Hierarchically Structured Materials

Selected Publications

Thumbnail of figure from publication
Removing Surface Contaminants from Silicon Wafers to Facilitate EUV Optical Characterization
R. E. Robinson, R. L. Sandberg, D. D. Allred, J. E. Johnson, W. Evans, T. Doughty, K. Adamson, and A. Jacquier (et al.)

The extreme ultraviolet (EUV) is becoming increasingly important. Principal applications include orbital space-based astronomy and lithography for integrated circuit computer chips. A main impediment to further development of efficient mirrors is the lack of reliable optical constants for various materials in this region of the electromagnetic spectrum. One reason for the unreliability of the optical constants is that the sample surfaces are often contaminated with foreign material, especially organic compounds, when exposed to laboratory air. Several cleaning techniques were evaluated, namely: 1) strippable solid optical cleaner (Opticlean®); 2) oxygen plasma etch; 3) high energy UV light/ozone; 4) strippable coating followed by oxygen plasma etch, 5) strippable coating followed by high intensity UV light and, 6) exposure to cold pressurized carbon dioxide (CO2 snow). These processes are compared experimentally based on effectiveness, cleaning time and ease of use. DADMAC (polydiallyldimethyl-ammonium chloride), which forms a layer of known, uniform thickness on silicon wafers is used as a “stand in” for organic contamination. Effectiveness is judged on how well the surface is cleaned. Ellipsometry is used to determine the thicknesses of surface layers. XPS (X-ray Photoelectron Spectroscopy) is used to look for trace contaminants, particularly carbon from the DADMAC. We find that the strippable cleaning coat leaves a residue. Oxygen plasma rapidly removes contaminants, but can quickly oxidize the silicon surface. Exposure to the UV light/ozone for five minutes leaves the surface clean with little additional oxidation. Oxygen plasma or UV light effectively removes the strippable coat residue. Exposure to cold pressurized carbon dioxide has reduced oxide level in one case. The recommended procedure for cleaning bare silicon wafers is strippable coat application followed by 2.5 minutes of exposure to high intensity UV light.

Thumbnail of figure from publication
A comparison of uranium oxide and nickle as single-layer reflectors from 2.7 to 11.6 nm
We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized uranium and naturally oxidized nickel thin films from 2.7 to 11.6 nm at 5°, 10°, and 15° 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 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. There are important discrepancies between UO2"s actual reflectance with those predicted by the atomic scattering factor model indicative of the need to determine the actual constants of UO2.
Thumbnail of figure from publication
What the Universe Means to People Like Me
Thumbnail of figure from publication
Highly Reflective Uranium Mirrors for Astrophysics Applications
David D. Allred, Matthew B. Squires, and R. Steven Turley (et al.)
The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of uranium-nickel alloys for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (C Kα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 44.7 Å (C Kα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. The initial oxidation behavior of sputtered uranium-nickel alloys is similar to pure U so their reflectance was not determined. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required
Thumbnail of figure from publication
Mars Jars
We have prepared our own very small, Martian test tubes and flasks which possess many of the conditions of Mars’ surface (up to the first three of five conditions listed below) and at a cost well below $100**. We did several experiments of interest to us, namely: lightening on Mars in a jar (12-13 year olds) and what might happen to ice cream on Mars (an 8 year old). Student, amateur, and professional researchers alike need Mars-like conditions in which to test their ideas and to help answer their questions. It has been proposed that vacuum stations like those used in industry and university labs could be employed to produce Mars-like conditions to test student ideas for whole classrooms of children.1 It is our goal to find simpler and lower cost ways to help researchers set up their own small test stations where they can test out their ideas. The test stands need only meet those characteristics of Mars which are pertinent for testing the ideas.
Thumbnail of figure from publication
The Optical Constants of Sputtered U and a Si at 30.4 and 58.4 nm
M. B. Squires, David D. Allred, and R. Steven Turley