EUV Multilayer Optics: Design, Development and Metrology

Design and realization of ML coatings needs the understanding of the optical working principles of these structures as well as to take into consideration the technological problems related to the materials intrinsic properties and to their deposition in thin films form. In this review we have tried to illustrate these various aspects through an introduction on the basic physics of ML coatings, the deposition and characterization techniques and then reporting on some examples of more peculiar applications. After a short review on the EUV lithography for integrated circuits production, it has been discussed and explained the conceptual design of different multilayers: for reflection of ultrashort EUV pulses and for space applications. These structures can be a-periodic, computed with optimization procedures in order to get wide bandwidth and high reflectivity and suitable phase, or making use of capping layers tailored by taking into account the stationary e.m. field configuration in order to get high reflectivity coupled with high rejection ratio in specific spectral bands. Furthermore, the problem of surviving in harsh environment like that one encountered in space close to the sun has been also addressed. Not less important is the problem of realization and testing of ML samples, this has been discussed with particular emphasis also on new metrological techniques like the measurement of the reflectance phase of the multilayer.