Chemistry of Atomic Layer Deposition

Seán Thomas Barry

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9783110712599
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

This book will help chemists and non-chemists alike understand the fundamentals of surface chemistry and precursor design, and how these precursors drive the processes of atomic layer deposition, and how the surface-precursor interaction governs atomic layer deposition processes. The underlying principles in atomic layer deposition rely on the chemistry of a precursor with a surface.

Chapter 1 Introduction

Notes: In this chapter, I would like to acknowledge the work done by Prof. Riikka Puurunen, both in bringing to light the historical contributions of Russia to ALD, and for continuing to keep the fundamental descriptions of ALD phenomenon discussed by the community. Both her blog and “History of ALD” web pages are invaluable resources.

This textbook is written to be a wide-ranging course in the chemistry of atomic layer deposition (ALD). I envision this text to be used for courses in early graduate school, to introduce enough chemistry to new researchers that it helps clarify ALD, without overburdening them with too much unfamiliar knowledge. It comprises two main sections: the fundamentals of the ALD process, and selected reaction mechanisms from ALD, molecular layer deposition (MLD), and atomic layer etching (ALE).

The fundamentals of ALD include the essential chemistry of saturation and thermolysis, as well as the nature of ligands and precursors. These sections require a solid background in chemistry, and the underlying chemical concepts can be found in any university-level textbooks on physical chemistry and inorganic chemistry. The necessary background ranges from understanding structure and bonding, thermodynamics, and kinetics.

The sections on reaction mechanisms are based on examples to highlight particularly important or broadly applicable reactivities. These sections by their nature are not comprehensive and are not meant to be a review of the ALD process literature. Understanding several mechanisms can give a researcher insight when trying to understand new chemistry in their own ALD process. In my experience, proposing and studying mechanisms is a creative endeavor that mostly relies on postulating incorrect mechanistic pathways. Chemical reactivity occurs along many widely varied pathways, and a reaction mechanism is an attempt to make a linear narrative of these events. Ideally the sections on reaction mechanisms will provide enough of a foundation to spur better understanding of ALD processes.

1.1 Atomic layer deposition

ALD is a thin film deposition technique that has found its place in microelectronics manufacturing as well as a myriad of other applications. This technique is inherently a nanoscale technique since it allowed the fabrication of thin films developing – in a bottom-up fashion – from a molecular monolayer at a surface to a film of a target material. It is famous as a layer-by-layer, self-limiting, thin film growth method that relies on monolayer saturation of a surface through adsorption of a chemical precursor. The definition of the vocabulary of ALD is important: it is helpful to the field in general if the practitioners of ALD all understand the necessary terminology:

1.2 Precursor

In general, a precursor is any volatile chemical compound that participates in the surface chemistry of an ALD process. Sometimes “reactant” is used, which is proper from a chemistry point of view, but this can also mean any chemical participating in any reaction, and so is too general. “Co-reactant” or “co-precursor” is less appropriate: this implies that the chemical compound is added simultaneously with other precursors; the prefix “co-” should be avoided except when multiple, different chemical compounds are added at the same time.

1.3 Adsorption

This general term covers both chemisorption (which implies a lack of reversibility) and physisorption (which implies easy reversibility). I prefer it as a general term until a surface mechanism is understood sufficiently to be more specific. Chemisorption is often accompanied by a change in bonding in the precursor such that it cannot easily reverse along the same reaction pathway, perhaps through loss of a ligand as a volatile by-product. Physisorption often describes a low-energy interaction between a surface and a molecule, as a first mechanistic step that could lead to chemisorption.

1.4 Surface vs. substrate

Substrate (in ALD) is a borrowed definition from microelectronics. In general, the substrate is the surface on which deposition is initiated. Many processes start on silicon, making this surface the partner in the adsorption reaction. However, as deposition progresses, the “surface” becomes different from the “substrate.” Once the substrate is entirely coated by the target film, ALD proceeds on this new surface. In some cases, the chemistry can drastically change when the surface changes. It bears remembering that the precursors and surface together comprise a thermodynamic system. Changing the surface (from initial substrate, to growing target film) naturally changes the thermodynamics of deposition.

1.5 Monolayer

Most ALD processes are considered to form a monolayer. This might conjure an image of a surface that is completely filled with adsorbed precursor molecules, but it truly means that the surface will not accommodate the further adsorption, regardless of whether the monolayer is densely packed or not. In most cases, the size of a precursor, as well as the number of nucleation sites, roughness of the surface, and other confounding factors all affect how densely made a monolayer is.

