Electrical Reliability of Twinned Metallic Nanowires

In today’s technology, the transistors have reached such a small size that it is increasingly costly to shrink them more. Therefore, the industry is leaning towards increased functionality of the devices rather than miniaturizing. Thus, new technologies are emerging e.g., stretchable, and wearable electronics. In addition to the high current density (~107 A/cm2) requirement due to smaller size, these technologies require excellent mechanical and optical properties. Metallic nanowires are excellent candidates for these devices meeting all the requirements. However, as surface to volume ratio increases, in nanoscale, the energy carriers- phonons (heat) and electrons (electricity), are scattered increasingly. Additionally, diffusion through the grain boundaries also increases. Moreover, a significant portion of these devices use random networks of nanowires, where conduction is not uniform, leading to more current density in some of its members, and hence localized failure of individual nanowires. Thus, assessing the electrical reliability of individual nanowires is needed. The reliability can be assessed by characterizing- 1) Joule heating and 2) electromigration. Studies show that the presence of a twin boundary across the path of an atomic vacancy slows down its diffusion. Since electromigration is a vacancy diffusion process, we are interested in studying the failure of twinned nanowires. However, silver nanowires are more prone to degradation over time and electromigration tests can go on for weeks. Hence, silver nanowires were selected for Joule heating study, which does not require longevity, and gold nanowires were selected for electromigration study. Thus, the goals of this dissertation are as follows:

1. To quantify the failure current densities of silver nanowires in relation to diameter with theoretical validation.
2. To quantify the median time to failures by electromigration of twinned gold nanowires at different temperatures.
3. To compare the electromigration data between twinned gold nanowires and other interconnects with grain boundaries, to observe the effect of twins on electromigration. In the Joule heating study of silver nanowires, we established the behavior of their failure current density against diameter for 93 samples. Heat transfer modelling was employed to explain the results, and Weibull statistics were used to quantify failure probabilities. The scatter observed in the measurements was attributed to surface roughness variations. The results quantify the Joule heating electrical reliability of silver nanowires and highlight the importance of heat transfer in increasing it. In the electromigration study of gold nanowires, we studied 30 samples at 3 temperatures. The median time to failure at each temperature has a lognormal distribution that can be described with maximum likelihood estimation. Electromigration activation energy was determined using Black’s equation. This is an important indicator of the modes of diffusion and the results highlight the importance of passivating the surface of nanowires to reduce electromigration. The study is divided into 5 chapters. Chapter 1 provides a background. Chapter 2 describes the experimental procedures. The results from Joule heating test are shown in Chapter 3. Chapter 4 shows the results from electromigration test. Chapter 5 shows the direction of future work.