Thermal atomic layer etching of cobalt using plasma chlorination and chelation with hexafluoroacetylacetone

In this study, a thermal atomic layer etching process for Co comprising two steps––plasma chlorination and chelation with hexafluoroacetylacetone (Hhfac)––was developed. The Co surface was chlorinated with BCl₃ plasma to form CoCl₂ in the plasma chlorination step, and the thickness of CoCl₂ was measured using secondary ion mass spectrometry. In the chelation step, CoCl₂ was removed from the surface by forming a volatile etch product at temperatures ≥ 150 °C, and the Etch per cycle (EPC) of Co was constant in the temperature range of 150–175 °C. Above 200 °C, the EPC of Co decreased owing to Hhfac decomposition at high process temperatures. The EPC of Co increased from 1.1 to 2.1 nm as the chlorination time increased from 30 to 90 s; however, it saturated with increasing Hhfac exposure time. Density functional theory calculations showed that the adsorption of Hhfac on the chlorinated Co surface is the rate-determining-step in the chelation process of chlorinated cobalt surfaces. The final reaction product of the chelation reaction is suggested to be CoCl₂(hfac). The Hhfac adsorption step is the rate-determining-step in the chelation process of chlorinated cobalt surfaces.