Building a Better ALD: Use of Plasma-Enhanced ALD to Construct Efficient Interference Filters for the FUV Prepared by: Paul Scowen (PI; Arizona State University, ASU); Daniel Messina, Zhiyu Huang, Brianna Eller, Robert J. Nemanich, and Hongbin Yu (ASU); Tom Mooney (Materion); and Matt Beasley (Southwest Research Inc., Boulder) Summary The goal of this work is to use plasma-enhanced atomic layer deposition (PEALD) to synthesize mirrors and filters compatible with near-ultraviolet (UV) and far-UV optics. The development of this technology will ultimately provide diagnostic tools to access a range of topics for study, including protostellar and protoplanetary systems, intergalactic-medium (IGM) gas from galactic star formation, and the most distant of objects in the early universe. Since the beginning of the project, our team, from the School of Earth and Science Exploration and the Physics Department at ASU, Materion, and Planetary Resources, has been working to initiate this research. Our most significant progress to date has been the design, development, and assembly of the equipment needed to complete this research. In this period, the assembly of the PEALD has been completed, PEALD Al has been deposited, and AlF3 deposition is in progress. Background Atomic layer deposition (ALD) is a layer-by-layer deposition technique that synthesizes ultra-thin, uniform, and conformal films (Fig. 1). The high quality of these films has consequently resulted in augmented coatings and optical elements. At the same time, major advances have been made in optical design and detector technologies. As a result, measurement of far-UV and near-UV bands has improved dramatically. The development of this technology ultimately allows access to emission and absorption lines in the UV emitted from a range of targets, including protostellar and protoplanetary systems, IGM gas from galactic star formation, and the most distant of objects in the early universe. These diagnostic tools require the implementation of stable optical layers, including high-UV- reflectivity coatings and UV-transparent films [1].

Fig. 1. Schematic representation of the layer-by-layer deposition process of thermal and PEALD. During the reactant or plasma exposure, the surface is exposed to a reactant gas or plasma. In this work, we will use a range of materials to implement stable protective overcoats with high UV reflectivity and unprecedented uniformity, and use that capability to leverage innovative UV/optical filter construction to enable the science mentioned above. The materials we will use include aluminum oxide, hafnium oxide, and silicon oxide (as an intermediary step for development only) and a range of fluoride-based compounds (for production). These materials will be deposited in a

Page 1 of 11 multilayer format over a metal base to produce a stable construct. Specifically, we will use PEALD for deposition and construction of reflective layers that protect bare aluminum for mirror use in the UV. Our designs indicate that by using PEALD we can reduce adsorption and scattering in the optical films because of the lower concentration of impurities and increased stoichiometric control, yielding vastly superior quality and performance over comparable traditional thermal ALD techniques [1-16] currently being developed by other NASA-funded groups [17-20]. These capabilities will allow us to push the blue edge in usable UV reflectivity for magnesium-fluoride-protected aluminum below the current 115-nm limit. This work will demonstrate for the first time whether loss-free oxides of materials such as Al, Hf, and Si can be deposited using ALD to lower the cutoff reflectivity in the UV to as low as 90 nm. We will also demonstrate the use of PEALD to deposit low-loss thin films of fluoride-based materials, and aluminum metal. Using these techniques, we will then demonstrate our proof-of-concept of using these techniques together to construct thin-film, multilayer metal-dielectric cavities with a reflective surface as the foundation, that can be tuned to isolate specific emission lines of astronomical importance. The resulting optical technologies will advance the coating stability, thickness, and performance of thin films in the far-UV sought by NASA, to match recent UV detector advances. Such improvement will enable next-generation space-based far-UV missions, opening access to the wealth of diagnostic information the far-UV offers for exoplanet, star formation, and cosmological/IGM science. Objectives and Milestones Our research seeks to demonstrate several objectives: • Films of material can be deposited to demonstrate the approach using PEALD techniques to produce loss-free films of e.g. silicon, hafnium, and aluminum oxide; the resulting coatings will be of a thickness and a purity far higher than can be delivered by current techniques that involve sputtering deposition; • Using the same deposition techniques, PEALD can deposit thin (tens of nm), low-loss films of fluorides of aluminum and magnesium as well as e.g. lithium, lanthanum, calcium, and beryllium, that can serve as protective overcoats for materials which would otherwise be easily oxidized by exposure to air; • Aluminum deposition, protective-layer deposition, and characterization can be completed in-situ in a controlled environment that minimizes contamination, improving the reflectivity of the resulting films and their interfaces by reducing scattering and adsorption; • Deposition of such protective overcoats over aluminum can be achieved with PEALD to provide a sufficiently crystalline, uniform, and stable structure, pushing blueward the currently observed 115-nm cutoff in efficient reflectivity from atomic sputtering deposition of magnesium fluoride, thereby extending the range of diagnostic emission and absorption lines available for science; • Extend the metal-dielectric overcoat process to concave mirrors to demonstrate the performance of reflective surfaces in an optical test-bed environment; • Use our PEALD approach to apply alternating layers of metals and dielectrics, producing multi- cavity structures exhibiting very high performance; this goal is currently limited by the inability to deposit very thin layers with great accuracy, while demonstrating film toughness and ‘bulk’ thin- film material losses; • Apply the multilayer approach to the construction of multilayer dielectric mirrors to act as reflection filters or high reflectors in narrow band systems; and • Similarly construct multilayer broadband mirrors, thought to exhibit higher performance than metal-based mirrors (using a short-wave extension to prototype dichroics our group is already developing for space applications). Table 1 summarizes the timeline to achieve these objectives, modified to accommodate equipment fabrication.

