ALD

Atomic Layer Deposition (ALD) is a vapor-phase thin-film deposition technique that enables growth of highly uniform, conformal films with atomic-level thickness control. It relies on sequential, self-limiting chemical reactions between gaseous precursors on a substrate surface.

In a typical ALD cycle, a metal-containing precursor is introduced and reacts with the surface, followed by a purge to remove excess precursor and byproducts. A second precursor or co-reactant is then introduced to react with the adsorbed species, followed by another purge. Each cycle deposits a sub-nanometer increment of material, and the growth per cycle is characteristic of the film chemistry and substrate, commonly in the range of about 0.5–2 Å.

Key advantages of ALD include superb control of film thickness, exceptional conformality on complex 3D structures and high aspect ratio features, and compatibility with relatively low to moderate temperatures. Variants such as plasma-enhanced ALD (PA-ALD) use reactive plasma to drive surface reactions, enabling lower-temperature processing, while spatial ALD separates steps spatially to increase throughput.

Common materials deposited by ALD include aluminum oxide (Al2O3), hafnium oxide (HfO2), titanium dioxide (TiO2), and zinc oxide (ZnO), among others. ALD films are used in semiconductor dielectric layers, barrier coatings, catalysis supports, energy storage devices, sensors, and protective coatings.

History and context: the approach originated from atomic layer epitaxy work in the 1960s–1970s by Tuomo Suntola, with the modern ALD framework and widespread adoption expanding in the 1990s and beyond.

Other uses of the acronym ALD exist, including Adrenoleukodystrophy and alcoholic liver disease; in scientific and industrial literature, ALD most commonly refers to atomic layer deposition.