High Aspect Ratio 3D Microstructures

Selected Publications

There is a natural tendency for students to act first (e.g. - build and conduct experiments) and think later (e.g. - outline goals, identify challenges, predict outcomes, etc.). This is often apparent in labs that include student design components. We have developed a lab course structure that teaches students how to develop their ideas and make plans before beginning an experiment by providing multiple opportunities for peer and instructor feedback. As a result, we have seen significant improvements in the success rate and quality of student-designed experiments and presentations. We provide a detailed explanation of the course structure and rubrics and evidence of the impacts of this course structure.
Michael Greenburg, David D. Allred, and R. Steven Turley

We report on the results of computationally designing and optimizing

multilayer mirrors for broadband reflectivity in a region spanning from

the vacuum ultraviolet to the infrared. Such a mirror would open up

new possibilities for future space observatories. Because of the

immense number of possible layer combinations and thicknesses for a

multilayer mirror, we automated the mirror selection process with a

genetic algorithm. Starting with a random object population within the

simulation, a genetic algorithm iteratively selects and mutates the best

portion of a population of objects that fit given design criteria to create

a new population; this can be repeated as many times as desired. Our

genetic algorithm yielded a high broadband reflectance mirror, which

was then optimized via gradient search within the program. We found

that placing a few layers under an aluminum coating can significantly

increase extreme ultraviolet reflectivity, which would give access to

important spectral lines such as that of the dominant He-II transition.

David D. Allred, R. Steven Turley, Stephanie M. Thomas, Spencer G. Willett, Michael J. Greenburg, and Spencer B. Perry
Protective layers on aluminum mirror surfaces which can be removed via the use of atomic hydrogen or hydrogen plasmas at the point of use in space may allow an expansion of broad-band mirrors into the EUV. LUVOIR (large, UV-optical-IR telescope) is a potential NASA flagship space-based observatory of the 2020’s or 30’s. It would utilize the largest mirrors ever flown1 . Their reflective coating will almost certainly be aluminum, since such telescopes would profit from truly broad-band mirrors. To achieve reflectance over the broadest band, the top surface of such aluminum mirrors, however, needs to be bare, without the oxide layers that naturally form in air. This will open the 11 to 15 eV band. Since thin aluminum films are largely transparent between 15 and 70 eV an EUV mirror under the aluminum could make EUV bands such as 30.4 nm available for space-based astrophysics without sacrificing mirror IR, visible and UV reflectance. The local space environment for the observatory is sufficiently oxygen-free that the surface should remain bare for decades. We discuss protecting as-deposited aluminum mirrors with robust, oxygenimpenetrable, barrier layers applied in vacuo to the aluminum immediately after deposition and before air contact. The goal is that the barrier could also be cleanly, and relatively easily, removed once the mirror is in space. We propose hydrogen atoms as the means for removing the overcoat, since they can be expected to meet the criteria that the means is gentle enough to not roughen the mirror surface, and does not redeposit material on the mirror or other spacecraft components. We have investigated both organic and inorganic (such as, a-Si) hydrogen-removable films that can be applied to the aluminum immediately after its deposition have been investigated. We also examined the REVAP technique, using Cd and Zn. Agglomeration limited their effectiveness as barrier layers. That and dealing with the reevaporated atoms may limit their utility as barrier materials.

Oxidation of a ∼1000 Å sputter-deposited thorium thin film at 150 °C in 100 ppm of flowing oxygen in argon produces the long-sought solid form of thorium monoxide. Changes in the scattering length density (SLD) distribution in the film over the 700-min experiment measured by in-situ, dynamic neutron reflectometry (NR) shows the densities, compositions and thickness of the various thorium oxides layers formed. Screened, hybrid density-functional theory calculations of potential thorium oxides aid interpretation, providing atomic-level picture and energetics for understanding oxygen migration. NR provided evidence of the formation of substoichiometric thorium oxide, ThOy (y < 1) at the interface between the unreacted thorium metal and its dioxide overcoat which grows inward, consuming the thorium at a rate of 2.1 Å/min while y increases until reaching 1:1 oxygen-to-thorium. Its presence indicates that kinetically-favored solid-phase ThO can be preferentially generated as a majority phase under the thermodynamically-favored ThO2 top layer at conditions close to ambient.

Matthew R. Jones, Dale R. Tree, and David D. Allred (et al.)
An optical fiber thermometer consists of an optical fiber whose tip is coated with a highly conductive, opaque material. When heated, this sensing tip becomes an isothermal cavity that emits like a blackbody. This emission is used to predict the sensing tip temperature. In this work, analytical and experimental research has been conducted to further advance the development of optical fiber thermometry. An inexpensive optical fiber thermometer is developed by applying a thin coating of a high-temperature cement onto the tip of a silica optical fiber. An FTIR spectrometer is used to detect the spectral radiance exiting the fiber. A rigorous mathematical model of the irradiation incident on the detection system is developed. The optical fiber thermometer is calibrated using a blackbody radiator and inverse methods are used to predict the sensing tip temperature when exposed to various heat sources.
Lawrence K. Barrett, Dallin J. Barton, Steven G. Noyce, David D. Allred, Richard R. Vanfleet, and Robert C. Davis
High-aspect-ratio metallic microstructures have a variety of potential applications in sensing and actuation. However, fabrication remains a challenge. We have fabricated nickel microstructures with over 20:1 aspect ratios by electroplating patterned carbon-coated carbon-nanotube forests using a nickel chloride bath. Pulse plating allows nickel ions to diffuse into the interior of the forest during off portions of the cycle. Done properly, this solves the problem of the formation of an external crust, which otherwise blocks nickel deposition in the interior of the structures. Thus, densities of 86 ± 3% of bulk Ni for the composite structures are achieved. Cantilever structures do not yield under load, but break. Measurements of the material properties of this composite material indicate an elastic modulus of ~42 GPa and a strength of 400 MPa. We demonstrate the utility of this method with an external field magnetic actuator consisting of a proof mass and two flexures. We achieved 1-mN actuation forces. [2014-0274]