Atomic Layer Deposition of AlF3 Using Trimethylaluminum and Hydrogen Fluoride

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.