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revised on 06.04.2020 and posted on 07.04.2020by Ji Liu, hongliang lu, david wei zhang, Michael Nolan
Cobalt is a potential
candidate in replacing copper for interconnects and has been applied in the
trenches and vias in semiconductor industry. A non-oxidizing reactant is
required in plasma-enhanced atomic layer deposition (PE-ALD) of thin films of metals
to avoid O-contamination. PE-ALD of Co has been demonstrated experimentally,
but the growth mechanism and key reactions are not clear. In this paper, the reaction
mechanism of metal cyclopentadienyl (Cp, C5H5) precursors
(CoCp2) and NHx-terminated Co surface is studied by
density functional theory (DFT) calculations. The Cp ligands are eliminated by
CpH formation via a hydrogen transfer step and desorb from metal surface. The surface
facet plays an important role in the reaction energies and activation barriers.
The results show that on the NHx-terminated surfaces corresponding
to ALD operating condition (temperature range 550K to 650K), the two Cp ligands
are eliminated completely on Co(100) surface during the metal precursor pulse,
resulting in Co atom deposited on the Co(100) surface. But the second Cp ligand
reaction of hydrogen transfer is thermodynamically unfavourable on the Co(001)
surface, resulting in CoCp fragment termination on Co(001) surface. The final
terminations after metal precursor pulse are 3.03 CoCp/nm2 on NHx-terminated
Co(001) surface and 3.33 Co/nm2 on NHx-terminated Co(100)
surface. These final structures after metal precursor pulse are essential to
model the reaction during the following N-plasma step..