Optical Constants of Uranium Nitride Thin Films in

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)? Ø Astronomy l l Ø Lithography l Ø Energetic Objects IMAGE Satellite Mirror Projection Imagining Medicine l High Resolution Imaging Microscopes Images courtesy of http: //euv. lpl. arizona. edu/euv/, www. schott. com/magazine/english/info 99/ and www. schott. com/magazine/english/info 99/.

Optical Constants Ø 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. Ø For maximum reflection for multilayers, we want high change in n and low k. Ø For a given material, optical constants are different for different wavelengths.

Why Uranium? Ø Most things, including air, are highly absorptive (big k) in the EUV. Ø Uranium has high theoretical reflectivity for the wavelengths of interest. Ø Problem: Oxidation

Reflectance computed using the CXRO Website: http: //www-cxro. lbl. gov/optical_constants/mirror 2. html

Reflectance computed using the CXRO Website: http: //www-cxro. lbl. gov/optical_constants/mirror 2. html

Making Thin Films Ø Sputtering l l 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 Ø Samples l l l Samples deposited on silicon wafers, quartz slides, polyimide films, Si. N 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 Ø Pressures l l l ® Different partial pressures result in different compounds. * Above 1 x 10 -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. L. Black, et al. , Journal of Alloys and Compounds, 315 (2001) 36 -41.

N 2 Partial Pressure vs. N/U Ratio

Learning About the Samples Ø X-Ray Photoelecton Spectroscopy (XPS) l Ø X-Ray Diffraction (XRD) l l Ø To find composition To find thickness mλ = 2 d 2 sinθ sin Atomic Force Microscopy (AFM) l To measure roughness Images courtesy of http: //www. weizmann. ac. il/surflab/peter/afmworks/, and http: //volta. byu. edu/adamson 03. pdf.

Finding Optical Constants Ø Ellipsometry l Ø Optical constants are different for different polarizations of light Advanced Light Source at Berkeley l Measures reflectance at different angles and wavelengths Images courtesy of http: //www. lbl. gov/ and http: //www. swt. edu/~wg 06/manuals/Gaertner 117/ellipsometer. Home. htm.

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

XRD Data

TEM Data

TEM Data

SAMPLES UN 002 UN 003 UN 004 N 2 Pressure >1 e-4 torr ~1 e-5 torr Suspected Phase U 2 N 3 UN Lattice Size (nm) (Lit. ) 0. 534 0. 489 XRD (nm) (Lattice Size) 50 40 1% in 10 Thickness Change 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