Conformality in atomic layer deposition: Current status overview of analysis and modelling

Abstract

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.

From Precursor Chemistry to Gas Sensors: Plasma‐Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications

“…Atomic layer deposition (ALD) can fulfill all the mentioned criteria by using sequential self‐limiting surface reactions of a gaseous precursor and a coreactant with the solid surface, separated by inert gas purges, in order to deposit the desired material with atomic scale precision. [ 24–26 ] Since ALD is a chemical vapor phase technique, process conditions and thin film properties can be fine‐tuned with the right precursor choice. Thus, precursors with different volatilities and reactivities toward different coreactants are of utmost importance.…”

<p>Zirconium Oxide Thin Films Obtained by Atomic Layer Deposition Technology Abolish the Anti-Osteogenic Effect Resulting from miR-21 Inhibition in the Pre-Osteoblastic MC3T3 Cell Line</p>

“…11 The conformal growth of ALD coatings allows the formation of homogeneous layers on complex surfaces. 12,13 This is a huge advantage of ALD technology for applications in biology and medicine. Another advantage of ALD is the possibility of obtaining biocompatible films.…”

Saturation profile based conformality analysis for atomic layer deposition: aluminum oxide in lateral high-aspect-ratio channels

“…Conformality refers to the ability to coat a three-dimensional object with a homogeneous coating of uniform thickness. 1,2 Conformal coatings are required in the fabrication of state-ofthe-art and future electronics, [2][3][4] as well as in various applications related to heterogeneous catalysts, [5][6][7][8] biosensors, 9 and energy industries. 10,11 Atomic layer deposition (ALD), [12][13][14][15] a variant of chemical vapor deposition (CVD), was invented independently over forty years ago under the names atomic layer epitaxy [16][17][18][19] and molecular layering.…”

“…23 ALD is gaining increasing interest as the method of choice for conformal thin film growth. 1,2 The unique features of ALD are the systematic use of self-terminating gas-solid reactions and precise film thickness control at subnanometer level. 13,14,24 When the gas-solid reactions are completed, they stop by themselves.…”