EUV Multilayer Optics: Design, Development and Metrology

Extreme ultraviolet (EUV) multilayer coatings are presently widely used in both science and technology. The most characteristic technological application of multilayers is in lithography (EUVL) for large volume production of electronic chips. According to the well known Moore’s law the density of components in integrated circuits doubles every about 18 months; the resolution in the processing of the wafer has to improve correspondingly. Since the lithographic process consists of the projection of a mask on the wafer, according to the optics diffraction limit in order to improve the resolution the wavelength of radiation has to decrease. Accordingly, effort is concentrated on the development of a lithographic tool at 13.5 nm in order to reach a resolution down to about 30 nm.1 In science the applications of multilayers are widely spread among beam lines of large scale facilities such as synchrotrons and free electron lasers (FELs), and for astronomy. At large scale facilities multilayer optics are used in order to select bandwidth or polarization and to focus the radiation beam. In astronomy multilayers are used in Solar imagers working at selected wavelengths for spectroscopic diagnostic purposes. More recently multilayer coatings for ultrashort pulses in the sub-femtosecond regime have been developed. By designing multilayers capable of reflecting or even compressing ultra-short pulses the “magic” door to atto-physics has been opened. To give a classical perspective a few tens of attoseconds correspond approximately to the time an electron takes to complete one orbit around the nucleus of a hydrogen atom; in the same time the electric field of an optical pulse makes a small fraction of its oscillation. Understanding the physics at such short time scales represents a great challenge both for theory and experiment and can provide amazing results. In this review after a short general discussion of the basic theory of multilayers coatings some recent results in the development of multilayers for ultrashort pulses and astronomical applications will be presented. In the EUV spectral range transparent materials do not exist; only fluorides have relatively good transmittance at short wavelengths, with LiF presenting the shortest absorption edge at about 105 nm. This means that all materials have complex refractive indices with non- negligible imaginary coefficients, and that it is not possible to make use of refractive optics, such as lenses, prisms, plates or windows, in order to focus or steer the radiation in experimental set-up. Thus, in principle, it is only possible to use optics working in reflection, but in this case a further problem arises as a result of the low normal incidence reflectivity of standard metal coatings such as gold, and platinum. This is because at such short wavelengths the real parts of the refractive indices are very close to 274 Short Wavelength Laboratory Sources: Principles and Practices unity and, consequently, in order to get efficient reflection the coatings have to be used at very small glancing angles, below the critical angle for total reflection. This means that the optical apertures are very small, affecting the final throughput of the system and giving aberrations that are considerably larger than at near-normal incidence, thus further affecting the final performances. For these reasons, the design of optical coatings with high reflectivity at nearly normal incidence is strongly required. Multilayer coatings (multilayers or ML) consist basically of structures obtained through the repetition of bilayers made of two materials. The materials are selected in order to get the highest reflectivity at the interfaces and in addition their thickness is optimized in order to provide coherent superposition of the various reflected components. The working principle is similar to that of dielectric coatings widely used in the optical range, for example filters and laser mirrors. However since at such short wavelengths the thickness of the layers scales down to a few nm’s new technological problems need to be solved.