ALD is a self-limiting adsorption reaction process, i.e. the amount of deposited precursor molecules is determined only by the number of reactive surface sites and is independent of the precursor exposure after saturation. In theory, the maximum growth-per-cycle (GPC) is exactly one monolayer per cycle, however in most cases the GPC is limited to 0.25-0.3 of a monolayer due to steric hindrance between the absorbed precursor molecules.
Starting with a conditioned surface, the ALD cycle is composed of four steps (pictured in Figure 1 for ALD of Al2O3 using trimethylaluminum (TMA) precursor and water):Step 1: During the precursor dosing, adsorption of precursor molecules occurs on reactive surface sites and reaction products are formed.Step 2: The excess precursor and reaction products are purged out of the deposition chamber and a (sub)monolayer of precursor remains adsorbed on the substrate surface.Step 3: The co-reactant (in this case water) is introduced into the chamber and reacts with the adsorbed TMA molecules to form a (sub)monolayer of the desired material (Al2O3).Step 4: Un-reacted co-reactant molecules and by-products are purged out.
The cycle is repeated to deposit additional monolayers to achieve the targeted film thickness.
Another advantage of ALD is that a layer can be deposited at lower substrate temperatures compared to CVD. Since the adsorption of the precursor to the surface is mainly thermally driven, there exist a so-called ALD temperature window in which the ALD process takes place. Figure 2 shows a general temperature window of an ALD process. The substrate temperature must be high enough (higher than T1) to prevent condensation of any of the reactants. If condensation occurs during an ALD cycle, undesirable or uncontrollable reactions might happen, resulting in the formation of porous and impure films. Moreover, in several types of surface reactions there is an activation energy that has to be exceeded. Therefore, a minimum substrate temperature (T1) is also required to proceed with the ALD process. However, an undesirable decomposition of a reactant will happen if the temperature is too high (higher than T2) . Furthermore, CVD reactions will start to occur, which leads to an uncontrolled deposition of the film. Re-evaporation is another effect which becomes more likely at higher temperatures and which may result in a decreasing GPC versus temperature. Due to the above characteristics, ALD finds applications in the preparation of high-quality thin films, especially when excellent step coverage and/or low processing temperatures are required.