Formation of AlF_x Gaseous Phases during High Temperature Etching: A Reactive Force Field Based Molecular Dynamics Study

The progress of research focused upon the etching of metal films or substrates using fluorine gases has been restricted by limited information regarding etching reactants and byproducts. Indeed, aspects of the etching mechanism itself remain unclear. In this study, a new reactive force field (ReaxFF) for Al-F was developed to describe the interaction and reactions in Al-F materials. The ReaxFF accurately reproduces the quantum mechanics derived training set for structures and energies of gaseous AlF_x molecules and Al-F crystals. Based on this Al-F ReaxFF, the effects of chemical source (F/Al = 1-6) and temperature (1000-1500 K) on the etching product and rate were studied. The formation of gaseous AlF_x was revealed in five steps with the fluorine concentration being the prime factor affecting the etching products. Below the critical concentration ratio of F/Al = 3, where the chemical driving force is insufficient, only four of the five steps occur and a AlF_x cluster is formed without significant gaseous species; above this critical concentration, a fifth step happens, and isolated AlF_x gaseous phases with much more negative formation energies, such as AlF₄, AlF₅, and AlF₆, can be formed. Besides this concentration ratio, external parameters such as elevated temperature or higher voltage dischage may be an important energetic factor affecting the product quantity. These results may provide insights into controlling the formation kinetics of specific AlF_x compounds or gaseous phases for the preparative chemistry of Al-F porous catalyst, and the Al-F ReaxFF provides a useful tool for studying the interaction and reaction of Al-F materials at the atomic scale.