(Invited) Fundamentals of Thin Film Growth and Etching: How Small Atomic Processes Govern Macroscopic Effects
Authors: Rost Marcel J. 
Published: February 2024

In order not to have our children inherit a huge problem in the future, we quickly have to realize the energy transition to renewable sources, which requires also energy storage devices and, thus, Li ion batteries. Although significant progress has been made during recent years regarding battery capacitance, cycle life time, deterioration, and fast charging capabilities, any further improvement becomes increasingly more difficult.

The reason for this is manifested in the top-down approach, which quickly delivers results, and thus products, by screening the parameter space and evaluating available options. At this point, however, further improvements often require a deep, fundamental understanding of all processes, and a change to a bottom-up approach is highly recommended, if not even essential.

Using Scanning Tunneling Microscopy movies, I will show in my talk how a small change in adatom formation energy and adatom diffusion barrier leads to a huge morphology change of the growing film. After providing the insights to the well-known homoepitaxial growth modes, I will introduce one specific atomic barrier that leads to an instability and triggers 3D growth. The 2D analogous of this barrier at a step edge, leads to dendritic island shapes, and the combination of both, the 2D and 3D instability, leads to dendrites as observed also in the batteries. With this in mind, I will show the dualism between adatoms and vacancies, which naturally explains also the equivalent, inverse growth modes during etching or dissolution including their instabilities.

From here we will switch gears and focus on polycrystalline film aspects, and, thus, the grain boundary network. I will show, again on the atomic scale, how the grain boundary energy determines the surface structure from extremely rough to flat and smooth. Quite surprisingly, I will further demonstrate how a tiny variation in the dilute adatom/vacancy equilibrium-pressure on the surface, during growth or dissolution, can setup pressures variations in the grain boundary network that exceed the pressure of the deepest point of all our world seas: the Mariana Trench.

These enormous compressive (and tensile) stresses are to a large extent reversible. However, due to the differences in diffusion constants in combination with the huge reservoir of grain boundary volume in the film, complications arise on the atomic scale at the triple point/lines, where the grain boundaries penetrate the surface. Mass accumulation or depletion leads to hillocks and grooves. Moreover, the remaining accumulated intrinsic film stress can also trigger whisker growth, which, on the basis of the involved atomic processes, clearly has to be distinguished from dendritic growth.

Although fundamentally still not understood, whisker growth leads to many failures, like short circuits in electronic devices and batteries. Their penetrating forces can be enormous, pushing through even hard oxide layers and films (and the SEI).

As many of the described effects play a major and critical role in Li ion batteries, and while I realize that I am adding seemingly hopeless, additional complexity, I will finally provide some basic strategies and concepts that we learned on how to tune and influence the previously mentioned effects efficiently on the atomic scale by using additives.