Competing reactions during metalorganic deposition: Ligand-exchange versus direct reaction with the substrate surface

Abstract

Surface-mediated reactions of metalorganic compounds on solid substrates are key processes in film deposition technology, especially in atomic layer deposition (ALD) or chemical vapor deposition. Since most applications of thin films require high purity, understanding and controlling the mechanisms of desired and undesired surface reactions are of the utmost importance. This work outlines a general approach to understand potential surface reactions during deposition through density functional theory calculations, considering precursors containing the most commonly used types of ligands, namely alkyl (Al(CH3)3), alkoxide (Ti[OC3H7]4), alkylamide (Hf[N(CH3)2]4), diketonate (Cu(acac)2), amidinate (Ni[Pr-amd]2), and cyclopentadienyl (Hf(Cp)2(CH3)2). In all cases, the “desired” ligand-exchange reaction (the basis of most ALD processes) is compared to “undesired” surface reactions, where the ligands of the precursor interact with reactive surface sites and can undergo uncontrolled decomposition pathways, incorporating undesired elements into the growing film. To be able to make an effective comparison across precursor types, all calculations were made considering the same surface model, that of a Si(100) surface, and the same level of theory. Our results show that the undesired ligand-mediated adsorption on reactive sites can often compete (both thermodynamically and kinetically) with the desired ligand-exchange reaction, particularly in the case of alkoxides, alkylamides, and diketonates. The intrinsic reactivity of each precursor (based on their frontier molecular orbitals) is found to determine the manner in which it will react with the surface. This article emphasizes that undesired reactions can often be predicted and evaluated based on the chemical reactivity of each precursor. This approach, applied to specific cases, will be important for probing the chemical performance of a deposition precursor.