On Determining the Optical Constants of Sputtered U and a-Si at 304 and 584 Å

The optical constants of uranium (U) and amorphous silicon (a-Si) at 304 and 584 Å in the extreme ultraviolet (EUV) are a source of error in the design of multilayer optics. Measured reflectivities of multilayer mirror coatings do not agree with calculated reflectivities using existing optical constants. The optical properties of materials depend on the electronic structure of the atom or compound being measured. In the low energy EUV it is uncertain how bonding affects the optical properties of elements and compounds. I have calculated the magnitude and the direction of the shift in the optical constants of U and a-Si from reflectivity measurements of DC magnetron sputtered U/a-Si multilayers at 304 and 584 Å. The reflectivity of the multilayers were measured using a UV hollow cathode plasma light source, a 1 meter VUV monochromator, a back-thinned CCD camera, and a channeltron detector. The reflectivities of the multilayer coatings were measured at 14.5 degrees from the normal surface of the mirror. The optical constants were calculated using IMD which uses CURVEFIT to fit the optical constants to reflectivity measurements of a range of multilayer mirrors that varied over a span of 150-250 Å layer thickness. The effects of surface oxide and roughness, interdiffusion, and interfacial roughness were numerically subtracted in fitting the optical constants. The optical constants at 304 Å of a-Si calculated from multilayers terminated with uranium oxide are k = 0.037±0.015, a positive 400% shift, and n = 0.961± 0.025, a positive 3.5% shift, and the optical constants of U are k = 0.67, a positive 380% shift, and n = 0.69, a positive 2.4% shift. The refractive index at 304 Å of a-Si calculated from multilayers terminated with amorphous silicon is n = 0.939, a negative 1% shift, and the refractive index of U is n = 0.705, a positive 4% shift. The refractive index at 584 Å of a-Si calculated from multilayers terminated with amorphous silicon is n = 0.806, a positive 2% shift, and the refractive index of U is n = 0.55, a negative 6.8% shift. The refractive index at 584 Å of U calculated from multilayers terminated with uranium oxide is n = 0.49, a negative 17% shift. These shifts and magnitudes should only be used as the basis of further determination of the optical constants of U and a-Si at 304 and 584 Å.