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

Abstract

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 AlF 4 , AlF 5 , and AlF 6 , 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.

Atomic-Scale Etching Mechanism of Aluminum with Fluorine-Based Plasma

“…These calculations provide another avenue of explanation of why etching of alumina is more difficult than aluminum. Liu et al 16 conducted a molecular dynamic study on the formation of AlF 3 from bulk Al and F 2 molecules. They have reported the formation of AlF 3 at temperatures 1000−1500 K. Such a high temperature requirement may result from the fact that they considered Al bulk.…”

“…More specifically, it is not known how aluminum interacts differently than alumina, whether the neutral F atom or F 2 molecules interact with aluminum/alumina, the role of IPA, etc. More recently, Liu and co-workers 16 presented a force field molecular dynamic study on the formation of AlF x from aluminum and F 2 gas. They reported that various AlF x were formed from aluminum and F 2 gas at 1000−1500 K, which are well above the plasma operation temperature.…”

Morphology evolution of the aluminum surface in a fluorine-containing environment

“…ReaxFF potential developed by Liu et al 13 was used to describe the kinetics and reactivity characteristics of F and O adatoms on the Al(111) surface. The Al(111) surface was modeled by 8 Â 8-15 layer supercells.…”

“…Their results identify that there is a stable and nonvolatile layer of AlF 3 on the Al 2 O 3 surface, indicating that this reaction is self-limiting. Liu et al 13 studied the effects of the chemical source and temperature (1000–1500 K) on the etching product and etching rate by reactive force field (ReaxFF) based molecular dynamics (RMD). When the concentration ratio of F over Al is lower than three, the gaseous species cannot be formed; above this concentration, AlF 4 , AlF 5 and AlF 6 gaseous phases will be formed.…”