ALD processing using HF to grow AlF3

Here is a interesting paper from Steven George in on growing AlF3 using HF. I haven´t seen anyone growing ALD films with HF before and usually you think of etching or surface hydrogen termination and growth passivation when HF is employed. As usual a lot of QCM growth characterization is used by the Boulder guys. The authors states that AlF3 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.

Atomic Layer Deposition of AlF3 Using Trimethylaluminum and Hydrogen Fluoride 

Younghee Lee , Jaime W. DuMont , Andrew S. Cavanagh , and Steven M. George
J. Phys. Chem. C, Just
Accepted Manuscript
DOI: 10.1021/acs.jpcc.5b02625 
Publication Date (Web): May 27, 2015

The atomic layer deposition (ALD) of AlF3 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 AlF3 ALD. The AlF3 ALD film growth was examined at temperatures from 75C to 300C. Both the TMA and HF reactions displayed self-limiting behavior. The maximum mass gain per cycle (MGPC) of 44 ng/(cm2 cycle) for AlF3 ALD occurred at 100C. The MGPC values decreased at higher temperatures. The MGPC values were negative at T> 250C when TMA and HF were able to etch the AlF3 films. Film thicknesses were also determined using ex situ X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) measurements. The AlF3 ALD growth rate determined by the ex situ analysis was 1.43 Å/cycle at 100C. 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 500-800 cm-1 during AlF3 ALD. In addition, absorption peaks were observed that were consistent with AlF(CH3)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 AlF3 with an oxygen impurity of only ~2 at%. AlF3 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.