Optical exposure characterization and comparisons for Sn EUV systems

Abstract

A critical issue for EUVL is the minimization of collector degradation from intense plasma erosion, debris deposition and hydrocarbon/oxide contamination. Collector optics reflectivity and lifetime heavily depends 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 investigation of DPP and LPP Xe plasma interactions with collector optics surfaces, the University of Illinois has analyzed collector samples before and after exposure in a Sn-upgraded Xtreme Technologies EUV source. Sn DPP post-exposure 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 co-sputtering system at UIUC. Pre and post exposed sample characterization studies actually investigates the 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, texture, and grain sizes as well as analysis of the implantation of energetic Sn ions, Sn diffusion, and mixing of multi-layers. 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). Implantation, diffusion, and mixing effects are analyzed with depth profiles using AES. Materials characterization on samples, removed after varying exposure times in the XTS source, together with in-situ EUV reflectivity measurements, 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 and interesting facts were noticed. First, hot mirrors exposed to SnCl4 will cause decomposition of the gas and will result into a contamination layer build up on the mirror surface. Secondly, erosion is mitigated to some extent by the simultaneous deposition of material. Third and most important is that the Gibbsian segregation concept works and a thin Au layer is maintained during the entire exposure, even though overall erosion took place. This phenomenon could be very useful in the design and development of a collector optics surface. In addition, this paper will present Sn DPP collector erosion mechanisms, source debris spectra and provide insight into plasma-facing optics lifetime as HVM tool conditions are approached.