Optical constants of materials in the EUV/soft x-ray region for multilayer mirror applications

The response of a given material to an incident electromagnetic wave is described by the energy dependent complex index of refraction {dollar}n=1-delta+ibeta.{dollar} In the extreme ultraviolet (EUV)/ soft x-ray spectral region, the need for accurate determination of n is driven by activity in areas such as synchrotron based research, EUV/x-ray lithography, x-ray astronomy and plasma applications. Knowledge of the refractive index is essential for the design of the optical components of instruments used in experiments and applications. Moreover, measured values of n may be used to evaluate solid state models for the optical behavior of materials. The refractive index n of Si, Mo and Be is investigated in the EUV/soft x-ray region. In the case of Si, angle dependent reflectance measurements are performed in the energy range 50-180 eV. The optical constants {dollar}delta,beta{dollar} are both determined by fitting to the Fresnel equations. The results of this method are compared to the values in the 1993 atomic tables. Clean surfaces, which are critical for the studies, were obtained by UV irradiation and HF:ethanol dipping to H-passivate the surface. It is found that the values of {dollar}delta{dollar} in the 1993 atomic tables, which are derived through dispersion (Kramers-Kronig) calculations, are 8% to 15% high in the region 50-90 eV. This is attributed to missing oscillator strength in the tabulated absorption coefficient for Si. The energy region below the Si edge is significant for the optimization of the performance of Si-based multilayer mirrors operating in this range. It is found that the method of least squares fitting reflectance data to obtain optical constants is most effective for energies well below the edge where {dollar}delta>beta,{dollar} while for a range of energies around and above the edge where {dollar}delta