Surface chemistry and atomic layer etching of metal alloys in aqueous solutions

Over the past decades, the semiconductor industry managed to continuously increase performance at an impressive, steady rate by scaling the dimensions of all parts of the integrated devices to nanometer-size dimensions, thereby increasing the number of transistors on the chip as well as decreasing the size of metallization lines enormously.  

For next-generation advanced interconnects, the material properties of Cu render it unreliable for producing ever more performant chips. Recently, various alternative metals, such as ruthenium, rhodium, and molybdenum, have been studied and start to be implemented by the industry to tackle resistive losses due to the small dimensions of the metal structures.

To further lower the impact of resistivity at even more aggressively scaled dimensions, metal alloys are currently being explored. One of the grand challenges to implement these materials at an industrial scale is related to the surface chemical properties. For instance, differences in diffusion rates of the individual metal atoms during surface oxidation results in a deviating stoichiometry of the alloy in the near-surface region. Even minor changes in the alloy’s composition have a dramatic impact on the resistivity of nanoscale structures. Both the underlying chemical and physical processes involved are, to date, not well understood and characterized.

In this PhD proposal, wet-chemical atomic layer etching (ALE) will be explored as a method to obtain insight into the surface chemistry of metal alloys. ALE processes are typically based on a 2-step mechanism where self-limiting surface oxidation and oxide dissolution are time-separated. Depending on the surface oxide chemistry, also a 1-step mechanism can be used to achieve wet-ALE conditions. The proposed PhD topic will explore the etching of metal alloys in aqueous solutions, using Ni­₃Al as a model system. As a next step, other alloys (e.g. RuAl) can be investigated. Fundamental insight into the surface chemistry are key to achieve the requirement of wet-ALE; the ability to selectively control oxidation and dissolution rates at atomic-scale precision. Of special interest is the use of wet-ALE to control and/or restore surface stoichiometry.   

In the project, the PhD student will learn to work in a challenging, highly dynamic, multicultural environment in which industry-relevant work is performed. For the development of wet-chemical etching processes, surface (chemical) analysis methods (e.g. in situ XPS and high-resolution synchrotron radiation photoemission spectroscopy (SRPES)) will be used. For studying material dissolution rates at the Angstrom-level, inductively coupled mass spectrometry (ICP-MS) is indispensable. Surface morphology can be analysed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Eventually, the fundamental learning developed should be demonstrated on device structures.

Required background

We are looking for candidates which have a background in materials science with a strongly developed knowledge of chemistry and physics.