The Surface Chemistry of ALD Precursors

Francisco Zaera

Department of Chemistry, University of California, Riverside, California 92521, USA

Email: zaera@ucr.edu

Atomic layer deposition (ALD) is one of the most promising methodologies available for the growth of solid thin films conformally on complex topographies and with atomic-level control on thickness. However, as a chemical process, ALD can lead to the incorporation of impurities and to the growth of poor-quality films. Surprisingly, little is known about the mechanisms of the chemical reactions involved in ALD; even the most basic information, such as the stoichiometry of the overall process, is in many instances unknown. Limited understanding is also available on the redox chemistry that affords the growth of metallic and other types of films from inorganic compounds containing elements in different initial oxidation states. The role of co-reactants in ALD is often misinterpreted: in many instances, these may not be the reducing agents they are set out to be, but rather are needed to remove the auxiliary moieties formed upon adsorption of the main precursor from the surface. These auxiliary surface species may be the original ligands in the ALD precursors, but quite often are new surface species formed upon thermal activation of the original compounds, a conversion that usually follows complex reaction networks. Reactivity in ALD is also controlled by the nature of the substrate, where specific nucleation sites are often responsible for the initial deposition and where a change in chemistry may take place as the first layer of the growing film is formed. Finally, solid-state chemical reactions may take place after deposition, leading to the formation of new layered structures.

Examples from our own laboratory will be used in this presentation to illustrate the relevance of all these issues and to exemplify the type of surface-science experiments that can be performed to shine light on them. We contend that a basic molecular-level understanding of the surface chemistry that underpins ALD processes should afford a better approach for the selection of ALD precursors and co-reactants and for the optimization of the ALD operating conditions. At the same time, our examples will be used to introduce the battery of surface-sensitive techniques that can be used to elucidate the mechanism and other details of the chemical reactions associated with ALD (Figure 1), which include X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and infrared absorption spectroscopy. Those can be complemented with techniques aimed at the detection and analysis of gas-phase products, which are often based on mass spectrometry: gas-chromatography/mass-spectrometry (GC/MS), temperature-programmed desorption (TPD), and molecular beams. Quantum mechanics calculations can also add to our understanding of this chemistry.

A number of concrete examples will be presented to illustrate the chemical issues listed above. First, the ALD of metal nitride films will be discussed in terms of the reactions that may lead to the reduction of the metal. In the case of TiN deposition using TiCl4 and ammonia, for instance, XPS data indicated that a Ti3N4 layer is always present on top of the expected TiN, and that some of the titanium atoms are reduced to the +3 state immediately upon the activated adsorption of the TiCl4 agent, before any ammonia is added. The data suggest that the reduction of the Ti4+ species may therefore occur during the TiCl4, not NH3, dosing step. As per the surface reactions that may lead to such metal reduction upon adsorption, GC/MS experiments with metal amido precursors suggest reductive-elimination or insertion steps, to produce hydrazines and other nitrogen containing organic fragments.

A second case, the deposition of copper films using copper acetamidinate precursors, will be presented in connection with the complex chemistry that the ligands in many precursors can follow on the surface. In particular, our XPS, LEIS, and TPD studies have indicated that, upon activated adsorption on solid metal surfaces, copper(I)-N,N'-di-sec-butylacetamidinate undergoes a series of decomposition steps, including an initial dimer-to-monomer conversion, dissociation to a smaller acetamidinate around 200 K (which is then hydrogenated to N-sec-butylacetamidine at 300 K), and further conversion to acetonitrile and butene above 400 K. The mechanistic work with this compound identified the external beta position from the nitrogens in the acetamidinate complex as the weak points for decomposition. Further research with variations of the original ligand has been directed to block those positions in order to stabilize the ALD precursor. Ultimately, our results show how a synergistic approach between synthetic and surface characterization efforts can lead to the development of new, better ALD precursors. Also to be discussed in the context of the Cu ALD chemistry is the differences in reactivity encountered on different substrates and the importance of considering the changing nature of the surface as the films grow in designing ALD processes.

A third example, regarding the deposition of Mn films on SiO2/Si(100) substrates, will be introduced to illustrate the possible complexity of the chemical composition of the deposited films. In this case, sequential thin layers of manganese silicide, mixtures of manganese and silicon oxides, and manganese silicate form on top of the underlying silicon substrate. All these examples will be placed in context within the general questions identified above.

Figure 1. Example of spectroscopic data that can be combined to elucidate the mechanism of the chemical reactions associated with ALD.