Role of the Oxidizing Co-Reactant in Pt Growth by Atomic Layer Deposition Using MeCpPtMe₃ and O₂/O₃/O₂-Plasma

Abstract

Atomic layer deposition (ALD) of Pt using MeCpPtMe 3 and the O 2 /O 3 /O 2 -plasma (O 2 *) at 300 °C is investigated with in vacuo X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to gain a better understanding of the Pt growth mechanism. Most notably, the chemical state of the surface Pt atoms and the role of surface O species in Pt growth are revealed. In the MeCpPtMe 3 /O 2 process, the surface Pt atoms remain in a metallic Pt 0 state throughout the ALD cycle, and the surface O species generated by the O 2 exposure only exist as unstable adatoms, desorbing in vacuum. As for the O 3 /O 2 * processes, the surface Pt layer is oxidized to a mixture of Pt 0 , Pt 2+ O and Pt 4+ O 2 upon O 3 /O 2 * exposure and then fully reduced to Pt 0 during the precursor exposure. Surface Pt oxides are stable in a vacuum but can be reduced by hydrocarbon vapors. Quantification analysis shows that the O 3 /O 2 * processes have a much higher surface O species content than the O 2 process after the coreactant exposure, favoring precursor ligand combustion over dehydrogenation in the next precursor exposure and leading to lower surface C density after the precursor pulse. DFT reveals differences in the combustion mechanism for Me vs Cp species, during the metal precursor and coreactant pulses. Importantly, the differences in the surface O content do not significantly affect the growth per cycle. Moreover, the MeCpPtMe 3 /O 2 process with surface O species and a tailored MeCpPtMe 3 /O 2 process without surface O species, both at 300 °C, yield nearly identical growth rates and as-deposited Pt with the same chemical state. This indicates that surface O species present before the precursor exposure have little impact on the overall Pt growth, in contrast to a previous assumption.

The surface chemistry of the atomic layer deposition of metal thin films

“…This approach is particularly promising for cases where the ALD temperature is high enough to lead to the initiation of the decomposition of the ligands in the metalorganic precursors used in the first ALD half, because, as indicated above, such chemistry often leads to the formation of irreversibly and hard-tohydrogenate surface species that then need to be burned to CO 2 and H 2 O (and possibly CO) [96]. Molecular oxygen is often used in these cases [20,148,149], but here a problem similar to that with H 2 emerges because the O=O double bond in O 2 is hard to activate; relatively high temperatures are required for that, and such high temperatures are in general to be avoided in ALD. Like in the case of hydrogen-containing ALD coreactant, atomic oxygen can be generated in the gas phase with the use of plasmas, an approach that is more common than with H 2 [28,149].…”

“…Molecular oxygen is often used in these cases [20,148,149], but here a problem similar to that with H 2 emerges because the O=O double bond in O 2 is hard to activate; relatively high temperatures are required for that, and such high temperatures are in general to be avoided in ALD. Like in the case of hydrogen-containing ALD coreactant, atomic oxygen can be generated in the gas phase with the use of plasmas, an approach that is more common than with H 2 [28,149]. On the negative side, the surface chemistry of oxygen plasmas may not be clean, and some surface etching may occur.…”