Coating development for the far and extreme ultraviolet based on material characterization

Juan I. Larruquert; Luis Rodríguez-de Marcos; Sergio García-Cortés; Manuela Vidal-Dasilva; Antonio P. Pérez-Marín; José A. Aznárez; José A. Méndez

Abstract

Little development on coatings has been available in the 50-120-nm spectral range until recently. One main reason for this is the large absorption of most materials in nature in this range. Our group has followed a research towards the development of novel coatings for this spectral range. This research has been based on the search and characterization of new materials mainly with low absorption. For many materials we have performed their optical characterizations in a large spectral range to reduce common inconsistencies that arise when combinations of data from different sources are used. We summarize our research on the characterization of many lanthanides, among other materials. Lanthanides are particularly interesting because they have a relatively low absorption in the spectral range of interest. Self-consistent characterization of other materials, such as SiC and B₄C, has been performed for their interest as candidate materials for coatings involving the EUV to the visible. The discovery in lanthanides of a wealth of materials with relatively low absorption has enabled the development of multilayers based on the low absorption of Yb and Eu lanthanides. This resulted in the first narrowband multilayers with a peak wavelength in the 70 to 100 nm. We also report recent research on the development of multilayers with a peak reflectance above 100 nm; these multilayers address two targets: a) narrowband performance; b) zero reflectance at a wavelength slightly longer (such as 121.6 nm) than the peak wavelength. As for a), a promising preliminary result is obtained with a narrowband multilayer peaked at 101 nm. Regarding b), multilayers with a high reflectance at 102.6 nm and a low reflectance at 121.6 nm were prepared and they displayed a successful performance when measured in situ (not exposed to the atmosphere); however, the minimum at 121.6 nm was lost after a short exposure to air. The latter research is still underway and we plan to experiment with new designs. Our group has also prepared efficient narrowband transmittance coatings peaked at wavelengths longer than 120 nm. They are based on the classical combination of Al and MgF₂.