Open Access
Add to BibSonomy
Abstract
The development of optical systems operating in the far ultraviolet range (FUV, λ=100-200 nm) is limited by the efficiency of passivated aluminum (Al) mirrors. Although it is presently possible to obtain high-reflectivity FUV mirrors through physical vapor deposition, the process involves deposition with substrates at high temperatures, which is technically challenging for large optical elements. A novel passivation procedure for bare Al mirrors is reported. The treatment consisted of using a low-temperature electron-beam generated plasma produced in a gas mixture of Ar and SF6 to etch away the native oxide layer from the Al film, while simultaneously promoting the generation of a thin aluminum tri-fluoride (AlF3) layer on the Al surface. In the first section we analyze the effect of varying both ion energy and SF6 concentration on the FUV reflectance, thickness, composition, and surface morphology of the resulting AlF3 protective layers. In the second section, the reflectivity of samples is optimized at selected important FUV wavelengths for astronomical observations. Notably, samples attained state-of-the-art reflectances of 75% at 108.5 nm (He Lyman γ), 91% at 121.6 nm (H Lyman α), 90% at 130.4 nm (OI), and of 95% at 155.0 nm (C IV). The stability over time of these passivated mirrors is also investigated.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Efficient mirrors are crucial components in space-based observatories particularly for far-ultraviolet astronomy. Al mirrors with a fluoride-based protective layer are the baseline FUV coatings with high reflectivity from ≈120 nm towards the mid-infrared. While these coatings have proven to be stable and reliable, with a long flight heritage, [1,2,3] this approach remains insufficient for the sensitivity requirements of envisioned future optical systems, particularly for space telescopes in which several reflections will be required. To this point, NASA’s technology roadmaps have emphasized that future observatories operating in the FUV range will require mirrors with higher reflectivity in the FUV [4,5]. Improved mirrors would yield dramatically more sensitive instruments and permit more instrument design freedom [6].
Aluminum is the coating of choice for application in the FUV since it provides the highest possible reflectance in this spectral region. However, the surface of bare Al films oxidizes immediately upon contact with air; the thin (≈2 nm) but highly absorbing oxide layer strongly degrades Al reflectance below wavelengths of 200 nm [7]. Recently, there have been notable advances in the production of Al coatings protected with metal fluoride dielectric films. Quijada et al. [8] reported a novel deposition procedure in which an unprecedented reflectivity was obtained, demonstrating a reflectance over ≈0.90 across a large portion of the FUV spectrum. The latter improvement was accomplished by the introduction of a novel physical vapor deposition (PVD) process, which consists of depositing the protective dielectric layer on a heated substrate (≈250 °C), whilst the Al is deposited at room temperature. The reflectance increase of mirror coatings deposited at high substrate temperatures is attributed to a densification of the protective layer along with a reduction in scattered light due to reduced surface roughness. This novel procedure has been tested with several FUV dielectrics, MgF2, LiF, [6,9,10] and AlF3, [11] and it has been reproduced in other laboratories with similar results [12,13,14]. A variation has been also successfully demonstrated where the hot-deposition of the protective layer was replaced by a room-temperature deposition followed by post-annealing in vacuum [12,15]. All these coatings, either produced on heated substrates or post-annealed in vacuum, have proven to be superior in terms of optical performance and stability compared to standard Al mirrors. Although these recent results associated with the utilization of the novel hot deposition procedure represent a breakthrough, they may not be applicable for large missions as it is unrealistic to uniformly heat a large mirror while simultaneously maintaining the original mirror figure [16]. Hence, a new passivation procedure for Al is needed which avoids high temperatures while keeping the intrinsic high reflectivity of Al.
In this work, we developed state-of-the-art Al mirror test samples following a novel passivation procedure [17]. Bare Al mirrors produced at the Goddard Space Flight Center (GSFC) coating facilities have been treated using the Large Area Plasma Processing System (LAPPS), [18] developed at the U.S. Naval Research Laboratory (NRL). LAPPS is based on a low-temperature, electron beam-generated plasma, which allows for precise control over the ion flux and ion energy distribution at surfaces adjacent to the plasma [19] as well as the decoupling of ion production and reactive neutral production [20,21]. Here, we use Ar/SF6 plasma chemistry to etch and controllably fluorinate Al surfaces in a single-step process, with minimal changes to surface morphology. Indeed, the conversion of the native Al2O3 layer into AlF3 was realized while maintaining a highly uniform smooth surface (root-mean-square (rms) roughness ≈1.8 nm). Through proper control of processing conditions, we were able to generate high optical quality AlF3 protective layers with tunable thickness. This capability allowed us to fabricate high-reflectivity mirrors optimized at any FUV wavelength starting from ≈110 nm, which is the intrinsic cut-off wavelength of AlF3.
Although there are several reports with the similar goal of low-temperature passivation of Al FUV mirrors, no directly analogous experiments were found in the literature in the context of optical coatings. Larruquert and Keski-Kuha [22] reported the ion-etching of native oxide in Al mirrors and subsequent passivation with PVD deposited-MgF2 for FUV reflectance monitor purposes. They demonstrated that the thin Al oxide layer can be completely removed with Ar+ ions (100 eV), although the use of high energy ions in the etch step produced noticeable changes in the Al surface morphology. Hennessy et al. [23,24,25] reported low- and high-temperature atomic layer etching of native oxide in Al mirrors followed with atomic layer deposition (ALD) of a protective layer of LiF or AlF3; in some cases they utilized the same reaction chemistry to accomplish both the etch and deposition. Wilbrandt et al. [26] reported the PVD-protection of Al mirrors with a combination of 2 or 3 fluorides among MgF2, AlF3, and LiF.
Additional efforts to passivate Al films have been reported, although the effect in FUV reflectivity was not investigated. Roodenko et al. [27,28] accomplished the removal of Si capping layers on Al and its subsequent passivation with the formation of AlFx on the surface by treatment with xenon-di-fluoride (XeF2) gas, as XeF2 is another well-known source of F- radicals. Klemperer and Williams [29] attempted the passivation of Al films by induced surface amorphization through bombardment with heavy Xe ions. Li et al. [30] examined the role of NF3-based plasmas in fluorinating the surface of aluminum vacuum vessels used for semiconductor processing. Interestingly, that work also demonstrated that the conversion of native aluminum oxide layers into fluorinated layers was highly dependent on plasma chemistry and ion flux at the surface.
