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

Abstract

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 .

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

“…However, the HF molecules may be able to fluorinate the Al 2 O 3 surface and form an AlF 3 layer according to Al 2 O 3 + 6HF → 2AlF 3 + 3H 2 O. 5,9,11 The next TMA exposure does not encounter many HF * species on the surface. Instead, the TMA can accept fluorine from the AlF 3 layer in a ligandexchange reaction.…”

“…4 In contrast, thermal processes have been developed only recently for ALE. [5][6][7][8][9][10] The recently developed thermal ALE processes are based on sequential fluorination and ligand-exchange reactions. 5,9,11 Fluorination converts the metal compound, such as a metal oxide, to a metal fluoride.…”

“…During ligandexchange, the metal precursor accepts fluorine from the metal fluoride and donates one of its ligands to the surface and produces volatile reaction products that lead to etching. 5,9,11 HF has been a successful fluorine precursor for Al 2 O 3 , HfO 2 , AlF 3 , and AlN ALE. [5][6][7][8][9][10] HF is also an effective fluorine precursor for the ALD of a variety of metal fluoridess such as AlF 3 , ZrF 4 , HfF 4 , MnF 2 , MgF 2 , ZnF 2 , and LiF.…”

“…As a result, HF can then act to fluorinate the underlying Al 2 O 3 substrate to form an AlF 3 layer according to Al 2 O 3 + 6HF → 2AlF 3 + 3H 2 O. 5,9,11 During the TMA exposures, TMA then accepts fluorine from the newly formed AlF 3 layer in a ligand-exchange process. 8 TMA also donates CH 3 ligands to the surface to form volatile AlF(CH 3 ) 2 species.…”

(Invited) Atomic Layer Etching Using Thermal Reactions: Atomic Layer Deposition in Reverse

“…Thermal Al2O3 ALE has been demonstrated using in situ quartz crystal microbalance (QCM) and Fourier transform infrared spectroscopy (FTIR) techniques [25,26]. The thermal Al2O3 ALE was observed using sequential Sn(acac)2 and HF exposures.…”