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

Summary

A dedicated Al–F ReaxFF is trained on QM energies/structures for gaseous AlF_x species and Al–F condensed phases, then applied to high-temperature etching MD spanning F/Al = 1–6 and 1000–1500 K. The work emphasizes a five-step pathway to volatile AlF_x products whose onset depends critically on fluorine chemical potential, with a F/Al ≈ 3 threshold separating cluster-retaining vs gas-phase product regimes. Plasma etching of Al interconnects and barrier films must balance volatile halide formation against residue clusters; the abstract frames atomistic MD as a way to rank F-rich regimes that volatilize Al versus those that trap fluoroaluminate clusters on surfaces.

Methods

  • Force field: New ReaxFF for Al–F interactions fit to QM training sets covering gas-phase AlF_x and crystalline Al–F references (abstract).
  • MD campaigns: Etching simulations scanning F/Al from 1 to 6 and temperature from 1000 to 1500 K (abstract).
  • Observables: Product stoichiometry, formation energies of gaseous AlF4, AlF5, AlF6-like fragments (as named in abstract), and qualitative rate trends with T and effective plasma overpotential (mentioned qualitatively).
  • Integration: velocity-Verlet MD with ReaxFF charges updated each step as standard; full timestep/thermostat choices appear in the journal article (proof PDF in corpus).

Force-field training. New Al–F ReaxFF terms are fit to QM energies and structures for gaseous AlF_x fragments and condensed Al–F references named in the abstract.

MD application (proof §3+ simulation setup). Engine / code: ReaxFF molecular dynamics on an Al etching supercell (software label N/A from this PDF text layer—confirm in the SI/VOR if you need a package string). System / composition: Al slab models with F₂ molecules added in integer multiples so F/Al runs from 1 to 6 (see Methods around the etching model description in the PDF). Boundaries / periodicity: 3D PBC for the slab supercell as defined in the article Methods. Ensemble: NVT. Temperature: etching temperatures in the 1000–1500 K window explored in the article (a representative ~1250 K case is motivated by typical etching T and the cryolite melting point in the text). Duration / timestep: 250 ps trajectories with 0.25 fs time step for the main ReaxFF etching runs quoted in the Methods. Barostat / pressure: N/A — NVT constant volume; plasma overpressure is not modeled beyond thermal kinetics. Engine / code: the proof PDF text layer does not clearly name a standalone MD package (only ReaxFF + DFT references such as CASTEP/Gaussian for training); confirm any ReaxFF driver call in the version-of-record SI if you need a software string. Electric field / enhanced sampling: N/A — not used in the thermal etching MD sweeps described.

Findings

  • Mechanistic steps: Five sequential steps toward gaseous AlF_x; fluorine concentration is the dominant control on which products appear (abstract).
  • F/Al threshold: For F/Al < 3, only four steps occur and condensed AlF_x clusters remain without substantial gas evolution; for F/Al > 3, a fifth step unlocks volatile AlF4/AlF5/AlF6-like species with more negative formation energies (abstract).
  • Kinetic drivers: Higher T (and factors analogous to higher discharge voltage) increase product yields (abstract-level claim).
  • Process reading: etch recipes that raise effective F chemical potential (higher F/Al flux) favor volatile halide evacuation, while lean-F conditions risk cluster accumulation that blocks steady etch fronts.

Limitations

Proof PDF; verify stoichiometric labels and energies in the published JPCC article. Plasma environments are only partially captured by thermal MD. Ion bombardment, sheath electric fields, and nonthermal electron distributions require multiscale treatments not included in the ReaxFF gas-phase etch campaigns summarized from the abstract. Surface roughness and chlorine residue from real chamber walls can alter effective F/Al ratios at the wafer interface.

Relevance to group

Extends ReaxFF coverage into Al–F plasma etching chemistry relevant to microfabrication and fluoroaluminate materials science.

Citations and evidence anchors