Page 2 of 11 Activity Name Start Date Finish Date Upgrade in-situ reflectivity to 120 nm PEALD of oxides on evaporated aluminum PEALD of Al2O3 and SiO2 on aluminum Upgrade PEALD for aluminum deposition Install precursors for PEALD of fluorides PEALD of aluminum PEALD of aluminum fluoride on aluminum 10/31/17 7/30/19 Oxides on PEALD aluminum 10/3/17 7/30/19 PEALD of magnesium fluoride on aluminum 1/2/17 7/30/19 PEALD of aluminum and magnesium fluoride 1/2/17 7/30/19 Upgrade in-situ reflectivity to 90 nm 1/2/17 8/31/19 Magnesium fluoride/aluminum filters 2/1/18 8/31/19 Oxide-fluoride multilayers and protective layers 4/1/18 9/31/19 Multilayer broad band mirrors 4/1/18 9/31/19 Table 1. Summary of timeline for objectives and milestones. Progress and Accomplishments The initial stage of this research has largely focused on developing the equipment necessary to synthesize and characterize the oxygen-free structures required to achieve the aforementioned objectives. Assembly of the systems has been completed, where the chambers added to the ultra-high vacuum system are highlighted in Fig. 2. Specifically, the two systems added to the setup are: • A fluoride PEALD system for both the aluminum metal and metal fluorides needed for this work; and • A visible and UV (VUV) optical system, which will be used to characterize reflectance of the films deposited without atmospheric contamination.

Fig. 2. Photo (top) and schematic (bottom) of ASU in-situ ultra-high-vacuum system; blue shows new systems in progress (UPS, UV Photoelectron Spectroscopy; AES, Auger electron Spectroscopy; XPS, X-ray Photoelectron Spectroscopy; MBE, Molecular Beam Epitaxy; iPlas, Innovative Plasma Chemical Vapor Deposition; ECR, Electron Cyclotron Resonance). A key component of this work is related to the in-situ nature of the deposition and characterization soon to be enabled. Since aluminum oxidation presented a significant challenge in previous research [17], this work will allow deposition of aluminum and metal fluorides without exposure to atmosphere.

