Atomic layer deposition (ALD) relies on alternated, self-limiting reactions between gaseous reactants and an exposed solid surface to deposit highly conformal coatings with a thickness controlled at the submonolayer level. These advantages have rendered ALD a mainstream technique in microelectronics and have triggered growing interest in ALD for a variety of nanotechnology applications, including energy technologies. Often, the choice for ALD is related to the need for a conformal coating on a 3D nanostructured surface, making the conformality of ALD processes a key factor in actual applications. In this work, we aim to review the current status of knowledge about the conformality of ALD processes. We describe the basic concepts related to the conformality of ALD, including an overview of relevant gas transport regimes, definitions of exposure and sticking probability, and a distinction between different ALD growth types observed in high aspect ratio structures. In addition, aiming for a more standardized and direct comparison of reported results concerning the conformality of ALD processes, we propose a new concept, Equivalent Aspect Ratio (EAR), to describe 3D substrates and introduce standard ways to express thin film conformality. Other than the conventional aspect ratio, the EAR provides a measure for the ease of coatability by referring to a cylindrical hole as the reference structure. The different types of high aspect ratio structures and characterization approaches that have been used for quantifying the conformality of ALD processes are reviewed. The published experimental data on the conformality of thermal, plasma-enhanced, and ozone-based ALD processes are tabulated and discussed. Besides discussing the experimental results of conformality of ALD, we will also give an overview of the reported models for simulating the conformality of ALD. The different classes of models are discussed with special attention for the key assumptions typically used in the different modelling approaches. The influence of certain assumptions on simulated deposition thickness profiles is illustrated and discussed with the aim of shedding light on how deposition thickness profiles can provide insights into factors governing the surface chemistry of ALD processes. We hope that this review can serve as a starting point and reference work for new and expert researchers interested in the conformality of ALD and, at the same time, will trigger new research to further improve our understanding of this famous characteristic of ALD processes.

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Articles you may be interested in Electronic structure investigation of atomic layer deposition ruthenium(oxide) thin films using photoemission spectroscopy J. Appl. Phys. 118, 065306 (2015) Due to its excellent conformality, atomic layer deposition (ALD) has become a key method for coating and functionalizing three dimensional (3D) large surface area structures such as anodized alumina (AAO), silicon pillars, nanowires, and carbon nanotubes. Large surface area substrates often consist of arrays of quasi-one-dimensional holes (into which the precursor gas needs to penetrate, e.g., for AAO), or "forests" of pillars (where the precursor gas can reach the surface through the empty 3D space surrounding the pillars). Using a full 3D Monte Carlo model, the authors compared deposition onto an infinite array of holes versus an infinite array of pillars. As expected, the authors observed that the required exposure to conformally coat an array of holes is determined by the height to width ratio of the individual holes, and is independent of their spacing in the array. For the pillars, the required exposure increases with decreasing center-to-center distance and converges in the limit to the exposure of an array of holes. Our simulations show that, when targeting a specific surface area enhancement factor in the range 20-100, a well-spaced pillar geometry requires a 2-30 times smaller precursor exposure than a hole geometry and is therefore more ALD friendly. The difference in required exposure is shown to depend on the initial sticking probability and structural dimensions.

Pinhole free, uniform and conformal oxidation barrier on Cu and Fe powder.• Proper agitation of the powder is required and performed by a rotating ALD reactor.• Al2O3 coating of 8 nm on Cu powder causes a shift of oxidation temperature of 200 °C • Al2O3 coating of 25 nm on Fe powder causes a shift of oxidation temperature of 400 °C

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