Mechanistic Study of Nucleation Enhancement in Atomic Layer Deposition by Pretreatment with Small Organometallic Molecules

Thermal atomic layer deposition (ALD) of metals on metal oxide surfaces typically suffers from nucleation delays that result in poor-quality films. The poor nucleation may be caused by a lack of suitable chemisorption sites on the oxide surface, which are needed for metal nucleation to occur. In this work, we demonstrate that prefunctionalizing the surface with a single monolayer of small organometallic molecules from the vapor phase can lead to a significant increase in surface coverage of the metal deposited by ALD. This process is demonstrated for Pt ALD from (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) and O-2, with nucleation enhanced almost 3-fold at 100 ALD cycles after the pretreatment. We hypothesize that the high coverage of the organometallic molecule provides an alternative chemisorption mechanism for the platinum precursor and thus leads to an increase in its uptake. The proposed chemisorption mechanism is robust across several organometallic molecule pretreatments and could potentially be exploited for other organometallic-based metal ALD processes. This chemisorption mechanism was probed using in situ quadrupole mass spectrometry (QMS). The growth of the platinum deposits was investigated in depth through scanning electron microscopy (SEM) and grazing incidence small-angle X-ray scattering (GISAXS). These studies show that the pretreatment also results in the improved wettability of Pt nanoparticles (NPs). The improved wettability is likely to affect the Pt diffusion properties, further contributing to the enhancement observed on the treated substrates. In addition, GISAXS and SEM studies indicate the growth of larger, denser, and more highly ordered Pt NPs at early cycle numbers, which subsequently coalesce into continuous and pinhole-free films. Surface pretreatment by organometallic molecules therefore introduces a potential route to achieve improved nucleation and growth of ultrathin films.