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Surface recombination of plasma radicals is generally considered to limit film conformality during plasma-assisted atomic layer deposition (ALD). Here, we experimentally studied film penetration into high-aspect-ratio structures and demonstrated that it can give direct information on the recombination probability r of plasma radicals on the growth surface. This is shown for recombination of oxygen (O) atoms on SiO2, TiO2, Al2O3, and HfO2 where a strong material dependence has been observed. Using extended plasma exposures, films of SiO2 and TiO2 penetrated extremely deep up to an aspect ratio (AR) of ∼900, and similar surface recombination probabilities of r = (6 ± 2) × 10–5 and (7 ± 4) × 10–5 were determined for these processes. Growth of Al2O3 and HfO2 was conformal up to depths corresponding to ARs of ∼80 and ∼40, with r estimated at (1–10) × 10–3 and (0.1–10) × 10–2, respectively. Such quantitative insight into surface recombination, as provided by our method, is essential for modeling radical-surface interaction and understanding for which materials and conditions conformal film growth is feasible by plasma-assisted ALD.

Atomic layer deposition (ALD) is a gas‐phase method to grow layers of solid materials with subnanometer precision. It has been invented independently in the Soviet Union in the 1960s under the name molecular layering, and in the 1970s in Finland under the name atomic layer epitaxy. ALD relies on alternatingly exposing a surface to gaseous reactants—separated by a purge step—that react in a self‐terminating manner. This article introduces the fundamentals of the surface chemistry of ideal ALD, including saturating and irreversible reactions, growth per cycle, monolayer concepts relevant to ALD, typical surface reaction mechanisms, saturation‐limiting factors, growth modes, area‐selective ALD, growth kinetics, and conformality. It also discusses typical deviations from ideal ALD. Over the years, many different ALD process chemistries have been developed. A range of reactor systems is available, depending on the type of substrate and required productivity. ALD is broadly applicable in practice since it couples nanoscale precision with a good scalability and can be used to deposit a large variety of materials. In recent years, the interest in ALD has been growing strongly. The most important sector regarding commercial applications of ALD is currently the semiconductor industry.