Development and Metrology of Extreme Ultraviolet and soft X-ray Multilayer Mirrors

This thesis addresses research works on the development and metrology of multilayer thin-film coatings designed for Extreme Ultraviolet (EUV) and soft X-ray spectral regions. While the development part is limited to reflective multilayers at 6.x nm wavelength, significant part of the thesis is devoted to the metrology of multilayers (MLs) in broader spectral range. The development part focuses on the design and fabrication of MLs of high reflectivity around 6.x nm wavelengths to support below 10 nm half-pitch (HP) patterning of the next generation EUV lithography and tabletop reflectometer for below 10 nm EUV sources. Specific activities under this topic include searching of new candidate materials, numerical design, deposition of the MLs, and performance testing at wavelengths below 10 nm. The second major topic of the thesis focuses on the comprehensive characterization of MLs using innovative approaches. Most challenging questions in ML coatings such as characterization of buried interfaces, determination of layer and interlayer thicknesses, uncertainties of optical constants near EUV and soft X-ray absorption edges are pressing issues. Synchrotron based Grazing-Incidence EUV Reflectivity (GI-EUVR) near absorption edges of one of the component elements in the ML structure is proposed as a major technique to achieve the metrology demands. The strength of GI-EUVR analysis of MLs lies on its simultaneous sensitivity to optical and structural parameters. The proposed EUV metrology method is also capable of evaluating damage analysis of high intensity photons and/or energetic particles on ML performances. In combination with traditional ML characterization techniques, the GI-EUVR enabled characterization of the most challenging buried–interface diffusion processes during multilayer growth. Analysis of chemical composition and corresponding optical properties of inter-diffusion layers are performed near Si LIII - edge for Mo/Si and near elemental boron (B) edge for B4C/CeO2 MLs to demonstrate the robustness of the proposed method. Changes of structural and optical parameters of MLs for applications in astronomical telescopes due to the bombardment of helium particles (4 keV He++ ) that are dominant in the solar corona can be analyzed using the current techniques. Lastly, the thesis deals with the development of tabletop scatterometer experiment around 13.5 nm EUV wavelength based on Xenon gas discharge produced plasma source. EUV scattering measurements at 13.5 nm central wavelength and 2% bandwidth can be simple and fast surface roughness analysis in the mid-spatial frequency range. Surface patterns in the mid-spatial frequency ranges are resolved in the scattered signals from multilayer surfaces.