This paper is organized as follows: Section 2 details the fabrication of bare Al mirrors by conventional PVD at GSFC, and their successive passivation with the LAPPS system at NRL, with a detailed description of the LAPPS facility. The instruments for the optical and surface characterization are also described. Section 3 analyzes the effects of plasma treatment on FUV reflectivity and thickness of the AlF3 protective layer through the variation of two process parameters: ion energy at the Al surface and Ar/SF6 gas flow ratio. The stoichiometry and the topography of the protective layer are also respectively investigated through XPS and AFM. Finally, several mirrors with optimized reflectivity at relevant spectral lines within the FUV range are presented in section 4. Their stability over time under controlled storage conditions is also reported.
2. Fabrication of Al mirrors
This section details the process of fabricating the bare Al mirrors at GSFC coating facilities and its subsequent passivation in the LAPPS facility at NRL.
2.1 Preparation of bare Al samples
Bare Al Samples were deposited in a 50-cm diameter, 100-cm height cylindrical stainless-steel deposition chamber at GSFC. The chamber was pumped with a cryogenic system, and the fore vacuum was made with a rough pump. Base pressure of the chamber was ∼6·10−8 Torr. Pure Al was evaporated from tungsten filaments. The average Al deposition rate was ≈80-100 Å/s per second. Film thickness was measured in situ with a quartz-crystal monitor that had been previously calibrated by ellipsometry. Substrates were 50 mm x 50 mm pieces of BK7 glass (microscopy slides) with a rms roughness of ≈0.3 nm. Prior to the loading in the deposition chamber, substrates were cleaned with a low-residue detergent (ALCONOX), then rinsed with de-ionized water, and finally rinsed with double-distilled acetone. During the evaporation, the pressure in the chamber momentarily increased to ∼10−6 Torr. All processes were carried out at room temperature.
2.2 Plasma passivation with LAPPS
Upon completion of the aluminum deposition the samples were transported to the LAPPS facility at the U. S. Naval Research Laboratory. The LAPPS facility was developed nearly two decades ago as a means to generate large area (>1 m2) high-density plasmas for the purposes of materials processing [18, 31]. The system makes use of hollow cathode electron sources to generate electron beams with typical current densities of 1-5 mA/cm2, and beam energies between 1-5 keV. The beam is magnetically collimated using Helmholtz coils producing a 100-200 Gauss magnetic field within the processing chamber. This magnetic field intensity is chosen to effectively collimate the electron beam while leaving the ions de-magnetized [32]. In this work, a cylindrical hollow cathode source enabling independent control of the beam current, beam energy, and operating pressure [33] was used to maximize flexibility in processing conditions. A schematic is shown below in Fig. 1. Importantly, changing from a cylindrical to linear geometry for the hollow cathode enables a large area planar [34] plasma geometry to be formed, which is well-suited for large substrates.
Fig. 1. A schematic of the plasma processing system used for native oxide etch and fluorine passivation of the aluminum mirror samples. The system possesses a hollow cathode electron source allowing independent control of electron beam energy and current over a wide range of operating pressures. The auxiliary ICP radical source also offers control over the ion/radical ratio present in the processing system.
Electron beam generated plasmas are known to possess a number of important properties relevant to this work. First, they generally exhibit very low electron temperatures (Te≤ 1 eV) [35,36] and consequently offer the potential for very low ion energies at surfaces (1-5 eV) [37]. With DC or RF biasing methods these ion energies can then be adjusted over a broad range (5–300 eV) [19] making them a highly flexible source of ions for mirror processing. Second, electron beam generated plasmas also preferentially ionize the background gas rather than dissociating it [20]. This allows for control over the ion/radical ratio within the system when an electron beam generated plasma is augmented with an auxiliary radical source [21]. In this work an inductively coupled plasma (ICP) through which the Ar/SF6 process gas flows is used to control the dissociation fraction of SF6 entering the process chamber. This configuration allows for control over the relative flux of ions and atomic F radicals at the oxidized aluminum surface. Finally, electron beam generated plasmas in Ar/SF6 are known to produce ion-ion plasmas, a state in which the formation of negative ions, through electron attachment, [38] drives the ratio of negative ions/electrons past the critical value (∼100-1000) [39] necessary for negative ion extraction from the system. The relative SF6 flow employed in this work is sufficient to meet this criterion in electron beam driven plasmas [40]. Thus, either negative or positive ions from the Ar/SF6 mixture can be extracted [41] toward the surface. In this work, the ability to extract negative ions proved to be an important ingredient in obtaining high quality AlF3 layers. This may be due to fact that highly reactive F- ions compose a large fraction of the negative ion mixture in electron beam generated Ar/SF6 plasmas [40].
For all samples investigated in this work the electron beam acceleration voltage was nominally held constant at 3 kV. The discharge current within the hollow cathode was also held constant at 100 mA. The beam current resulting from this setting is ≈21 mA. Neutral gas pressure within the reactor was monitored with a capacitance manometer and maintained at 15 mTorr regardless of gas flow conditions. Gas flow into the processing reactor was controlled with mass flow controllers. A small additional gas flow, controlled with a needle valve, is directed into the hollow cathode to maintain pressure within the electron beam source. The gas mixture being flowed into the diagnostics reactor is passed through a cylindrical quartz tube enclosed with a helical antenna which is constructed of hollow copper tubing and water cooled. Applying 400W of RF power to the antenna produces an inductively coupled plasma within the quartz tube which dissociates the SF6 flowing through the source in proportion to both the applied RF power and SF6 fraction in the flow [21]. The axial magnetic field within the processing reactor effectively limits diffusion of plasma from the source into the processing reactor. Radicals, however, are free to diffuse into the processing chamber unhindered. The exit of the radical source is positioned ≈40 cm above the beam axis.
Bias voltage on the processing stage, on which mirrors were mounted, was maintained using a bi-polar DC power supply to extract only negative ions from the plasma, meaning the stage was held at a positive potential with respect to the plasma. The mirror mounts also served to electrically connect the aluminum surface of the mirrors to the processing stage to ensure a uniform bias across the sample and stage. The potential difference between the stage bias and the plasma potential (monitored using an independent floating potential measurement) was used to determine the ion energy at the surface of the sample. Before and after treatment the samples were stored at ambient in polypropylene wafer cases.
2.3 Mirror characterization
Reflectivity of the mirrors was measured in the FUV range with a McPherson Vacuum Ultraviolet (VUV) 225 spectrophotometer. This spectrometer has a one-meter length high-vacuum monochromator with a 1200 lines/mm grating operating at near-normal incidence in the spectral range from 30 nm to 325 nm. The spectrometer is equipped with a windowless hydrogen-purged light source, which provides discrete H2 emission lines between 90 nm and 160 nm and a continuum above these wavelengths. The detector, which is housed inside a sample-holder compartment, consists of a photomultiplier cathode tube connected to a light-pipe for feeding the light signal coming out of the monochromator. The light pipe has a fluorescence and high quantum efficiency coating of sodium salicylate that is used to convert the FUV radiation into visible light. Absolute reflectance was obtained by alternately measuring the incident intensity and the intensity of the light beam reflected off the sample. Reflectance measurements were performed at 10° from normal incidence. FUV reflectance was initially measured a few days after the passivation, and again after several months of storage in a cabinet with a stable relative humidity of 30%.
