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).

采用交替暴露五氟化铌（NbF5）和四氯化碳（CCl4）的方法，对非晶态氧化铝（Al2O3）进行了热原子层刻蚀（ALEt）。在380至460°C的温度范围内，观察到Al2O3的ALEt现象。通过光谱椭偏法确定了刻蚀厚度和刻蚀速率，并利用X射线反射率进行了验证。在460°C时，获得了最大刻蚀速率约为1.4 Å/周期，以及随着刻蚀循环次数的增加，去除的薄膜厚度呈线性增加。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，从而刻蚀掉材料被转化的部分。与此一致，当通过X射线光电子能谱对厚NbF5处理后的Al2O3层的元素深度剖面进行分析时，发现表面氟含量约为55 at. %。利用原子力显微镜和明场透射电子显微镜分析了参考、刻蚀前和刻蚀后Al2O3表面的形貌。此外，发现该过程化学能够选择性地刻蚀Al2O3，而非二氧化硅（SiO2）和氮化硅（Si3N4）。