Novel ALD Process for Depositing CaF₂ Thin Films

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Abstract

Metal fluorides, like CaF2, are interesting dielectric materials which are optically transparent over a wide wavelength range down to the vacuum ultraviolet regime. In addition, CaF2 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 TiF4 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 CaF2 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 TiF4 is likely more general and applicable also to other metal fluorides.

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