1.6 Self-limiting

This is the crux of ALD: that the monolayer, once formed, will stop adsorption, and remain stable until it is further reacted. In a perfect system, this would be a dense and chemisorbed monolayer that was thermodynamically stable under the conditions in which it was formed. But truly, an ALD process can exhibit self-limiting growth if the monolayer is kinetically stable, meaning that it persists long enough for the next chemical reaction to occur.

1.7 Layer-by-layer growth

The chemisorbed monolayer comprises a “layer,” and the reaction of it with a second precursor further represents a “layer.” The meaning of layer in layer-by-layer growth in the context of ALD is generally taken to mean a full layer of the target film. But, in a great majority of ALD processes, the growth-per-cycle is lower than what might be considered the thickness of one layer of material. This indicates that several different cycles might contribute to the same layer of target film. Given the variety of ways in which “layer” can be used in ALD, it is always worth further explaining what about a layer is being discussed.

It is commonly stated that ALD is a type of chemical vapor deposition (CVD), and the similarities are obvious. Both techniques require valving, furnaces, substrates and substrate holders, vacuum pumps, and the like for deposition to occur. But ALD is fundamentally different than CVD in the control that is exhibited over the precursor chemistry. In CVD, the principal reactants can react in the gas phase, at the surface, or in the continuum between those two different thermodynamic states. This allows for complex deposition chemistry to help produce the desired target film. An ideal ALD process does not allow for gas-phase interaction of the precursors, and idealized atomic layer deposition considers the reactivity at the surface to be paramount in the formation of the target film. This fundamental difference sets ALD apart as its own method. It is quite common for one precursor to be useful both as a CVD precursor and as an ALD precursor, and the insights in either deposition technique can influence the understanding of both.

It is in the chemistry of the precursor that the differences between ALD and CVD are highlighted. In CVD, more than one precursor is commonly mixed in the gas phase. Chemical reactions can occur in the gas phase and are commonly of benefit to the deposition of the intended target film. For example, boron carbide (B_xC) films can be deposited by CVD using triethylboron(III) as a single source precursor. However, the deposition temperature of this CVD process can be lowered to between 600–800 °C when dihydrogen (H₂) is used as a carrier gas and co-precursor. This is due to the reaction of triethylboron(III) with dihydrogen in the gas phase to produce highly reactive BH₃:

$\mathrm{B}{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3+3{\mathrm{H}}_2\to \mathrm{B}{\mathrm{H}}_3+3\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{e}$

It is the subsequent reactivity of the ethane that provides carbon for the B_xC films; this type of reactivity is precluded in ALD. In ALD, the precursors are necessarily introduced separately, where each can react to fully saturate a surface. When all precursors are available to the growth surface continually, the growth is also continual (i.e., CVD-type growth): there is not saturation of a monolayer of one precursor.

In the above case, the triethylboron(III) can act as a single-source precursor: all of the elemental components required for the deposition of the target film (B, C) are available in the precursor. The thermal reaction of the triethylboron(III) produces two precursors in situ, which then allows the continual deposition of B_xC as the target film. Naturally, single-source precursors are not viable in ALD. Self-limiting growth requires each independent precursor to undergo chemistry with the surface to further the deposition of the target film.

ALD relies entirely on serial chemical reactions for growth to occur, and therefore can be carried out in a robust fashion with simple equipment. At its heart, ALD is a cyclical process where two or more precursors are sequentially entrained into a reactor, separated by steps that eliminate the by-products of reaction, and other reactants from the gas phase (Figure 1.1). This allows each precursor to interact with the surface independently and achieve a chemical reaction that is (ideally) unhindered by the complication of a variety of chemical pathways to continue. This separation of species allows control over the surface chemistry that drives ALD. This is why ALD can be used to deposit such a wonderfully diverse number of materials.

Figure 1.1: The classic depictions of an ALD cycle, where a) shows a stoichiometric balanced cycle, and b) shows a cartoon depiction.

The classic idea of an ALD cycle bears some scrutiny: in Figure 1.1a, both “n” and “m” are used as variables to demonstrate that the surface reactions do not have to proceed through a loss of a single group (as is generally shown, like in Figure 1.1b). It also demonstrates that, in a two-precursor process, that the precursors must have a stoichiometric balance to eliminate all ligands from both precursors. Further chapters show specific ALD cycles, and it would be a good exercise to try to compare them to Figure 1.1a and consider what the values of “n” and “m” are in real, applied ALD processes. In Figure 1.1b, the cartoon depiction of ALD is much more familiar, and (while maintaining a stoichiometric balance of “circles”) it more easily depicts the concepts of chemisorption, by-product fo...