Atomic layer deposition of aluminum fluoride using Al(CH₃)₃ and SF₆ plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF₃ films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH₃)₃] and SF₆ plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm⁻³ was found to be relatively close to the bulk value of 3.1 g cm⁻³. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140–2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of <10⁻⁴ above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF₆ plasma exposure step of the ALD cycle revealed the formation of C₂H₂ and CF₂ species, resulting from the interaction of the plasma with the surface after Al(CH₃)₃ exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF₆ plasma participate in the surface reactions. Overall, this work demonstrates that SF₆ plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.