Novel ALD Process for Depositing CaF₂ Thin Films

Abstract

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), timeof-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.

Metal Fluorides as Lithium-Ion Battery Materials: An Atomic Layer Deposition Perspective

“…However, it has been reported in another publication that MgF2 does not readily adsorb HF during the ALD growth process [147]. [147,148,153,154]; (b) [141,155,160]; (c) [143,145,149,156]; (d) [148,153,154].…”

“…Generally, metal fluorides deposited using TiF 4 as the fluorine source show a decrease in growth rate as the deposition temperature is increased (Figure 12) [155]. This decrease has been proposed to be due to a decrease in the TiF x adsorption density but this has not been verified experimentally.…”

“…Of these materials, lithium fluoride is of special interest because of its band gap of approximately 14 eV and low refractive index of 1.39 at 580 nm, much like AlF3 as discussed previously [168,169]. For the deposition of these fluorides, HF was generated from a solution containing 30 wt.% of pyridine and 70 wt.% HF ("Olah's reagent") to mitigate the safety [147,148,153,154]; (b) [141,155,160]; (c) [143,145,149,156]; (d) [148,153,154].…”

“…The deposition of many metal fluorides has been studied at the University of Helsinki using TiF 4 as the fluorine source, including materials such as MgF 2 , LaF 3 and CaF 2 [153,155,157,158]. TiF 4 is a relatively safe alternative to HF, since it is a solid at room temperature.…”

“…This decrease has been proposed to be due to a decrease in the TiF x adsorption density but this has not been verified experimentally. Using TiF 4 as the fluorine source leads to higher growth rates compared to the use of HF, possibly due to the formation of the fluoride in question during both precursor pulses [155,156]. When using TiF 4 as the fluorine source, the film growth usually shows saturation with respect to the fluorine precursor (Figure 13b) but the metal precursor can show either poor [150,153,157] or good saturation [158], depending on the material deposited (Figure 13a).…”