Magnetic materials: atomic layer etching and passivation

Transition metals are becoming increasingly important in the field of spintronics. The technology is recognized as a new paradigm that can replace and be adopted in conventional electronic devices for semiconductor, storage, biomedical, and automotive applications. Moreover, spintronic devices are finding further utilization for the internet of things (IoT) and the wireless industry owing to the remarkable performance characteristics such as nonvolatility, high-speed read and write operation, and cost benefit in term of productivity. Its low power consumption, compared to traditional electron-based Si CMOS, will make our e-life more energy-efficient and therefore more environmentally sustainable.

For spintronic device fabrication, patterning of stacked metal layers is a critical step. Various nanofabrication techniques have been investigated of which ion beam etching has proven successful. However, this dry etching technique can cause shorting across a tunnel barrier due to redeposition of metal atoms because of the formation of nonvolatile etching products. An ion beam impinging on the sample at a tilted angle can be used to remove these atomic residues, but this technique does not scale well with the ever-increasing density of device structures. Halogen plasmas have also been investigated, and they are known to yield high etch rates. However, the etched metal byproducts produced by the plasma tend to be also nonvolatile at low temperatures, which degrades device performance. By contrast, atomic layer etching (ALE) offers a simple and attractive solution which can avoid these problems. Atomic layer etching processes are typically based on a two-step mechanism where self-limiting surface oxidation and oxide product removal are time-separated. Depending on the surface oxide chemistry, also a 1-step mechanism can be used to achieve wet-ALE conditions. The target materials will be at start pure elements such as Ru, Ni, Co, Fe and will be then extended to metal alloys such as RuAl and Ni₃Al. Of special interest is the use of solvent-based chelating chemistries to enable surface modification and evaporation of oxide product layers. Such a thermal ALE process is essential to maintain the magnetic properties of the device due to the hygroscopic nature of the MgO/CoFeB stack. Fundamental insights in the surface chemical mechanisms are key to selectively control oxidation and etch rates with atomic-scale resolution. Finally, it is of tremendous importance, after etching, to passivate in-situ the device so as to avoid ambient oxidation of the junction – this will be also explored in the frame of the PhD work.

As a PhD student, you will learn to work in a highly dynamic and multicultural environment and be exposed to a large variety of analytical techniques and experimental methods. Inductively coupled plasma mass spectrometry (ICP-MS) will be used to study etching kinetics in both the liquid and gas phase. These experiments are complemented with electrochemical measurements to study the metal/electrolyte interface in the parameter space. Surface chemistry and physics will be studied both ex situ and post operando by x-ray photoelectron spectroscopy (XPS) and high-resolution synchrotron radiation photoemission spectroscopy (SRPES). Other complementary physical characterization techniques like elastic recoil detection analysis (ERDA), electrical measurements, atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM, TEM) are available to support your mechanistic studies on wet-ALE processing. The magnetic characteristics of selected samples will be measured by vibrating sample magnetometry (VSM) and magneto-optic Kerr effect (MOKE).