Novel ALD Process for Depositing CaF₂ Thin Films

Metal fluorides, like CaF 2, are interesting dielectric materials which are optically transparent over a wide wavelength range down to the vacuum ultraviolet regime. In addition, CaF 2 has a low refractive index and is therefore an attractive material for optical filters. In this study thin films of calcium fluoride were deposited by atomic layer deposition (ALD) at a temperature range of 300−450 °C using TiF 4 as a new convenient fluoride precursor. Ca(thd) 2 was used as a calcium precursor. Films were analyzed by X-ray diffraction/reflection (XRD/XRR), field emission scanning electron microscopy (FESEM), time-of-flight elastic recoil detection analysis (TOF-ERDA), and UV−vis spectrophotometer. The growth rate of the film was 1.6 Å/cycle, which is 4 times higher than that published before for CaF 2 ALD. All films were polycrystalline and the impurity levels were low. The refractive index was 1.43 and the permittivity 6.6. This novel ALD process using the new fluoride precursor TiF 4 is likely more general and applicable also to other metal fluorides.