Reaction mechanism of the Me₃AuPMe₃-H₂ plasma-enhanced ALD process

The reaction mechanism of the recently reported Me₃AuPMe₃-H₂ plasma gold ALD process was investigated using in situ characterization techniques in a pump-type ALD system. In situ RAIRS and in vacuo XPS measurements confirm that the CH₃ and PMe₃ ligands remain on the gold surface after chemisorption of the precursor, causing self-limiting adsorption. Remaining surface groups are removed by the H₂ plasma in the form of CH₄ and likely as PH_xMe_y groups, allowing chemisorption of new precursor molecules during the next exposure. The decomposition behaviour of the Me₃AuPMe₃ precursor on a Au surface is also presented and linked to the stability of the precursor ligands that govern the self-limiting growth during ALD. Desorption of the CH₃ ligands occurs at all substrate temperatures during evacuation to high vacuum, occurring faster at higher temperatures. The PMe₃ ligand is found to be less stable on a gold surface at higher substrate temperatures and is accompanied by an increase in precusor decomposition on a gold surface, indicating that the temperature dependent stability of the precursor ligands is an important factor to ensure self-limiting precursor adsorption during ALD. Remarkably, precursor decomposition does not occur on a SiO₂ surface, in situ transmission absorption infrared experiments indicate that nucleation on a SiO₂ surface occurs on Si-OH groups. Finally, we comment on the use of different co-reactants during PE-ALD of Au and we report on different PE-ALD growth with the reported O₂ plasma and H₂O process in pump-type versus flow-type ALD systems.