Coupling between surface reaction and subsurface mass transport for atomic layer etching of cobalt by hexafluoroacetylacetone

Growth and etch rates of functional films are often controlled by the interplay between surface reaction and mass transport. For the thermally driven hexafluoroacetylacetone (hfacH)-assisted removal process of chlorinated cobalt films during cobalt atomic layer etching, we present how the interplay between surface reaction and mass transport governs the etch characteristics. To this end, we investigate atomic-scale etching mechanisms through comprehensive thermodynamic and kinetic analyses based on a series of ab initio calculations. Screening reaction energies at multiple reaction sites on amorphous chlorinated cobalt surfaces reveals hfac adsorption on a Co with formation of HCl gas followed by CoCl₂(hfac) cluster desorption as the thermodynamically most favorable reaction pathway. The preferential formation of CoCl₂(hfac) highlights that Cl-deficient etching front surfaces result from the stoichiometric mismatch between the desorbed species (Co:Cl = 1:3) and the original chlorinated cobalt layer (Co:Cl = 1:2). Then, we demonstrate through mean-square-displacement analysis that Cl atoms are sufficiently mobile within chlorinated cobalt layers to enable compensatory Cl transport to the etching front surface. Finally, the identified coupling between surface reactions and subsurface Cl diffusion provides the mechanistic basis for the experimentally observed etch thickness profiles in cobalt atomic layer etching, which increase sublinearly approaching saturation.