Thermal atomic layer etching (ALE) is typically defined by sequential surface reactions. Thermal ALE can be viewed as the reverse of atomic layer deposition (ALD). Thermal ALE is usually performed with two steps: modification of the surface and removal of the modified surface. The modification can be oxidation, fluorination, or chlorination. The removal step is often a ligand-exchange reaction, creating stable volatile species that leave the surface, leading to a net removal of material. For example, aluminum oxide can be etched using thermal ALE with HF to fluorinate the surface and trimethylaluminum (TMA) to exchange methyl ligands for surface fluoride ligands, resulting in volatile Al(CH₃)_xF_y species.

Thermal ALE of metals is particularly challenging because the oxidation state of the metal must be changed to match the oxidation state of the volatile metal etch product. This oxidation is performed in the modification step of thermal ALE. After changing the oxidation state, the metal then produces a stable complex during the volatile release reaction. In this work, Ni and Co ALE are developed by modifying the metal via chlorination using SO₂Cl₂. Subsequently, the metal is removed by addition of an L ligand to the metal center to create a stable, volatile etch product, MCl_xL_y (Figure 1). The L ligands explored in this work were PMe₃ and tetramethyl ethylenediamine (TMEDA).

The chlorination and ligand-addition approach for metal ALE is based on the Covalent Bond Classification (CBC) method. X ligands are one-electron donors like Cl and other halogens. L ligands are two-electron donors like PMe₃. TMEDA is a bidentate ligand. One TMEDA ligand acts as two L ligands since TMEDA binds to the metal center through the lone electron pairs on the two nitrogen groups. According to the CBC method, nickel complexes typically have NiX₂L₃ or NiX₂L₂ configurations. Likewise, cobalt complexes are typically found in the CoX₂L₄, CoX₂L₂, or CoX₃L₃ configurations. The goal for Ni or Co ALE is to create these volatile metal compounds using sequential SO₂Cl₂ and L ligand surface reactions.

Both Ni and Co ALE have been studied with in situ quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) to determine the etch rates. Quadrupole mass spectrometry (QMS) was also employed to determine the volatile metal-containing etch species. Using SO₂Cl₂ and PMe₃, Ni etch rates varied from 0.1-3 Å/cycle at temperatures from 75-200 °C. QMS measurements also identified NiCl₂(PMe₃)₂ (NiX₂L₂) as the stable volatile etch product. Cobalt ALE using SO₂Cl₂ and TMEDA displayed etch rates from 1-12 Å/cycle at temperatures from 175-300 °C. QMS measurements also detected CoCl₂(TMEDA) (CoX₂L₂) as the stable volatile etch product. In addition, cobalt can be etched using SO₂Cl₂ and PMe₃ from 70-200 °C, with etch rates between 2-4 Å/cycle. These two Co ALE pathways with TMEDA and PMe₃ are complementary with TMEDA useful at higher temperatures and PMe₃ useful at lower temperatures.

Many other X and L ligands could be used for the halogenation and ligand-addition reactions during metal thermal ALE. The CBC method can be used as a guide to develop the X ligands for surface modification and the L ligands for volatile release. This approach should lead to various other metal ALE processes. Selective metal thermal ALE should also be possible using different X and L ligands.