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).

在380至460°C的温度范围内，通过交替暴露于五氟化铌（NbF5）和四氯化碳（CCl4）中，对非晶态氧化铝（Al2O3）进行了热原子层刻蚀（ALEt）。利用光谱椭偏仪测定了刻蚀厚度和刻蚀速率，并通过X射线反射率进行验证。在460°C的温度下，获得了约为1.4Å/周期的最大刻蚀速率和随着刻蚀循环次数线性增加的去除薄膜厚度。借助密度泛函理论计算，提出了一种刻蚀机制，其中NbF5将部分Al2O3表面转化为三氟化铝（AlF3）或铝氧氟化物层，该层与CCl4反应后转化为挥发性卤化物副产物，从而刻蚀掉材料的转化部分。与此一致，当使用X射线光电子能谱对厚NbF5处理的Al2O3层的元素深度剖析时，揭示了约55原子%的显著表面氟含量。采用原子力显微镜和明场透射电子显微镜对参考、刻蚀前和刻蚀后Al2O3表面的形貌进行了分析。此外，发现此过程化学能够选择性地在二氧化硅（SiO2）和氮化硅（Si3N4）上刻蚀Al2O3。