Abstract
Lithium fluoride is an important material for ultraviolet optical systems, possessing among the largest optical bandgaps of dielectric materials. We report on the development of an atomic layer deposition (ALD) process for lithium fluoride that is capable of depositing thin films in a self-limiting manner, with an approximate deposition rate of approximately 0.15 Å per ALD cycle at a substrate temperature of 150 °C. Films are characterized by spectroscopic ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, and far ultraviolet reflectometry. For substrate temperatures of 150 °C and greater, films showed significant microroughness with a correlated reduction in effective refractive index. This behavior was mitigated by a reduction in substrate temperature to as low as 100 °C. Films deposited on silicon substrates were subjected to long-term storage testing to evaluate the environmental sensitivity of the deposited layers. Protected aluminum mirrors were also fabricated with ALD LiF overcoats, yielding a reflectance of 84% at a wavelength of 125 nm. The performance relative to state-of-the-art LiF thin films deposited by physical vapor deposition methods is discussed, along with the prospects for future optimization.
Highlights
The use of lithium fluoride (LiF) as an ultraviolet optical coating material is motivated by its large optical bandgap, exceeding that of other metal fluoride materials used for this application such as magnesium fluoride (MgF2 ) and aluminum fluoride (AlF3 )
We have demonstrated that thin films of LiF can be deposited in a self-limiting manner over a temperature range of 100–250 ◦ C, with potential applications related to FUV optical coatings
LiF deposited on Si substrates indicated an aging effect that was similar to previous studies on films deposited by physical vapor deposition (PVD) methods
Summary
The use of lithium fluoride (LiF) as an ultraviolet optical coating material is motivated by its large optical bandgap, exceeding that of other metal fluoride materials used for this application such as magnesium fluoride (MgF2 ) and aluminum fluoride (AlF3 ). Al mirrors typically involves immediate encapsulation of the metal layer with a protective transparent coating to preserve performance at the shortest wavelength possible. In this case, loss in the protective coating dominates the short wavelength reflectance of the mirror; MgF2 /Al can typically provide meaningful reflectance (R > 50%) down to ~115 nm, AlF3 /Al down to ~110 nm, and LiF/Al down to ~100 nm. Loss in the protective coating dominates the short wavelength reflectance of the mirror; MgF2 /Al can typically provide meaningful reflectance (R > 50%) down to ~115 nm, AlF3 /Al down to ~110 nm, and LiF/Al down to ~100 nm This has motivated the use of LiF/Al mirrors on optical telescope systems in space applications when the desired scientific observations require this FUV throughput [6,7]. Measurements of FUV reflectance are demonstrated for LiF deposited on Si substrates as well as Al layers to evaluate the potential of this material for FUV mirror applications
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