Ab Initio Calculations of the Reaction Mechanisms for Metal−Nitride Deposition from Organo-Metallic Precursors onto Functionalized Self-Assembled Monolayers

An atomistic mechanism has been derived for the initial stages of the adsorption reaction for
metal−nitride atomic layer deposition (ALD) from alkylamido organometallic precursors of Ti and Zr on
alkyltrichorosilane-based self-assembled monolayers (SAMs). The effect of altering the terminal functional
group on the SAM (including −OH, −NH2, −SH, and −NH(CH3)) has been investigated using the density
functional theory and the MP2 perturbation theory. Reactions on amine-terminated SAMs proceed through
the formation of a dative-bond complex with an activation barrier of 16−20 kcal/mol. In contrast, thiol-terminated SAMs form weak hydrogen-bonded intermediates with activation barriers between 7 and 10
kcal/mol. The deposition of Ti organometallic precursors on hydroxyl-terminated SAMs proceeds through
the formation of stronger hydrogen-bonded complexes with barriers of 7 kcal/mol. Zr-based precursors
form dative-bonded adducts with near barrierless transitions. This variety allows us to select a kinetically
favorable substrate for a chosen precursor. The predicted order of reactivity of differently terminated SAMs
and the temperature dependence of the initial reaction probability have been confirmed for Ti-based
precursors by recent experimental results. We predict that the replacement of methyl groups by
trifluoromethyl groups on the SAM backbone decreases the activation barrier for amine-terminated SAMs
by 5 kcal/mol. This opens a route to alter the native reactivities of a given SAM termination, in this case
making amine termination energetically viable. The surface distribution of SAM molecules has a strong
effect on the adsorption kinetics of Ti-based precursors. Unimolecular side decomposition reactions were
found to be kinetically competitive with adsorption at 400 K.