Atomic Layer Deposition of Aluminum Fluoride For Use in Optical Devices
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Thin metal fluoride films are useful as protective and anti-reflection coatings for optical instruments. Aluminum trifluoride (AlF3), which in the bulk exhibits dielectrical properties of a high band gap (>10 eV) and low refractive index (~1.35 at 632 nm), makes it an appropriate coating for ultraviolet applications. In this study, AlF3 atomic layer deposition (ALD) thermochemistry and surface reaction mechanisms between precursors trimethyl aluminum (TMA) and titanium (IV) tetrafluoride (TiF4), is studied as a means of fabricating these highly conformal films. Based on the hypothesized ALD reaction mechanism, ideal growth per cycle (GPC) is predicted. This work encompasses the design and construction of two ALD reactors for fabricating AlF3 thin films. A cross-flow reactor design will be shown to successful produce thin films with correct Al to F stoichiometry and refractive index (1.38 at 632 nm). Characterization techniques include X-ray photoelectron spectroscopy (XPS), variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), and spectrophotometry. Because films produced by the cross-flow reactor exhibited film spatial thickness gradients, film impurities, and particle formation, a next-generation showerhead type reactor was designed to reduce precursor flow asymmetries. The simplified design also was subject to fewer sources of leaks and with a modified gas delivery system based on the direct draw of TiF4 operating at 110°C and TMA operating at room temperature. Self-limiting growth was demonstrated for this ALD process. A collaboration between the University of Maryland and NASA-Goddard Space Flight Center was conducted to overcoat physically vapor deposited Al-LiF mirrors with ALD AlF3. Detailed reflectivity and maintained performance for these optical mirrors are presented.