Nature of Hydrophilic Aluminum Fluoride and Oxyaluminum Fluoride Surfaces Resulting from XeF₂ Treatment of Al and Al₂O₃

Abstract

XeF2 treatment of aluminum and alumina surfaces is known to produce hydrophilic surfaces. There is however poor knowledge of the chemical nature of these surfaces. Using infrared absorption and X-ray photoelectron spectroscopy, the formation of highly hydrophilic AlF3 and AlOxFy surface layers is identified upon XeF2 exposure, along with strongly bound H2O and other related surface species formed by interactions with trace H2O under typical vacuum conditions (≈10–4 Torr). Surfaces resulting from XeF2 etching of oxide-free aluminum covered by a sacrificial Si layer have a strong affinity for H2O, with a contact-angle of ca. 5–10°. First-principles simulations offer new insight into details of the AlFx surface structure, based on the surface IR characterization by providing reliable assignments for associated AlFx···H2O infrared bands and showing that fluorine is strongly bound to Al, preventing further Al oxidation. The formation of hydrophilic AlF3 surface layers upon fluorine-based etching may pose a fundamental limitation for the use of Al in microelectromechanical (MEMs) applications, precluding the release of low-stiction, low-capillary force components.