Ellipsometry measurements in the 200-2500 nm range were performed with a HORIBA Jovin-Yvon spectroscopic ellipsometer in a fixed configuration of 70°. Information about thin film thickness and refractive index of AlF3-protected Al mirrors was obtained from a model based on Lorentz oscillators fitted to experimental data.
X-ray photoelectron (XPS) spectra were obtained in a Thermo Scientific NEXSA system with a monochromatic Al source (Kα = 1486.6 eV). A low-energy flood gun was utilized for charge compensation and when possible, spectra were calibrated to the adventitious carbon, C1s peak at 284.9 eV to account for any additional charging effects. Prior to collecting spectra, samples were etched for 30s with a 1 keV, low current, monoatomic Ar ion beam to remove any surface contamination resulting from transfer in atmospheric environment. Depth profiles were conducted in select samples using a monoatomic Ar etch at 3 keV and low current for 30s per depth step until the interface with the underlying Al is detected. Peak fitting and quantitative analysis were done using the Thermo Scientific Advantage software that accounts for the NEXSA geometry for surface sensitivity factors. All fits utilized a smart background and deconvolution was performed using a product of mixed Gaussian and Lorentzian peak shapes where the ratio of the two shapes was allowed to float for the best fit.
AFM was performed using a Bruker Dimension FastScan in tapping mode with 512 samples/line and a line scan rate of 3.92 Hz. The topography of each sample was acquired in two fields of size 500 nm x 500 nm and 5µm x 5µm.
3. Parameter optimization
In this section, two important parameters in the passivation process have been explored: the mean energy of ions delivered to the Al surface and the SF6 concentration in the Ar/SF6 mixture. The choice of the process parameters used in the experiments reported here were driven by the competing needs to completely remove the native oxide layer from the surface of the Al thin film and passivate the film, while leaving the reflective qualities of the Al thin film unchanged. Thus, a wide range of ion bombardment energies (∼20-250 eV) were explored. The variation of the Ar/SF6 explores the effect varying the atomic F density within the reactor and its corresponding flux to the surface. Previous work [21] has demonstrated a linear relation between SF6 partial pressure and atomic F concentration downstream of the auxiliary ICP source.
A set of 8 samples were prepared to analyze the effect of the aforementioned parameters on the FUV reflectivity of Al mirrors. First, a set of bare optical glasses were coated with Al in a single deposition run in the GSFC coatings facility following the procedure described in the previous section. Aluminum thickness was set at 80 nm instead of the typical 60-65 nm required for high FUV reflectivity, since a reduction of the effective Al thickness due to conversion of Al to AlF3 is expected. By using a thicker Al layer, we ensured that there is enough Al to prevent transmission losses. Afterwards, mirrors were sent to the NRL LAPPS facility where each mirror was processed independently using a different set of parameters. Table 1 summarizes the relevant process parameters used for each passivated Al mirror. A first set of 4 samples were processed with an Ar/SF6 gas mixture comprised of 13 sccm Ar and 3.5 sccm SF6 (LowSF6), whereas a second set of samples were prepared with an Ar/SF6 flow of 4 sccm Ar and 8 sccm SF6 (HighSF6). For each set of samples, four different incident negative ion energies were explored, ranging from ≈20 eV to ≈250 eV. The remaining parameters were fixed for all samples: plasma RF power (400W), cathode voltage (3 kV), fluorination time (960s), total pressure in the reactor (≈15 mTorr), and standoff distance between the beam center and sample stage (26 mm). Nominal reactor base pressure varied between 2·10−6 and 4·10−6 Torr. Substrate temperature was generally constant at 28 °C, monitored with an in situ thermocouple immediately before and after the passivation. In addition to the aforementioned parameters, the effect of the electron beam current, which is directly correlated with the negative ion flux at the Al surface [38,40] was also investigated, although it is not shown. We concluded that, for a fixed Ar/SF6 ratio, ion energy, and exposure time, the optimum ion beam current is within the range 14-25 mA. Beam currents below 14 mA resulted in incomplete oxygen removal, whereas beam currents exceeding 25 mA resulted in excessively thick AlF3 passivation layers. Therefore, all samples presented in this work have been fabricated with an ion beam current of 21 mA.
Table 1. Parameters used in the passivation process and FUV reflectivity of samples.
It should to be pointed out that, although our optimization process focuses on plasma ion energy and on Ar/SF6 ratio, modifications in the parameters that were left fixed might have a significant effect in the final quality of the Al mirror. These include applied RF power, operating pressure during processing, as well as separation distances between the beam and substrate and radical source and the substrate.
The reflectivity of the 8 samples was measured after few days of storage in a dry cabinet and it is displayed in Fig. 2.
Fig. 2. Reflectivity as a function of the incident wavelength of samples processed at different plasma ion energies with a SF6 flow of 3.5 sccm (a) or 8 sccm (b). The reference wavelength of 121.6 nm is highlighted.
Several trends have been extracted from Fig. 2, and are plotted separately in Fig. 3. The average FUV reflectivity (100-180 nm) of mirrors decreases as the incident negative ion energy increase for both Ar/SF6 ratios, as shown in Fig. 3(a). The same trend applies to the reflectivity at 121.6 nm (not plotted). A second trend can be detected if the reflectivity of samples processed with relatively similar ion energy but with different Ar/SF6 ratios as compared in Fig. 3(a); for a given incident ion energy, the higher SF6 concentration yields consistently higher average FUV reflectivity than the lower SF6 concentration. The third observed trend is that AlF3 thickness decreases with the incident ion energy, and also decreases at lower SF6 concentration as shown in Fig. 3(b). Preliminary X-ray reflectivity data (not shown) indicates etching becomes significant as the ion energy increases. When ion energies are low (≈25 eV), the total coating (Al + AlF3) thickness is found to increase compared to the original Al thickness of 80 nm, corresponding to the development of the fluoride layer. When ion energies are large (≈250 eV) however, the total coating thickness decreases, as the etch rate exceeds the growth rate of the fluoride layer.
Fig. 3. (a) Average FUV reflectivity of Al mirrors in the 100-180 nm range and (b) AlF3 thickness as a function of the mean incident ion energy.