Page 3 of 11 1. Plasma-Enhanced Atomic Layer Deposition The fluoride PEALD system, which has been completed, is based on the previous oxide system available in the lab, with only a few alterations. Specifically, the plasma is designed with a remote configuration to aid in reducing ion bombardment of the sample, mitigating potential ion damage. Plasmas are ignited with a 13.56-MHz RF excitation, applied at 100 W to a helical copper coil wound around a 32-mm-diameter quartz tube and maintained at ~100 mTorr with a flow rate of ~35 sccm (standard cubic centimeters per minute). This system has a background pressure of 5×10-8 Torr and a processing base pressure of ~10 mTorr. To achieve these pressures, we use a Pfeiffer HiPace 80 turbo pump, with a pumping speed of 300 liters/sec and an ACP15 dry backing pump, enabling lower pressures; however the chemicals used during deposition are too harsh, reducing the lifetime of the turbo and backing pumps. Therefore, when operating, the turbo and backing pumps are isolated with a gate valve, and a corrosion-resistant Ebara A70W multi-staged dry pump, with a pumping speed of 140 liters/sec is used. The A70W is configured to provide a nitrogen dilution flow at ~14.2 SLM (standard liters per minute) during operation to further ensure system longevity. In addition, the gas-flow mechanisms are designed to deliver precursors to the chamber with the correct timing sequence using mass flow controllers, pneumatically actuated valves, and a custom LabView program. Fine-metering valves are in line with the precursor-delivery lines to further control the amount of precursor released into the chamber. An MKS 235B Exhaust Throttling Valve is used before the A70W dry pump to maintain the required pressure during processing. To ensure the abatement of all unreacted processing gases and byproducts, a Mass Vac Multi-Trap, equipped with activated charcoal and stainless-steel-mesh filters, is used to abate pyrophorics and large molecules, while a NovaSafe Acid Dry Bed Abatement System is used for hydrogen fluoride. The Multi-Trap is placed directly after the throttling valve while the acid dry bed is installed after the A70W dry pump. Additionally, the acid-dry bed is fitted with a Bionics SH-10030-WAD Gas Detector and configured for hydrogen fluoride. A schematic of the PEALD system is shown in Fig. 3.

Fig. 3. Schematic of the PEALD system. Fluoride processes have not been developed extensively. In fact, to date, there is only one published process for fluoride PEALD [16], in which trimethylaluminum [Al(CH3)3] and SF6 plasma are employed to obtain AlF3 thin films. However, there has been much more work on thermal ALD

Page 4 of 11 processes, which use HF pyridine [22-24], anhydrous HF [17, 19, 25], TiF4 [26-28], and TaF5 [29] as the fluorine sources. While TiF4 and TaF5 come with significantly less safety hazards, the films deposited with these materials are typically characterized by some metal contamination, which would likely cause absorption at the desired wavelengths. A fluoride-based plasma was also considered for this work but dismissed due to concerns that it might lead to etching rather than deposition. Therefore, an HF-pyridine mixture seems to be the best option at this point. To take advantage of the desirable properties of PEALD, the HF step will be coupled with a hydrogen plasma step to ensure film purity. For aluminum and magnesium fluoride, trimethylaluminum and bis- methylcyclopentadienyl magnesium have been chosen as the respective metal precursors. To date, there have been two reports on the PEALD [21, 32] and one reports on thermal ALD [33] of Al metal films. Lee et al. showed that aluminum could be deposited using a trimethylaluminum and a hydrogen plasma [21, 32] while Blakeney et al. used a thermally stable Al hydride reducing agent [33]. PEALD of Aluminum on Phosphorus Doped Si (100). We have successfully deposited aluminum metal thin films on p-type Si (100) wafers. Wafers received an ultrasonic solvent bath (acetone and methanol) followed by a DI water rinse and drying with N2. After ex-situ cleaning, the wafers were loaded into the UHV system and treated with an RF hydrogen plasma, at 100 W and 250°C, for 30 minutes prior to surface chemical characterization with XPS. Aluminum films were deposited at 250°C for 90 cycles, using alternating exposures of trimethylaluminum and hydrogen plasma. A single cycle consists of a 3-second trimethylaluminum pulse, 10-second exposure, 7.5-second vacuum purge, 20-second H2 plasma, and a 15-second Ar purge. A schematic of the process in shown in Fig. 4.

Fig. 4. Schematic of the PEALD Al cycle. Due to the effectiveness of the A70W dry pump, a gas purge is not needed after the TMA pulse.

= Equation 1 𝑰𝑰𝒊𝒊 Film thickness was determined by XPS using𝒕𝒕 − Equation𝝀𝝀 𝐥𝐥𝐥𝐥 𝑰𝑰𝒐𝒐 1 [33], where t is the thin film thickness, λ is the inelastic mean free path of Si 2p electrons in the over-layer material, and Io and Ii are the integrated intensity of the Si 2p peak before and after deposition. XPS was performed using an Al K-alpha X-ray source (VG Scienta R3000, Al K E = 1482.35 eV). The inelastic mean free path of Si 2p electrons in Al is

31.58 Å [34]. Using the XPS spectraα shown in Fig. 5, a metallic Al thickness of 1.89 nm was deposited, yielding a growth rate per cycle, GPC, of 0.21 Å/cycle. Additional XPS spectra, of the C, N, and O lines are shown in Fig. 6, showing the oxygen- and nitrogen-free nature of our system and process.

