To enable expansion of ALD into more complex classes of materials, it is critical to understand its underlying chemical mechanisms. Using a range of custom-built in situ tools, synchrotron-based experiments, and theory, our group studies the chemical reactions driving ALD growth phenomena to better inform process design, precursor chemistry, and materials development. We are investigating new growth mechanisms for pure and multicomponent materials whereby reactive species can be stored superstoichiometrically within the film and be catalytically activated by metals. We want to increase our understanding on generation mechanisms for active species at the film-substrate interface, and investigate how these species are stored in, and diffuse through a range of materials. Because these mechanisms deviate from typically held assumptions that ALD reactions participate solely at the film surface, insight in the sub-surface reactions present in these systems may open the door for new deposition strategies and chemistries.

Upper: Mechanism of reactive oxygen species from ozone in superstoichiometric oxygen reservoirs. Lower: Reactive oxygen species from catalytic reactions on active metal surfaces facilitating inherently area-selective deposition.

Passivating species also play a key role in ALD mechanisms, and our group studies their behavior in ALD. For example, proper release of precursor ligands ensures that growth processes are self-limiting and uniform. By bridging theoretical kinetic modeling and in situ experimental measurements, we can draw conclusions about likely reaction pathways.

Above: Comparing in situ growth measurements to theoretical modeling to understand surface passivation mechanism in Ru ALD.

Nucleation is one of the greatest challenges in metal ALD. Many metal ALD processes exhibit island growth on low surface energy substrates, so we are also modifying surface chemistry to enhance nucleation, enabling deposition of high-quality continuous metal films at low thicknesses. We carry out in situ studies of the initial stages of ALD to elucidate the mechanisms of film nucleation and growth using a combination of laboratory- and synchrotron-based experiments and theory.

Top: Schematic of nucleation enhancement process for Pt ALD. Bottom: Comparison of Pt film morphologies, with less sparse depositions achieved using treatments that can participate in the mechanism in the top figure.