A lower density of SF6 in the mixture has little effect on the ion flux to the surface, as the total chamber pressure during the passivation was kept constant at ≈15 mTorr regardless of Ar/SF6 flow mixture. Rather, the SF6 neutral concentration has a far more dramatic effect on the atomic F density in the reactor. As was demonstrated by Petrov, [32] the negative ion flux is dominated by F-, SF5-, and SF6- ions at both the SF6 concentrations chosen, and the total ion flux was found to vary only slightly with SF6 flow fraction. In contrast, F atom density within the reactor is strongly affected by the flow fraction of SF6 due to the linear relationship between SF6 concentration in the ICP source and SF6 dissociation. This means that at higher SF6 concentrations, higher F atom flux can be expected at the Al surface, thus promoting the more rapid growth of the AlF3 layer.
Now consider the reflectivity of samples at the reference wavelength of 121.6 nm, shown in Table 1. The higher reflectivity at the latter wavelength is presented by samples 27/HighSF6 and 57/HighSF6 (0.89 and 0.88, respectively), because their fluoride thicknesses are closer to the optimum to maximize the interferencial effect at 121.6 nm. As a reference, to reach a maximum reflectivity at 121.6 nm, the optimum AlF3 optical thickness has to be close to a quarter wave, which translates into a physical thickness of ≈24-28 nm, depending on the packing density of the film. Samples 209/LowSF6 and 246/HighSF6, whose average ion energy during the process was >200 eV, have AlF3 layers thinner than 10 nm, which might not be thick enough as to protect the underlying Al from oxidation when exposed to atmosphere. This hypothesis is supported by the calculated refractive indexes (see Table 1) at 250 nm wavelength of the protective layer in samples 209/LowSF6 and 246/HighSF6, which are respectively 1.71 and 1.76. These values are closer to the refractive index of Al2O3 (≈1.76) than to AlF3 (≈1.40). This result agrees with what is typically seen in MgF2, a material relatively similar to AlF3 in terms of optical and structural properties, where thin layers of MgF2 also do not protect Al from oxidation [42]. The thin AlF3 layers of samples 209/LowSF6 and 246/HighSF6 may be related to the use of elevated ion energies, which may unbalance the passivation procedure shifting it towards a net etching regime. In the present work, we estimate that the zero net fluorination might occur at ion energies of ≈300-350 eV for AlF3, by extrapolating the data in Fig. 3(b). While high energy ions seem to have clear deleterious effects, low energy ion fluence (<60 eV) appear to produce highly desirable FUV optical properties in the film. Samples 27/HighSF6 and 57/HighSF6 for example, exhibit FUV reflectances near 0.9 at 121.6 nm wavelength. This could be the result of film densification via low energy ion flux. Reference [43], for instance, has noted that the densification of MgF2 films happens at ion energies between 4 and 18 eV. This densification range, extrapolated to AlF3, might explain the high FUV reflectivity of the aforementioned samples.
Most of the samples shown in Fig. 2 present a broadband reflectivity dip at ≈150-155 nm, which may be a fingerprint of the excitation of surface plasmons at the Al/AlF3 interface. Surface plasmons are evanescent electromagnetic waves that propagate along the surface of Al. Mainly Al roughness components with spatial wavelength shorter than the radiation wavelength can contribute to the generation of surface plasmons [44]. Similar plasmonic absorptions can be found in PVD-AlF3 protected Al mirrors, such as in Ref. [11], Fig. 3, and in Ref. [13], Fig. 1. As mentioned in Ref. [13], sometimes the discrimination between the dip due to surface plasmon and the dip produced by the destructive interferences due to the thickness of the protective dielectric layer is complicated as both might be superimposed in wavelength. Because of the wide range of AlF3 thicknesses across all samples (see Table 1) in the present experiment, it is expected that the main source of the dip is the excitation of Al surface plasmons instead of interferencial effects. As shown in Fig. 2, all dips exhibit similar depth and bandwidth, which agrees with the fact that Al mirrors were coated in a single run, so that all samples are expected to have the same components of roughness relevant for the plasmonic effect, as is demonstrated below in the AFM analysis. The dip shape also seems to be independent of the fluorination parameters, thus one can conclude, at least for ion energies <200 eV, that plasma exposure did not have a evident impact in the spatial wavelengths of roughness shorter than radiation wavelength.
A sequence of XPS measurements combined with several etching steps (sputtering with Ar ions at 3 keV, 30” per step) for depth profiling were performed on samples 23/LowSF6 and 27/HighSF6 (samples with the higher FUV reflectivity for each Al/SF6 ratio) to gain a better insight of the chemical composition throughout the AlF3 layer, so that the following items could be investigated: i) AlF3 stoichiometry, ii) uniformity in composition of the AlF3 layer, iii) oxygen distribution throughout AlF3 layer, iv) the presence of oxygen in the Al-AlF3 interface, and v) the presence of a surface contaminant layer. Measurements were done after more than 8 months of sample storage in dry boxes (Relative Humidity ≈30%). Figures 4(a)–4(b) show the Al 2p peak survey for 7 different sputtering times (from 0s to 180s) to obtain a depth profile. If we focus on sample 23/LowSF6, where the thickness of the AlF3 layer is 19.4 nm and where the Al-AlF3 interface is reached at after 180s of sputtering time, we can roughly estimate that the etching rate was ≈1 Å/s.
Fig. 4. Survey of the Al 2p peak with depth profile for samples 23/LowSF6 (a) and 27/HighSF6 (b). Depth profile of the relative concentration of C, O, Al, and F species for samples 23/LowSF6 (c) and 27/HighSF6 (d).
The Al 2p peak has multiple components which can be deconvolved to yield information regarding the local binding environments of the film. The peak associated with the Al-F bonding is located at 77.2-78 eV, which is similar to that of AlF3, 76.9-77.1 eV, from literature [45]. Similarly, the Al-F bond in the F1s peak is located at 687.9 eV, where AlF3 should be at 687.5-687.8 eV [45]. Beyond the surface we cannot use the adventitious carbon peak to account for any charging not compensated by the flood gun. Thus, while there may be slight shifts in the local binding energies, the core level separation between Al 2p and F1s for AlF3 should be 610.6 eV. In this layer, the core level separation was 610 ± 0.5 eV. These results indicate that the passivation layer has a stoichiometry of AlF3 or close to it, and the small differences between tabulated and experimental binding energies could be attributed to the small presence of Al-F-O or Al-O-F bonds. An alternative way to determine the stoichiometry is taking the ratio of Al/F of the areas under the peaks Al 2p and F 1s, which have been adjusted by the surface sensitivity factor related to the cross section of each core level. Considering only the portions of the passivation layer in which the Al2p and F1s peaks are associated with the film (thus, neglecting data obtained at 0s and 180s for sample 23/LowSF6, and 0s for 27/HighSF6), we obtain the stoichiometry of AlFx, where x=2.6 for 23/LowSF6, and x=2.7 for 27/HighSF6. Even though 2.6-2.7 is slightly lower than 3 (likely due to the presence of oxygen), for simplicity we have nominally expressed the passivation layer as AlF3 throughout the text.
