Thermal Atomic Layer Etching of Aluminum Oxide (Al₂O₃) Using Sequential Exposures of Niobium Pentafluoride (NbF₅) and Carbon Tetrachloride (CCl₄): A Combined Experimental and Density Functional Theory Study of the Etch Mechanism

Thermal atomic layer etching (ALEt) of amorphous Al₂O₃ was performed by alternate exposures of niobium pentafluoride (NbF₅) and carbon tetrachloride (CCl₄). The ALEt of Al₂O₃ is observed at temperatures from 380 to 460 °C. The etched thickness and the etch rate were determined using spectroscopic ellipsometry and verified by X-ray reflectivity. The maximum etch rate of about 1.4 Å/cycle and a linear increase of the removed film thickness with the number of etch cycles were obtained at a temperature of 460 °C. With the help of density functional theory calculations, an etch mechanism is proposed where NbF₅ converts part of the Al₂O₃ surface into an AlF₃ or aluminum oxyfluoride layer, which upon reacting with CCl₄ is converted into volatile halide-containing byproducts, thus etching away the converted portion of the material. Consistent with this, a significant surface fluorine content of about 55 at. % was revealed when the elemental depth profile analysis of a thick NbF₅-Treated Al₂O₃ layer was performed by X-ray photoelectron spectroscopy. The surface morphology of the reference, pre-, and postetch Al₂O₃ surfaces was analyzed using atomic force microscopy and bright-field transmission electron microscopy. Moreover, it is found that this process chemistry is able to etch Al₂O₃ selectively over silicon dioxide (SiO₂) and silicon nitride (Si₃N₄).