REFLECTANCE MEASUREMENTS ON CLEAN SURFACES FOR THE DETERMINATION OF OPTICAL-CONSTANTS OF SILICON IN THE EXTREME ULTRAVIOLET-SOFT-X-RAY REGION

The refractive index n = 1 - delta + i beta of Si in the energy range 50-180 eV is investigated with angle-dependent reflectance measurement s. The optical constants delta and beta are both determined by fitting to the Fresnel equations. The results of this method are compared wit h the values in the atomic tables derived from experimental data for b eta and implementation of the Kramers-Kronig for relations for delta. The samples were prepared by UV irradiation and HF:ethanol dipping to H passivate the surface. It is found that the values of delta in the a tomic tables are 8-15% too high in the region 50-90 eV. This is attrib uted to missing oscillator strength in the tabulated absorption coeffi cient for Si. The measured values of beta for crystalline Si exhibit s tructure below the L-2,L-3 edge (99.8 eV), as was previously observed in transmission measurements of Si(111). It is also found that the met hod of least-squares fitting reflectance data to obtain optical consta nts is most effective for energies well below the edge, where delta > beta, while for a range of energies around and above the edge, where d elta < beta, the optical constants are determined with large uncertain ties. This behavior is not unique to the Si L-2,L-3 edge. (C) 1997 Opt ical Society of America.