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 Energetic Objects IMAGE Satellite Mirror Project IMAGE Satellite Mirror Project  Lithography Projection Imagining Projection Imagining  Medicine High Resolution Imaging Microscopes High Resolution Imaging Microscopes Images courtesy of and

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

Reflectance computed using the CXRO Website:

Making Thin Films  Sputtering Bombard target, a large piece of uranium, with argon ions from glow discharge Bombard target, a large piece of uranium, with argon ions from glow discharge Uranium atoms leave target due to collisions Uranium atoms leave target due to collisions Nitrogen partial pressure in plasma creates N atoms Nitrogen partial pressure in plasma creates N atoms U and UN molecules deposit on our samples 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 Samples deposited on silicon wafers, quartz slides, polyimide films, SiN membranes, and carbon coated TEM grids UN x films nm thick UN x films nm thick Low pressure sputtering allows for smooth, dense, low stress films because of the increased mean free path U atoms 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.* Different partial pressures result in different compounds.* Above 1x10 -4 torr partial pressure N 2, we create U 2 N 3. With lower pressures we can make UN. Above 1x10 -4 torr partial pressure N 2, we create U 2 N 3. 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. 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)

N 2 Partial Pressure vs. N/U Ratio

Learning About the Samples  X-Ray Photoelecton Spectroscopy (XPS) To find composition To find composition  X-Ray Diffraction (XRD) To find thickness To find thickness mλ = 2d sinθ  Atomic Force Microscopy (AFM) To measure roughness To measure roughness Images courtesy of and

Finding Optical Constants  Ellipsometry Optical constants are different for different polarizations of light Optical constants are different for different polarizations of light  Advanced Light Source at Berkeley Measures reflectance at different angles and wavelengths Measures reflectance at different angles and wavelengths Images courtesy of and

Problem!!  Is it really UN or is it UN 2-x ?  Our samples change with time. The peaks seen in XRD move. The peaks seen in XRD move.  Continuing research in this area.

XRD Data

TEM Data

SAMPLESUN002UN003UN004 N2 Pressure>1e-4 torr ~1e-5 torr Suspected PhaseU2N3 UN Lattice Size (nm) (Lit.) XRD (nm) (Lattice Size) 5040 Thickness Change 1% in 10 days TEM (nm) (Thickness) Ratio (measured/lit)

Acknowledgements Thanks to: Dr. David D. Allred Dr. R. Steven Turley Kristi R. Adamson Luke J. Bissell Winston Larsen Richard L. Sandberg Mindy Tonks