BYU EUV Optics

1. BYU EUV Optics August 4, 2004 Optical Properties and Application of Uranium-based Thin-Films for the Extreme Ultraviolet and Soft X-ray Region Richard L. Sandberg, David D. Allred, Marie K. Urry, Shannon Lunt, R. Steven Turley Thanks to Fellow EUV Members: Jed E. Johnson, Winston Larson, 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 August 4, 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 August 4, 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 August 4, 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 August 4, 2004 Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

6. BYU EUV Optics August 4, 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). UO2 deposited in two ways. Reactively sputtered (DC) in oxygen partial pressure (Lunt at oxygen partial pressure of 3x10-4 torr) or as pure uranium (RF) and then allowed to oxidize in ambient air. Uranium nitride was reactively sputtered (RF) in nitrogen partial pressure of about 10-5 torr. • 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 August 4, 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)

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9. BYU EUV Optics August 4, 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 August 4, 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/

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14. Calculating Optical Properties of Uranium Oxide and Uranium Nitride from Reflectance Lunt and Urry both used Kohn and Parratt’s equations to calculate reflectance from previously published value of δ and β. The values of δ and β were then adjusted until the difference between the measured reflectance and calculated reflectance were minimized. The measured reflectance scans were angle scans from the ALS. Urry studied uranium nitride and Lunt studied uranium oxide. where V.G. Kohn. Phys. Stat. Sol. 185(61), 61-70 (1995), L.G. Parratt. Physical Review 95 (2), 359-369 (1954).

15. Optical Properties of Uranium Oxide and Uranium Nitride Lunt found that her samples were mostly UO2 with a top layer of an slightly higher oxidation state.

16. BYU EUV Optics August 4, 2004 Measured Data compared with CXRO Previously Published Constants Measured reflectance features do not agree with CXRO published constants. More work need to be done on measuring uranium’s optical constants.

17. BYU EUV Optics August 4, 2004 XANES (X-Ray Absorption Near Edge Spectroscopy) XANES at ALS show additional absorption resonances not accounted for in Published 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.

18. BYU EUV Optics August 4, 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 previously published uranium optical properties. • 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 current uranium optical properties. Questions? EUV Group Contact Dr. David Allred allred@byu.edu (801) 422-3489 THANK YOU!!