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Fig. 5. XPS spectra for silicon (left) and aluminum (right) before and after Al deposition on a p-type Si wafer. Left: Si spectra with the SiO2 2p peak at 104.6 eV and the 2p1/2 and 2p3/2 doublets at 100.3 eV and 99.7 eV, respectively. Right: Al 2p peak at 75.9 eV with no detected Al prior to deposition.

Fig. 6. XPS spectra of nitrogen (left), oxygen (middle), and carbon (right). Left: There are no signs of nitrogen contamination from deposition. Center: The O 1s binding energy shift from 533.8 eV to 533 eV indicates the formation of Al2O3 on the native SiO2 layer, while the difference of intensity implies oxygen removal. Right: After deposition there are two carbon peaks indicating incomplete removal of the Al precursor methyl groups. In comparison to the Al films grown in [21, 32], the growth rates reported here are an order of magnitude lower. We believe this could be due to a number of issues, possibly including non-ideal growth temperature or insufficient precursor per pulse, and are taking steps to resolve this issue. Additional work is underway to find the optimal growth parameters for PEALD Al. 2. Visible and Ultraviolet Optical System We have also established in-situ reflectivity to 120 nm to characterize PEALD oxides, fluorides, and aluminum. In the future, the VUV optical system can be upgraded for shorter wavelengths (to ~ 90 nm), which will be accomplished by replacing the deuterium lamp with a windowless hollow cathode lamp using inert gases or mixtures. This system, integrated with a McPherson 234/302 Monochromator (Fig. 7), has also been designed and fabricated, and is undergoing calibration and alignment.

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Fig. 7. Illustration of the custom-designed UV spectrometer unit. Left: Optical system, which can currently measure from 400 nm to 120 nm. Right: Outline drawing of the optical system. Deuterium-UV-lamp emission spectra. The vacuum of the optical system was improved to 2×10-6 Torr. The initial calibration of diffraction grating and beam alignment was completed, and the deuterium-UV-lamp emission spectra were scanned by our optical system, detecting the characteristic molecular lines, e.g. 121 nm (Fig. 8).

100 159.5 161.4

126.3 80 139.6 131.8

60 156.8

40

121.9

Relative output intensity (%) 20

0 110 120 130 140 150 160 170 Wavelength (nm) Fig. 8. Hamamatsu L1835 Deuterium-lamp emission spectra: Left: Scanned by McPherson 234/302 Monochromator shown in Fig. 4. Right: Reference from Hamamatsu Photonics.

PEALD oxides on B-doped Silicon substrate. Some PEALD oxides (HfO2 and Al2O3) are employed for testing and verification of our optical system. The in-situ reflectance (R%) for HfO2 was measured in our optical system, and further compared with the ex-situ measurements in Perkin Lambda 950 UV/Vis Spectrophotometer. The disparity (Fig. 9) between the two results may be due to the difference of the incident angle and pressure during measurements.

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Fig. 9. UV reflectance of PEALD HfO2 on B-doped Si substrate. Some additional work is needed to ensure accuracy and consistency. Determination of optical constants (n,k) from optical measurements. It has been reported that (n, k) can be determined by reflectance measurements at various thin-film thicknesses [35, 36]. Reflectance and thickness measurements can be done in-situ with our optical system and XPS. Thus, (n, k) can be determined from single wavelength reflectance contours technique described in [35, 36]. Here, (n, k) can be calculated using a modified Python program based on the transfer-matrix method [37]. An example (n, k) determination is shown in Fig. 10. The (n, k) and thickness were pre-given to generate reflectance data for the calculation. The (n, k) can then be determined from the single-wavelength reflectance contours after measuring the reflectance for several thicknesses of the thin film layer. The accuracy of this calculation method depends on the precision of the reflectance measurements, and the agreement between the simulation results and the experiments. This method works well when k is large; for AlF3, it means the wavelength should be lower than 124 nm [35]. When the wavelength is longer than 124 nm, k will be around 0, simplifying the calculation, as we can assume k=0.

