Electromigration-induced resistance switching in indented Al microstrips

The phenomenon of electromigration, i.e. the phenomenon of current-induced atomic migration in solids, is well known since the 60’s for causing failures in circuits’ interconnections through the formation of atomic depletions and/or accumulations. Initially, one of the major objectives of the study of this phenomenon was focused on preventing it. Later on, control methods were developed and allowed to master the destructive character of electromigration in order to create nanogaps, or modify the geometry or the material properties to fabricate point contacts.
One particularly appealing aspect that makes controlled electromigration to stand out, is the possibility to heal a previously electromigrated sample, by simply inverting the direction of the current (anti-electromigration). This unique feature has naturally fostered the idea of fabricating monolithic memory devices based on switching between two resistance states. Johnson et al. introduced a two-terminal hysteretic switch (memristor) in which the state variable of the resistance was the system's physical geometry.
However, the successive application of electromigration/anti-electromigration allowing to switch the state of a memristive device is not guaranteed since many factors such as the granular structure of the material or the toggle frequency have to be taken into account. Among them, the effects of thermal fatigue can also alter the initial material structure and cause undesired premature switching.
In this work, we investigate the electromigration-induced resistance switching in pre-indented Al microstrips. In order to guarantee a large switching endurance, we limited the on-to-top ratio to a minimum readable value. Two switching protocols were tested, (i) a variable current pulse amplitude adjusted to ensure a precise change of resistance, and (ii) a fixed current pulse amplitude. Both approaches exhibit an initial training period where the mean value of the device's resistance drifts in time, followed by a more stable behavior. Electron microscopy imaging of the devices, supported by simulations using finite elements methods of the local temperature, show irreversible changes of the material properties from the early stages of the switching process. High and low resistance states show retention times of days and endurances of ~103 min.