Atomic Layer Deposition of Cobalt Using H₂-, N₂-, and NH₃-Based Plasmas: On the Role of the Co-reactant

This work investigates the role of the co-reactant for the atomic layer deposition of cobalt (Co) films using cobaltocene (CoCp₂) as the precursor. Three different processes were compared: an AB process using NH₃ plasma, an AB process using H₂/N₂ plasma, and an ABC process using subsequent N₂ and H₂ plasmas. A connection was made between the plasma composition and film properties, thereby gaining an understanding of the role of the various plasma species. For NH₃ plasma, H₂ and N₂ were identified as the main species apart from the expected NH₃, whereas for the H₂/N₂ plasma, NH₃ was detected. Moreover, HCp was observed as a reaction product in the precursor and co-reactant subcycles. Both AB processes showed self-limiting half-reactions and yielded similar material properties, that is, high purity and low resistivity. For the AB process with H₂/N₂, the resistivity and impurity content depended on the H₂/N₂ mixing ratio, which was linked to the production of NH₃ molecules and related radicals. The ABC process resulted in high-resistivity and low-purity films, attributed to the lack of NH species during the co-reactant exposures. The obtained insights are summarized in a reaction scheme where CoCp₂ chemisorbs in the precursor subcycle and NH species eliminate the remaining Cp in the consecutive subcycle.