Efficient and uniform Aluminum-based broadband mirrors are essential components for far-ultraviolet (FUV) astronomy. Plasma-enhanced atomic layer deposition (PEALD) is a low temperature, highly conformal coating process that has previously been demonstrated to produce high quality AlF3 films, although little has been reported on their performance in FUV applications. An ongoing collaboration between the US Naval Research Laboratory (NRL) and NASA Goddard Space Flight Center (GSFC) focuses on optimizing material properties of PEALD AlF3 coatings on Al mirrors to enhance FUV optical performance.
PEALD AlF3 films were deposited using trimethylaluminum and SF6 plasma precursors in a modified Veeco Fiji G2 reactor. ALD growth windows (the range of process parameters resulting in ideal growth) were established using an in situ ellipsometer to monitor the fluoride growth rate directly on Al substrates and supplemented with post-deposition x-ray photoelectron spectroscopy to elucidate process-structure property relationships. Optimal AlF3 films had a growth rate of 0.75-0.8Å/cycle, F/Al ratio of ≈3, < 2 at% O, indicating that PEALD is a beneficial process technique towards achieving optical coatings on a variety of potential mirror materials. The influence of PEALD parameters on the FUV optical performance of Al mirrors overcoated with PEALD-AlF3 will be also discussed.
Astronomical space telescopes to study astrophysical phenomena from the far ultraviolet (FUV) to the near infrared (NIR) will require mirror coatings with high reflectance over this entire spectral region. While coatings for the optical and NIR part of the spectrum are fairly well developed with proven performance, the FUV range has presented significant challenges, particularly below 120nm. Recent developments in electron-beam (e-Beam) generated plasma treatment in a SF6 environments has enabled the effective passivation of aluminum (Al) coatings for applications in the FUV, by native oxide removal and the formation of a AlF3 passivation layer which could be tuned to any desired AlF3 thickness. These results have been produced through a collaboration between the Goddard Space Flight Center (GSFC) and the Naval Research Laboratory (NRL). The passivation experiments have been carried out using the Large Area Plasma Processing System (LAPPS) at NRL using bare aluminum samples and provided by the coating group at GSFC. This novel procedure has demonstrated improved Al mirrors with state-of-the-art FUV reflectivity (e.g. R=91% at 121.6nm). In this paper, we will be reporting on environmental testing, micro-roughness, as well as polarization studies of these E-beam treated samples. These characterizations are being done in order to advance the Technology Readiness Level (TRL) for these Al+AlF3 mirror coatings produced at LAPPS. The ultimate goal is to demonstrate the promise of using this coating technology to deliver reflectance performance plus stability and uniformity over a large area for a future IR/O/UV space telescope observatory.
In previous work we demonstrated the feasibility of a new plasma process based on electron beam-generated plasmas in SF6 environments that effectively passivates the surface of aluminum mirror samples for applications in the UV/O/IR (ultraviolet/optical/infrared) by removing the native oxide layer and producing an AlF3 passivation layer with tunable thickness. This process provides good results in terms of far ultraviolet reflectivity, environmental stability, uniformity, polarization aberration, surface roughness, and does not require elevated substrate temperatures or ultra-high vacuum conditions. In this communication we show that, in addition to these characteristics, Al mirrors can be passivated faster with NF3 than with SF6 over a wide range of process parameters without the loss of optical performance independent of the working-gas choice adopted for the plasma.
Efficient mirrors with high reflectivity over the ultra-violet, optical, and infra-red (UVOIR) spectral range are essential components in future space-based observatories. Aluminum mirrors with fluoride-based protective layers are commonly the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their optical performance to date, their reflectivity is still insufficient for future large telescope instrumentation in which several reflections are required.
