Paper PS+TF-WeM12
Conformality of Thermal Al₂O₃ Atomic Layer Etching in High Aspect Ratio Structures

Wednesday, November 9, 2016, 11:40 am, Room 104C

Thermal atomic layer etching (ALE) is the reverse of atomic layer deposition (ALD). Conformal deposition in high aspect ratio structures is one of the key features of ALD. The conformality of etching in high aspect ratio structures will also be important for thermal ALE. In this study, the conformality of thermal Al₂O₃ ALE was investigated in channels with high aspect ratios ranging from 60 to 200. Al₂O₃ ALD was used to deposit the initial Al₂O₃ films in the channels. The Al₂O₃ ALE was performed at 300°C using HF and Al(CH₃)₃ as the reactants. HF is known to fluorinate Al₂O₃ and form an AlF₃ layer on the Al₂O₃ surface. The Al(CH₃)₃ then undergoes a ligand-exchange transmetalation reaction with the AlF₃ layer. Al(CH₃)₃ accepts fluorine and donates methyl ligands to the surface. This ligand-exchange allows the Al in the AlF₃ layer to leave as a volatile reaction product such as AlF(CH₃)₂ or Al(CH₃)₃. The conformality of Al₂O₃ etching was examined in high aspect ratio channels defined by stainless steel foil spacers between silicon substrates. Spectroscopic ellipsometry was used to measure the Al₂O₃ film thickness in the channels. Increasing the aspect ratio increased the reactant exposure and purge times necessary to maintain conformal etching. Longer times were required to allow the reactants and products to diffuse in and out of the high aspect ratio channels. Increasing the reactant pressures also lowered the required reactant exposure times. However, increasing the reactant pressures from 0.1 to 9 Torr also increased the Al₂O₃ etching rate. The higher etching rates were attributed to a thicker AlF₃ layer formed at higher reactant partial pressures. Using longer reactant exposure or purge times or higher reactant pressures, conformal Al₂O₃ etching was obtained in the high aspect ratio channels.