Thorium-Based Thin Films as Highly Reflective Mirrors in the EUV
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Thorium-Based Thin Films as Highly Reflective Mirrors in the EUV Jed E. Johnson*, David D. Allred*+, R. Steven Turley*, William R. Evans*, and Richard L. Sandberg** *Brigham Young University, **now Univ. of Colorado. [email protected]; phone: (801)-422-3489; www.physics.byu.edu/faculty/allred/cv/05CV.htm 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.
INTRODUCTION Technological advances in recent years have increased the demand for the controlled use of higher energy radiation. The extreme ultraviolet (EUV) and soft x-ray portions of the electromagnetic spectrum (~0.5 to ~50 nm) in particular have the potential for a number of important applications. Smaller computer chips could be fabricated using lithography techniques, which are presently limited by longer optical wavelengths.1 The biological community also recognizes the advantages of the EUV. Soft x-ray microscopy is promising because water is somewhat transparent below 4.4 nm, whereas carbon’s higher absorption helps resolve fine organic structures.2 Additionally, distant astronomical data could be analyzed more completely with increased reflection capabilities.3 Uranium has long been identified by researchers as an element which would theoretically exhibit high reflectance in this range because the deviation of the real part of its index of refraction from unity is significantly larger than any other element. Since the late 1990’s, the BYU XUV Group has produced uranium-containing multilayers (U/Si specifically) and uranium compound films for space applications and theoretical reflectance verification. While it is still a promising material deserving of additional attention, there are critical issues which limit its practical effectiveness. Uranium oxidizes readily. When exposed to air for o
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