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 degrees 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 A/cycle and a linear increase of the removed film thickness with the number of etch cycles were obtained at a temperature of 460 degrees 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 brightfield 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).