Fig. 10. Illustration of the calculation method for determining optical constants (n, k) from reflectance measurements of several different thicknesses. Left: Optical model used for the calculation; interface roughness can be introduced to improve the calculation accuracy. Right: Single-wavelength reflectance contours used for the determination of (n, k), the common point of all three curves is the final result of (n, k). Filter Design - The key contributions of Tom Mooney and Matthew Beasley Matthew Beasley inspected the facilities in progress and made specific suggestions for which materials should be prioritized for ALD deposition.

The PEALD system is capable of Al and AlF3, and will have functionality for MgF2 by July 2019. The implementations of lanthanum and calcium precursors are being considered for LaF3 and CaF2, as suggested by Dr. Beasley. Tom Mooney created a trial run of a filter using optical constants, from literature, and analyzed the tolerance to varying film thickness. This provided the direction that measuring the optical constants, of the as-deposited films, is the next key step to ensure a filter with desired performance can be designed for fabrication and testing. Mooney retired and Dr. Robert Sprague at Materion has taken his place.

Page 8 of 11 Path Forward The plan of work is structured around the remaining goals and milestones stated above. In general, the plan is to focus on optimizing the growth of PEALD Al as soon as possible while continuing the calibration of the UV spectrometer and preparation for the addition of Mg to the PEALD. This will enable the use and fabrication of bandpass filters. More specifically, the following will be necessary. 1. Demonstrate the use of PEALD to deposit loss-free oxides of silicon and aluminum on evaporated and e-beam-deposited aluminum surfaces. These oxides were selected based on their promise for stable, high-performance oxides. This is motivated because current methods produce lossy films, which are not usable below 190 nm. 2. Optimize the capability for PEALD aluminum films and characterize the UV reflectivity of the films and the surface contamination using in-situ characterization tools. These aluminum surfaces will be the basis of the structures pursued in this program. 3. Demonstrate the feasibility of using PEALD to deposit low-loss films of fluoride compounds on ALD aluminum. The fluorides of interest include most significantly aluminum and magnesium fluoride, but others will be considered as well. We will need to demonstrate stability, uniformity, and performance before advancing. 4. Extend the reflectivity characterization to 90 nm and characterize the fluoride-aluminum structures. This is a critical step, as the aluminum-oxide/fluoride layer will serve as the foundation for more complicated reflective surfaces. For filters, we intend to show proof-of- concept in the next segment. The quality of aluminum and interface will define the required performance, which will be verified before advancing to the next stages. In this system development phase, work will be conducted by Dr. Nemanich and his team, focusing on developing processes that result in optimized films. A number of complexities in the system design and construction have delayed the development phase, which is now completed. Before moving on to the next sequence, the optics design team of Drs. Scowen, Mooney, and Beasley will revisit the demonstrated performance of the three classes of product (above) and compare them to the models that initiated the work. Stability will be measured using vacuum exposure and spectral retest, and similarly humidity exposure followed by spectral retest. If there are differences in stability and reflectivity, we need to understand their origins before we can build prototype filters, since these parameters are critical for tuning an individual filter for a particular bandpass. Once this has been done, we will implement the techniques used above for the deposition of multilayer construction. The next phase will then address the following goals. 5. Optimize far-UV reflectivity of fluorides on aluminum to deliver a wide bandpass, far-UV- optimized mirror. The optimization will focus on minimizing interface contamination and optimizing material-growth parameters and film thickness for far-UV reflectivity. We will demonstrate performance as far as possible into the far-UV with our new VUV system. 6. Construct metal-dielectric Fabry-Perot bandpass filters using aluminum and magnesium fluoride, leading to multi-cavity structures that will exhibit very high performance—based on our models. This design approach is currently limited by the ability to deposit very thin layers with great accuracy, film toughness, and ‘bulk’ thin-film material losses. We believe only two to five layers will be needed to demonstrate proof-of-concept, with each layer ~10-nm thick. 7. Fine-tune approach and models to produce designs for multilayer dielectric (narrowband) mirrors to act as reflection filters or high reflectors in narrowband systems. We will demonstrate construction and performance. 8. Demonstrate the construction of multilayer broadband mirrors, which we believe—based on models—will exhibit higher performance than metal-based mirrors.

