Atomic Layer Deposition of LiF Thin Films from Lithd and TiF₄ Precursors

Abstract

ithium fluoride (LiF) is an important optical material with a low refractive index and a large band gap. In this study, thin films of LiF are deposited using atomic layer deposition (ALD). Lithd and TiF 4 are used as precursors, and they produce crystalline LiF in the temperature range 250-350 8C. The films are studied with UV-Vis spectrometry, field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and elastic recoil detection analysis (ERDA).Adhesion of the films is tested by a Scotch tape test. This ALD process results in LiF films with a growth rate of approximately 1 Å per cycle at 325 8C. According to ERDA measurements, the films are pure LiF with only small O, C, and H impurities.

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

“…Unlike the Lithd + TiF 4 process of ref. [150], this sequence shows both an ALD window between 325 and 350 • C and saturation with respect to both Lithd and TiF 4 ( Figure 14). The growth rate at 325 • C was 1.4 Å/cycle, as opposed to 1.0 Å/cycle in the previous LiF process.…”

“…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). Pilvi et al postulated that the reason for the non-saturative behaviour might be either slow kinetics or an enhancement of metal precursor decomposition caused by the TiF x -surface groups [153].…”

“…Films deposited with TiF 4 are generally very close to stoichiometric, as determined with ERDA ( Figure 12d). Titanium is often found as an impurity in the deposited films but usually in only very small amounts [150,153,155]. Still, this impurity can limit the UV transmittance of these films when optical applications are the goal.…”

“…Following the work of Pilvi et al, a process for the deposition of LiF was developed [150]. Lithd and TiF4 were used as precursors and the deposition temperature was varied between 250 and 350 °C.…”

“…After the removal of magnesium, Lithd adsorbs onto the formed lithium fluoride (Equation (5)) and is converted to LiF during the last TiF4 pulse (Equation (6)). [150,153,155,157].…”