Fundamentals of ALD
Atomic Layer Deposition
Atomic layer deposition (ALD) is a relatively new and low-temperature growth method, which is capable of depositing a variety of materials in virtually any substrate. Its working principle is based on self-terminating surface reactions between volatile precursors and the substrate surface, which offer unique assets, namely, high thickness control to the sub-nanometer scale, pinhole-free films with the unrivaled capability of uniformly coating on high-aspect-ratio structures. Thanks to the highly reactive nature of ALD precursors, the technique allows to deposit high-quality materials at relatively low temperatures (≤ 200 °C), which makes the use of polymeric, flexible substrates possible.
Figure 1 shows a schematic representation of ZnO deposition via conventional ALD, in which the sequential exposures of the substrate to metal precursor (diethylzinc, DEZ) and oxidant (H2O) are separated by purging steps (thus separated in time). An ALD cycle would ideally result in a monolayer, however, due to steric effects, only a distinct factor of a monolayer is usually deposited per ALD cycle.
Figure 1: Schematic representation of an ALD process of ZnO
There are several technical requirements in the development of ALD, one of the most critical is the choice of organometallic precursor. Some general requirements for ALD precursors include:
- Good volatility at the deposition temperature;
- No self-decomposition or reaction on itself at the deposition temperature;
- Precursors must adsorb or react with the surface sites;
- Sufficient reactivity toward the other precursors;
- No etching of the substrate or the growing film
After the mentioned requirements, the precursor compatibility with substrate, the price as well as the toxicity are also to be considered.
 Van Bui, H.; Grillo, F.; van Ommen, J. R. Atomic and Molecular Layer Deposition: Off the Beaten Track. Chemical Communications 2017, 53 (1), 45–71.
 Putkonen, M. Precursors for ALD Processes. In Atomic Layer Deposition of Nanostructured Materials; John Wiley & Sons, Ltd, 2012; pp 41–59.