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

Atomic layer etching (ALE) is a thin film removal technique based on sequential, self-limiting surface reactions.  ALE is the reverse of atomic layer deposition (ALD).  ALD has developed rapidly over the last 10-15 years to meet many technological needs such as the miniaturization of semiconductor devices.  In contrast, ALE is relatively new and is only starting to receive serious attention.  Initial work in ALE has focused on plasma ALE using halide adsorption and energetic ion bombardment to obtain anisotropic etching.  There is a need for thermal approaches to ALE based on spontaneous reactions that would produce isotropic etching.

This talk will present a new thermal approach to ALE based on sequential, self-limiting thermal surface reactions.  Examples will be presented for Al₂O₃ [1, 2], HfO₂ [3] and AlF₃ ALE.  The ALE is achieved using sequential, self-limiting thermal reactions.  For the metal oxides, the ALE process is based on fluorination and ligand-exchange reactions.  Using HF and Sn(acac)₂ as the reactants, Al₂O₃ and HfO₂ ALE are examined using quartz crystal microbalance, x-ray reflectivity and Fourier transform infrared spectroscopy measurements.  These studies show that controlled, atomic level removal of Al₂O₃ and HfO₂ is possible at temperatures from 150-250°C [1-3]. 

The surface reaction mechanism involves the formation of a metal fluoride reaction intermediate from fluorination of the metal oxide by HF.  The HF reactant also allows H₂O to leave as a reaction product.  The Sn(acac)₂ reactant then accepts fluorine from the metal fluoride and donates acac to the substrate.  The donated acac ligand can produce Al(acac)₃ or Hf(acac)₄ as volatile reaction products.  AlF(acac)₂ and HfF(acac)₃ may also be volatile reactions products.  Sequential exposures of HF and Sn(acac)₂ produce Al₂O₃ ALE etching rates of 0.14 Å/cycle- 0.61 Å/cycle per cycle at temperatures from 150- 250°C [1, 2].  Sequential exposures of HF and Sn(acac)₂ produce HfO₂ ALE etching rates of 0.070 Å/cycle - 0.117 Å/cycle per cycle at temperatures from 150- 250°C [3]. 

This ALE reaction mechanism based on fluorination and ligand-exchange should be applicable to other materials such as metal nitrides, metal phosphides, metal arsenides and elemental metals.  This ALE reaction mechanism is also general and works with other fluorine metal acceptors such as Al(CH₃)₃ and fluorination precursors such as XeF₂.  In addition, metal fluorides are candidates for ALE.  Fourier transform infrared spectroscopy reveals that Al₂O₃ is converted to AlF₃ during the HF exposure.  Sn(acac)₂ can then remove the AlF₃ by the ligand-exchange process.  If AlF₃ is a reaction intermediate during Al₂O₃ ALE, then AlF₃ ALE should also be possible using AlF₃ substrates.  We have demonstrated AlF₃ ALE using AlF₃ ALD thin films grown using Al(CH₃)₃ and HF.  The AlF₃ ALE was performed using Sn(acac)₂ and HF as the ALE reactants.  

1.  Younghee Lee and Steven M. George, “Atomic Layer Etching of Al₂O₃ Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)₂ and HF”, ACS Nano 9,2061 (2015).

2.  Younghee Lee, Jaime W. DuMont and Steven M. George, “Mechanism of Thermal Al₂O₃ Atomic Layer Etching Using Sequential Reactions with Sn(acac)₂ and HF”  Chem. Mater. (In Press).

3.  Younghee Lee, Jaime W. DuMont and Steven M. George, “Atomic Layer Etching of HfO₂ Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)₂ and HF”, ECS J. Solid State Sci. Technol. 4, N5013 (2015).