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Optical Constants of Uranium Nitride in the EUV

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					   Optical Constants of
Uranium Nitride Thin Films
  in the EUV (7-15 nm)

           Marie K. Urry
       EUV Thin Film Group
     Brigham Young University
Why Extreme Ultraviolet (EUV)?
   Astronomy
         Energetic Objects
         IMAGE Satellite Mirror
          Project
   Lithography
         Projection Imagining
   Medicine
         High Resolution
          Imaging Microscopes


Images courtesy of http://euv.lpl.arizona.edu/euv/, www.schott.com/magazine/english/info99/ and www.schott.com/magazine/english/info99/.
           Optical Constants
 Index  of refraction:
                 N=n+i k,
  where n is the real part of the index of
  refraction and k , the imaginary part, is
  called the coefficient of absorption.
 For maximum reflection for multilayers, we
  want high change in n and low k .
 For a given material, optical constants are
  different for different wavelengths.
              Why Uranium?
 Mostthings, including air, are highly
 absorptive (big k) in the EUV.

 Uraniumhas high theoretical reflectivity for
 the wavelengths of interest.

 Problem:   Oxidation
                                Computed Reflectance at 10 degrees of various
                                                materials

                      0.9

                      0.8

                      0.7

                      0.6
        Reflectance




                      0.5

                      0.4

                      0.3

                      0.2

                      0.1

                       0
                            2   4        6           8         10        12              14           16   18   20
                                                             Wavelength (nm)

                                                                 Au        Ni       U



Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
                                Computed Reflectance at 10 degrees of various
                                                materials

                      0.9

                      0.8

                      0.7

                      0.6
        Reflectance




                      0.5

                      0.4

                      0.3

                      0.2

                      0.1

                       0
                            2          4                     6               8                        10   12
                                                             Wavelength (nm)

                                                        ThO2          UO2         UN         U



Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
                Making Thin Films
   Sputtering
       Bombard target, a large
        piece of uranium, with
        argon ions from glow
        discharge
       Uranium atoms leave
        target due to collisions
       Nitrogen partial pressure in
        plasma creates N atoms
       U and UN molecules
        deposit on our samples
            Making Thin Films
 Samples
     Samples deposited on silicon wafers, quartz
      slides, polyimide films, SiN membranes, and
      carbon coated TEM grids
     UNx films 10-30 nm thick
     Low pressure sputtering allows for smooth,
      dense, low stress films because of the
      increased mean free path U atoms
                 Making Thin Films
 Pressures
     Different partial pressures result in different
      compounds.*
     Above 1x10-4 torr partial pressure N2, we create
      U2N3. With lower pressures we can make UN.
     Our system can’t measure partial pressures in
      this range, so we don’t know which we made.

     L. Black, et al., Journal of Alloys and Compounds, 315 (2001) 36-41.
N2 Partial Pressure vs. N/U Ratio
         Learning About the Samples
   X-Ray Photoelecton
    Spectroscopy (XPS)
       To find composition
   X-Ray Diffraction
    (XRD)
       To find thickness
       mλ = 2d sinθ
   Atomic Force
    Microscopy (AFM)
       To measure roughness


     Images courtesy of http://www.weizmann.ac.il/surflab/peter/afmworks/, and http://volta.byu.edu/adamson03.pdf .
                      Finding Optical Constants
        Ellipsometry
               Optical constants are
                different for different
                polarizations of light
        Advanced Light
         Source at Berkeley
               Measures reflectance
                at different angles and
                wavelengths



Images courtesy of http://www.lbl.gov/ and http://www.swt.edu/~wg06/manuals/Gaertner117/ellipsometerHome.htm.
                   Problem!!
   it really UN or is it UN2-x?
 Is
 Our samples change with time.
      The peaks seen in XRD move.
 Continuing    research in this area.
XRD Data
TEM Data
TEM Data
SAMPLES              UN002      UN003      UN004
N2 Pressure         >1e-4 torr >1e-4 torr ~1e-5 torr
Suspected Phase      U2N3       U2N3         UN
Lattice Size (nm)
  (Lit.)              0.534      0.534      0.489
XRD (nm)
  (Lattice Size)                  50         40
Thickness Change                           1% in 10
                                             days
TEM (nm)
  (Thickness)         0.546                 0.498
Ratio
  (measured/lit)      1.022                 1.018
Acknowledgements
      Thanks to:
  Dr. David D. Allred
 Dr. R. Steven Turley
  Kristi R. Adamson
    Luke J. Bissell
   Winston Larsen
 Richard L. Sandberg
     Mindy Tonks

				
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