Uranium Oxide and Uranium Nitride as Highly Reflective Coatings from 2.7 to 11.6 Nanometers

1. BYU EUV Optics April 19, 2004 Uranium Oxide and Uranium Nitride as Highly Reflective Coatings from 2.7 to 11.6 Nanometers Richard L. Sandberg, Marie K. Urry, Shannon Lunt David D. Allred, R. Steven Turley Thanks to Fellow EUV Members: Jed E. Johnson, Luke J. Bissel, Kristi R. Adamson, Nikki Farnsworth, William R. Evans, and others from EUV Group, Andy Aguila & Eric Gullickson at ALS/CXRO Funding: SPIE Scholarship, BYU Physics Dept. Funding, BYU ORCA Scholarship

2. Why Extreme Ultraviolet (EUV) and Soft X-Rays? Thin Film or Multilayer Mirrors EUV Lithography (making really small computer chips) EUV Astronomy BYU EUV Optics Soft X-Ray Microscopes April 19, 2004 The Earth’s magnetosphere in the EUV Images from www.schott.com/magazine/english/info99/ and www.lbl.gov/Science-Articles/Archive/xray-inside-cells.html.

3. Why Uranium? BYU EUV Optics April 19, 2004 • Pros: high density and many electrons (92) for absorption, high theoretical reflectivity: low absorption and high index of refraction • Con: chemically reactive (oxidizes in air to most abundant natural oxide UO2 at STP) • We study different compounds of uranium, such as uranium dioxide (UO2) and uranium mononitride (UN), in search of compounds with the highest reflectance and most chemical stability. • Previous Success: IMAGE Satellite Mirror Project—BYU uranium based mirrors (Launched March 25, 2000)

4. 4.48nm BYU EUV Optics April 19, 2004 Delta vs. beta plot for several elements at 4.48 nm Note: Nickel and its neighboring 3d elements are the nearest to uranium in this area.

5. BYU EUV Optics April 19, 2004 Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

6. BYU EUV Optics April 19, 2004 Sample Preparation • The UO, UN, Ni, and Au samples were deposited on pieces polished silicon test wafers (100 orientation). Quartz crystal monitors were used to measure the sputtering and evaporation rates. • U DC Magnetron/RF Sputtering • The uranium sputter targets used here were of depleted uranium metal (less than 0.2% U-235). After sputtering, the uranium oxide was allowed to oxidize naturally in laboratory air. Uranium nitride was reactively sputtered (RF) in nitrogen partial pressure of about 10-5 torr, bellow 10-4 torr as suggested by L. Black et al., Journal of Alloys and Compounds 315, 36-41 (2001). • Ni/Au Resistive Thermal Evaporation • Evaporated Ni wire/Au beads from a resistively heated tungsten boat (RD Mathis Co.) in a large, cryopumped, stainless steel “bell jar” coater. • Ir Sample Prepared at Goddard Space Flight Center on Glass Slides Schematic of DC magnetron sputtering system at BYU.

7. BYU EUV Optics April 19, 2004 Thickness Determined by XRD m λ = 2d sin θ • XRD Sample Thickness • -UO2 30.0 nm (ρ=10.97 g/cm3) • -UN 38.0 nm (ρ=10. g/cm3) • -NiO on Ni 49.7 nm (ρ=6.67g/cm3) • -Au 29.5 nm (ρ=19.3g/cm3) • -Ir ?? (ρ=22.42 g/cm3)

8. BYU EUV Optics April 19, 2004

9. BYU EUV Optics April 19, 2004 Studying Our Samples Scanning/Transmission Electron Ellipsometry Microscopes (SEM/TEM) X-ray Photoelectron Spectroscope (XPS) Atomic Force Microscopy (AFM) 5 Images courtesy of www.weizmann.ac.il/surflab/peter/afmworks, www.mos.org/sln/SEM/works/ http://volta.byu.edu/adamson03.pdf, and http://www.swt.edu/~wg06/manuals/Gaertner117/ellipsometerHome.htm .

10. BYU EUV Optics April 19, 2004 Taking Reflectance Measurements at the ALS (Advance Light Source) • Beamline 6.3.2 Reflectometer • Bright synchrotron radiation • 1-24.8 nm range • High spectral purity • Energy/wavelength or θ-2θ scan capability Sample of Data from the ALS • Small Discrepancies arise from one region to another with the use of different filters. • XANES Capability • Normalization given by R=(Idetector-Idark)/(Ibeam-Idark) Inage courtesy of http://www.lbl.gov/

11. BYU EUV Optics April 19, 2004

12. BYU EUV Optics April 19, 2004

13. BYU EUV Optics April 19, 2004

14. Optical Properties of Uranium Oxide and Uranium Nitride V.G. Kohn. Phys. Stat. Sol. 185(61), 61-70 (1995). L.G. Parratt. Physical Review 95 (2), 359-369 (1954).

15. BYU EUV Optics April 19, 2004 Measured Data compared with CXRO Atomic Scattering Factor Model • Photons are scattered principally off electrons. More electrons = higher reflection. Measured reflectance features do not agree with CXRO atomic scattering factors. More work need to be done on measuring uranium’s optical constants.

16. BYU EUV Optics April 19, 2004 XANES (X-Ray Absorption Near Edge Spectroscopy) XANES at ALS show additional absorption resonances not accounted for in ASF Data at CXRO. U NVIOIV @ 286.3 eV * *D. R. Lide (ed.), CRC Handbook of Chemistry and Physics, 71st edition, CRC Press, Boca Raton, 1990-91, p.10-256.

17. BYU EUV Optics April 19, 2004 Conclusions • UO2 and UN reflect significantly more than Ni, Ir, and Au, the current materials with highest reflectance, between 4 and 9 nm. • U reflectance differs from the reflectance predicted by the atomic scattering factor model (ASF). • Current preparation of UN is not stable in ambient air (oxidizes to UO2). Need to test oxidation of heated UN sample Goals • Determine the optical properties of UO2 below Shannon’s data (4.5 nm) and fill out UN optical properties data. • Work with CXRO to amend the existing uranium atomic scattering factor data. Questions? EUV Group Contact Dr. David Allred allred@byu.edu (801) 422-3489 THANK YOU!!