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^[1]

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:^[2]

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.

References:

[1] 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.

[2] Putkonen, M. Precursors for ALD Processes. In Atomic Layer Deposition of Nanostructured Materials; John Wiley & Sons, Ltd, 2012; pp 41–59.

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