Brief introduction to ALD [page]
The ALD process
A prototypical atomic layer deposition (ALD) process consists of self-terminating reactions* of at least two gaseous compounds on a reactive surface, separated by purge/evacuation steps. At room temperature, the compounds (here called "reactants"**) may be solid, liquid or gaseous; at the ALD reaction temperature, typically elevated from room temperature, the reactants need to be gaseous. Together, the four reaction and purge/evacuation steps form an ALD reaction cycle. Despite the widely adopted name of the technique (atomic layer deposition), the growth per cycle*** (GPC) is typically a fraction of a monolayer of the targeted material. ALD reaction cycles are repeated as many times as necessary to obtain the desired amount of material on the surface.
Figure 1 below shows a cartoon illustration of an ALD reaction cycle, adapted from a recent review (Appl. Phys. Rev. 6 (2019) 021302, https://doi.org/10.1063/1.5060967). While the cartoon has been created specifically to describe the prototypical trimethylaluminium-water ALD process to grow aluminium oxide, it also works as a general scheme to describe the ALD processes. The surface chemistry characteristics of ALD have been discussed in detail in surface chemistry reviews (J. Appl. Phys. 97 (2005) 121301, https://doi.org/10.1063/1.1940727; J. Chem. Phys. 152 (2020) art. 040902, https://doi.org/10.1063/1.5133390).
Figure 1. Schematic illustration of an ALD reaction cycle, consisting of (Step 1) a self-terminating reaction of a gaseous molecule ("Reactant A") with a surface, (Step 2) a purge/evacuation step to remove the excess reactants and gaseous by-products, (Step 3) a self-terminating reaction of another compatible gaseous molecule ("Reactant B") with the surface left behind by Reactant A, and (Step 4) a purge/evacuation step to remove the excess reactants and gaseous by-products. (Image adapted by Puurunen (2019) from Cremers et al., Applied Physics Reviews 6, 021302 (2019); https://doi.org/10.1063/1.5060967; CC BY 4.0.)
Other ALD cartoons in published reviews
Most ALD reviews and many original ALD publications present some kind of cartoon illustration of the characteristic ALD process. An evolving (unfinished and unpublished) collection of cartoon illustrations of the ALD process is presented here (a Google Slides file). Suggestions for additions are welcome.
* "Self-terminating" in common scientific terminology is "saturating" and "irreversible". In some works, what is here called "self-terminating" is referred to e.g. as "self-limiting", "adsorption-controlled", or sometimes simply just "saturating".
** In some works, what is here called "Reactant A" is called "precursor" and sometimes what is here called "Reactant B" is called "co-reactant" or just "reactant" (when Reactant A is "precursor").
*** In some works, what is here called "growth per cycle" (GPC) is called "growth rate" or "growth rate per cycle".