Atomic layer deposition of MgF2 thin films using TaF5 as a novel fluorine source

Magnesium fluoride is one of the most important optical thin film materials due to the good light transparency down to the vacuum ultraviolet (UV) range. A novel atomic layer deposition (ALD) process was developed for depositing MgF2 thin films. Instead of the previously used fluorine sources, namely HF or TiF4, we used TaF5 as the fluorine precursor for the first time in ALD. The films were grown in a temperature range of 225-400 degrees C. The films were characterized by X-ray diffraction, X-ray reflection, atomic force microscopy, scanning electron microscopy, spectrophotometer, and elastic recoil detection analysis. Optical and electrical properties of the films were also studied. The films grew in columnar fashion onto silicon and all the films were polycrystalline. The film densities were close to bulk MgF2. Films with a good stoichiometry and low impurity levels were achieved. The refractive indices were between 1.36 and 1.38 at lambda = 580 nm. The permittivity of a film grown at 300 degrees C was 5.0. The transmittance of a film deposited at 350 degrees C was good even in the deep UV range. This type of novel ALD process using the new fluorine precursor TaF5 is a convenient method also for depositing other metal fluoride thin films.