Competition between Al2O3 atomic layer etching and AlF3 atomic layer deposition using sequential exposures of trimethylaluminum and hydrogen fluoride

Abstract

The thermal atomic layer etching (ALE) of Al₂O₃ can be performed using sequential and self-limiting reactions with trimethylaluminum (TMA) and hydrogen fluoride (HF) as the reactants. The atomic layer deposition (ALD) of AlF₃ can also be accomplished using the same reactants. This paper examined the competition between Al₂O₃ ALE and AlF₃ ALD using in situ Fourier transform infrared (FTIR) vibrational spectroscopy measurements on Al₂O₃ ALD-coated SiO₂ nanoparticles. The FTIR spectra could observe an absorbance loss of the Al-O stretching vibrations during Al₂O₃ ALE or an absorbance gain of the Al-F stretching vibrations during AlF₃ ALD. The transition from AlF₃ ALD to Al₂O₃ ALE occurred versus reaction temperature and was also influenced by the N₂ or He background gas pressure. Higher temperatures and lower background gas pressures led to Al₂O₃ ALE. Lower temperatures and higher background gas pressures led to AlF₃ ALD. The FTIR measurements also monitored AlCH₃* and HF^* species on the surface after the TMA and HF reactant exposures. The loss of AlCH₃* and HF^* species at higher temperatures is believed to play a vital role in the transition between AlF₃ ALD at lower temperatures and Al₂O₃ ALE at higher temperatures. The change between AlF₃ ALD and Al₂O₃ ALE was defined by the transition temperature. Higher transition temperatures were observed using larger N₂ or He background gas pressures. This correlation was associated with variations in the N₂ or He gas thermal conductivity versus pressure. The fluorination reaction during Al₂O₃ ALE is very exothermic and leads to temperature rises in the SiO₂ nanoparticles. These temperature transients influence the Al₂O₃ etching. The higher N₂ and He gas thermal conductivities are able to cool the SiO₂ nanoparticles more efficiently and minimize the size of the temperature rises. The competition between Al₂O₃ ALE and AlF₃ ALD using TMA and HF illustrates the interplay between etching and growth and the importance of substrate temperature. Background gas pressure also plays a key role in determining the transition temperature for nanoparticle substrates.