Reflectance measurements on clean surfaces for the determination of
optical constants of materials in the EUV/soft X-ray region 

R. Soufli, E. M. Gullikson and J. H. Underwood 

Center for X-ray Optics
Lawrence Berkeley National Laboratory Berkeley, CA 94720
regina@rose.lbl.gov 

The refractive index n = 1 - delta + ibeta of materials in the EUV/soft X-ray
energy range is being investigated using reflectance measurements.  The
material of choice in this work is Si and the spectral region considered
includes the L_2,3 edge of Si (99.8eV) where Si exhibits relatively low
absorption, thus being useful as a filter or spacer material in multilayer
mirrors for EUV lithography applications.  The reflectance data vs.  angle at
each energy were fitted in order to obtain the optical constants delta ; beta
by means of a least squares fitting algorithm, using the Fresnel equation.
This method is of particular interest, because it determines experimen tally
both delta and beta, as opposed to the transmission method which is using the
Kramers-Kronig relations for the calculation of delta from experimental data
on beta , and suffers from systematic errors at low energies.  The samples
investigated include crystalline silicon (111) and (100) orientations and
amorphous silicon.  The reliabilit y of reflectance data depends on the
quality of the sample surface.  The most serious sources of error are:  1)
surface roughness, 2) the presence of native oxide and organic contamination
on the sample surface.  For these reasons, particularly smooth samples were
used and a cleaning treatment was applied to the sample surfaces, prior to the
measurements.  The cleaning procedure includes UV cleaning (for organic
contamination removal) and subsequent HF:ethanol dipping of the samples, which
results in H-passivated surfaces.  The samples were investigated before and
after cleaning was applied, using the X-ray Photo electron Spectroscopy (XPS)
method, in order to determine the effectiveness of the H-passivation
technique.

It is shown that the beta of crystalline Si exhibits fine structure below the
L 2 ; 3 edge similar to that observed in transmission data for Si(111) The
fine structure is not observed in amorphous Si.  also found that the method of
fitting reflectance data to obtain optical constants is most effective for
energies away from the L_2,3 edge of Si, while for a range of energies around
the edge the optical constants are determined with large uncertainties.  This
behavior is not unique to the silicon L_2,3 edge.