Figures 4(c)–4(d) show the concentration of Al, O, F, and C as a function of the etching time that can be translated into layer depth as previously mentioned. Both Al and F concentrations remain relatively stable after the topmost surface has been etched (t=0) which indicates that the AlF3 stoichiometry is uniform in composition throughout it thickness. At 180s etching time the concentration of Al in sample 23/LowSF6 starts increasing while F decreases as the measurement begins to probe the Al-AlF3 interface; the same effect is less prominent with 27/HighSF6 as the AlF3 layer in that sample is thicker. Moreover, both Figs. 4(c)–4(d) show the presence of oxygen within the AlF3 layer, decreasing from top to bottom, and it is bound to Al atoms as Al-O or Al-OH as shown in Figs. 4(a)–4(b). The presence of O in the films is attributed to the aging of samples; had it been due to an incomplete etching of the native Al2O3 layer in bare Al mirrors, the O distributions would have been concentrated near the Al-AlF3 interface. Finally, the significant amount of C on both 23/LowSF6 and 27/HighSF6 on the topmost part of the AlF3 layer suggests the presence of hydrocarbon contaminants on the samples surface.
The surface topography of all samples was investigated with AFM. The rms roughness of samples was calculated from the radially averaged power spectral density, and the grain size from a cross section of the autocorrelation function, [46] using Gwydion software [47]. Fig. 5 shows two examples (23/LowSF6 and 209/LowSF6) of measured surface topography, and the roughness and grain size of all samples as a function of the processing ion energy.
Fig. 5. Surface topography of sample 23/LowSF6 (a) and 209/LowSF6 (b). rms surface roughness (c) and grain size (d) as a function of the mean incident ion energy for samples processed at different SF6 concentrations.
The surface roughness as a function of the ion energy is relatively constant and in the range of 1.6-2 nm for samples processed with ion energies ranging from 23 eV to 209 eV, independendlty of SF6 concentration, but at 246 eV it dramatically increases to 5.8 nm. A similar trend can be observed with the grain size, with values between 39 nm and 49 nm in the 23-209 eV ion energies range, and then 74 nm at 246 eV. Both the surface roughness and grain size of all samples processed with ion energies below 210 eV are constant [see Figs. 5(a)–5(b)] and thus, independent of both AlF3 layer thickness and processing ion energy. Additionally, the Al surface plasmon is similar in all samples processed with ion energies <200 eV as mentioned before, indicating that the grain size and roughness of the Al surface is fairly similar among all samples. The combined facts in the two last sentences lead to the hypothesis that what we are probing with AFM is the topography of the Al layer, and that AlF3 layer is merely conformal to the Al topography. To support this hypothesis, we compared our data with Larruquert et al. [48], who investigated the correlation between film thickness and grain size (derived from the autocorrelation length) of Al mirrors deposited at room temperature. Interpolating the thickness of all Al samples in the present work (around 80 nm) in Fig. 5 from Ref. [48], we obtain a predicted Al grain size of around 53 nm, which is compatible with the values obtained in this work.
As mentioned in the introduction, authors in Ref. [22] used both 100 eV and 300 eV Ar ions to remove either the native oxide on Al samples or a sacrificial MgF2 protective layer; in both cases, they noticed a short-range surface roughening of the Al layer induced by the ion bombarment. In the present work, samples processed at with ion energies <200 eV did not exhibit signficant surface roughening. Sample 246/HighSF6, which was processed with the highest ion energy (246 eV), leads to roughness and grain size values out of the trend. Furthermore, the 2D autocorrelation function of sample 246/HighSF6 (not shown) showed “canal-like” structures following a prefered direction on the mirror surface. In all other samples the autocorrelation function was isotropic. We conclude that the use of energetic ions damaged the Al surface in 246/HighSF6 and promoted roughness in a prefered orientation, and thus severely deteriorating the optical properties of the 246/HighSF6 mirror. This could be due to high energy ions penetrating further into the material and creating defects during processing.
A mirror surface roughness within the 1.6-2 nm range (ignoring sample 246/HighSF6) does not have a significant impact to the FUV reflectivity, as it is ≈λ/60, taking λ=121.6 nm as reference wavelength. As an example, using the well-known Debye [49] and Waller [50] equation, a 2-nm surface roughness at 121.6 nm wavelength may reduce reflectivity only by ≈0.1%. However, if the surface roughness is the result of underlying Al topography leading to roughness at the Al-AlF3 interface, then the impact on the FUV reflectance will be more significant due to the high refractive index contrast between Al and AlF3. As an example, also taking λ=121.6 nm as reference, an Al-AlF3 interfacial roughness of 2 nm reduces the reflectivity of the mirror by ≈3%.
4. Mirrors peaked at key FUV wavelengths
In this section, several mirrors have been fabricated with a different set of parameters to optimize the AlF3 thickness and reach high reflectivity at key wavelengths in astrophysics. These lines are: He Lyman γ (108.5-164.0 nm), H Lyman α (121.6 nm), O I (130.4 -135.6 nm), and the C IV doublet (centered at 155.0 nm), in their rest frame. EUV diagnostic lines strongly red-shifted into the FUV could also benefit from the present technology. Samples M109, M122, M130 and M155 were fabricated with the ion energies and Ar/SF6 ratios indicated in Table 2. The remaining parameters are fixed and relatively similar to the ones used in the previous section. Figure 6 shows the experimental reflectivity of the samples fabricated with the parameters in Table 2. Reflectivity data was obtained after few days of storage. The thicknesses the passivation layers were determined by ellipsometry, and are 23.9 nm (M109), 27.2 nm (M122), 30.5 nm (M130), and 40.2 nm (M155). With this new set of samples we have demonstrated that by an adequate selection of the process parameters, the thickness of the AlF3 can be tailored to application. Particularly, sample M155 shows that protective AlF3 layers as thick as 40 nm can be obtained with the present procedure.
Fig. 6. Reflectance of Al mirrors as a function of wavelength optimized at several key FUV diagnostics for astronomy (indicated with same color). The reflectivity of aged mirrors is also displayed in dashed lines. The post-deposition reflectance of sample M130 was not measured prior to aging.
