Thermal and mechanical behavior of plasma-synthesized, nanometer scale aluminum fluoride passivation layers

Aluminum is highly valued in ultraviolet (UV) optics for its exceptional reflectivity at wavelengths as short as 90 nm, but its effectiveness is compromised by rapid formation of a native oxide layer that reduces its UV reflectivity. To mitigate this issue, fluorine-containing protective layers, such as aluminum fluoride, are applied to inhibit oxidation while maintaining high transmission rates. Additionally, an aluminum fluoride layer serves as an effective barrier coating in advanced lithium-ion battery designs, preventing failures like significant temperature increases and thermal runaway. Despite these important applications, the thermal and mechanical properties of aluminum fluoride thin films have not been thoroughly investigated. In this study, optical pump–probe techniques are used to measure the thermal conductivity and elastic moduli of aluminum fluoride passivation layers with thicknesses ranging from 4 to 48 nm. The passivation layers are produced using a plasma-based process that removes the native oxide while converting the aluminum into aluminum fluoride. The results show that both thermal conductivity and elastic moduli increase with film thickness, indicating a thickness-dependent change in physico-chemical composition. Energy-dispersive x-ray spectroscopy reveals that thicker layers contain a higher proportion of fluorine relative to aluminum. Moreover, x-ray photoelectron spectroscopy and infrared variable-angle spectroscopic ellipsometry indicate that the chemical structure stabilizes in thicker samples, confirming the formation of AlF3. These findings provide valuable insights into the process–structure–property relationships of plasma-produced fluorinated layers, which are critical for optimizing UV optical coatings and enhancing lithium-ion battery safety.