Atomic layer deposition of LiF using LiN(SiMe3)2 and SF6 plasma

Lithium fluoride films were prepared by atomic layer deposition (ALD) using a new route in which LiN(SiMe3)2 is used as a precursor and SF6 plasma as a coreactant. Conformal LiF films were deposited at 150 °C at a growth rate of ∼0.4 A per cycle. All deposited films were polycrystalline and slightly lithium-rich with a composition of LiF0.8, independently of the plasma conditions (e.g. exposure time, pressure and power). The levels of H, C, N, O, Si, and S were all <1 at%. Spectroscopic ellipsometry measurements were carried out over the wavelength range of 140-2480 nm and showed a refractive index of 1.37 at 633 nm for films deposited using 1 s plasma exposure time. We conclude that short plasma exposures are preferred, since a prolonged exposure time leads to an increase in optical absorption and lower growth per cycle values. Furthermore, mass spectrometry measurements revealed the formation of SiMe3F species during both half-cycles, originating from the reaction between the precursor ligands and fluorine species present either at the surface or in the plasma. Moreover, the SF6 plasma step led to the formation of fluorocarbon species, suggesting that dissociation and recombination in the plasma takes place. Overall, this work demonstrates that SF6 plasma offers a promising alternative to other coreactants for ALD of high purity lithium fluoride.