Table 2. Summary of the results obtained in this subsection.
The reflectance of newly deposited samples at the target wavelengths are 0.75 at 108.5 nm and 0.85 at 164 nm (sample M109), 0.91 at 121.6 nm (M122), and 0.94 at 155.0 nm (M155). Sample M130 was not measured immediately following coating. Sample M109 is particularly relevant considering that it is tuned at 108.5 nm whereas the cut-off wavelength of AlF3 is ≈110 nm. Hence, the passivation procedure is effective in allowing the operation of Al + AlF3 mirrors below the intrinsic cut-off limit of AlF3.
All samples discussed in this section have characteristics that are commensurate with state-of-the-art mirrors. To put these results into context, the present samples are compared to available AlF3-protected Al mirrors data. For example, Al mirrors optimized at 121.6 nm with a protective layer deposited at ≈250 °C demonstrated a reflectance of ≈0.90 [11,13]. These results are compatible in terms of reflectivity, but in the present work the process temperature did not exceed 28 °C. Reference [26] reported room-temperature AlF3-protected Al mirrors, with an average FUV reflectivity of ≈0.73 (reflectance at 121.6 nm was not shown), and Al protected with a combination of AlF3, LiF and MgF2 with ≈0.81 reflectance at 121.6 nm. References [23] and [25] reported atomic layer etching and ALD protection (using temperatures lower than 200 °C) of Al mirrors with AlF3 and LiF + ALF3, reaching reflectance at 121.6 nm of ≈0.76 and ≈0.82, respectively. Therefore, we report a recipe to produce mirrors tuned at different FUV wavelengths whose optical performance surpassed the best PVD-produced AlF3-protected Al, with a scalable process at low temperature.
The present technology is expected to have an impact on future space observatories. Most of large astronomical telescopes have two or three mirrors to bring the image of the object being observed in focus, and hence the fields are picked off into several individual assemblies that often require more reflective surfaces to control the image in order to meet the specific scientific imaging requirements. Thus, future large observatories are expected to require a large number of mirrors. Let us consider a telescope intended to image at H Lyman α wavelength. Then, a telescope whose mirrors were coated with the present technology (M122) would present a (relative) increased throughput of ${\left( {\frac{{0.91}}{{0.86}}} \right)^n} - 1$, in comparison with mirrors coated with standard Al/MgF2, where n is the total number of reflective surfaces. As an example, for a 7-mirrors architecture as intended for LUVOIR-LUMOS or HabEx-UVS instruments, the sensitivity enhancement would be ≈48%.
Fluorides evaporated at room temperature without any assistance are known to have a columnar structure with densities lower than their respective bulk crystals, leaving room for contaminants such as water or hydrocarbons. The accumulation of these contaminants, particularly of water which is a molecule of small size and with a high absorption coefficient in the FUV, [51] is believed to be one of the main causes of the reflectivity drop over time. It is known that AlF3 films deposited at temperatures >150 °C are amorphous with no evident columnar structure [52] and with densities similar to bulk AlF3 [13], thus leaving less room for contaminants and presenting a better stability over time. Reflectivity of all samples was re-measured after 6-10 months (see Table 2) of storage in a cabinet with a relative humidity of ≈30%. In this work, the average reflectivity drop of all samples shown in Fig. 4 at their respective target wavelengths is ≈4.2% relative to the as processed reflectivity, which is compatible with the aged data of samples deposited at high temperatures reported in Ref. [13]. This degradation is attributed to the oxidation of Al within the AlF3 layer and to the presence of a hydrocarbon-based contamination layer on the surface of all mirror, as detected on samples 23/LowSF6 and 27/HighSF6 through the XPS analysis. Hence, the passivation procedure presented in this work, in addition to producing state-of-the art Al mirrors, results in environmental stability similar to hot-deposited AlF3- protected Al mirrors.
Although it is beyond the scope of the present work, the restoration of the high intrinsic FUV reflectance in aged MgF2-protected Al mirrors has been demonstrated in earlier reports [17] using the LAPPS system with relative short treatment times. It is reasonable to expect similar improvements on aged AlF3 coatings as well.
5. Conclusions
In this work, the feasibility of using the LAPPS facility to passivate bare Al mirrors for the FUV range has been investigated. LAPPS, in conjunction with an auxiliary atomic F radical source, provides a highly flexible source of low energy F containing ions that etches the native oxide from exposed Al mirrors and promotes a controlled fluorination of the Al surface in a single-step process, and at ambient temperature (≈28 °C).
A set of 8 bare Al mirrors prepared at GSFC coating facilities were passivated at LAPPS with different processing parameters. The effect of the mean ion energy and of the SF6 concentration in the Ar/SF6 mixture on the average FUV (100-180 nm) reflectivity and on the thickness of the protective layer was investigated. The average FUV reflectivity decreased with negative ion energy and increased with the SF6 concentration in the Ar/SF6 mixture. As for the thickness, samples processed with low plasma energies and with higher SF6 content in the Ar/SF6 mixture had thicker protective layers on Al. Samples whose protective layer was thinner than 10 nm were found to be oxidized. XPS analysis of aged samples passivated with low ion energies indicated that the passivation layer remained uniform in composition throughout the layer with a stoichiometry close to AlF3, and also indicated the presence of oxygen within the AlF3 layer, and a significant quantity of carbon on the mirror surface. AFM analysis on mirrors showed a relatively constant surface roughness (1.6-2.0 nm) and grain size (39-49 nm) for all samples processed with plasma energies ≤210 eV, compatible with as-grown Al. Several factors indicated that the latter values were representative of the Al layer, and that the AlF3 layers in all samples were conformal.
With the knowledge acquired from the first set of samples, a new set of samples were produced with the AlF3 thickness optimized at different key FUV wavelengths: He Lyman γ (108.5-164.0 nm), H Lyman α (121.6 nm), O I (130.4 -135.6 nm), and the C IV doublet (centered at 155.0 nm). The AlF3 protective layer thicknesses of the new samples ranged from 23.9 nm to 40.2 nm, demonstrating a controllable process of simultaneous oxide removal and fluorine passivation of aluminum. As displayed in Table 2, all samples presented state-of-the-art reflectance at the target wavelengths, including the sample M109 optimized at 108.5 nm, in spite of being optimized at a wavelength shorter than the intrinsic AlF3 cut-off wavelength of ≈110 nm. Aging tests performed in the second set of samples showed a moderate average decay in reflectivity of ≈4.2% (relative) after 6 to 10 months of sample storage in a controlled humidity cabinet.
