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

1. Optical Constants ofUranium Nitride Thin Filmsin the EUV (7-15 nm) Marie K. Urry EUV Thin Film Group Brigham Young University

2. 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/.

3. 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.

4. 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

5. Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

6. Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

7. 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

8. 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

9. 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.

10. N2 Partial Pressure vs. N/U Ratio

11. 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.

12. 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.

13. 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.

14. XRD Data

15. TEM Data

16. TEM Data

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