Volatile Etch Species Produced during Thermal Al2O3 Atomic Layer Etching

The thermal atomic layer etching (ALE) of Al 2 O 3 can be achieved using sequential fluorination and ligand-exchange reactions. Although previous investigations have characterized the etch rates and surface chemistry, no reports have identified the volatile etch products. This study explored the volatile etch species during thermal Al 2 O 3 ALE at 300 °C using quadrupole mass spectrometry (QMS). HF was the fluorination reactant; Al(CH 3 ) 3 (trimethylaluminum (TMA)) and AlCl(CH 3 ) 2 (dimethylaluminum chloride, (DMAC)) were the metal precursors for ligand exchange. When TMA was used as the metal precursor after the fluorination of Al 2 O 3 powder, the QMS measurements revealed that the main ion species were consistent with dimers of AlF(CH 3 ) 2 (dimethylaluminum fluoride (DMAF)) with itself (DMAF + DMAF) or with TMA (DMAF + TMA). These ion species were observed after loss of a methyl group as Al 2 F 2 (CH 3 ) 3 + at m / z = 137 and Al 2 F(CH 3 ) 4 + at m / z = 133, respectively. In addition, an ion species consistent with a trimer was also observed as Al 3 F 3 (CH 3 ) 5 + at m / z = 213. Very similar results were observed for TMA exposures on AlF 3 powder. Comparable results were also obtained using DMAC as the metal precursor for ligand exchange. In contrast, SiCl 4 and TiCl 4 are not successful metal precursors because they do not lead to thermal Al 2 O 3 ALE at 300 °C. QMS measurements revealed no Al-containing etch species after SiCl 4 and TiCl 4 exposures on AlF 3 powder. However, SiF x Cl y + and TiF x Cl y + species were observed which suggested that ligand-exchange reactions can occur without the release of Al-containing etch species. Density functional theory (DFT) and coupled cluster singles, doubles, and perturbative triples (CCSD(T)) calculations were performed to support the preference for dimer products. The theoretical results confirmed the stability of the dimer products and showed that dimers with two Al–F–Al bridging bonds are the most stable and dimers with two Al–CH 3 –Al bridging bonds are the least stable. In addition, the calculations suggested that dimers with terminal CH 3 ligands are most able to desorb from the surface because these dimers need to break weak Al–CH 3 –Al bridging bonds. Transmission electron microscopy (TEM) studies confirmed the thermal Al 2 O 3 ALE of Al 2 O 3 films on W powders. The TEM images revealed that the etch process was uniform and conformal after various numbers of thermal Al 2 O 3 ALE cycles using HF and TMA as the reactants.