Mechanism of Thermal Al₂O₃ Atomic Layer Etching Using Sequential Reactions with Sn(acac)₂ and HF

Thermal Al 2 O 3 atomic layer etching (ALE) can be performed using sequential, self-limiting reactions with tin(II) acetylacetonate (Sn(acac) 2 ) and HF as the reactants. To understand the reaction mechanism, in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) measurements were conducted versus temperature. The mass change per cycle (MCPC) increased with temperature from −4.1 ng/(cm 2 cycle) at 150 °C to −18.3 ng/(cm 2 cycle) at 250 °C. Arrhenius analysis of the temperature-dependent MCPC values yielded an activation barrier for Al 2 O 3 ALE of E = 6.6 ± 0.4 kcal/mol. The mass changes after the individual Sn(acac) 2 and HF exposures also varied with temperature. The mass changes after the Sn(acac) 2 exposures were consistent with more Sn(acac) 2 surface reaction products remaining at lower temperatures. The mass changes after the HF exposures were consistent with more AlF 3 species remaining at higher temperatures. The FTIR spectroscopic analysis observed Al 2 O 3 etching by measuring the loss of absorbance of Al–O stretching vibrations in the Al 2 O 3 film. The infrared absorbance of the acetylacetonate vibrational features from Sn(acac) 2 surface reaction products was also smaller at higher temperatures. The correlation between the MCPC values and the acetylacetonate infrared absorbance suggested that the Al 2 O 3 ALE rate is inversely dependent on the acetylacetonate surface coverage. In addition, the QCM and FTIR measurements explored the nucleation of the Al 2 O 3 ALE. A large mass gain and loss of infrared absorbance of Al–O stretching vibrations after the initial HF exposure on the Al 2 O 3 film was consistent with the conversion of Al 2 O 3 to AlF 3 . FTIR experiments also observed the formation of AlF 3 after the initial HF exposure and the presence of AlF 3 on the surface after each HF exposure during Al 2 O 3 ALE. In the proposed reaction mechanism, AlF 3 is the key reaction intermediate during Al 2 O 3 ALE. HF converts Al 2 O 3 to AlF 3 prior to removal of AlF 3 by Sn(acac) 2 .