Recently, a novel passivation procedure based on the exposure of bare Al to a fluorine containing electron beam generated plasma has been presented [1,2]. This research is framed in a collaboration between Goddard Space Flight Center (GSFC) and the U.S. Naval Research Laboratory (NRL), with plasma treatment carried out in NRL’s large area plasma processing system (LAPPS) using aluminum coated glass samples produced at GSFC coating facilities. The passivation of the bare Al is accomplished by using an electron-beam generated plasma produced in a fluorine-containing background to simultaneously remove the native oxide layer while promoting the formation of an AlF3 passivation layer with tunable thickness. Importantly, this new treatment uses benign precursors (SF6) and is performed at room temperature. In this work, details of the plasma process and in situ surface monitoring with spectroscopic ellipsometry are discussed. This novel procedure has demonstrated improved Al mirrors with state of the art far-ultraviolet (FUV) (λ = 90-200 nm) reflectivity (e.g. R=91% at 121.6 nm) paired with an excellent thickness control of the Al protective layer.
Efficient ultraviolet (UV) mirrors are essential components in space observatories for UV astronomy. Aluminum mirrors with fluoride-based protective layers are commonly the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their acceptable optical performance, the single-bounce reflectance values are still too low for use in optical systems in which several reflections are required. Recently, a novel passivation procedure based on the self-fluorination of bare Al has been presented [1, 2]. This research is framed in a collaboration between the Goddard Space Flight Center (GSFC) and the Naval Research Laboratory (NRL), and the experiments are carried out in the Large Area Plasma Processing System (LAPPS) at NRL using bare aluminum samples coated at GSFC coating facilities. The passivation of the oxidized Al is accomplished by using an electron-beam generated plasma produced in a fluorine-containing background to simultaneously remove the native oxide layer while promoting the formation of an AlF3 passivation layer with tunable thickness. Importantly, this new treatment uses benign precursors (SF6) and does not require high substrate temperatures. This novel procedure has demonstrated improved Al mirrors with enhanced FUV reflectivity. Examples of mirrors tuned at several key FUV wavelengths are provided. The LAPPS has been recently upgraded to include a new spectroscopic ellipsometer for real-time, in situ measurements of film thickness and optical constants of the fluoride layer during the plasma treatment. Since this new capability requires precise knowledge of the complex refractive index (n,k) of AlF3, we present optical constants in the 90-2500 nm range obtained from Al mirrors previously prepared using the LAPPS process. The derived optical properties from the AlF3 passivation layer show similar optical properties in the FUV when compared with PVD- and ALD- hot-deposited AlF3.
Astronomical space telescopes to study astrophysical phenomena from the far-ultraviolet (FUV) to the near infrared (NIR) will require mirror coatings with high reflectance over this entire spectral region. While coatings for the optical and NIR part of the spectrum are fairly well developed with proven performance, the FUV presents significant challenges. The U.S. Naval Research Laboratory (NRL) has developed a processing system based on an electron beam-generated plasma that provides for controlled fluorination and/or etching of surfaces with near monolayer precision and minimal changes to surface morphology. In this paper, we report recent results of samples treated in the NRL Large Area Plasma Processing System (LAPPS) where restoration of the high intrinsic reflectance in the FUV spectral range have been observed of aluminum (Al) mirrors protected with a magnesium di-fluoride (MgF2) overcoat. This paper will also extend these studies to other un-protected Al mirrors protected to demonstrate the capability of LAPPS to simultaneously etch the native oxide layer from bare Al and passivate the surface with fluorine, leading to marked enhancements in FUV reflectance. Laboratory test data and optical diagnostic techniques used to verify surface scattering and durability of selected coatings will be presented. Finally, we will discuss the scalability of the LAPPS etching process in order to realize these high-reflectivity coatings on mirror segments as large as those proposed for the Large Ultraviolet, Optical, and Infrared (LUVOIR) astronomical telescope system (1+meter class).
This paper will describe efforts at developing broadband mirror coatings with high performance that will extend from infrared wavelengths down to the Far-Ultraviolet (FUV) spectral region. These mirror coatings would be realized by passivating the surface of freshly made aluminum coatings with fluorine ions in order to form a thin AlF3 overcoat that will protect the aluminum from oxidation and, hence, realize the high-reflectance of this material down to its intrinsic cut-off wavelength of 90 nm. Improved reflective coatings for optics, particularly in the FUV region (90-120 nm), could yield dramatically more sensitive instruments and permit more instrument design freedom.
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