Prospects for Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions

Abstract

Thermal atomic layer etching (ALE) of Al2O3 and HfO2 using sequential, self-limiting fluorination and ligand-exchange reactions was recently demonstrated using HF and tin acetylacetonate (Sn(acac)2) as the reactants. This new thermal pathway for ALE represents the reverse of atomic layer deposition (ALD) and should lead to isotropic etching. Atomic layer deposition and ALE can together define the atomic layer growth and removal steps required for advanced semiconductor fabrication. The thermal ALE of many materials should be possible using fluorination and ligand-exchange reactions. The chemical details of ligand-exchange can lead to selective ALE between various materials. Thermal ALE could produce conformal etching in high-aspect-ratio structures. Thermal ALE could also yield ultrasmooth thin films based on deposit/etch-back methods. Enhancement of ALE rates and possible anisotropic ALE could be achieved using radicals or ions together with thermal ALE.

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

“…[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. A metal precursor then undergoes a ligand-exchange transmetalation reaction.…”

“…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.…”

Review Article: Plasma–surface interactions at the atomic scale for patterning metals

“…54 Since first formalized for Si etch using Cl 2 /Ar chemistry, 8,9 ALE has been shown to be viable in patterning SiO 2 , 7 Si 3 N 4 , 55-58 as well as other semiconductors and metal oxides. 59,60 While all these reported results show some level of efficacy in etching targeted materials, there is currently limited literature demonstrating ALE of metals. This is partly because metallic bonding, caused by the sharing of free electrons across the metal lattice, makes it challenging to direct the bond formation between metals and the gasphase chemistries to realize self-limiting reactions that are essential to ALE.…”

Anisotropic atomic layer etching of W using fluorine radicals/oxygen ion beam

“…Recently, for the fabrication of nanoscale devices, in addition to the anisotropic ALE, isotropic ALE or also known as thermal ALE, where, materials are etched the same in all direction isotropically, is actively investigated. The thermal ALE uses an elevated temperature similar to atomic layer deposition (ALD) and, for the thermal ALE, a reactive molecule is chemisorbed on the surface to form a compound, and then the surface compound is removed by converting into a volatile compound with a second reactive molecule …”

“…The thermal ALE uses an elevated temperature similar to atomic layer deposition (ALD) and, for the thermal ALE, a reactive molecule is chemisorbed on the surface to form a compound, and then the surface compound is removed by converting into a volatile compound with a second reactive molecule. [16] For semiconductor device fabrication, various metals such as aluminum (Al), copper (Cu), tungsten (W), titanium (Ti), cobalt (Co), molybdenum (Mo), etc have been investigated and applied for interconnecting devices. Among these metals, due to the very high conductivity (the highest next to Ag), Cu is currently used most commonly for metallization.…”