Interface-engineered multilayer mirrors
Most applications of Mo/Si multilayer optics in EUVL require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ lambda = 13.5 nm. Different interface-engineered Mo/X/Si/X multilayers with maximum reflectivity of 69.6 % at 13.5 nm were developed. These new multilayer mirrors consist of molybdenum and silicon layers separated by different interdiffusion barriers (X = C and SiC). Microstructure and optical properties of the multilayers have been investigated by small and Zarge angle Cu-Ka scattering, AFM and characterized by EUV reflectometry. A concept for material selection, thickness optimization of interdiffusion barriers and perspectives for their wide application in imaging EUVL optics is discussed. Some applications of multilayer mirrors in EUVL require not only the highest possible normal incidence reflectivity but also a long-term and thermal stability at the operating temperatures. The Mo/C/Si/C interface-engineered were optimized in terms of high peak reflectivity at a wavelength near 13.5 nm (R(ind p) 60.0 %) and broad operated temperature range (T = 20 - 500°C). The best results were obtained with 0.8 nm thickness of carbon interlayers on both interfaces. Annealing in vacuum was carried out at elevated temperatures up to 650 °C for up to 100 hours. The combination of good optical properties and high thermal stability of interface - engineered Mo/C/Si/C multilayer mirrors underlines their potential for their use in EUVL optics.