Design, development, and characterization of thin-film filters for high brilliance sources in the EUV-soft x-ray spectral range

This thesis addresses research work on the design, fabrication development, and characterization of thin-film transmittance filters for high brilliance radiation sources in the Extreme Ultraviolet (EUV) and soft X-ray spectral regions. This development and fabrication are strongly required in many applications, by instance in third and fourth generation light sources, and high order harmonic generation (HHG) sources. Thin-film filters are used to remove multiple-order radiations; In addition to high transmittance and spectral purity, those filters must be suited to face the high peak power of this kind of source. In the EUV and soft-X ray spectral range, materials have high absorption; so finding proper materials or a combination of materials that fulfill the suitable requirements is quite challenging. By using simulations of the transmittance performance for a variety of materials, based on the theoretical values of the optical constants, and taking into account their mechanical properties, Nb and Zr were chosen as core elements for the fabrication and study of free-standing filters. The first development part of this thesis is focused on the bottom-up fabrication technique of Nb, Zr, Nb/Zr, and Zr/Nb, 100 nm thick, thin films, deposited on silicon nitrate membrane windows by magnetron sputtering technique, Nb and Nb/Zr free-standing filters were produced after reactive ion etching of the substrate. These samples were characterized using Rutherford backscattering, AFM images and transmittance measurements in the EUV between 4-20 nm using synchrotron radiation at Bear Beamline, ELETTRA, Italy and at Optical Beamline at BESSY synchrotron, Berlin, Germany. Also, the samples were characterized in the same range using a laser-plasma source based on puff gas target as a secondary technique for transmittance characterization. The second part is devoted to the structural characterization of the filters by using TEM, SEM, HRSTEM, and EDX analysis. The third development part of this thesis is focused on the study of high-density EUV radiation damage of Nb, Zr, Zr/Nb, and Nb/Zr 100 nm thick free-standing filters. For this part of the experiment, the samples were deposited on silicon nitrite windows using e-beam deposition technique, showing more stable structures, free-standing filters of each type were achieved after reactive ion etching. Samples are intended to be characterized before and after radiation exposure by using x-ray photoelectron spectroscopy (XPS) technique, AFM images, x-ray diffraction technique, and transmittance characterization in the EUV spectral range. For the radiation exposure, a high-density set-up based on a pulsed plasma discharge source at 13.5 nm wavelength was used. Also, transmission electron microscopy and scanning electron microscopy characterization were used as complementary techniques to study both the sample structures and interface properties.