High Aspect Ratio 3D Microstructures

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF₂, are used to protect aluminum mirrors against oxidation. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometric (SE) studies of aluminum oxidation as a function of MgF₂ overlayer thickness (thickness 0-5 nm). Dynamic SE tracks the extent of oxide growth every ca. 2s over a period of several hours after the evaporated Al + MgF₂ bilayer is removed from the deposition chamber. Aluminum oxidation changes under the fluoride layer were quantitatively verified with XPS. Changes in chemical state from Al metal to Al oxide were directly observed. Oxide growth is computed from relative XPS peak areas as corrected for electron attenuation through the MgF₂ overlayer. An empirical formula fits time-dependent data for aluminum surfaces protected by MgF₂ as a function of MgF₂ layer thickness: aluminum-oxide thickness = k_SE*log(t)+b_SE. The slope depends only on MgF₂ thickness, decreasing monotonically with increasing MgF₂ thickness. This method of employing SE coupled with XPS can be extendable to the study of other metal/overlayer combinations.

Lithium fluoride (LiF) is difficult to work with because of its hygroscopic nature (it pulls water out of air). The stability limits of LiF thin films and the nature of their failure when exposed to humid air are poorly understood. We show that LiF films undergo irreversible changes in optical properties and microstructure as determined by ellipsometry and SEM when exposed to dew points greater than 6 C. On the other hand, samples stored at a dew point of -22 C (4% RH at room temperature), showed only small changes in ellipsometric parameters. The ones stored at intermediate humidity 6 C (21% RH at room temperature) showed larger changes in ellipsometric parameters. SEM shows that deliquescence as well as efflorescence is important in LiF thin films. In situ spectroscopic ellipsometric measurements using a controlled variable humidity environment illuminates the changes in LiF thin films moving from moisture absorption to complete deliquescence.

Aluminum is the best choice of material for broadband mirrors. However, once an oxide layer forms on the surface of the mirrors the reflectance in the far ultraviolet range decreases. The study of Al mirrors is difficult because they oxidize so quickly in the air. This makes reproducibility and joint work between laboratories difficult because the mirrors will oxidize and make successive measurements inaccurate. We have found that storing aluminum thin-film mirrors in low oxygen environments (such as liquid nitrogen, dry ice, and hexane) retards mirror oxidation. We examined the retardation of the growth of aluminum oxide during storage in these environments. This oxidation retardation was most pronounced when mirrors were stored in liquid nitrogen. In comparing the growth rate of oxide out of storage to that while it was in storage, we found that the apparent growth of aluminum oxide, is 1/500 in liquid nitrogen, 1/200 in hexane and 1/40 in dry ice.

We report on a large-area, high-aspect-ratio, carbon nanotube (CNT) forest structure produced at BYU acting as a window/separator for a hollow cathode EUV lamp. The structure has large-surface-area, high light trans-mission, and differential pumping. CNT fabrication allows for variable dimensions, which allows various EUV distributions and pressure gradients to be possible. Theory is presented for predicting such distributions and gradients. Several structures have been fabricated; their dimensions, properties, and predicted distributions and gradients are given.

U-6Nb is a uranium alloy containing 6 wt% niobium that possesses high corrosion resistance. The structure and composition of the passivating oxide layer formed on air-aged U-6Nb, which gives the material its corrosion resistant properties, was characterized using surface scattering techniques. Stable oxide layers formed on the surface of a set of U-6Nb alloy thin films under ambient conditions were investigated using neutron reflectometry (NR), x-ray reflectometry (XRR) and grazing incidence x-ray diffraction (GIXD). The passivating oxide was composed of approximately 27% U, 5% Nb, and 68% O, primarily consisting of a thin niobium oxide layer (5.5 ± 0.4 nm) separating a thicker UO2 layer (27.1 ± 2.3 nm) from the underlying U-6Nb alloy. A critical density of enriched niobium oxide at the metal-oxide interface is hypothesized to limit oxygen diffusion and confer high corrosion resistance to the alloy.

Uranium and thorium oxides have critical roles as fuels in existing nuclear power plants, as well as in proposed reactor concepts. The thermal conductivity of these materials determines their ability to transfer heat from the reactor core to the surrounding coolant. Additionally, these actinide compounds are of interest in condensed matter physics because of the 5f orbitals and unique electron interaction, coupling, and scattering events that can occur. Because of the radioactivity of thorium and uranium, thin film measurements of actinide materials are used to limit the amount of operator exposure, but standard thermal characterization methods are not well suited for thin films. This paper presents the process of depositing thin film UO_x and ThO_x samples of nm-μm thicknesses and the results of thermal property measurements. Thin films were deposited on silicon and glass substrates via dc-magnetron sputtering using an argon/oxygen mixture as the working gas. The thermal properties of the films were measured by the Thermal Conductivity Microscope (TCM). This uses one laser to generate thermal waves and a second laser to measure the magnitude and phases of the thermal waves to obtain the conductivity of materials. The results of the research show that the UO_x film properties are lower than bulk values and that the role of the substrate has a considerable effect on determining the measured properties. Future work aims at improving the deposition process. Epitaxial film growth is planned. Additional understanding of thermal property measurements is targeted.