Selected Publications

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By Jed E. Johnson, David D. Allred, R. Steven Turley, William R. Evans, and Richard L. Sandberg
Abstract: As applications for extreme ultraviolet (EUV) radiation have been identified, the demand for better optics has also increased. Thorium and thorium oxide thin films (19 to 61 nm thick) were RF-sputtered and characterized using atomic force microscopy (AFM), spectroscopic ellipsometry, low-angle x-ray diffraction (LAXRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure (XANES) in order to assess their capability as EUV reflectors. Their reflectance and absorption at different energies were also measured and analyzed at the Advanced Light Source in Berkeley. The reflectance of oxidized thorium is reported between 2 and 32 nm at 5, 10, and 15 degrees from grazing. The imaginary component of the complex index of refraction, β, is also reported between 12.5 and 18 nm. Thin films of thorium were found to reflect better between 6.5 and 9.4 nm at 5 degrees from grazing than all other known materials, including iridium, gold, nickel, uranium dioxide, and uranium nitride. The measured reflectance does not coincide with reflectance curves calculated from the Center for X-Ray Optics (CXRO) atomic scattering factor data. We observe large energy shifts of up to 20 eV, suggesting the need for better film characterization and possibly an update of the tabulated optical constants.
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By Guillermo Acosta, David D. Allred, and Robert C. Davis
Abstract: We describe a technique which allows for atomic force microscopy to be used to make a physical measurement of the thickness of thin film samples. When dealing with a film which is ultrathin (<100 nm), standard measurement techniques may become difficult to apply successfully. The technique developed involves the fabrication of a distinct, abrupt step on the film surface, using a device we call the Abruptor. This step can be scanned with an atomic force microscope, revealing the height of the step. Films from 6-15 nm are now routinely measured in this way, though it is possible to apply this measurement technique to thinner and thicker films. The thinnest film we measured was 3.6 nm.
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By Kristi Adamson, Shannon Lunt, Richard Sandberg, Elke Jackson, David Allred, and R. Steven Turley
Abstract: We have studied thin films (100-200Ǻ) of uranium oxide created through DC magnetron sputtering. The oxidation of the uranium surface has been examined through x-ray photoelectron spectroscopy (XPS). This work shows that the surface does not oxidize immediately, but over a period of several weeks. By comparison with the work of Teterin [1] (“A Study of Synthetic and Natural Uranium Oxides by X-Ray Photoelectron Spectroscopy.” J. Phys. Chem. Minerals. 1981), our thinner samples are a mixture of UO2 and γ-UO3, with γ-UO3 becoming more prominent as the sample has more time to oxidize. The surface of the sample oxidizes more quickly than the rest of the sample.
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By Luke J. Bissel, David d. Allred, R. Steven Turley, William R. Evans, and Jed. E. Johnson
Abstract: Ruthenium is one material that has been suggested for use in preventing the oxidation of Mo/Si mirrors used in extreme ultraviolet (EUV) lithography. The optical constants of Ru have not been extensively studied in the EUV. We report the complex index of refraction, 1 - δ + iβ, of sputtered Ru thin films from 11-14 nm as measured via reflectance and transmission measurements at the Advanced Light Source at Lawrence Berkley National Laboratory. Constants were extracted from reflectance data using the reflectance vs. incidence angle method and from the transmission data by Lambert"s law. We compare the measured indices to previously measured values. Our measured values for delta are between 14-18% less than those calculated from the atomic scattering factors (ASF) available from the Center for X-ray Optics (CXRO). Our measured values of beta are between 5-20% greater than the ASF values.
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By Richard L. Sandberg, David d. Allred, Shannon Lunt, Marie K. Urry, and R. Steven Turley
Abstract: Uranium oxide and uranium nitride thin films reflect significantly more than all previously known/standard reflectors (e.g., nickel, gold, and iridium) for most of the 4-10 nm range at low angles of incidence. This work includes measurements of the EUV/soft x-ray (2-20 nm) reflectance of uranium-based thin films (~20 nm thick) and extraction of their optical constants (δ and β). We report the reflectances at 5, 10, and 15 degrees grazing incidence of air-oxidized sputtered uranium, reactively sputtered (O2) uranium oxide, and reactively sputtered (N2) uranium nitride thin films measured at Beamline 6.3.2 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL). Additionally, we report optical constants of reactively sputtered uranium oxide at nine wavelengths from 4.6 to 17.5 nm derived from ALS angle-scan reflectance measurements. We also report optical constants of uranium nitride at 13 and 14 nm. We compare the reflectance of these uranium-compound thin films to gold, nickel (and nickel oxide), and iridium thin films from 2.5 to 11.6 nm. These metal thin films were chosen for comparison due to their wide use in EUV/soft x-ray applications as low-angle, thin-film reflectors. The uranium compounds can exhibit some surface oxidation in ambient air. There are important discrepancies between UO2"s and UN"s actual thin-film reflectance with those predicted from tabulated optical constants of the elemental constituents of the compounds. These differences are also demonstrated in the optical constants we report. Uranium-based optics applications have important advantages for zone plates, thin-film reflectors, and filters.
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By Richard L. Sandberg, David D. Allred, Luke J. Bissell, Jed E. Johnson, and R. Steven Turley
Abstract: We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized uranium and naturally oxidized nickel thin films from 100-460 eV (2.7 to 11.6 nm) at 5 and 15 degrees grazing incidence. These show that uranium, as UO2, can fulfill its promise as the highest known single surface reflector for this portion of the soft x-ray region, being nearly twice as reflective as nickel in the 124-250 eV (5-10 nm) region. This is due to its large index of refraction coupled with low absorption. Nickel is commonly used in soft x-ray applications in astronomy and synchrotrons. (Its reflectance at 10° exceeds that of Au and Ir for most of this range.) We prepared uranium and nickel thin films via DC-magnetron sputtering of a depleted U target and resistive heating evaporation respectively. Ambient oxidation quickly brought the U sample to UO2 (total thickness about 30 nm). The nickel sample (50 nm) also acquired a thin native oxide coating (<2nm). Though the density of U in UO2 is only half of the metal, its reflectance is high and it is relatively stable against further changes.