(Invited) Atomic Layer Etching Using Thermal Reactions: Atomic Layer Deposition in Reverse
Authors: Lee Younghee , DuMont Jaime W. , George Steven M. 
Published: February 2020

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).