Gibbsian segregating alloys: a potential solution to minimize collector degradation

Abstract

A critical issue for EUV lithography is the minimization of collector degradation from intense plasma erosion and debris deposition. Reflectivity and lifetime of the collector optics will be dependent on surface chemistry interactions between fuels and various mirror materials, in addition to high-energy ion and neutral particle erosion effects. An innovative Gibbsian segregation (GS) concept has been proposed to create self-healing, erosion-resistant collector optics. A Mo-Au GS alloy was developed on silicon using a DC dual-magnetron co-sputtering system. A thin Au segregating layer was maintained through segregation during exposure, even though overall erosion was taking place. The reflective material, Mo, underneath the segregating layer remains protected by the sacrificial layer, which is lost due to preferential sputtering. The two dominant driving gradients are temperature and surface concentration (surface removal flux). Both theoretical and experimental efforts were performed to test the suitability of the GS alloy as EUV collection optics, and to elucidate the underlying physics behind it. Results show a strong enhancement effect of temperature, while only a weak effect of surface removal rate on GS performance. When segregation-erosion equilibrium is reached, the surface remains smooth while showing good erosion resistance and maintaining a substantially better reflectivity as compared to the continuous degradation observed in a pure Mo mirror. Further research is recommended.