We have developed a recipe to produce state-of-the-art FUV mirrors without the use of high substrate temperatures, whose FUV performance has surpassed the best PVD-produced AlF3-protected Al mirrors published up to date. The success of this novel procedure opens a new path to increase the sensitivity of future space observatories and ultimately enable new scientific imaging capabilities in the FUV.
Funding
Fund for Astrophysics Research and Analysis (15-APRA15-0103); NASA Strategic Astrophysics Technology (17-SAT17-0017).
Acknowledgments
This work has been performed through a joint collaboration between the Optics Branch (Code 551) at the Goddard Space Flight Center (GSFC) and the Plasma Physics Division at the Naval Research Laboratory. DRB, SGW, VDW, JMW and ACK were also supported via the Naval Research Laboratory Base Program and the Office of Naval Research. JMW, ACK and SGR appreciate the support of the American Society for Engineering Education (ASEE) postdoctoral research associate program.
Disclosures
The authors declare no conflict of interest.
References
1. R. E. Carter, Space Telescope Systems Description Handbook (Lockheed Missiles & Space Company, 1985).
2. W. E. McClintock, G. M. Lawrence, R. A. Kohnert, and L. W. Esposito, “Optical design of the ultraviolet imaging spectrograph for the Cassini mission to Saturn,” Opt. Eng. 32(12), 3038 (1993). [CrossRef]
3. R. Lundin, G. Haerendel, and S. Grahn, The Freja Mission (Springer, 1995).
4. LUVOIR Final Report, 2019. Available at https://asd.gsfc.nasa.gov/luvoir/reports/
5. HabEx Final report, 2019. Available at https://www.jpl.nasa.gov/habex/documents/
6. J. Tumlinson, A. Aloisi, G. Kriss, K. France, S. McCandliss, K. Sembach, A. Fox, T. Tripp, E. Jenkins, and M. Beasley, “Unique Astrophysics in the Lyman Ultraviolet.” ArXiv-prints (2012). Available online: https://arxiv.org/abs/1209.3272
7. R. P. Madden and G. Hass, Physics of Thin Films, G. Hass, ed. (Academic Press, 1964), Chapter 3.
8. M. A. Quijada, S. Rice, and E. Mentzell, “Enhanced MgF2 and LiF over-coated Al mirrors for FUV space astronomy,” Proc. SPIE 8450, 84502H (2012). [CrossRef]
9. M. A. Quijada, J. Del Hoyo, and S. Rice, “Enhanced far-ultraviolet reflectance of MgF2 and LiF over-coated Al mirrors,” Proc. SPIE 9144, 91444G (2014). [CrossRef]
10. B. Fleming, M. Quijada, J. Hennessy, A. Egan, J. Del Hoyo, B. A. Hicks, J. Wiley, N. Kruczek, N. Erickson, and K. France, “Advanced environmentally resistant lithium fluoride mirror coatings for the next generation of broadband space observatories,” Appl. Opt. 56(36), 9941–9950 (2017). [CrossRef]
11. J. Del Hoyo and M. Quijada, “Enhanced aluminum reflecting and solar-blind filter coatings for the far-ultraviolet,” Proc. SPIE 10372, 1037204 (2017). [CrossRef]
12. L. Rodríguez-de Marcos, J. I. Larruquert, J. A. Méndez, N. Gutiérrez-Luna, L. Espinosa-Yáñez, C. Honrado-Benítez, J. Chavero-Royán, and B. Perea-Abarca, “Optimization of MgF2-deposition temperature for UV Al mirrors,” Opt. Express 26(7), 9363–9372 (2018). [CrossRef]
13. N. Gutiérrez-Luna, B. Perea-Abarca, L. Espinosa-Yáñez, C. Honrado-Benítez, T. de Lis, L. Rodríguez-de Marcos, J. A. Aznárez, and J. I. Larruquert, “Temperature dependence of AlF3 protection on far-UV Al mirrors,” Coatings 9(7), 428 (2019). [CrossRef]
14. C. Guo, C. B. Li, M. Kong, and D. Lin, “Effects of deposition temperature on optical properties of MgF2 over-coated Al mirrors in the VUV,” Chinese Phys. B 28(11), 117801 (2019). [CrossRef]
15. J. Wang, J. Zhang, H. Jiao, X. Cheng, and Z. Wang, "The reflectance of the Al+MgF2 film in the far-ultraviolet," in OSA’s Optical Interference Coatings Conference, FB.3 (2019).
16. M. A. Quijada, D. A. Sheikh, J. G. Del Hoyo, and J. G. Richardson, “ZERODUR substrates for application of high-temperature protected-aluminum far-ultraviolet coatings,” Proc. SPIE 11116, 111160T (2019). [CrossRef]
17. M. A. Quijada, D. R. Boris, J. del Hoyo, E. J. Wollack, A. C. Kozen, S. Walton, and V. Dwivedi, “E-beam generated plasma etching for developing high-reflectance mirrors for far-ultraviolet astronomical instrument applications,” Proc. SPIE 10699, 106992X (2018). [CrossRef]
18. R. A. Meger, R. F. Fernsler, M. Lampe, and W. Manheimer, “Large area plasma processing system,” U.S. Patent US5874807A (1999).
