Optical Exposure Characterization and Comparisons for Discharge Produced Plasma Sn Extreme Ultraviolet System

J. Microlithography, Microfabrication, and Microsystems, 5(3), 033007(1-11) (2006).

Qiu, H., Thompson, K. C., Srivastava, S. N., Antonsen, E. L., Alman, D. A., Jurczyk, B. E., Ruzic, D. N.

A critical issue for EUV lithography (EUVL) is the minimization of collector degradation from intense plasma erosion, debris deposition, and hydrocarbon/oxide contamination. Collector optics reflectivity and lifetime heavily depend on surface chemistry and interactions between fuels and various mirror materials, such as silicon, in addition to high-energy ion and neutral particle erosion effects. As a continuation of our prior investigations of discharge-produced plasma (DPP) and laser-produced plasma (LPP) Xe plasma interactions with collector optics surfaces, the University of Illinois at Urbana–Champaign (UIUC) has analyzed collector samples before and after exposure in a Sn-upgraded Xtreme Technologies EUV source. Sn DPP postexposure characterization includes multiple samples, Si/Mo multilayer film with normal incidence, 200-nm-thick Ru film with grazing incidence, as well as a Gibbsian segregated (GS) Mo-Au alloy developed on silicon using a dc dual-magnetron cosputtering system at UIUC for enhanced surface roughness properties, erosion resistance, and self-healing characteristics to maintain reflectivity over a longer period of mirror lifetime. Surface analysis draws heavily on the expertise of the Center for Microanalysis of Materials at UIUC, and investigates mirror degradation mechanisms by measuring changes in surface roughness and film thickness as well as analysis of deposition of energetic Sn ions, Sn diffusion, and mixing of multilayer. Results from atomic force microscopy (AFM) and auger electron spectroscopy (AES) measurements show exposure effects on surface roughness and contamination. The best estimates of thickness and the resultant erosion measurements are obtained from scanning electron microscopy (SEM). Deposition, diffusion, and mixing effects are analyzed with depth profiles by AES. Material characterization on samples removed after varying exposure times in the XTS source can identify the onset of different degradation mechanisms within each sample. These samples are the first fully characterized materials to be exposed to a Sn-based DPP EUV source. Several valuable lessons are learned. First, hot mirrors exposed to SnCl₄ gas will cause decomposition of the gas and build up a contamination layer on the surface. Second, erosion is mitigated to some extent by the simultaneous deposition of material. Third, and most important, Gibbsian segregation works and a thin Au layer is maintained during exposure, even though overall erosion is taking place. This phenomenon could be very useful in the design of a collector optics surface. In addition, we present Sn DPP collector erosion mechanisms and contamination and provide insight into plasma-facing optics lifetime as high-volume manufacturing (HVM) tool conditions are approached.