E-Beam Generated Plasma Etching for Developing High-Reflectance Mirrors for Far-Ultraviolet Astronomical Instrument Applications

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 Al mirrors protected with aluminum tri-fluoride (AlF3) in order to realize the high intrinsic reflectance Al down to FUV wavelengths (100200 nm), while still maintaining the high reflectance in the optical and NIR spectral regions. 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)

Atomic Layer Deposition of the Solid Electrolyte LiPON

We demonstrate an atomic layer deposition (ALD) process for the solid electrolyte lithium phosphorousoxynitride (LiPON) using lithium <i>tert</i>-butoxide (LiO^tBu), H₂O, trimethylphosphate (TMP), and plasma N₂ (^PN₂) as precursors. We use in-situ spectroscopic ellipsometry to determine growth rates for process optimization to design a rational, quaternary precursor ALD process where only certain substrate–precursor chemical reactions are favorable. We demonstrate via in-situ XPS tunable nitrogen incorporation into the films by variation of the ^PN₂ dose and find that ALD films over approximately 4.5% nitrogen are amorphous, whereas LiPON ALD films with less than 4.5% nitrogen are polycrystalline. Finally, we characterize the ionic conductivity of the ALD films as a function of nitrogen content and demonstrate their functionality on a model battery electrodea Si anode on a Cu current collector

Atomic Layer Deposition and <i>in Situ</i> Characterization of Ultraclean Lithium Oxide and Lithium Hydroxide

We demonstrate the ultraclean atomic layer deposition (ALD) of Li₂O and LiOH using lithium <i>tert</i>-butoxide (LiO^<i>t</i>Bu) precursor with H₂O and plasma O₂ as oxidants, along with conversion of Li₂O and LiOH products to Li₂CO₃ upon CO₂ dosing. Using LiO^<i>t</i>Bu and H₂O results in LiOH below 240 °C and Li₂O above 240 °C for otherwise identical process parameters. Substituting plasma O₂ as the oxidation precursor results in a combination of Li₂CO₃ and Li₂O products, indicating modification of the ALD reaction preventing volatilization of the C from the Li precursor. The chemistry of the films is definitively characterized for the first time with XPS utilizing an all-UHV transfer procedure from the ALD reactor. We use <i>in situ</i> UHV gas dosing to investigate the reaction mechanisms of ALD Li₂O and LiOH with H₂O and CO₂ to simulate reactions upon air exposure. Lastly, we employ <i>in situ</i> spectroscopic ellipsometry to determine the reaction kinetics of thermal LiOH decomposition, and we report an activation energy of 112.7 ± 0.6 kJ/mol

Stabilization of Lithium Metal Anodes by Hybrid Artificial Solid Electrolyte Interphase

Li metal is among the most attractive anode materials for secondary batteries, with a theoretical specific capacity > 3800 mAh g^–1. However, its extremely low electrochemical potential is associated with high chemical reactivity that results in undesirable reduction of electrolyte species on the lithium surface, leading to spontaneous formation of a solid electrolyte interphase (SEI) with uncontrolled composition, morphology, and physicochemical properties. Here, we demonstrate a new approach to stabilize Li metal anodes using a hybrid organic/inorganic artificial solid electrolyte interphase (ASEI) deposited directly on the Li metal surface by self-healing electrochemical polymerization (EP) and atomic layer deposition (ALD). This hybrid protection layer is thin, flexible, ionically conductive, and electrically insulating. We show that Li metal protected by the hybrid protection layer gives rise to very stable cycling performance for over 300 cycles at current density 1 mA/cm² and over 110 cycles at current density 2 mA/cm², well above the threshold for dendrite growth at unprotected Li. Our strategy for protecting Li metal anodes by hybrid organic/inorganic ASEI represents a new approach to mitigating or eliminating dendrite formation at reactive metal anodesillustrated here for Liand may expedite the realization of a “beyond-Li-ion” battery technology employing Li metal anodes (e.g., Li–S)

Next-Generation Lithium Metal Anode Engineering <i>via</i> Atomic Layer Deposition

Lithium metal is considered to be the most promising anode for next-generation batteries due to its high energy density of 3840 mAh g^–1. However, the extreme reactivity of the Li surface can induce parasitic reactions with solvents, contamination, and shuttled active species in the electrolyte, reducing the performance of batteries employing Li metal anodes. One promising solution to this issue is application of thin chemical protection layers to the Li metal surface. Using a custom-made ultrahigh vacuum integrated deposition and characterization system, we demonstrate atomic layer deposition (ALD) of protection layers directly on Li metal with exquisite thickness control. We demonstrate as a proof-of-concept that a 14 nm thick ALD Al₂O₃ layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. Using Li–S battery cells as a test system, we demonstrate an improved capacity retention using ALD-protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles