Optical exposure characterization and comparisons for discharge produced plasma Sn extreme ultraviolet system

Abstract

A critical issue for EUV lithography ͑EUVL͒ 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 highenergy ion and neutral particle erosion effects. As a continuation of our prior investigations of discharge-produced plasma ͑DPP͒ and laserproduced plasma ͑LPP͒ Xe plasma interactions with collector optics surfaces, the University of Illinois at Urbana-Champaign ͑UIUC͒ 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 ͑GS͒ Mo-Au alloy developed on silicon using a dc dualmagnetron 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 ͑AFM͒ and auger electron spectroscopy ͑AES͒ 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 ͑SEM͒. 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 SnCl 4 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 ͑HVM͒ tool conditions are approached.

Time exposure performance of Mo-Au Gibbsian segregating alloys for extreme ultraviolet collector optics

“…An innovative idea using the Gibbsian segregation (GS) concept as the potential grazing incident EUV collector optics is developed and explored [10][11][12][13]. GS processes have been defined [14,15] as the tendency of certain solute elements in a homogenously interspersed solid solution to accumulate at imperfections, such as grain boundaries and interfaces in the bulk lattice that then may segregate to the free sur-faces.…”

“…If the regenerative transport processes (bulk, grain, interface diffusion) and surface renormalization are faster than the erosion time scale (i.e., the average time between large energy sputtering events), then the collector optics would be self-repairing. Early works have been published to assess the suitability of the GS alloy technology for early adoption of EUV lithography and test the validity with the theoretical estimate [10][11][12][13]. This work describes further experimental efforts to discover if GS alloys effectively ameliorate and self-heal the damage in grazing incidence EUV collector optics.…”