produced plasmas (DPP). Both configurations require collection of EUV light with very stringent specifications. The EUV wavelength of 13.5-nm uses Xe, Sn or Li as candidate EUV radiator fuels [3]. Due to this wavelength level, reflective optics is required. For highvolume manufacturing (HVM) operation (about 100 wafers-per-hour), the collector optics must not lose more than about 10% of absolute EUV reflectivity over 1011 pulses
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