Growth Per Cycle in Atomic Layer Deposition: A Theoretical Model

TLDR

Atomic layer deposition builds nanometer‑scale films by repeating saturating reactions of two gaseous reactants on a solid substrate, with the growth per cycle determined by reactant chemistry, temperature, and substrate. The study derives a model that predicts ALD growth per cycle based on the chemistry of compound reactants. The model uses a mass‑balance approach that accounts for ligand‑exchange or dissociation/association chemisorption, linking reactant size and steric hindrance to the growth per cycle. The model shows that steric hindrance limits growth per cycle to below one monolayer, and its validity is demonstrated with several theoretical examples.

Abstract

Abstract Atomic layer deposition (ALD) is used in advanced applications where thin layers of materials with precise thickness down to the nanometer scale are needed. Growth of materials by ALD takes place through repeating the separate, saturating reactions of at least two gaseous reactants with a solid substrate. When surface saturation is systematically utilized, the growth obtained per ALD reaction cycle is a well‐defined quantity that depends on i) the reactants used, ii) the ALD processing temperature, and iii) sometimes the substrate material. A model is derived to describe the growth per cycle in ALD as a function of the chemistry of the growth when compounds are used as reactants. Two main types of chemisorption may occur: i) ligand exchange reaction of the ML n reactant with surface “a” groups, where ligands are removed from the surface as gaseous aL, and ii) dissociation or association, where all parts of the ML n reactant are attached to the surface. A simple mathematical model based on the mass balance of chemisorption relates the growth per cycle to the size of the ML n reactant and the chemisorption mechanisms involved. Steric hindrance of the ligands causes saturation of chemisorption if a limited number of bonding sites does not cause it. Because of the steric hindrance, the growth per cycle remains less than a monolayer. The applicability of the model is illustrated through several theoretical examples.