ALD of YF₃ Thin Films from TiF₄ and Y(thd)₃ Precursors

Abstract

Yttrium fluoride (YF 3 ) is a dielectric material with good light transmittance between the ultraviolet (UV) and infrared (IR) range of wavelengths. In this paper we introduce the first use of the atomic layer deposition (ALD) of YF 3 thin films. The films are grown at 175-325 8C. Y(thd) 3 (thd ¼ 2,2,6,6-tetramethyl-3,5-heptanedionato) is used as a cation source and TiF 4 as a fluorine precursor. YF 3 film growth characteristics, together with structural, optical, and electrical properties, are studied. Various methods, such as spectrophotometry, X-ray diffractometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and elastic recoil detection analysis (ERDA) are applied to characterize the films. Electrical properties are analyzed from Al/YF 3 /indium-tin-oxide capacitor structures at room temperature. The growth rates of the films are between 1.1 and 1.7 Å per cycle. The films grown below 225 8C are amorphous, otherwise they are polycrystalline. The impurities detected in the YF 3 film are H, C, O, and Ti. The amount of all of these tends to decrease with increasing deposition temperature, and is only 3.0 at.-% at 325 8C. Permittivities of the films are around 6. The refractive indices are 1.51-1.59 (at l ¼ 580 nm), and high light transmittance is achieved from the UV to IR regions with the sample grown at 300 8C.

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

“…With longer pulse times, the growth rate increased linearly. Such an increase has not been observed with other processes utilizing TiF4, however in these studies the TiF4 pulse lengths have been limited to 2 s or less (Figure 13b) [153,155,157,158].…”

“…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.…”

“…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].…”

“…Still, this impurity can limit the UV transmittance of these films when optical applications are the goal. The impurity level decreases as the deposition temperature is increased but at the same time film roughness increases resulting in more scattering of UV-light [153,157,158].…”

“…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].…”