High Aspect Ratio 3D Microstructures

Hollow cathode plasmas are common extreme ultraviolet (EUV) lamps used for material characterization. However, the relatively high pressure of the plasma can affect downstream instruments, as well as absorb the EUV. EUV windows are difficult to fabricate due to EUV’s strong interaction with all materials. We present a carbon nanotube (CNT) microfabricated window composed of multiple high aspect-ratio columns in parallel. The open areas allow wide bandpass transmission, while the walls restrict gas flow. We model the CNT window transmission as a weight function on the light from of a Mcpherson 629-like hollow cathode helium plasma in visible wavelengths. We model the CNT window differential pumping as a series of columns between two chambers of different pressures.

Four evaporated, thin-film Al samples protected by a thin (29±2 nm) aluminum fluoride (AlF3) overcoat stored in dry (dew point 276K ), 327 K air over a period of 2500 hours exhibited no significant changes in the thickness of the protective AlF3 layer nor growth in aluminum oxide as observed by variable-angled, spectroscopic ellipsometry. Two of the samples had AlF3 evaporated at T>200°C, two without substrate heating. No difference in aging was noted amongst the samples. Since many months may elapse between fabrication and launch of the completed observatory, this result contributes to understanding the boundaries in temperature and humidity separating negligible changes in fluoride-containing optical components from unacceptable degradation. While negligible changes in thicknesses were observed, there were changes in the ellipsometric data, psi and delta, with time. In this study, we also present our use of an effective medium approximation model in understanding changes in the fluoride layer with aging. The observed changes in SE parameters are here interpreted as changes in void fraction, though the presence of some water was not ruled out. Apparent void fraction fell by a factor of two by the end of the 2500 hours. The decreasing void fraction suggests that the films might be becoming more compact with time. Other surface sensitive techniques such as AFM are needed to narrow down possible explanations for observed changes.

Aluminum mirrors protected by metal fluoride overcoats are crucial for FUV observations. Many contemplated missions specify optics elements with high reflectance down to 103 nm (Lyman beta). Lithium fluoride (LiF) has the highest band gap of any solid material and thus finds applications in FUV optics. However, LiF is difficult to work with because of its hygroscopic nature. The instability of these films was investigated by evaporating LiF onto silicon wafers and aging in environments with different relative humidities and temperatures. Samples were characterized using variable-angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM). From these methods we found that storing LiF in a hot environment improved sample surface stability, and that in dry, hot environments, surfaces became more smooth after many hours in storage.

This chapter considers the future of wide-field, plasmaspheric extreme ultraviolet (EUV) imaging, including reviews of previous work as well as some new material. We begin with a review of the technological and scientific progress made by the EUV imager that operated onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft from 2000 to 2005. Analysis of the future of plasmaspheric EUV imaging is organized into three major topics. The first major topic is 30.4 nm imaging of terrestrial He+ ions. We consider two improved 30.4 nm camera designs, and their use to study important science topics such as fine-scale structure, erosion, and refilling. We analyze the benefits of three notional mission designs: continuous and/or stereo imaging from a high-inclination circular orbit, side-view imaging from geosynchronous orbit, and side-view imaging from the Moon. The second major topic is 83.4 nm imaging of terrestrial O+ and O++ ions. We review the use of EUV imaging to provide much-needed system-level measurements of the dense oxygen torus—whose origin and global distribution remain unknown after decades of in situ observations. Simulated 83.4 nm images demonstrate the scientific value of macroscale information about the oxygen torus. The third major topic is near-68 nm EUV imaging of the S++ ions in the Io plasma torus around the planet Jupiter. We review the Io torus's central role in driving convection in the Jovian magnetosphere, and the need for imaging to capture fundamental elements of this process. To achieve Io torus imaging we introduce a new EUV camera identical to IMAGE EUV except for a new multilayer mirror coating optimized for 68 nm. We present simulated images to illustrate EUV imaging's enormous potential to explore, observe, and understand the Io plasma torus.

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.