In situ transmission electron microscopy investigations of electromigration in metals

In situ transmission electron microscopy investigations of electromigration in metals

Kozlova, T.

Zandbergen, H.W. (promotor)

Applied Sciences

Quantum Nanoscience

2015-09-25

Electromigration is a process in which a metallic contact line is thinned by passing a current through it; which occurs due to a gradual displacement of atoms, ultimately leading to destruction of the wire. Despite the active investigations on electromigration for over fifty years, until now there is no general theory of the process and many open questions in understanding this process at a fundamental level still remain. In order to understand the material transport under an applied electric field, many parameters should be considered: size of the structure, material, temperature, stress, microstructure (size of the grains, type of grain boundaries), and surface effects. In this thesis, transmission electron microscopy (TEM) together with in situ electrical/heating measurements (which require special MEMS chips and TEM holders) was used for investigating electromigration in thin metallic (Pt, Pd–Pt alloy) nanobridges. This technique allows obtaining information about material under an applied electric field at atomic level, helping in understanding the process dynamics. In our experiments morphological transformations were recorded in real time at the nanoscale, along with the corresponding I–V while passing an electric current. By using scanning transmission electron microscopy (STEM) imaging with high angle annular dark ?eld (HAADF) detector, electromigration in Pd–Pt alloy nanobridges was shown to be quite different from the pure elements Pt and Pd. The electromigration of Pt and Pd is very similar: after recrystallization (also observed in Pd–Pt alloy), the bridge gradually becomes narrower and breaks. In case of alloy, the outer shape of the bridge is maintained while the material transport from the cathode to the anode is guided by the direction of electric field and is fully reversible. Material transport corresponds to the electron-wind force, indicating a negative effective charge. This principle of controllable reversible material transport in Pd–Pt alloy with the change in field polarity can be implemented in memristor applications. The material properties under both dynamic conditions, i.e. heating and electric current passage, were investigated in situ. We found increased resistivity in Pt nanobridges and correlated the variation in resistance with morphological changes while heating the polycrystalline Pt films. During electromigration experiments, the bridge resistance changes due to two processes, Joule heating and electromigration. By measuring the resistance variation upon heating the substrate, these processes can be decoupled. This study also yielded the importance of surrounding temperature on the nanobridge: Despite the same tendency of material transport from the cathode to anode, higher temperature of the surroundings enhances the nanobridge breakage. The higher temperature adds potential energy to the atoms so they can easily overcome the crystal lattice energy barrier and leave their original positions. In general, besides describing the failure due to electromigration extensively in this thesis, the scope of expanding these in situ heating/electrical measurements in the TEM to other studies has been presented. For instance, developed system can be used in molecule electronic device studies of electrical properties of nanoparticles or molecules in combination with morphological changes visualizations at atomic level. It can be also implemented in memory-switching devices investigations or in one of the hot topics of lithium-ion batteries.

electromigration
Transmission Electron Microscopy
in situ

https://doi.org/10.4233/uuid:10b2fb31-9352-4527-a0dd-7bded605196b

9789085932260

Institutional Repository

doctoral thesis

(c) 2015 Kozlova, T.