Mechanisms of surface reactions in thin solid film chemical deposition processes

In this review, key aspects of the surface chemistry associated with atomic layer deposition (ALD) are discussed. It is argued that, in spite of its central role in defining the efficacy of these film deposition processes, little is known about the mechanisms of the chemical reactions involved in ALD. Even the most basic information, the stoichiometry of the overall process in particular, is in many instances unknown. Limited understanding is also available on the redox chemistry that affords the growth of metallic and other types of films from inorganic compounds containing elements in different initial oxidation states. The role of co-reactants in ALD is often misinterpreted: in many instances, these may not be the reducing agents they are set out to be, but rather are needed to remove the auxiliary moieties formed upon adsorption of the main precursor from the surface. These auxiliary surface species may be the original ligands in the ALD precursors, but quite often are new surface species formed upon thermal activation of the original compounds, a conversion that usually follows complex reaction networks. Reactivity in ALD is also controlled by the nature of the substrate, where specific nucleation sites are often responsible for the initial deposition and where a change in chemistry may take place as the first layer of the growing film is formed. Finally, solid-state chemical reactions may take place after deposition, leading to the formation of new layered structures. Examples from our own laboratory are used in this review to illustrate all these issues and to exemplify the type of surface-science experiments that can be performed to shine light on them. We contend that a basic molecular-level understanding of the surface chemistry that underpins ALD processes should afford a better approach for the selection of ALD precursors and co-reactants and for the optimization of the ALD operating conditions. One of the objectives of this review is to encourage the surface-science community to take on this challenge.