Page 9 of 11 9. We expect this design and construction phase to occupy most of Q3 and Q4 of 2019. To achieve all this work, we will demonstrate our approach, designs, and methodologies deliver the necessary improvements in thickness control, lower ‘bulk’ losses, absence of color centers, smoother films (lower scatter), ability to deposit very thin films (with bulk-like optical properties), durability, and lower stress. The above plan, executed in this manner, will demonstrate all these goals. References [1] J.M. Shull, B.D. Smith, and C.W. Danforth, “The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still be Missing,” The Astrophysical J. 759, 23 (2012) [2] M. Putkonen et al., “Thermal and plasma enhanced atomic layer deposition of SiO2 using commercial silicon precursors,” Thin Solid Films, 558, 93 (2014) [3] S.J. Won, S. Suh, M.S. Huh, and H.J. Kim, “High-Quality Low-Temperature Silicon Oxide by Plasma- Enhanced Atomic Layer Deposition Using a Metal–Organic Silicon Precursor and Oxygen Radical,” IEEE Electron Device Lett., 31, 8 (2010) [4] D. Shin, H. Song, M. Lee, H. Park, and D.H. Ko, “Plasma-enhanced atomic layer deposition of low temperature silicon dioxide films using di-isopropylaminosilane as a precursor,” Thin Solid Films, 660, 572 (2018) [5] K. Pfeiffer et al., “Comparative study of ALD SiO2 thin films for optical applications,” Optical Materials Express, 6, 660 (2016) [6] J. Yang, B.S. Eller, M. Kaur, and R.J. Nemanich, “Characterization of plasma-enhanced atomic layer deposition of Al2O3 using dimethylaluminum isopropoxide,” Journal of Vacuum Science & Technology A 32, 021514 (2014) [7] P. Singh, R.K. Jha, R.K. Singh, and B.R. Singh, “Preparation and characterization of Al2O3 film deposited by RF sputtering and plasma enhanced atomic layer deposition,” Journal of Vacuum Science & Technology B 36, 04G101 (2018) [8] S.E. Potts, G. Dingemans, C. Lachaud, and W.M.M. Kessels, “Plasma-enhanced and thermal atomic layer deposition of Al2O3 using dimethylaluminum isopropoxide, [Al(CH3)2(µ-OiPr)]2, as an alternative aluminum precursor,” Journal of Vacuum Science & Technology A 30, 021505 (2012) [9] J. Koo, S. Kim, S. Jeon, H. Jeon, Y. Kim, and Y. Won, “Characteristics of Al2O3 Thin Films Deposited Using Dimethylaluminum Isopropoxide and Trimethylaluminum Precursors by the Plasma- Enhanced Atomic-Layer Deposition Method,” J. Korean Phys. Soc. 48, 131 (2006) [10] S. Shestaeva et al., “Mechanical, structural, and optical properties of PEALD metallic oxides for optical applications,” Applied Optics 56, 4 (2017) [11] B. Hoex, J. Schmidt, P. Pohl, M.C.M. van de Sanden, and W.M.M. Kessels, “Silicon surface passivation by atomic layer deposited Al2O3,” J. Appl. Phys. 104, 044903 (2008) [12] P.K. Park and S.W. Kang, “Enhancement of dielectric constant in HfO2 thin films by the addition of Al2O3,” Appl. Phys. Lett. 89, 192905 (2006) [13] A. Sharma, V. Longo, M.A. Verheijen, A.A. Bol and W.M.M. Kessels, “Atomic layer deposition of HfO2 using HfCp(NMe2)3 and O2 plasma,” J. Vac. Sci. Technol. A 35, 01B130 (2017) [14] J. Provine et al., “Atomic layer deposition by reaction of molecular oxygen with tetrakisdimethylamidometal precursors,” Journal of Vacuum Science & Technology A 34, 01A138 (2016) [15] X.Y. Zhang et al., “Surface Passivation of Silicon Using HfO2 Thin Films Deposited by Remote Plasma Atomic Layer Deposition System,” Nanoscale Res Lett. 12, 324 (2017) [16] M.F.J. Vos, H.C.M. Knoops, R.A. Synowicki, W.M.M. Kessels, and A.J.M. Mackus, “Atomic layer deposition of aluminum fluoride using Al(CH3)3 and SF6 plasma,” Appl. Phys. Lett. 111, 113105 (2017) [17] J. Hennessy, A.D. Jewell, F. Greer, M.C. Lee, and S. Nikzad, “Atomic layer deposition of magnesium fluoride via bis(ethylcyclopentadienyl)magnesium and anhydrous hydrogen fluoride,” J. Vac. Sci. Technol. A 33, 01A125 (2015)

