Molecular mechanisms of atomic layer etching of cobalt with sequential exposure to molecular chlorine and diketones

The mechanism of thermal dry etching of cobalt films is discussed for a thermal process utilizing sequential exposures to chlorine gas and a diketone [either 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone, hfacH) or 2,4-pentanedione (acetylacetone, acacH)]. The process can be optimized experimentally to approach atomic layer etching (ALE); a sequential exposure to Cl₂ and hfacH dry etchants at 140 °C is shown to proceed efficiently. The use of acacH as a diketone does not result in ALE with chlorine even at 180 °C, but the decrease of surface chlorine concentration and chemical reduction of cobalt is noted. However, thermal desorption analysis suggests that the reaction of chlorinated cobalt surface exposed to the ambient conditions (oxidized) with hfacH does produce volatile Co-containing products within the desired temperature range and the products contain Co³⁺. The effect of adsorption of ligands on the energy required to remove surface cobalt atoms is evaluated using the density functional theory.