Atomic Layer Deposition of AlF₃ Using Trimethylaluminum and Hydrogen Fluoride

Abstract

The atomic layer deposition (ALD) of AlF 3 was demonstrated using trimethylaluminum (TMA) and hydrogen fluoride (HF). The HF source was HF-pyridine. In situ quartz crystal microbalance (QCM), quadrupole mass spectrometer (QMS), and Fourier transform infrared (FTIR) spectroscopy measurements were used to study AlF 3 ALD. The AlF 3 ALD film growth was examined at temperatures from 75 to 300°C. Both the TMA and HF reactions displayed self-limiting behavior. The maximum mass gain per cycle (MGPC) of 44 ng/(cm 2 cycle) for AlF 3 ALD occurred at 100°C. The MGPC values decreased at higher temperatures. The MGPC values were negative at T > 250°C when TMA and HF were able to etch the AlF 3 films. Film thicknesses were also determined using ex situ X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) measurements. The AlF 3 ALD growth rate determined by the ex situ analysis was 1.43 Å/cycle at 100°C. These ex situ measurements were in excellent agreement with the in situ QCM measurements. FTIR analysis monitored the growth of infrared absorbance from Al−F stretching vibrations at 500−900 cm −1 during AlF 3 ALD. In addition, absorption peaks were observed that were consistent with AlF(CH 3 ) 2 and HF species on the surface after the TMA and HF exposures, respectively. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) measurements revealed that the deposited films were nearly stoichiometric AlF 3 with an oxygen impurity of only ∼2 at %. AlF 3 ALD may be useful for a number of applications such as ultraviolet optical films, protective coatings for the electrodes of Li ion batteries, and Lewis acid catalytic films.

Vapor deposition of copper(I) bromide films via a two-step conversion process

“…However, metal fluorides are the only halide compounds with known CVD growth processes. [17][18][19] Previous research in this lab indicates that direct CVD of metal halides is not well controlled when highly acidic hydrogen halide precursors (HX, with X = Cl, Br, I) are used. The reaction between HX and a metalorganic precursor does in fact produce the desired metal halide material by transferring a proton from HX to the anionic ligand of the metalorganic precursor, thereby releasing the neutral form of the ligand to be evacuated.…”

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

“…In addition, as already mentioned, AlF 3 is a potential artificial SEI-layer for protecting both cathodes and anodes [47,143,145,[165][166][167]. Thus, AlF 3 has become a much studied ALD material in the past few years [143,145,147,149,156]. Magnesium fluoride showed growth rates of 0.6 to 0.3 Å/cycle in the deposition temperature range of 100 to 250 • C (Figure 12a) [148].…”

“…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].…”

“…Aluminum fluoride is a material with a similar variety of possible optical applications [149,[162][163][164]. In addition, as already mentioned, AlF 3 is a potential artificial SEI-layer for protecting both cathodes and anodes [47,143,145,[165][166][167]. Thus, AlF 3 has become a much studied ALD material in the past few years [143,145,147,149,156].…”

“…For the first two decades, metal fluorides were studied mainly because of their optical properties, namely low refractive indices and low absorption in the UV range [142]. However, with the rise of lithium-ion battery related ALD research, the potential of ALD metal fluorides in batteries has also been recognized [51,[143][144][145]. Still, very few results on using atomic layer deposited metal fluoride thin films in lithium-ion batteries is available at this time.…”