Page 10 of 11 [18] 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, 041206 (2016) [19] J. Hennessy and S. Nikzad, “Atomic Layer Deposition of Lithium Fluoride Optical Coatings for the Ultraviolet,” Inorganics 6, 46 (2018) [20] B. Fleming et al., “Advanced environmentally resistant lithium fluoride mirror coatings for the next generation of broadband space observatories,” Applied Optics 56, 36 (2017) [21] Y.J. Lee and S.W. Kang, “Atomic Layer Deposition of Aluminum Thin Films Using an Alternating Supply of Trimethylaluminum and a Hydrogen Plasma,” Electrochem. Solid-State Lett. 5, 10 (2002) [22] Y. Lee, H. Sun, M.J. Young, and S.M. George, “Atomic Layer Deposition of Metal Fluorides Using HF– Pyridine as the Fluorine Precursor,” Chem. Mater. 28 [23] Y. Lee, J.W. DuMont, A.S. Cavanagh, and S.M. George, “Atomic Layer Deposition of AlF3 Using Trimethylaluminum and Hydrogen Fluoride,” J. Phys. ,Chem. 2022−2032 C, 119 (2016), 25, 14185-14194 (2015) [24] Lee, Younghee et al., “Atomic Layer Deposition of LiF and Lithium Ion Conducting (AlF3)(LiF)x Alloys Using Trimethylaluminum, Lithium Hexamethyldisilazide and Hydrogen Fluoride,” ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.5459653.v1. (2017) [25] J. Hennessy, A.D. Jewell, K. Balasubramanian, and S. Nikzad, “Ultraviolet optical properties of aluminum fluoride thin films deposited by atomic layer deposition,” J. Vac. Sci. Technol. A 34, 01A120 (2016) [26] M. Mäntymäki, J. Hämäläinen, E. Puukilainen, F. Munnik, M. Ritala, and M. Leskelä, “Atomic Layer Deposition of LiF Thin Films from Lithd and TiF4 Precursors,” Chem. Vap. Dep. 19, 111 (2013) [27] M. Mäntymäki, M.J. Heikkilä, E. Puukilainen, K. Mizohata, B. Marchand, J. Räisänen, M. Ritala, and M. Leskelä, “Atomic Layer Deposition of AlF3 Thin Films Using Halide Precursors,” Chem. Mater. 27, 604 (2015) [28] R. Swanepoel, “Determination of the thickness and optical constants of amorphous silicon,” J. Phys. E: Sci. Instrum. 16, 1214 (1983) [29] J. Aarik, H. Mandar, M. Kirm and L. Pung, “Optical characterization of HfO2 thin films grown by atomic layer deposition,” Thin Solid Films, 466, 41 (2004) [30] M. F. Al-Kuhaili, “Optical properties of hafnium oxide thin films and their application in energy- efficient windows,” Opt. Mater. 27, 383 (2004) [31] J.L. Hernández-Rojas et al., “Optical analysis of absorbing thin films: application to ternary chalcopyrite semiconductors,” Applied Optics, 31, 10, 1606-1611 (1992) [32] F.L. Martinez et al., “Optical properties and structure of HfO2 thin films grown by high pressure reactive sputtering,” J. Phys. D: Appl. Phys. 40, 5256 (2007) [33] A. Jablonski and J. Zemek, “Overlayer thickness determination by XPS using the multiline approach,” Surf. Interface Anal., 41 193-204 (2009) [34] C.J. Powell and A. Jabloski, “NIST Electron Inelastic-Mean-Free-Path Database,” Version 1.2, SRD 71, National Institute of Standards and Technology, Gaithersburg, MD (2010) [35] F. Bridou et al., “Experimental determination of optical constants of MgF2 and AlF3 thin films in the vacuum ultra-violet wavelength region (60–124 nm), and its application to optical designs,” Optics Communications, 283, 1351-1358 (2010) [36] F. Bridou et al., “Experimental determination of optical constants in the vacuum ultra violet wavelength region between 80 and 140 nm: A reflectance versus thickness method and its application to ZnSe,” Optics Communications, 271, 353-360 (2007) [37] S.J. Byrnes, “Multilayer optical calculations,” arXiv:1603.02720v3 [physics.comp- ph] (2018)

For additional information, contact Paul Scowen: [email protected]

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