High Aspect Ratio 3D Microstructures

The far-UV (FUV) reflectance of the state-of-the-art, broadband UV/optical/IR mirrors of XeF2-passivated LiF on Al (Al + XeLiF) is promising for future space telescope missions. To reach their potential, dependable cleaning procedures and storage methods for such reflective surfaces need to be developed. First Contact™ polymer (FCP) formulations have proven to be a reliable method for cleaning conventional mirror surfaces coated with oxides or bare metal and for protecting them in storage. We report here on studies of the cleaning and storage of Al + XeLiF samples using customized FCP formulations designed by Photonic Cleaning Technologies. Cleaning of such mirrors is demanding since fluoride coatings are softer than oxides and can be moisture sensitive. Any damage that marks the overcoat can lead to catastrophic loss of FUV reflectance due to surface roughening and formation of aluminum oxide, which is FUV opaque. We discovered that one formulation could be successfully applied to and removed from Al + XeLiF coatings multiple times. The coatings retained low roughness, minimal aluminum oxide thickness, and high far-UV reflectance. Another of the four FCP formulations successfully cleaned the Al + XeLiF coatings several times. Variable-angle, spectroscopic ellipsometry, tapping-mode atomic force microscopy, x-ray photoelectron spectroscopy, and FUV reflectance allowed us to observe any changes in reflectance and surface roughness, the formation of aluminum oxide, and damage to coating integrity. From the studies of the range of FCP-fluoride interactions, we noted that too much polymer-to-surface adhesion or exposure to trace water in the polymer can result in coating damage.

We present an undergraduate optics instructional laboratory designed to teach skills relevant to a broad range of modern scientific and technical careers. In this laboratory project, students image a custom aperture using coherent diffraction imaging, while learning principles and skills related to digital image processing and computational imaging, including multidimensional Fourier analysis, iterative phase retrieval, noise reduction, finite dynamic range, and sampling considerations. After briefly reviewing these imaging principles, we describe the required experimental materials and setup for this project. Our experimental apparatus is both inexpensive and portable, and a software application we developed for interactive data analysis is freely available.

A new deposition method developed by Goddard Space Flight Center fluorinates Al mirrors with XeF2 followed by a LiF coating to create what they term Al+XeLiF. This in-situ, room temperature process produces mirrors with high reflectivity in a broad spectral range, from the FUV to the IR, and is reported to be stable in relative humidities of 30% and lower. These mirrors are envisioned for missions requiring sensitivity down to 100 nm wavelength such as the habitable worlds observatory. Because most mission integration and testing campaigns require prolonged exposure to lab environments, and launch sites experience high relative humidities (RH) on average, some at 80% RH. We investigate Al+XeLiF stability in a wider range of temperatures and humidities along with employing additional characterization techniques including atomic force microscopy and x-ray photoelectron spectroscopy. We found that Al+XeLiF is stable in environments up to 82%RH when kept at cooler temperatures (3°C and 21°C). However, this material is unstable when stored at 60°C, experiencing roughening and loss in reflection from resulting Al surface plasmon excitation.

Astronomical instrumentation for measurements in the Far Ultraviolet (FUV, 90−200 nm) have historically considered aluminum (Al) thin film mirrors due to this material high reflectance over this wavelength range. However, the native aluminum oxide layer that forms on Al upon exposure to the atmosphere is strongly absorbing in this wavelength range, requiring that the films be protected with a dielectric that inhibits oxidation. Typically, magnesium fluoride (MgF₂) or lithium fluoride (LiF) coatings are used as protective layers, but each has shortcomings. For example, MgF₂ has an absorption cutoff at 115 nm that reduces performance below this wavelength, which is a critical part of the FUV spectrum for observational astrophysics. The use of LiF as a protection for Al provides a lower absorption cutoff at 100 nm, but it is hygroscopic and thus susceptible to degradation in humid conditions. Our team at GSFC has developed a new reactive Physical Vapor Deposition (rPVD) process that consists of a fluorination process with XeF₂ gas combined with our traditional PVD process. We have found that this new rPVD process produces Al+XeF₂+LiF (XeLiF) and Al+XeF₂+MgF₂ (XeMgF₂) mirror coatings with unprecedented reflectance. In addition, the rPVD process seems to produce much more environmentally stable coatings (when compared to the conventional process without the XeF₂ fluorination). We report on IR/Vis/UV reflectance of XeLiF and XeMgF₂ mirrors. The surface roughness as well as the FUV reflectance measured over a period of 8 months for a XeLiF sample with a relatively thin (≃ 30 nm) Al layer are also reported. We have also been investigating the compatibility of this rPVD coating process for potential efficiency enhancements of Si-based gratings. Since it is known that the XeF₂ vapor is a strong Si etchant, we are investigating if the native SiO₂ layer on Si is sufficient to protect the groove profile of E-beam-ruled Si gratings from degradation. Preliminary results indicate that the native SiO₂ layer is an effective barrier against etching of Si by XeF₂.

The ubiquity of particulate contamination requires dust mitigation techniques to provide low scatter surfaces and edges on sensitive optical devices in space. Poly(olefin sulfone)s have been shown to photodegrade with the assistance of a photobase generator when exposed to UV light (254 nm) and heat (120 °C). These may be applicable in minimizing dust on optical surfaces for space applications. However, their behavior in vacuum has not been fully characterized. We synthesized poly(2-methyl-1-pentene sulfone) (PMPS) and poly(1-hexene sulfone) (PHS) with and without a photobase generator. We studied the photodegradation (172 nm or 254 nm) of thin films in vacuum. Spectroscopic ellipsometry was used to quantify film thickness over time. The PMPS and PHS films both degraded when exposed to UV light in vacuum, though PHS to a lesser degree. We found that heat was not required to cause degradation, and that degradation occurred with UV irradiation even without a photobase generator. This investigation shows that poly(olefin sulfone)s could be used to protect optical surfaces until their deployment in space.

Windows for vacuum ultraviolet (VUV) sources are valuable for many applications but difficult to fabricate due to most materials being too absorptive at VUV wavelengths. We have designed, fabricated, and characterized a carbon nanotube (CNT) collimator as a window with high (VUV) transmission and significant differential pumping. The CNT collimators are arrays of square channels of various dimensions and height with sidewalls composed of vertically aligned CNT forests. The CNT collimators in this work exhibited peak intensity transmissions for VUV light (58.4 nm) of 18%–37% of that reported for the same system without a collimator present [S. Olsen, D. Allred, and R. Vanfleet, J. Vac. Sci. Technol. A (2024)]. Further analysis found that the peak intensity transmissions were lowered due to carbon deposition on the phosphor viewing screen from contaminants. The CNT collimator also had significant sidewall reflection in the VUV range (⁠R = 0.21 +/- 0.08) in the VUV range for angles 15.6 degrees and below). Pressure ratios (low pressure over high pressure) in the VUV transmission experiment were dominated by leaks in the alignment mechanism. Additional experiments demonstrated the CNT collimator’s reflection and superior differential pumping with pressure ratios less than 0.001.