Extreme ultraviolet reflectance of thin films deposited in situ

J.M. Slaughter and Charles M. Falco

Optical Sciences Center and Department of Physics
The University of Arizona, Tucson, Arizona  85721

C. Tarrio, R.N. Watts, and T.B. Lucatorto

Physics Laboratory, National Institute of Standards and Technology
Gaithersburg, Maryland  20899

     Typical thin-film samples that are made in vacuum and then
exposed to the atmosphere, undergo some surface oxidation and
contamination.  Figure 1 illustrates the importance of having
uncontaminated samples for determining optical constants.  The three
curves are calculated reflectance (R) at wavelength=310 � for a W
surface with and without surface oxide (WO2) and carbon contamination
as shown.  All roughnesses (�) are 15 � rms with error function
profiles.  Contamination has a large effect on the R vs. wavelength 
curves.  Fitting curves that are affected by contamination with a
model that assumes only surface roughness leads to large errors in the
fitted optical constants.  For the most contaminated example shown in
Figure 1, such a fit yields constants (n,k) of (0.94, 0.33) rather
than the correct constants that would be found from the clean-surface
curve (1.024, 0.163).  The effect is less pronounced at shorter
wavelengths, but is very significant throughout the EUV.

     We have built a sputter-deposition system that is attached
directly to the NIST EUV reflectometer at SURF-II.  The combined
apparatus allows us to make measurements on samples that have been
grown and transferred in vacuo to the NIST reflectometer.  This
procedure practically eliminates the problem of surface contamination
and oxidation that plagues measurements made on samples produced in
separate deposition chambers.  With this equipment, we are able to
acquire the most reliable data possible for the determination of EUV
optical constants.  Figure 2 shows an example of the data for a thick
W film, measured at wavelength=335 �, with the associated fit and
optical constants.  The complete study will be presented at the
meeting.  This work was supported by NIST.

Figure Captions:

Figure 1.  Calculated reflectance for a tungsten surface with and
without contamination.

Figure 2.  Measured data and a fit for a thick tungsten film deposited
and measured in situ.