Reaction Mechanisms during Atomic Layer Deposition of AlF₃ Using Al(CH₃)₃ and SF₆ Plasma

Metal fluorides generally demonstrate a wide band gap and a low refractive index, and they are commonly employed in optics and optoelectronics. Recently, an SF₆ plasma was introduced as a novel co-reactant for the atomic layer deposition (ALD) of metal fluorides. In this work, the reaction mechanisms underlying the ALD of fluorides using a fluorine-containing plasma are investigated, considering aluminum fluoride (AlF₃) ALD from Al(CH₃)₃ and an SF₆ plasma as a model system. Surface infrared spectroscopy studies indicated that Al(CH₃)₃ reacts with the surface in a ligand-exchange reaction by accepting F from the AlF₃ film and forming CH₃ surface groups. It was found that at low deposition temperatures Al(CH₃)₃ also reacts with HF surface species. These HF species are formed during the SF₆ plasma exposure and were detected both at the surface and in the gas phase using infrared spectroscopy and quadrupole mass spectrometry (QMS), respectively. Furthermore, QMS and optical emission spectroscopy (OES) measurements showed that CH₄ and CH _y F_4-y (y ≤ 3) species are the main reaction products during the SF₆ plasma exposure. The CH₄ release is explained by the reaction of CH₃ ligands with HF, while CH _y F_4-y species originate from the interaction of the SF₆ plasma with CH₃ ligands. At high temperatures, a transition from AlF₃ deposition to Al₂O₃ etching was observed using infrared spectroscopy. The obtained insights indicate a reaction pathway where F radicals from the SF₆ plasma eliminate the CH₃ ligands remaining after precursor dosing and where F radicals are simultaneously responsible for the fluorination reaction. The understanding of the reaction mechanisms during AlF₃ growth can help in developing ALD processes for other metal fluorides using a fluorine-containing plasma as the co-reactant as well as atomic layer etching (ALE) processes involving surface fluorination.