19. S. G. Walton, D. R. Boris, S. C. Hernández, E. H. Lock, T. B. Petrova, G. M. Petrov, and R. F. Fernsler, “Electron Beam Generated Plasmas for Ultra Low Te Processing,” ECS J. Solid State Sci. Technol. 4(6), N5033–N5040 (2015). [CrossRef]
20. D. R. Boris, T. B. Petrova, G. M. Petrov, and S. G. Walton, “Atomic fluorine densities in electron beam generated plasmas: A high ion to radical ratio source for etching with atomic level precision,” J. Vac. Sci. Technol., A 35(1), 01A104 (2017). [CrossRef]
21. D. R. Boris and S. G. Walton, “Precise control of ion and radical production using electron beam generated plasmas,” J. Vac. Sci. Technol., A 36(6), 060601 (2018). [CrossRef]
22. J. I. Larruquert and R. A. M. Keski-Kuha, “Removal of a protective coating on Al by ion etching for high reflectance in the far ultraviolet,” Appl. Opt. 47(29), 5253–5260 (2008). [CrossRef]
23. J. Hennessy, K. Balasubramanian, C. S. Moore, A. D. Jewell, S. Nikzad, K. France, and M. Quijada, “Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition,” J. Astron. Telesc. Instrum. Syst. 2(4), 041206 (2016). [CrossRef]
24. J. Hennessy, C. S. Moore, K. Balasubramanian, A. D. Jewell, C. Carter, K. France, and S. Nikzad, “Atomic layer deposition and etching methods for far ultraviolet aluminum mirrors,” Proc. SPIE 10401, 1040119 (2017). [CrossRef]
25. J. Hennessy, A. D. Jewell, C. S. Moore, A. G. Carver, K. Balasubramanian, K. France, and S. Nikzad, “Ultrathin protective coatings by atomic layer engineering for far ultraviolet aluminum mirrors,” Proc. SPIE 10699, 1069902 (2018). [CrossRef]
26. S. Wilbrandt, O. Stenzel, H. Nakamura, D. Wulff-Molder, A. Duparré, and N. Kaiser, “Protected and enhanced aluminum mirrors for the VUV,” Appl. Opt. 53(4), A125–A130 (2014). [CrossRef]
27. K. Roodenko, O. Seitz, Y. Gogte, J.-F. Veyan, X.-M. Yan, and Y. J. Chabal, “Modification of the adhesive properties of XeF2-etched aluminum surfaces by deposition of organic self-assembled monolayers,” J. Phys. Chem. C 114(51), 22566–22572 (2010). [CrossRef]
28. K. Roodenko, M. D. Halls, Y. Gogte, O. Seitz, J.-F. Veyan, and Y. J. Chabal, “Nature of hydrophilic aluminum fluoride and oxyaluminum fluoride surfaces resulting from XeF2 treatment of Al and AlF3,” J. Phys. Chem. C 115(43), 21351–21357 (2011). [CrossRef]
29. D. F. Klemperer and D. J. Williams, “Changes in the chemical reactivity of metals exposed to an inert gas glow discharge,” Vacuum 33(5), 301–305 (1983). [CrossRef]
30. X. Li, X. Hua, and G. S. Oehrlein, “Surface chemical changes of aluminum during NF3 -based plasma processing used for in situ chamber cleaning,” J. of Vac. Sci. & Tech. A 22(1), 158–164 (2004). [CrossRef]
31. W. M. Manheimer, R. F. Fernsler, M. Lampe, and R. A. Meger, “Theoretical overview of the large-area plasma processing system (LAPPS),” Plasma Sources Sci. Technol. 9(3), 370–386 (2000). [CrossRef]
32. G. M. Petrov, D. R. Boris, E. H. Lock, T. B. Petrova, R. F. Fernsler, and S. G. Walton, “The influence of magnetic field on electron beam generated plasmas,” J. Phys. D: Appl. Phys. 48(27), 275202 (2015). [CrossRef]
33. C. D. Cothran, D. R. Boris, C. S. Compton, E. M. Tejero, R. F. Fernsler, W. E. Amatucci, and S. G. Walton, “Continuous and pulsed electron beam production from an uninterrupted plasma cathode,” Surf. Coat. Technol. 267(15), 111–116 (2015). [CrossRef]
34. D. R. Boris, R. F. Fernsler, and S. G. Walton, “The spatial profile of density in electron beam generated plasmas,” Surf. Coat. Technol 241, 13–18 (2014). [CrossRef]
35. E. H. Lock, R. F. Fernsler, and S. G. Walton, “Experimental and theoretical evaluations of electron temperature in continuous electron beam generated plasmas,” Plasma Sources Sci. Technol. 17(2), 025009 (2008). [CrossRef]
36. D. Leonhardt, S. G. Walton, and R. F. Fernsler, “Fundamentals and applications of a plasma-processing system based on electron-beam ionization,” Phys. Plasmas 14(5), 057103 (2007). [CrossRef]
37. D. R. Boris, G. M. Petrov, E. H. Lock, T. B. Petrova, R. F. Fernsler, and S. G. Walton, “Controlling the electron energy distribution function of electron beam generated plasmas with molecular gas concentration: I. Experimental results,” Plasma Sources Sci. Technol. 22(6), 065004 (2013). [CrossRef]
38. G. M. Petrov, D. R. Boris, T. B. Petrova, and S. G. Walton, “One-dimensional Ar-SF6 hydromodel at low-pressure in e-beam generated plasmas,” J. of Vac. Sci. & Tech. A 34(2), 021302 (2016). [CrossRef]
39. J. Bredin, P. Chabert, and A. Aanesland, “Langmuir probe analysis of highly electronegative plasmas,” Appl. Phys. Lett. 102(15), 154107 (2013). [CrossRef]
40. D. R. Boris, R. F. Fernsler, and S. G. Walton, “Measuring the electron density, temperature, and electronegativity in electron beam-generated plasmas produced in argon/SF6 mixtures,” Plasma Sources Sci. Technol. 24(2), 025032 (2015). [CrossRef]
41. S. G. Walton, D. Leonhardt, R. F. Fernsler, and R. A. Meger, “Extraction of positive and negative ions from electron-beam-generated plasmas,” Appl. Phys. Lett. 81(6), 987–989 (2002). [CrossRef]
42. B. I. Johnson, T. G. Avval, G. T. Hodges, V. Carver, K. Membreno, D. D. Allred, and M. R. Linford, “Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF2 layers,” Proc. SPIE 11116, 111160O (2019). [CrossRef]
43. R. R. Wiley, K. Patel, and R. Kaneriya, “Improved magnesium fluoride process by ion-assisted deposition,” 53rd Annual Technical Conference Proceedings, 505/856-7188 (2010).
44. R. P. Feuerbacher and W. Steinmann, “Reflectance of evaporated aluminium films in the 1050-1600 Å region, and the influence of the surface plasmon,” Opt. Commun. 1(2), 81–85 (1969). [CrossRef]
45. https://srdata.nist.gov/xps/ElmComposition.aspx
46. J. C. Stover, “Optical Scattering, Measurements and Analysis,” 3rd edition, (SPIE Press, 2012)
48. J. I. Larruquert, J. A. Méndez, and J. A. Aznárez, “Empirical relations among scattering, roughness parameters, and thickness of aluminum films,” Appl. Opt. 32(31), 6341–6346 (1993). [CrossRef]
49. P. Debye, “Über der Intensitätetsverteilung in den mit Röntgenstrahlung erzeugten Interferenzbildern,” Verh. d. Deutsch. Phys. Ges. 15, 738 (1913).
50. I. Waller, “Zur Frage der Einwirkung der Wärmebewegung auf die Interferenz von Röntgenstrahlen,” Z. Physik 17(1), 398–408 (1923). [CrossRef]
51. D. J. Segelstein, “The complex refractive index of water,” Master Thesis (1981).
52. C.-C. Lee, M.-C. Liu, M. Kaneko, K. Nakahira, and Y. Takano, “Characterization of AlF3 thin films at 193 nm by thermal evaporation,” Appl. Opt. 44(34), 7333–7338 (2005). [CrossRef]
