Design of the chirped Mo/Si multilayer mirror with SiC capping layer in extreme ultraviolet region

The design procedures were discussed in detail for a normal incident chirped Mo/Si multilayer mirror with Group Delay Dispersion (GDD) of −3600 as2 in the wavelength region of 13–17 nm. The GDD was calculated using an analytical approach by fitting the optical constants of the coating materials in the wavelength range of 12.8–17.2 nm, respectively. The final structure of the chirped mirror was obtained by using the simplex algorithm from the initial structure obtained by the genetic algorithm based on structure generated by a random generator. After considering the interfacial roughness and layers thickness deviation, the effects on the reflectivity and the GDD were discussed. It was found that the average reflectivity decreases from 5.98% to 4.22% and the average GDD decreases from −3561.86 as2 to −3462.03 as2, the vibration of GDD were larger than that of the reflectivity. The reflectivity was affected greatly by the 9th layer and the GDD was affected greatly by the 25th layer when each layer thickness changes ±0.2 nm. Compared with the GDD, the reflectivity was affected greatly by the layer thickness error.