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



Lithography


Energetic Objects
IMAGE Satellite Mirror
Project
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?
 Most
things, including air, are highly
absorptive (big k) in the EUV.
 Uranium
has high theoretical reflectivity for
the wavelengths of interest.
 Problem:
Oxidation
Computed Reflectance at 10 degrees of various
materials
0.9
0.8
0.7
Reflectance
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
10
12
Wavelength (nm)
Au
Ni
14
U
Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
16
18
20
Computed Reflectance at 10 degrees of various
materials
0.9
0.8
0.7
Reflectance
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
Wavelength (nm)
ThO2
UO2
UN
10
U
Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
12
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)


X-Ray Diffraction
(XRD)



To find composition
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!!
 Is
it really UN or is it UN2-x?
 Our samples change with time.

The peaks seen in XRD move.
 Continuing
research in this area.
XRD Data
TEM Data
TEM Data
SAMPLES
N2 Pressure
Suspected Phase
Lattice Size (nm)
(Lit.)
XRD (nm)
(Lattice Size)
UN002
UN003
UN004
>1e-4 torr >1e-4 torr ~1e-5 torr
U2N3
U2N3
UN
0.534
Thickness Change
TEM (nm)
(Thickness)
Ratio
(measured/lit)
0.534
0.489
50
40
1% in 10
days
0.546
0.498
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