1 Uranium Oxide as a Highly Reflective Coating from 2.7 to 11.6 Nanometers William R. Evans, Richard L. Sandberg, David D. Allred*, Jed E. Johnson, R. Steven Turley Department of Physics and Astronomy Brigham Young University phone ; fax ; xuv.byu.edu BYU EUV Optics October 25, 2003

2 Overview RESULTS 1.Uranium reflects more than nickel, the current metal of choice for soft X-Ray applications, between 4 and 11 nm. 2.The atomic scattering factor model (ASF) needs revision for compounds. Oxidized uranium reflectance differs from that predicted by ASF. 3.Reflectance of air-oxidized uranium (UO 2 ) matches reactively sputtered UO 2. OUTLINE –Why EUV and soft X- Rays? –Why uranium? –What do the current models predict? –Methods. Sample preparation Reflectance measurement –Results. –Conclusions. BYU EUV Optics October 25, 2003

3 BYU EUV Optics October 25, 2003 Why Extreme Ultraviolet (EUV) and Soft X-Rays? Images from and EUV Lithography (making really small computer chips) Thin Film or Multilayer MirrorsEUV Astronomy The Earth’s magnetosphere in the EUV Soft X-Ray Microscopes

4 Why Uranium? Uranium has many electrons to interact with photons (light) and is more dense than many materials, causing them to interact with high energy EUV photons. High Theoretical Reflectivity: Low absorption and high index of refraction Previous Success: IMAGE Satellite Mirror Project (Launched March 25, 2000) BYU EUV Optics October 25, 2003

5 Note: Nickel and its neighboring 3d elements are the nearest to uranium in this area. Delta vs. beta plot for several elements at 4.48 nm 4.48nm BYU EUV Optics October 25, 2003

6 Reflectances for Ni, NiO, U, and UO2 predicted by the atomic scattering factor model from the Center for X-Ray Optics (CXRO) website (www-cxro.lbl.gov). BYU EUV Optics October 25, 2003

7 Schematic of DC magnetron sputtering system at BYU. Sample Preparation The uranium oxide and nickel 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 Sputtering The uranium sputter targets used here were of depleted uranium metal (less than 0.2% U-235). After sputtering, the uranium was allowed to oxidize naturally in laboratory air. Ni Resistive Thermal Evaporation Evaporated Ni wire from a resistively heated tungsten boat (RD Mathis Co.) in a large, cryopumped, stainless steel “bell jar” coater. BYU EUV Optics October 25, 2003

8 Taking Reflectance Measurements at the Advanced Light Source (ALS) Beamline Schematic of ALS beamline courtesy of Beamline Reflectometer Bright synchrotron radiation nm range High spectral purity Energy/wavelength or θ-2θ scan capability ALS Reflectance Measurements UOx means UO2, U most abundant natural oxide NiO on Ni means Ni sample on Si oxidized by UV cleaning lamp Ni on quartz means Ni on quartz slide * Discrepancies in data appear between wavelength scans because samples were removed and from chamber and then re-measured. BYU EUV Optics October 25, 2003

9 Measured reflectances of UOx, NiO on Ni, and Ni on quartz at 5 degrees from nm

10 Measured reflectance at 10 degrees of UO2, NiO on Ni, Ni on Quartz from nm

11 Measured reflectance at 15 degrees of UO2, NiO on Ni, and Ni on Quartz from nm.

12 [i] Sandberg et a., Advances in Mirror Technology for X-Ray, EUV Lithography, Laser, and Other Applications, Ali M. Khounsary, Udo Dinger, Kazuya Ota, Editors, Proc. SPIE 5193, SPIE, Bellingham, WA, [ii] Shannon Lunt, Determining the Indices of Refraction of Reactively Sputtered Uranium Dioxide Thing Films from 46 to 584 Angstroms, Masters Thesis, Dept. of Physics and Astronomy, BYU, Provo, UT BYU EUV Optics October 25, 2003

13 BYU EUV Optics October 25, 2003

14 BYU EUV Optics October 25, 2003 Acknowledgements Hollilyn Drury and Megan Rowberry (Provo High School) aided in sputtering the uranium films studied. An SPIE scholarship and department funding aided Richard Sandberg in the research. We also acknowledge gratefully the financial contributions of V. Dean and Alice J. Allred and Marathon Oil Company (US Steel) and Nan Ellen Ah You for gifts to Brigham Young University for thin film research. We would also like to thank the ALS for the beamtime used to make the optical measurements.

15 BYU EUV Optics October 25, 2003 Conclusions 1.Uranium oxide reflects significantly better than nickel, the current material with highest reflectance, between 4 and 11 nm. 2.Uranium oxide reflectance differs from the reflectance predicted by the atomic scattering factor model (ASF). 3.Reflectances of naturally oxidized uranium (UO 2 ) matches reactively sputtered UO 2 –Thus the material can be made in a number of different ways and is stable enough for practical use.